Python socket error class

18.1. socket — Low-level networking interface¶

Source code: Lib/socket.py

This module provides access to the BSD socket interface. It is available on all modern Unix systems, Windows, MacOS, and probably additional platforms.

Some behavior may be platform dependent, since calls are made to the operating system socket APIs.

The Python interface is a straightforward transliteration of the Unix system call and library interface for sockets to Python’s object-oriented style: the socket() function returns a socket object whose methods implement the various socket system calls. Parameter types are somewhat higher-level than in the C interface: as with read() and write() operations on Python files, buffer allocation on receive operations is automatic, and buffer length is implicit on send operations.

Module socketserver Classes that simplify writing network servers. Module ssl A TLS/SSL wrapper for socket objects.

18.1.1. Socket families¶

Depending on the system and the build options, various socket families are supported by this module.

The address format required by a particular socket object is automatically selected based on the address family specified when the socket object was created. Socket addresses are represented as follows:

The address of an AF_UNIX socket bound to a file system node is represented as a string, using the file system encoding and the ‘surrogateescape’ error handler (see PEP 383). An address in Linux’s abstract namespace is returned as a bytes-like object with an initial null byte; note that sockets in this namespace can communicate with normal file system sockets, so programs intended to run on Linux may need to deal with both types of address. A string or bytes-like object can be used for either type of address when passing it as an argument.

Changed in version 3.3: Previously, AF_UNIX socket paths were assumed to use UTF-8 encoding.

Changed in version 3.5: Writable bytes-like object is now accepted.

A pair (host, port) is used for the AF_INET address family, where host is a string representing either a hostname in Internet domain notation like ‘daring.cwi.nl’ or an IPv4 address like ‘100.50.200.5’ , and port is an integer.

For AF_INET6 address family, a four-tuple (host, port, flowinfo, scopeid) is used, where flowinfo and scopeid represent the sin6_flowinfo and sin6_scope_id members in struct sockaddr_in6 in C. For socket module methods, flowinfo and scopeid can be omitted just for backward compatibility. Note, however, omission of scopeid can cause problems in manipulating scoped IPv6 addresses.

AF_NETLINK sockets are represented as pairs (pid, groups) .

Linux-only support for TIPC is available using the AF_TIPC address family. TIPC is an open, non-IP based networked protocol designed for use in clustered computer environments. Addresses are represented by a tuple, and the fields depend on the address type. The general tuple form is (addr_type, v1, v2, v3 [, scope]) , where:

addr_type is one of TIPC_ADDR_NAMESEQ , TIPC_ADDR_NAME , or TIPC_ADDR_ID .

scope is one of TIPC_ZONE_SCOPE , TIPC_CLUSTER_SCOPE , and TIPC_NODE_SCOPE .

If addr_type is TIPC_ADDR_NAME , then v1 is the server type, v2 is the port identifier, and v3 should be 0.

If addr_type is TIPC_ADDR_NAMESEQ , then v1 is the server type, v2 is the lower port number, and v3 is the upper port number.

If addr_type is TIPC_ADDR_ID , then v1 is the node, v2 is the reference, and v3 should be set to 0.

A tuple (interface, ) is used for the AF_CAN address family, where interface is a string representing a network interface name like ‘can0’ . The network interface name » can be used to receive packets from all network interfaces of this family.

• CAN_ISOTP protocol require a tuple (interface, rx_addr, tx_addr) where both additional parameters are unsigned long integer that represent a CAN identifier (standard or extended).

A string or a tuple (id, unit) is used for the SYSPROTO_CONTROL protocol of the PF_SYSTEM family. The string is the name of a kernel control using a dynamically-assigned ID. The tuple can be used if ID and unit number of the kernel control are known or if a registered ID is used.

New in version 3.3.

AF_BLUETOOTH supports the following protocols and address formats:

BTPROTO_L2CAP accepts (bdaddr, psm) where bdaddr is the Bluetooth address as a string and psm is an integer.

BTPROTO_RFCOMM accepts (bdaddr, channel) where bdaddr is the Bluetooth address as a string and channel is an integer.

BTPROTO_HCI accepts (device_id,) where device_id is either an integer or a string with the Bluetooth address of the interface. (This depends on your OS; NetBSD and DragonFlyBSD expect a Bluetooth address while everything else expects an integer.)

Changed in version 3.2: NetBSD and DragonFlyBSD support added.

BTPROTO_SCO accepts bdaddr where bdaddr is a bytes object containing the Bluetooth address in a string format. (ex. b’12:23:34:45:56:67′ ) This protocol is not supported under FreeBSD.

AF_ALG is a Linux-only socket based interface to Kernel cryptography. An algorithm socket is configured with a tuple of two to four elements (type, name [, feat [, mask]]) , where:

• type is the algorithm type as string, e.g. aead , hash , skcipher or rng .
• name is the algorithm name and operation mode as string, e.g. sha256 , hmac(sha256) , cbc(aes) or drbg_nopr_ctr_aes256 .
• feat and mask are unsigned 32bit integers.

Availability Linux 2.6.38, some algorithm types require more recent Kernels.

New in version 3.6.

AF_VSOCK allows communication between virtual machines and their hosts. The sockets are represented as a (CID, port) tuple where the context ID or CID and port are integers.

Availability: Linux >= 4.8 QEMU >= 2.8 ESX >= 4.0 ESX Workstation >= 6.5

New in version 3.7.

Certain other address families ( AF_PACKET , AF_CAN ) support specific representations.

For IPv4 addresses, two special forms are accepted instead of a host address: the empty string represents INADDR_ANY , and the string ‘
‘ represents INADDR_BROADCAST . This behavior is not compatible with IPv6, therefore, you may want to avoid these if you intend to support IPv6 with your Python programs.

If you use a hostname in the host portion of IPv4/v6 socket address, the program may show a nondeterministic behavior, as Python uses the first address returned from the DNS resolution. The socket address will be resolved differently into an actual IPv4/v6 address, depending on the results from DNS resolution and/or the host configuration. For deterministic behavior use a numeric address in host portion.

All errors raise exceptions. The normal exceptions for invalid argument types and out-of-memory conditions can be raised; starting from Python 3.3, errors related to socket or address semantics raise OSError or one of its subclasses (they used to raise socket.error ).

Non-blocking mode is supported through setblocking() . A generalization of this based on timeouts is supported through settimeout() .

18.1.2. Module contents¶

The module socket exports the following elements.

18.1.2.1. Exceptions¶

A deprecated alias of OSError .

Changed in version 3.3: Following PEP 3151, this class was made an alias of OSError .

A subclass of OSError , this exception is raised for address-related errors, i.e. for functions that use h_errno in the POSIX C API, including gethostbyname_ex() and gethostbyaddr() . The accompanying value is a pair (h_errno, string) representing an error returned by a library call. h_errno is a numeric value, while string represents the description of h_errno, as returned by the hstrerror() C function.

Changed in version 3.3: This class was made a subclass of OSError .

A subclass of OSError , this exception is raised for address-related errors by getaddrinfo() and getnameinfo() . The accompanying value is a pair (error, string) representing an error returned by a library call. string represents the description of error, as returned by the gai_strerror() C function. The numeric error value will match one of the EAI_* constants defined in this module.

Changed in version 3.3: This class was made a subclass of OSError .

A subclass of OSError , this exception is raised when a timeout occurs on a socket which has had timeouts enabled via a prior call to settimeout() (or implicitly through setdefaulttimeout() ). The accompanying value is a string whose value is currently always “timed out”.

Changed in version 3.3: This class was made a subclass of OSError .

18.1.2.2. Constants¶

The AF_* and SOCK_* constants are now AddressFamily and SocketKind IntEnum collections.

New in version 3.4.

These constants represent the address (and protocol) families, used for the first argument to socket() . If the AF_UNIX constant is not defined then this protocol is unsupported. More constants may be available depending on the system.

socket. SOCK_STREAM ¶ socket. SOCK_DGRAM ¶ socket. SOCK_RAW ¶ socket. SOCK_RDM ¶ socket. SOCK_SEQPACKET ¶

These constants represent the socket types, used for the second argument to socket() . More constants may be available depending on the system. (Only SOCK_STREAM and SOCK_DGRAM appear to be generally useful.)

socket. SOCK_CLOEXEC ¶ socket. SOCK_NONBLOCK ¶

These two constants, if defined, can be combined with the socket types and allow you to set some flags atomically (thus avoiding possible race conditions and the need for separate calls).

Availability: Linux >= 2.6.27.

New in version 3.2.

Many constants of these forms, documented in the Unix documentation on sockets and/or the IP protocol, are also defined in the socket module. They are generally used in arguments to the setsockopt() and getsockopt() methods of socket objects. In most cases, only those symbols that are defined in the Unix header files are defined; for a few symbols, default values are provided.

Changed in version 3.6: SO_DOMAIN , SO_PROTOCOL , SO_PEERSEC , SO_PASSSEC , TCP_USER_TIMEOUT , TCP_CONGESTION were added.

Changed in version 3.7: TCP_NOTSENT_LOWAT was added.

Many constants of these forms, documented in the Linux documentation, are also defined in the socket module.

Availability: Linux >= 2.6.25.

New in version 3.3.

CAN_BCM, in the CAN protocol family, is the broadcast manager (BCM) protocol. Broadcast manager constants, documented in the Linux documentation, are also defined in the socket module.

Availability: Linux >= 2.6.25.

New in version 3.4.

Enables CAN FD support in a CAN_RAW socket. This is disabled by default. This allows your application to send both CAN and CAN FD frames; however, you one must accept both CAN and CAN FD frames when reading from the socket.

This constant is documented in the Linux documentation.

Availability: Linux >= 3.6.

New in version 3.5.

CAN_ISOTP, in the CAN protocol family, is the ISO-TP (ISO 15765-2) protocol. ISO-TP constants, documented in the Linux documentation.

Availability: Linux >= 2.6.25

New in version 3.7.

Many constants of these forms, documented in the Linux documentation, are also defined in the socket module.

Availability: Linux >= 2.6.30.

New in version 3.3.

Constants for Windows’ WSAIoctl(). The constants are used as arguments to the ioctl() method of socket objects.

Changed in version 3.6: SIO_LOOPBACK_FAST_PATH was added.

TIPC related constants, matching the ones exported by the C socket API. See the TIPC documentation for more information.

socket. AF_ALG ¶ socket. SOL_ALG ¶ ALG_*

Constants for Linux Kernel cryptography.

Availability: Linux >= 2.6.38.

New in version 3.6.

Constants for Linux host/guest communication.

Availability: Linux >= 4.8.

New in version 3.7.

Availability: BSD, OSX.

New in version 3.4.

This constant contains a boolean value which indicates if IPv6 is supported on this platform.

socket. BDADDR_ANY ¶ socket. BDADDR_LOCAL ¶

These are string constants containing Bluetooth addresses with special meanings. For example, BDADDR_ANY can be used to indicate any address when specifying the binding socket with BTPROTO_RFCOMM .

socket. HCI_FILTER ¶ socket. HCI_TIME_STAMP ¶ socket. HCI_DATA_DIR ¶

For use with BTPROTO_HCI . HCI_FILTER is not available for NetBSD or DragonFlyBSD. HCI_TIME_STAMP and HCI_DATA_DIR are not available for FreeBSD, NetBSD, or DragonFlyBSD.

18.1.2.3. Functions¶

18.1.2.3.1. Creating sockets¶

The following functions all create socket objects .

socket. socket ( family=AF_INET, type=SOCK_STREAM, proto=0, fileno=None ) ¶

Create a new socket using the given address family, socket type and protocol number. The address family should be AF_INET (the default), AF_INET6 , AF_UNIX , AF_CAN or AF_RDS . The socket type should be SOCK_STREAM (the default), SOCK_DGRAM , SOCK_RAW or perhaps one of the other SOCK_ constants. The protocol number is usually zero and may be omitted or in the case where the address family is AF_CAN the protocol should be one of CAN_RAW , CAN_BCM or CAN_ISOTP . If fileno is specified, the other arguments are ignored, causing the socket with the specified file descriptor to return. Unlike socket.fromfd() , fileno will return the same socket and not a duplicate. This may help close a detached socket using socket.close() .

The newly created socket is non-inheritable .

Changed in version 3.3: The AF_CAN family was added. The AF_RDS family was added.

Changed in version 3.4: The CAN_BCM protocol was added.

Changed in version 3.4: The returned socket is now non-inheritable.

Changed in version 3.7: The CAN_ISOTP protocol was added.

Build a pair of connected socket objects using the given address family, socket type, and protocol number. Address family, socket type, and protocol number are as for the socket() function above. The default family is AF_UNIX if defined on the platform; otherwise, the default is AF_INET .

The newly created sockets are non-inheritable .

Changed in version 3.2: The returned socket objects now support the whole socket API, rather than a subset.

Changed in version 3.4: The returned sockets are now non-inheritable.

Changed in version 3.5: Windows support added.

Connect to a TCP service listening on the Internet address (a 2-tuple (host, port) ), and return the socket object. This is a higher-level function than socket.connect() : if host is a non-numeric hostname, it will try to resolve it for both AF_INET and AF_INET6 , and then try to connect to all possible addresses in turn until a connection succeeds. This makes it easy to write clients that are compatible to both IPv4 and IPv6.

Passing the optional timeout parameter will set the timeout on the socket instance before attempting to connect. If no timeout is supplied, the global default timeout setting returned by getdefaulttimeout() is used.

If supplied, source_address must be a 2-tuple (host, port) for the socket to bind to as its source address before connecting. If host or port are ‘’ or 0 respectively the OS default behavior will be used.

Changed in version 3.2: source_address was added.

Duplicate the file descriptor fd (an integer as returned by a file object’s fileno() method) and build a socket object from the result. Address family, socket type and protocol number are as for the socket() function above. The file descriptor should refer to a socket, but this is not checked — subsequent operations on the object may fail if the file descriptor is invalid. This function is rarely needed, but can be used to get or set socket options on a socket passed to a program as standard input or output (such as a server started by the Unix inet daemon). The socket is assumed to be in blocking mode.

The newly created socket is non-inheritable .

Changed in version 3.4: The returned socket is now non-inheritable.

Instantiate a socket from data obtained from the socket.share() method. The socket is assumed to be in blocking mode.

New in version 3.3.

This is a Python type object that represents the socket object type. It is the same as type(socket(. )) .

18.1.2.3.2. Other functions¶

The socket module also offers various network-related services:

Translate the host/port argument into a sequence of 5-tuples that contain all the necessary arguments for creating a socket connected to that service. host is a domain name, a string representation of an IPv4/v6 address or None . port is a string service name such as ‘http’ , a numeric port number or None . By passing None as the value of host and port, you can pass NULL to the underlying C API.

The family, type and proto arguments can be optionally specified in order to narrow the list of addresses returned. Passing zero as a value for each of these arguments selects the full range of results. The flags argument can be one or several of the AI_* constants, and will influence how results are computed and returned. For example, AI_NUMERICHOST will disable domain name resolution and will raise an error if host is a domain name.

The function returns a list of 5-tuples with the following structure:

(family, type, proto, canonname, sockaddr)

In these tuples, family, type, proto are all integers and are meant to be passed to the socket() function. canonname will be a string representing the canonical name of the host if AI_CANONNAME is part of the flags argument; else canonname will be empty. sockaddr is a tuple describing a socket address, whose format depends on the returned family (a (address, port) 2-tuple for AF_INET , a (address, port, flow info, scope id) 4-tuple for AF_INET6 ), and is meant to be passed to the socket.connect() method.

The following example fetches address information for a hypothetical TCP connection to example.org on port 80 (results may differ on your system if IPv6 isn’t enabled):

Changed in version 3.2: parameters can now be passed using keyword arguments.

Return a fully qualified domain name for name. If name is omitted or empty, it is interpreted as the local host. To find the fully qualified name, the hostname returned by gethostbyaddr() is checked, followed by aliases for the host, if available. The first name which includes a period is selected. In case no fully qualified domain name is available, the hostname as returned by gethostname() is returned.

socket. gethostbyname ( hostname ) ¶

Translate a host name to IPv4 address format. The IPv4 address is returned as a string, such as ‘100.50.200.5’ . If the host name is an IPv4 address itself it is returned unchanged. See gethostbyname_ex() for a more complete interface. gethostbyname() does not support IPv6 name resolution, and getaddrinfo() should be used instead for IPv4/v6 dual stack support.

socket. gethostbyname_ex ( hostname ) ¶

Translate a host name to IPv4 address format, extended interface. Return a triple (hostname, aliaslist, ipaddrlist) where hostname is the primary host name responding to the given ip_address, aliaslist is a (possibly empty) list of alternative host names for the same address, and ipaddrlist is a list of IPv4 addresses for the same interface on the same host (often but not always a single address). gethostbyname_ex() does not support IPv6 name resolution, and getaddrinfo() should be used instead for IPv4/v6 dual stack support.

Return a string containing the hostname of the machine where the Python interpreter is currently executing.

Note: gethostname() doesn’t always return the fully qualified domain name; use getfqdn() for that.

socket. gethostbyaddr ( ip_address ) ¶

Return a triple (hostname, aliaslist, ipaddrlist) where hostname is the primary host name responding to the given ip_address, aliaslist is a (possibly empty) list of alternative host names for the same address, and ipaddrlist is a list of IPv4/v6 addresses for the same interface on the same host (most likely containing only a single address). To find the fully qualified domain name, use the function getfqdn() . gethostbyaddr() supports both IPv4 and IPv6.

socket. getnameinfo ( sockaddr, flags ) ¶

Translate a socket address sockaddr into a 2-tuple (host, port) . Depending on the settings of flags, the result can contain a fully-qualified domain name or numeric address representation in host. Similarly, port can contain a string port name or a numeric port number.

socket. getprotobyname ( protocolname ) ¶

Translate an Internet protocol name (for example, ‘icmp’ ) to a constant suitable for passing as the (optional) third argument to the socket() function. This is usually only needed for sockets opened in “raw” mode ( SOCK_RAW ); for the normal socket modes, the correct protocol is chosen automatically if the protocol is omitted or zero.

socket. getservbyname ( servicename [ , protocolname ] ) ¶

Translate an Internet service name and protocol name to a port number for that service. The optional protocol name, if given, should be ‘tcp’ or ‘udp’ , otherwise any protocol will match.

socket. getservbyport ( port [ , protocolname ] ) ¶

Translate an Internet port number and protocol name to a service name for that service. The optional protocol name, if given, should be ‘tcp’ or ‘udp’ , otherwise any protocol will match.

Convert 32-bit positive integers from network to host byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 4-byte swap operation.

Convert 16-bit positive integers from network to host byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 2-byte swap operation.

Deprecated since version 3.7: In case x does not fit in 16-bit unsigned integer, but does fit in a positive C int, it is silently truncated to 16-bit unsigned integer. This silent truncation feature is deprecated, and will raise an exception in future versions of Python.

Convert 32-bit positive integers from host to network byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 4-byte swap operation.

Convert 16-bit positive integers from host to network byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 2-byte swap operation.

Deprecated since version 3.7: In case x does not fit in 16-bit unsigned integer, but does fit in a positive C int, it is silently truncated to 16-bit unsigned integer. This silent truncation feature is deprecated, and will raise an exception in future versions of Python.

Convert an IPv4 address from dotted-quad string format (for example, ‘123.45.67.89’) to 32-bit packed binary format, as a bytes object four characters in length. This is useful when conversing with a program that uses the standard C library and needs objects of type struct in_addr , which is the C type for the 32-bit packed binary this function returns.

inet_aton() also accepts strings with less than three dots; see the Unix manual page inet(3) for details.

If the IPv4 address string passed to this function is invalid, OSError will be raised. Note that exactly what is valid depends on the underlying C implementation of inet_aton() .

inet_aton() does not support IPv6, and inet_pton() should be used instead for IPv4/v6 dual stack support.

socket. inet_ntoa ( packed_ip ) ¶

Convert a 32-bit packed IPv4 address (a bytes-like object four bytes in length) to its standard dotted-quad string representation (for example, ‘123.45.67.89’). This is useful when conversing with a program that uses the standard C library and needs objects of type struct in_addr , which is the C type for the 32-bit packed binary data this function takes as an argument.

If the byte sequence passed to this function is not exactly 4 bytes in length, OSError will be raised. inet_ntoa() does not support IPv6, and inet_ntop() should be used instead for IPv4/v6 dual stack support.

Changed in version 3.5: Writable bytes-like object is now accepted.

Convert an IP address from its family-specific string format to a packed, binary format. inet_pton() is useful when a library or network protocol calls for an object of type struct in_addr (similar to inet_aton() ) or struct in6_addr .

Supported values for address_family are currently AF_INET and AF_INET6 . If the IP address string ip_string is invalid, OSError will be raised. Note that exactly what is valid depends on both the value of address_family and the underlying implementation of inet_pton() .

Availability: Unix (maybe not all platforms), Windows.

Changed in version 3.4: Windows support added

Convert a packed IP address (a bytes-like object of some number of bytes) to its standard, family-specific string representation (for example, ‘7.10.0.5’ or ‘5aef:2b::8’ ). inet_ntop() is useful when a library or network protocol returns an object of type struct in_addr (similar to inet_ntoa() ) or struct in6_addr .

Supported values for address_family are currently AF_INET and AF_INET6 . If the bytes object packed_ip is not the correct length for the specified address family, ValueError will be raised. OSError is raised for errors from the call to inet_ntop() .

Availability: Unix (maybe not all platforms), Windows.

Changed in version 3.4: Windows support added

Changed in version 3.5: Writable bytes-like object is now accepted.

Return the total length, without trailing padding, of an ancillary data item with associated data of the given length. This value can often be used as the buffer size for recvmsg() to receive a single item of ancillary data, but RFC 3542 requires portable applications to use CMSG_SPACE() and thus include space for padding, even when the item will be the last in the buffer. Raises OverflowError if length is outside the permissible range of values.

Availability: most Unix platforms, possibly others.

New in version 3.3.

Return the buffer size needed for recvmsg() to receive an ancillary data item with associated data of the given length, along with any trailing padding. The buffer space needed to receive multiple items is the sum of the CMSG_SPACE() values for their associated data lengths. Raises OverflowError if length is outside the permissible range of values.

Note that some systems might support ancillary data without providing this function. Also note that setting the buffer size using the results of this function may not precisely limit the amount of ancillary data that can be received, since additional data may be able to fit into the padding area.

Availability: most Unix platforms, possibly others.

New in version 3.3.

Return the default timeout in seconds (float) for new socket objects. A value of None indicates that new socket objects have no timeout. When the socket module is first imported, the default is None .

socket. setdefaulttimeout ( timeout ) ¶

Set the default timeout in seconds (float) for new socket objects. When the socket module is first imported, the default is None . See settimeout() for possible values and their respective meanings.

socket. sethostname ( name ) ¶

Set the machine’s hostname to name. This will raise an OSError if you don’t have enough rights.

New in version 3.3.

Return a list of network interface information (index int, name string) tuples. OSError if the system call fails.

New in version 3.3.

Return a network interface index number corresponding to an interface name. OSError if no interface with the given name exists.

New in version 3.3.

Return a network interface name corresponding to an interface index number. OSError if no interface with the given index exists.

New in version 3.3.

18.1.3. Socket Objects¶

Socket objects have the following methods. Except for makefile() , these correspond to Unix system calls applicable to sockets.

Changed in version 3.2: Support for the context manager protocol was added. Exiting the context manager is equivalent to calling close() .

Accept a connection. The socket must be bound to an address and listening for connections. The return value is a pair (conn, address) where conn is a new socket object usable to send and receive data on the connection, and address is the address bound to the socket on the other end of the connection.

The newly created socket is non-inheritable .

Changed in version 3.4: The socket is now non-inheritable.

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Bind the socket to address. The socket must not already be bound. (The format of address depends on the address family — see above.)

Mark the socket closed. The underlying system resource (e.g. a file descriptor) is also closed when all file objects from makefile() are closed. Once that happens, all future operations on the socket object will fail. The remote end will receive no more data (after queued data is flushed).

Sockets are automatically closed when they are garbage-collected, but it is recommended to close() them explicitly, or to use a with statement around them.

Changed in version 3.6: OSError is now raised if an error occurs when the underlying close() call is made.

close() releases the resource associated with a connection but does not necessarily close the connection immediately. If you want to close the connection in a timely fashion, call shutdown() before close() .

Connect to a remote socket at address. (The format of address depends on the address family — see above.)

If the connection is interrupted by a signal, the method waits until the connection completes, or raise a socket.timeout on timeout, if the signal handler doesn’t raise an exception and the socket is blocking or has a timeout. For non-blocking sockets, the method raises an InterruptedError exception if the connection is interrupted by a signal (or the exception raised by the signal handler).

Changed in version 3.5: The method now waits until the connection completes instead of raising an InterruptedError exception if the connection is interrupted by a signal, the signal handler doesn’t raise an exception and the socket is blocking or has a timeout (see the PEP 475 for the rationale).

Like connect(address) , but return an error indicator instead of raising an exception for errors returned by the C-level connect() call (other problems, such as “host not found,” can still raise exceptions). The error indicator is 0 if the operation succeeded, otherwise the value of the errno variable. This is useful to support, for example, asynchronous connects.

Put the socket object into closed state without actually closing the underlying file descriptor. The file descriptor is returned, and can be reused for other purposes.

New in version 3.2.

Duplicate the socket.

The newly created socket is non-inheritable .

Changed in version 3.4: The socket is now non-inheritable.

Return the socket’s file descriptor (a small integer), or -1 on failure. This is useful with select.select() .

Under Windows the small integer returned by this method cannot be used where a file descriptor can be used (such as os.fdopen() ). Unix does not have this limitation.

Get the inheritable flag of the socket’s file descriptor or socket’s handle: True if the socket can be inherited in child processes, False if it cannot.

New in version 3.4.

Return the remote address to which the socket is connected. This is useful to find out the port number of a remote IPv4/v6 socket, for instance. (The format of the address returned depends on the address family — see above.) On some systems this function is not supported.

Return the socket’s own address. This is useful to find out the port number of an IPv4/v6 socket, for instance. (The format of the address returned depends on the address family — see above.)

socket. getsockopt ( level, optname [ , buflen ] ) ¶

Return the value of the given socket option (see the Unix man page getsockopt(2)). The needed symbolic constants ( SO_* etc.) are defined in this module. If buflen is absent, an integer option is assumed and its integer value is returned by the function. If buflen is present, it specifies the maximum length of the buffer used to receive the option in, and this buffer is returned as a bytes object. It is up to the caller to decode the contents of the buffer (see the optional built-in module struct for a way to decode C structures encoded as byte strings).

Return the timeout in seconds (float) associated with socket operations, or None if no timeout is set. This reflects the last call to setblocking() or settimeout() .

socket. ioctl ( control, option ) ¶

The ioctl() method is a limited interface to the WSAIoctl system interface. Please refer to the Win32 documentation for more information.

On other platforms, the generic fcntl.fcntl() and fcntl.ioctl() functions may be used; they accept a socket object as their first argument.

Currently only the following control codes are supported: SIO_RCVALL , SIO_KEEPALIVE_VALS , and SIO_LOOPBACK_FAST_PATH .

Changed in version 3.6: SIO_LOOPBACK_FAST_PATH was added.

Enable a server to accept connections. If backlog is specified, it must be at least 0 (if it is lower, it is set to 0); it specifies the number of unaccepted connections that the system will allow before refusing new connections. If not specified, a default reasonable value is chosen.

Changed in version 3.5: The backlog parameter is now optional.

Return a file object associated with the socket. The exact returned type depends on the arguments given to makefile() . These arguments are interpreted the same way as by the built-in open() function, except the only supported mode values are ‘r’ (default), ‘w’ and ‘b’ .

The socket must be in blocking mode; it can have a timeout, but the file object’s internal buffer may end up in an inconsistent state if a timeout occurs.

Closing the file object returned by makefile() won’t close the original socket unless all other file objects have been closed and socket.close() has been called on the socket object.

On Windows, the file-like object created by makefile() cannot be used where a file object with a file descriptor is expected, such as the stream arguments of subprocess.Popen() .

Receive data from the socket. The return value is a bytes object representing the data received. The maximum amount of data to be received at once is specified by bufsize. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero.

For best match with hardware and network realities, the value of bufsize should be a relatively small power of 2, for example, 4096.

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Receive data from the socket. The return value is a pair (bytes, address) where bytes is a bytes object representing the data received and address is the address of the socket sending the data. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero. (The format of address depends on the address family — see above.)

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Receive normal data (up to bufsize bytes) and ancillary data from the socket. The ancbufsize argument sets the size in bytes of the internal buffer used to receive the ancillary data; it defaults to 0, meaning that no ancillary data will be received. Appropriate buffer sizes for ancillary data can be calculated using CMSG_SPACE() or CMSG_LEN() , and items which do not fit into the buffer might be truncated or discarded. The flags argument defaults to 0 and has the same meaning as for recv() .

The return value is a 4-tuple: (data, ancdata, msg_flags, address) . The data item is a bytes object holding the non-ancillary data received. The ancdata item is a list of zero or more tuples (cmsg_level, cmsg_type, cmsg_data) representing the ancillary data (control messages) received: cmsg_level and cmsg_type are integers specifying the protocol level and protocol-specific type respectively, and cmsg_data is a bytes object holding the associated data. The msg_flags item is the bitwise OR of various flags indicating conditions on the received message; see your system documentation for details. If the receiving socket is unconnected, address is the address of the sending socket, if available; otherwise, its value is unspecified.

On some systems, sendmsg() and recvmsg() can be used to pass file descriptors between processes over an AF_UNIX socket. When this facility is used (it is often restricted to SOCK_STREAM sockets), recvmsg() will return, in its ancillary data, items of the form (socket.SOL_SOCKET, socket.SCM_RIGHTS, fds) , where fds is a bytes object representing the new file descriptors as a binary array of the native C int type. If recvmsg() raises an exception after the system call returns, it will first attempt to close any file descriptors received via this mechanism.

Some systems do not indicate the truncated length of ancillary data items which have been only partially received. If an item appears to extend beyond the end of the buffer, recvmsg() will issue a RuntimeWarning , and will return the part of it which is inside the buffer provided it has not been truncated before the start of its associated data.

On systems which support the SCM_RIGHTS mechanism, the following function will receive up to maxfds file descriptors, returning the message data and a list containing the descriptors (while ignoring unexpected conditions such as unrelated control messages being received). See also sendmsg() .

Availability: most Unix platforms, possibly others.

New in version 3.3.

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Receive normal data and ancillary data from the socket, behaving as recvmsg() would, but scatter the non-ancillary data into a series of buffers instead of returning a new bytes object. The buffers argument must be an iterable of objects that export writable buffers (e.g. bytearray objects); these will be filled with successive chunks of the non-ancillary data until it has all been written or there are no more buffers. The operating system may set a limit ( sysconf() value SC_IOV_MAX ) on the number of buffers that can be used. The ancbufsize and flags arguments have the same meaning as for recvmsg() .

The return value is a 4-tuple: (nbytes, ancdata, msg_flags, address) , where nbytes is the total number of bytes of non-ancillary data written into the buffers, and ancdata, msg_flags and address are the same as for recvmsg() .

Availability: most Unix platforms, possibly others.

New in version 3.3.

Receive data from the socket, writing it into buffer instead of creating a new bytestring. The return value is a pair (nbytes, address) where nbytes is the number of bytes received and address is the address of the socket sending the data. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero. (The format of address depends on the address family — see above.)

socket. recv_into ( buffer [ , nbytes [ , flags ] ] ) ¶

Receive up to nbytes bytes from the socket, storing the data into a buffer rather than creating a new bytestring. If nbytes is not specified (or 0), receive up to the size available in the given buffer. Returns the number of bytes received. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero.

socket. send ( bytes [ , flags ] ) ¶

Send data to the socket. The socket must be connected to a remote socket. The optional flags argument has the same meaning as for recv() above. Returns the number of bytes sent. Applications are responsible for checking that all data has been sent; if only some of the data was transmitted, the application needs to attempt delivery of the remaining data. For further information on this topic, consult the Socket Programming HOWTO .

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Send data to the socket. The socket must be connected to a remote socket. The optional flags argument has the same meaning as for recv() above. Unlike send() , this method continues to send data from bytes until either all data has been sent or an error occurs. None is returned on success. On error, an exception is raised, and there is no way to determine how much data, if any, was successfully sent.

Changed in version 3.5: The socket timeout is no more reset each time data is sent successfully. The socket timeout is now the maximum total duration to send all data.

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Send data to the socket. The socket should not be connected to a remote socket, since the destination socket is specified by address. The optional flags argument has the same meaning as for recv() above. Return the number of bytes sent. (The format of address depends on the address family — see above.)

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Send normal and ancillary data to the socket, gathering the non-ancillary data from a series of buffers and concatenating it into a single message. The buffers argument specifies the non-ancillary data as an iterable of bytes-like objects (e.g. bytes objects); the operating system may set a limit ( sysconf() value SC_IOV_MAX ) on the number of buffers that can be used. The ancdata argument specifies the ancillary data (control messages) as an iterable of zero or more tuples (cmsg_level, cmsg_type, cmsg_data) , where cmsg_level and cmsg_type are integers specifying the protocol level and protocol-specific type respectively, and cmsg_data is a bytes-like object holding the associated data. Note that some systems (in particular, systems without CMSG_SPACE() ) might support sending only one control message per call. The flags argument defaults to 0 and has the same meaning as for send() . If address is supplied and not None , it sets a destination address for the message. The return value is the number of bytes of non-ancillary data sent.

The following function sends the list of file descriptors fds over an AF_UNIX socket, on systems which support the SCM_RIGHTS mechanism. See also recvmsg() .

Availability: most Unix platforms, possibly others.

New in version 3.3.

Changed in version 3.5: If the system call is interrupted and the signal handler does not raise an exception, the method now retries the system call instead of raising an InterruptedError exception (see PEP 475 for the rationale).

Specialized version of sendmsg() for AF_ALG socket. Set mode, IV, AEAD associated data length and flags for AF_ALG socket.

Availability: Linux >= 2.6.38

New in version 3.6.

Send a file until EOF is reached by using high-performance os.sendfile and return the total number of bytes which were sent. file must be a regular file object opened in binary mode. If os.sendfile is not available (e.g. Windows) or file is not a regular file send() will be used instead. offset tells from where to start reading the file. If specified, count is the total number of bytes to transmit as opposed to sending the file until EOF is reached. File position is updated on return or also in case of error in which case file.tell() can be used to figure out the number of bytes which were sent. The socket must be of SOCK_STREAM type. Non-blocking sockets are not supported.

New in version 3.5.

Set the inheritable flag of the socket’s file descriptor or socket’s handle.

New in version 3.4.

Set blocking or non-blocking mode of the socket: if flag is false, the socket is set to non-blocking, else to blocking mode.

This method is a shorthand for certain settimeout() calls:

• sock.setblocking(True) is equivalent to sock.settimeout(None)
• sock.setblocking(False) is equivalent to sock.settimeout(0.0)

socket. settimeout ( value ) ¶

Set a timeout on blocking socket operations. The value argument can be a nonnegative floating point number expressing seconds, or None . If a non-zero value is given, subsequent socket operations will raise a timeout exception if the timeout period value has elapsed before the operation has completed. If zero is given, the socket is put in non-blocking mode. If None is given, the socket is put in blocking mode.

For further information, please consult the notes on socket timeouts .

socket. setsockopt ( level, optname, value: int ) ¶ socket. setsockopt ( level, optname, value: buffer ) socket. setsockopt ( level, optname, None, optlen: int )

Set the value of the given socket option (see the Unix manual page setsockopt(2)). The needed symbolic constants are defined in the socket module ( SO_* etc.). The value can be an integer, None or a bytes-like object representing a buffer. In the later case it is up to the caller to ensure that the bytestring contains the proper bits (see the optional built-in module struct for a way to encode C structures as bytestrings). When value is set to None , optlen argument is required. It’s equivalent to call setsockopt C function with optval=NULL and optlen=optlen.

Changed in version 3.5: Writable bytes-like object is now accepted.

Changed in version 3.6: setsockopt(level, optname, None, optlen: int) form added.

Shut down one or both halves of the connection. If how is SHUT_RD , further receives are disallowed. If how is SHUT_WR , further sends are disallowed. If how is SHUT_RDWR , further sends and receives are disallowed.

socket. share ( process_id ) ¶

Duplicate a socket and prepare it for sharing with a target process. The target process must be provided with process_id. The resulting bytes object can then be passed to the target process using some form of interprocess communication and the socket can be recreated there using fromshare() . Once this method has been called, it is safe to close the socket since the operating system has already duplicated it for the target process.

New in version 3.3.

Note that there are no methods read() or write() ; use recv() and send() without flags argument instead.

Socket objects also have these (read-only) attributes that correspond to the values given to the socket constructor.

The socket family.

The socket type.

The socket protocol.

18.1.4. Notes on socket timeouts¶

A socket object can be in one of three modes: blocking, non-blocking, or timeout. Sockets are by default always created in blocking mode, but this can be changed by calling setdefaulttimeout() .

• In blocking mode, operations block until complete or the system returns an error (such as connection timed out).
• In non-blocking mode, operations fail (with an error that is unfortunately system-dependent) if they cannot be completed immediately: functions from the select can be used to know when and whether a socket is available for reading or writing.
• In timeout mode, operations fail if they cannot be completed within the timeout specified for the socket (they raise a timeout exception) or if the system returns an error.

At the operating system level, sockets in timeout mode are internally set in non-blocking mode. Also, the blocking and timeout modes are shared between file descriptors and socket objects that refer to the same network endpoint. This implementation detail can have visible consequences if e.g. you decide to use the fileno() of a socket.

18.1.4.1. Timeouts and the connect method¶

The connect() operation is also subject to the timeout setting, and in general it is recommended to call settimeout() before calling connect() or pass a timeout parameter to create_connection() . However, the system network stack may also return a connection timeout error of its own regardless of any Python socket timeout setting.

18.1.4.2. Timeouts and the accept method¶

If getdefaulttimeout() is not None , sockets returned by the accept() method inherit that timeout. Otherwise, the behaviour depends on settings of the listening socket:

• if the listening socket is in blocking mode or in timeout mode, the socket returned by accept() is in blocking mode;
• if the listening socket is in non-blocking mode, whether the socket returned by accept() is in blocking or non-blocking mode is operating system-dependent. If you want to ensure cross-platform behaviour, it is recommended you manually override this setting.

18.1.5. Example¶

Here are four minimal example programs using the TCP/IP protocol: a server that echoes all data that it receives back (servicing only one client), and a client using it. Note that a server must perform the sequence socket() , bind() , listen() , accept() (possibly repeating the accept() to service more than one client), while a client only needs the sequence socket() , connect() . Also note that the server does not sendall() / recv() on the socket it is listening on but on the new socket returned by accept() .

The first two examples support IPv4 only.

The next two examples are identical to the above two, but support both IPv4 and IPv6. The server side will listen to the first address family available (it should listen to both instead). On most of IPv6-ready systems, IPv6 will take precedence and the server may not accept IPv4 traffic. The client side will try to connect to the all addresses returned as a result of the name resolution, and sends traffic to the first one connected successfully.

The next example shows how to write a very simple network sniffer with raw sockets on Windows. The example requires administrator privileges to modify the interface:

The next example shows how to use the socket interface to communicate to a CAN network using the raw socket protocol. To use CAN with the broadcast manager protocol instead, open a socket with:

After binding ( CAN_RAW ) or connecting ( CAN_BCM ) the socket, you can use the socket.send() , and the socket.recv() operations (and their counterparts) on the socket object as usual.

This last example might require special privileges:

Running an example several times with too small delay between executions, could lead to this error:

This is because the previous execution has left the socket in a TIME_WAIT state, and can’t be immediately reused.

There is a socket flag to set, in order to prevent this, socket.SO_REUSEADDR :

the SO_REUSEADDR flag tells the kernel to reuse a local socket in TIME_WAIT state, without waiting for its natural timeout to expire.

For an introduction to socket programming (in C), see the following papers:

• An Introductory 4.3BSD Interprocess Communication Tutorial, by Stuart Sechrest
• An Advanced 4.3BSD Interprocess Communication Tutorial, by Samuel J. Leffler et al,

both in the UNIX Programmer’s Manual, Supplementary Documents 1 (sections PS1:7 and PS1:8). The platform-specific reference material for the various socket-related system calls are also a valuable source of information on the details of socket semantics. For Unix, refer to the manual pages; for Windows, see the WinSock (or Winsock 2) specification. For IPv6-ready APIs, readers may want to refer to RFC 3493 titled Basic Socket Interface Extensions for IPv6.

Источник

Время прочтения
16 мин

Просмотры 2.8K

К старту курса по Fullstack-разработке на Python делимся заключительной частью руководства по программированию сокетов, эта часть посвящена устранению неполадок и справочным сведениям.

К концу руководства вы освоите основные функции и методы модуля Python socket, научитесь применять пользовательский класс для отправки сообщений и данных между конечными точками и работать со всем этим в собственных клиент-серверных приложениях.

Устранение проблем

Что-то будет «отказываться» работать, это неизбежно. И что тогда делать? Не переживайте, это случается с каждым. Надеемся, что с помощью этого руководства, отладчика и любимой поисковой системы вы сможете продолжить работу с частью исходного кода.

Иначе вам прямая дорога в модуль socket на Python. Обязательно прочитайте всю документацию по каждой вызываемой функции или методу. А чтобы почерпнуть какие-то идеи, загляните ещё в справочный раздел.

Иногда дело не в исходном коде. Он может быть правильным, а проблема связана с другим хостом, клиентом или сервером. Или сетью. А возможно, проводится «атака посредника», в качестве которого используется роутер, брандмауэр или какое-то другое сетевое устройство.

Для решения таких проблем нужны дополнительные инструменты. Ниже приводятся инструменты и утилиты, которые могут в этом пригодиться или хотя бы подвести к решению.

ping

Отправкой запроса проверки связи по ICMP в ping проверяется, работает ли хост и подключён ли он к сети. Взаимодействие ping со стеком протоколов TCP/IP прямое, поэтому его работа не зависит от каких бы то ни было приложений, запускаемых на хосте.

Вот пример запуска ping на macOS:

$ ping -c 3 127.0.0.1
PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.058 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.165 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.164 ms

--- 127.0.0.1 ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.058/0.129/0.165/0.050 ms

Статистика внизу может быть полезной, тчобы найти разрыв постоянного подключения и ответить на вопрос, теряются ли пакеты? Какова временнáя задержка? Можно посмотреть и время приёма-передачи.

Если между вами и другим хостом есть брандмауэр, запрос проверки связи в ping может быть запрещён. Политики соблюдения этого запрета реализуются администраторами брандмауэра, для которых обнаружение их хостов нежелательно. Если это ваш случай (и добавлены правила брандмауэра для взаимодействия хостов), убедитесь, что этими правилами также разрешается передача сообщений по ICMP между хостами.

ICMP — это протокол, применяемый в ping, а также протокол TCP и другие более низкоуровневые протоколы, используемые для передачи сообщений об ошибках. Причина странного поведения или медленных подключений может быть в ICMP.

ICMP-сообщения идентифицируются по типу и коду. Чтобы дать вам представление о важности их информации, приведём несколько таких сообщений:

Информацию о фрагментации и ICMP-сообщениях см. в статье об обнаружении наименьшего MTU на пути следования пакета в сети. Это пример того, чтó может быть причиной странного поведения.

netstat

В разделе «Просмотр состояния сокета» вы узнали, как можно использовать netstat для отображения информации о сокетах и их текущем состоянии. Эта утилита доступна на macOS, Linux и Windows.

В том разделе столбцы Recv-Q и Send-Q в выводе примера отсутствовали. В них показано количество байтов, которые хранятся в сетевых буферах и поставлены в очередь на передачу или приём, но по какой-то причине не считаны или не записаны в удалённом или локальном приложении.

То есть они остаются в ожидании в сетевых буферах, очередях ОС. Одна из причин — в приложении ограничено использование ресурсов ЦП либо нельзя вызвать socket.recv() или socket.send() и обработать байты. Или это проблемы с сетью, которые отражаются на передаче данных, например перегрузка сети или неисправное сетевое оборудование / кабели.

Посмотрим, сколько данных вы сможете отправить, прежде чем увидите ошибку. Попробуйте тестовый клиент с подключением к тестовому серверу и многократным вызовом socket.send(). В тестовом сервере никогда не вызывается socket.recv(), а лишь принимается подключение. Это становится причиной заполнения сетевых буферов на сервере, после чего в клиенте выдаётся ошибка.

Сначала запустите сервер:

$ python app-server-test.py 127.0.0.1 65432
Listening on ('127.0.0.1', 65432)

Затем, чтобы увидеть ошибку, запустите клиент:

$ python app-client-test.py 127.0.0.1 65432 binary test
Error: socket.send() blocking io exception for ('127.0.0.1', 65432):
BlockingIOError(35, 'Resource temporarily unavailable')

Вот вывод netstat, когда в клиенте и на сервере продолжается выполнение. При этом в клиенте многократно выводится приведённое выше сообщение об ошибке:

$ netstat -an | grep 65432
Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4  408300      0  127.0.0.1.65432        127.0.0.1.53225        ESTABLISHED
tcp4       0 269868  127.0.0.1.53225        127.0.0.1.65432        ESTABLISHED
tcp4       0      0  127.0.0.1.65432        *.*                    LISTEN

Первая запись — это сервер (у Local Address порт 65432):

Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4  408300      0  127.0.0.1.65432        127.0.0.1.53225        ESTABLISHED

Обратите внимание на 408300 в Recv-Q.

Вторая запись — это клиент (у Foreign Address порт 65432):

Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4       0 269868  127.0.0.1.53225        127.0.0.1.65432        ESTABLISHED

Обратите внимание на 269868 в Send-Q.

В клиенте, конечно, пытались записать байты, но на сервере они не считывались. Это стало причиной заполнения очереди сетевого буфера как на стороне приёма на сервере, так и на стороне отправки в клиенте.

Windows

Если работаете с Windows, обязательно ознакомьтесь с набором утилит Windows Sysinternals.

Одна из них — TCPView.exe. Это графический netstat для Windows. Кроме адресов, номеров портов и состояния сокета, в ней нарастающим итогом показывается число отправленных и полученных пакетов и байтов. Как и в случае с утилитой lsof в Unix, здесь вы получаете имя и идентификатор процесса. Другие параметры отображения см. в меню.

Wireshark

Иногда нужно увидеть, что происходит при передаче по сети. Забудьте, чтó там в журнале приложения или какое значение возвращается из библиотечного вызова. Вам нужно видеть, чтó отправляется или получается в сети на самом деле. Здесь как с отладчиками: когда нужно увидеть это, они незаменимы.

Wireshark — это анализатор сетевых протоколов и приложение для захвата трафика, запускаемое на macOS, Linux, Windows и других ОС. Существует две версии: wireshark с графическим интерфейсом и текстовая tshark для терминала.

Захват трафика — это отличный способ понаблюдать за поведением приложения в сети и узнать: чтó, как часто и сколько в нём отправляется и получается. А ещё увидеть, когда клиент или сервер закрывается, когда в них прерывается подключение или прекращается отправка ответов. Эта информация может очень пригодиться при устранении проблем.

В интернете много хороших руководств и других ресурсов по основам использования Wireshark и TShark.

Вот пример захвата трафика в интерфейсе «внутренней петли» с помощью Wireshark:

А вот тот же пример с tshark:

$ tshark -i lo0 'tcp port 65432'
Capturing on 'Loopback'
    1   0.000000    127.0.0.1 → 127.0.0.1    TCP 68 53942 → 65432 [SYN] Seq=0 Win=65535 Len=0 MSS=16344 WS=32 TSval=940533635 TSecr=0 SACK_PERM=1
    2   0.000057    127.0.0.1 → 127.0.0.1    TCP 68 65432 → 53942 [SYN, ACK] Seq=0 Ack=1 Win=65535 Len=0 MSS=16344 WS=32 TSval=940533635 TSecr=940533635 SACK_PERM=1
    3   0.000068    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [ACK] Seq=1 Ack=1 Win=408288 Len=0 TSval=940533635 TSecr=940533635
    4   0.000075    127.0.0.1 → 127.0.0.1    TCP 56 [TCP Window Update] 65432 → 53942 [ACK] Seq=1 Ack=1 Win=408288 Len=0 TSval=940533635 TSecr=940533635
    5   0.000216    127.0.0.1 → 127.0.0.1    TCP 202 53942 → 65432 [PSH, ACK] Seq=1 Ack=1 Win=408288 Len=146 TSval=940533635 TSecr=940533635
    6   0.000234    127.0.0.1 → 127.0.0.1    TCP 56 65432 → 53942 [ACK] Seq=1 Ack=147 Win=408128 Len=0 TSval=940533635 TSecr=940533635
    7   0.000627    127.0.0.1 → 127.0.0.1    TCP 204 65432 → 53942 [PSH, ACK] Seq=1 Ack=147 Win=408128 Len=148 TSval=940533635 TSecr=940533635
    8   0.000649    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [ACK] Seq=147 Ack=149 Win=408128 Len=0 TSval=940533635 TSecr=940533635
    9   0.000668    127.0.0.1 → 127.0.0.1    TCP 56 65432 → 53942 [FIN, ACK] Seq=149 Ack=147 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   10   0.000682    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [ACK] Seq=147 Ack=150 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   11   0.000687    127.0.0.1 → 127.0.0.1    TCP 56 [TCP Dup ACK 6#1] 65432 → 53942 [ACK] Seq=150 Ack=147 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   12   0.000848    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [FIN, ACK] Seq=147 Ack=150 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   13   0.001004    127.0.0.1 → 127.0.0.1    TCP 56 65432 → 53942 [ACK] Seq=150 Ack=148 Win=408128 Len=0 TSval=940533635 TSecr=940533635
^C13 packets captured

Далее приводим вам в помощь справочные материалы по программированию сокетов.

Справочный раздел

Этот раздел может использоваться в качестве справочника общего содержания с дополнительной информацией и ссылками на внешние ресурсы.

Документация Python

• Модуль socket.
• Практическое руководство по программированию сокетов.

Ошибки

Следующее взято из документации к модулю socket на Python:

«Все ошибки сопровождаются вызовом исключений. При недопустимых типах аргументов и нехватке памяти могут вызываться обычные исключения. Начиная с Python 3.3 ошибки, связанные с семантикой сокетов или адресов, сопровождаются вызовом OSError или одного из его подклассов». (Источник).

Вот типичные ошибки, с которыми вы можете столкнуться при работе с сокетами:

Семейства адресов сокетов

socket.AF_INET и socket.AF_INET6 — это семейства адресов и протоколов для первого аргумента в socket.socket(). В API предполагается адрес определённого формата — в зависимости от того, создан сокет с помощью socket.AF_INET или socket.AF_INET6.

Ниже приведена выдержка из документации Python к модулю socket о значении host в кортеже адресов:

«Для IPv4-адресов вместо адреса хоста принимаются две специальные формы: пустая строка — это INADDR_ANY, а строка '<broadcast>' — это INADDR_BROADCAST. Такое поведение несовместимо с IPv6, так что они вам, возможно, не понадобятся, если вы намерены поддерживать IPv6 в своих программах на Python». (Источник)

Подробнее см. в документации по семействам сокетов на Python.

В этом руководстве применяются сокеты IPv4. Если в вашей сети поддерживается IPv6, попробуйте по возможности протестировать и использовать эту версию. Это легко делается с функцией socket.getaddrinfo(). Её аргументы host и port преобразуются в последовательность из пяти кортежей со всеми аргументами, необходимыми для создания сокета, подключённого к этому сервису. В socket.getaddrinfo() принимаются и интерпретируются переданные IPv6-адреса и имена хостов, которые разрешаются не только в IPv4, но и в IPv6-адреса.

В следующем примере возвращается информация об адресе для TCP-подключения к example.org в порте 80:

>>> socket.getaddrinfo("example.org", 80, proto=socket.IPPROTO_TCP)
[(<AddressFamily.AF_INET6: 10>, <SocketType.SOCK_STREAM: 1>,
 6, '', ('2606:2800:220:1:248:1893:25c8:1946', 80, 0, 0)),
 (<AddressFamily.AF_INET: 2>, <SocketType.SOCK_STREAM: 1>,
 6, '', ('93.184.216.34', 80))]

На вашем компьютере результаты могут отличаться, если IPv6 не включена. Возвращаемые выше значения можно передавать в socket.socket() и socket.connect(). В документации Python к модулю socket, в разделе с примерами, содержится пример клиента и сервера.

Использование имён хостов

Кстати, этот раздел примени́м в основном к использованию имён хостов с .bind() и .connect() или .connect_ex(), когда задействуется интерфейс «внутренней петли» (localhost). Но примени́м он и в любой момент, когда вы используете имя хоста и ожидается его разрешение в определённый адрес. Причём имя хоста имеет особое значение для приложения, что отражается на его поведении или делаемых в нём допущениях. Это отличается от типичного сценария, когда в клиенте имя хоста используется для подключения к серверу, разрешённому при помощи DNS, например www.example.com.

Следующее взято из документации к модулю socket на Python:

«Если использовать имя хоста в хостовой части адреса сокета IPv4/v6, в программе может проявиться недетерминированное поведение, поскольку в Python используется первый адрес, возвращаемый из DNS-разрешения. Адрес сокета будет разрешён в фактический адрес IPv4/v6 по-разному в зависимости от результатов из DNS-разрешения и/или конфигурации хоста. Для детерминированного поведения используйте в хостовой части числовой адрес». (Источник).

Стандартным именем localhost предусматривается его разрешение в 127.0.0.1 или ::1, интерфейс «внутренней петли». В вашей системе наверняка так и будет. А может, и нет. Это зависит от того, как она сконфигурирована для разрешения имён. Как и во всём, что связано с ИТ, всегда имеются исключения, и нет никаких гарантий, что при использовании имени localhost произойдёт подключение к интерфейсу «внутренней петли».

Например, на Linux см. файл конфигурации man nsswitch.conf диспетчера службы имён. На macOS и Linux стóит ещё заглянуть в файл /etc/hosts. На Windows см. C:WindowsSystem32driversetchosts. В файле hosts имеется статическая таблица сопоставления имён и адресов в простом текстовом формате. DNS — это совершенно иная часть пазла.

Примечательно, что с июня 2018 года существует черновой вариант RFC Let ‘localhost’ be localhost («Пусть ‘localhost’ будет localhost»), где обсуждаются соглашения, допущения и вопросы безопасности, связанные с применением имени localhost.

Важно понимать: когда в приложении используются имена хостов, в качестве адресов может возвращаться буквально всё что угодно. Не делайте допущений относительно имени, если в приложении важна безопасность. Представляет это повод для беспокойства или нет — зависит от приложения и окружения.

Даже если в приложении важность безопасности неочевидна, соответствующие рекомендации всё равно применяются. Если в приложении имеется доступ к сети, оно должно сопровождаться и быть защищено. То есть это как минимум:

• Регулярные обновления системного ПО и исправления уязвимостей, в том числе на Python. Обязательно также проверяйте и обновляйте любые сторонние библиотеки.
• Чтобы подключаться только к доверенным системам, по возможности используйте выделенный брандмауэр или межсетевой экран узлов.
• Какие DNS-серверы сконфигурированы? Вы доверяете им и их администраторам?
• Убедитесь, что данные запроса максимально очищены и проверены, прежде чем вызывать другой код, в котором они обрабатываются. Используйте для этого фаззинг-тесты и запускайте их регулярно.

Независимо от того, используете вы имена хостов или нет, если в приложении нужна поддержка безопасных подключений через шифрование и аутентификацию, то вам, вероятно, стóит попробовать TLS. Но это — тема отдельной статьи и в настоящем руководстве не рассматривается. Чтобы начать с ним работу, см. документацию по модулю ssl на Python. Это тот же протокол, который применяется в браузерах для безопасного подключения к сайтам.

Интерфейсы, IP-адреса, разрешения имён… Переменных много. Что же делать? Если у вас нет процесса рассмотрения сетевых приложений, используйте эти рекомендации:

Для клиентов или серверов, если нужно аутентифицировать хост, к которому подключаетесь, стóит попробовать TLS.

Блокирующие вызовы

Блокирующий вызов — это функция или метод сокета, на которых работа приложения временно приостанавливается. Например, .accept(), .connect(), .send() и .recv() блокируются, то есть немедленного возвращения в них не происходит. Прежде чем от блокирующих вызовов возвращается значение, они находятся в ожидании завершения системных (ввод-вывод). Поэтому источник вызова блокируется до их завершения, или истечения тайм-аута, или возникновения другой ошибки.

Чтобы в блокирующих вызовах сокетов происходило немедленное возвращение значений, такие вызовы можно настроить на неблокируемый режим. При этом, чтобы обрабатывать готовую операцию сокета, нужно как минимум провести рефакторинг или перепроектирование приложения.

Поскольку в вызове значения возвращаются немедленно, данные могут быть не готовы. Вызываемая сторона находится в сети в ожидании — ей не хватило времени завершить свою работу. В этом случае текущий статус — это errno-значение socket.EWOULDBLOCK. Неблокируемый режим поддерживается с помощью .setblocking().

По умолчанию сокеты всегда создаются в блокируемом режиме. Описание трёх режимов см. в примечаниях по тайм-аутам сокетов.

Закрытие подключений

В TCP примечательно то, что в клиенте или на сервере совершенно допустимо закрывать свою сторону подключения, пока другая остаётся открытой. Это называется «полуоткрытым» подключением. Желательно оно или нет — решается в приложении. Вообще говоря, нежелательно. В этом состоянии со стороны, чей конец подключения закрыт, данные больше отправляться не смогут. Они будут только приниматься.

Этот подход рекомендуется не как обязательный, но в HTTP в качестве примера используется заголовок с именем Connection для стандартизации того, как в приложениях должны закрываться или сохраняться открытыми подключения. Подробнее см. в разделе 6.3 RFC 7230 о протоколе передачи гипертекста (HTTP/1.1), синтаксисе и маршрутизации сообщений.

При разработке и написании приложения и его протокола прикладного уровня рекомендуется заранее продумать закрытие подключений. Иногда это просто и очевидно, а иногда могут потребоваться создание прототипов и предварительное тестирование. Всё зависит от приложения и от того, как обрабатывается цикл сообщений с ожидаемыми данными. Просто убедитесь, что сокеты всегда своевременно закрываются после завершения работы.

Порядок следования байтов

Подробнее о том, как порядок байтов хранится в памяти разных ЦП, см. в соответствующей статье «Википедии». При интерпретации отдельных байтов это не проблема. Но при обработке нескольких байтов, считываемых и обрабатываемых как одно значение, например 4-байтовое целое число, порядок байтов нужно изменить на противоположный, если взаимодействовать с компьютером, на котором другой порядок следования байтов.

Порядок байтов важен и для текстовых строк, представленных в виде многобайтовых последовательностей, таких как «Юникод». Если только вы не используете всегда true, строгий ASCII и не контролируете реализацию клиента и сервера, вам наверняка лучше использовать «Юникод» с кодировкой вроде UTF-8 или той, которой поддерживается маркер последовательности байтов.

Важно явно определить кодировку, применяемую в протоколе прикладного уровня. Это можно сделать, установив в качестве обязательной для всего текста UTF-8 или использовав заголовок content-encoding, в котором эта кодировка указана. Так приложение избавляется от необходимости определять кодировку, которую по возможности следует избегать.

Это становится проблематичным, когда имеются данные, которые хранятся в файлах или базе данных, и недоступны метаданные, в которых указана их кодировка. Когда данные передаются на другую конечную точку, в ней придётся попробовать определить кодировку. Идеи для обсуждения см. в статье «Википедии» о «Юникоде», в которой упоминается о RFC 3629: UTF-8, формат преобразования ISO 10646:

«Однако в RFC 3629 (стандарт UTF-8) рекомендуется запрещать маркер последовательности байтов в протоколах с UTF-8, но обсуждаются случаи, когда это бывает невозможно. Кроме того, под большим ограничением на возможные шаблоны в UTF-8 (например, не может быть одиночных байтов с установленным старшим битом) подразумевается, что должна быть возможность отличать UTF-8 от других кодировок символов без применения маркера последовательности байтов». (Источник).

А вывод из этого такой: всегда сохранять кодировку, используемую для обрабатываемых в приложении данных, если она может меняться. То есть пытаться каким-то образом сохранять кодировку в виде метаданных, если это не всегда UTF-8 или какая-то другая кодировка с маркером последовательности байтов. Затем можно отправить её в заголовке вместе с данными, чтобы сообщить получателю, что это такое.

В TCP/IP применяется обратный порядок байтов, который называется сетевым. Сетевой порядок используется для представления целых чисел на нижних уровнях стека протоколов, таких как IP-адреса и номера портов. В модуль socket на Python включены функции, в которых целые числа преобразуются в порядок байтов сети и хоста и обратно:

Кроме того, чтобы с помощью строк формата упаковывать и распаковывать двоичные данные, можно использовать модуль struct:

import struct
network_byteorder_int = struct.pack('>H', 256)
python_int = struct.unpack('>H', network_byteorder_int)[0]

Заключение

В этом руководстве рассмотрено много вопросов! Сети и сокеты — это большие темы. Если они вам в новинку, пусть вас не смущают все эти термины и сокращения.

Чтобы понять, как здесь всё сочетается, нужно ознакомиться с кучей подробностей. Но в Python начинаешь лучше понимать целое, осваивая отдельные части и проводя с ними больше времени. Теперь вы можете задействовать свой пользовательский класс и, опираясь на него, проще и быстрее создавать собственные приложения с сокетами.

Ознакомиться с примерами можно по ссылке ниже:

исходный код из руководства для примеров в этом руководстве.

Поздравляем с тем, что добрались до конца! Вы уже готовы применять сокеты в собственных приложениях. Желаю вам успехов в разработке сокетов.

Научим вас аккуратно работать с данными, чтобы вы прокачали карьеру и стали востребованным IT-специалистом. Новогодняя акция — скидки до 50% по промокоду HABR:

• Профессия Fullstack-разработчик на Python (16 месяцев)
• Профессия Data Scientist (24 месяца)

Sockets and the socket API are used to send messages across a network. They provide a form of inter-process communication (IPC). The network can be a logical, local network to the computer, or one that’s physically connected to an external network, with its own connections to other networks. The obvious example is the Internet, which you connect to via your ISP.

In this tutorial, you’ll create:

• A simple socket server and client
• An improved version that handles multiple connections simultaneously
• A server-client application that functions like a full-fledged socket application, complete with its own custom header and content

By the end of this tutorial, you’ll understand how to use the main functions and methods in Python’s socket module to write your own client-server applications. You’ll know how to use a custom class to send messages and data between endpoints, which you can build upon and utilize for your own applications.

The examples in this tutorial require Python 3.6 or above, and have been tested using Python 3.10. To get the most out of this tutorial, it’s best to download the source code and have it on hand for reference while reading:

Networking and sockets are large subjects. Literal volumes have been written about them. If you’re new to sockets or networking, it’s completely normal if you feel overwhelmed with all of the terms and pieces.

Don’t be discouraged though. This tutorial is for you! As with anything Python-related, you can learn a little bit at a time. Bookmark this article and come back when you’re ready for the next section.

# echo-server.py

import socket

HOST = "127.0.0.1"  # Standard loopback interface address (localhost)
PORT = 65432  # Port to listen on (non-privileged ports are > 1023)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        while True:
            data = conn.recv(1024)
            if not data:
                break
            conn.sendall(data)

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    pass  # Use the socket object without calling s.close().

# echo-server.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    # ...

# echo-server.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    # ...

# echo-server.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        while True:
            data = conn.recv(1024)
            if not data:
                break
            conn.sendall(data)

# echo-client.py

import socket

HOST = "127.0.0.1"  # The server's hostname or IP address
PORT = 65432  # The port used by the server

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((HOST, PORT))
    s.sendall(b"Hello, world")
    data = s.recv(1024)

print(f"Received {data!r}")

# echo-server.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.bind((HOST, PORT))
    s.listen()
    conn, addr = s.accept()
    with conn:
        print(f"Connected by {addr}")
        while True:
            data = conn.recv(1024)
            if not data:
                break
            conn.sendall(data)

$ python echo-client.py 
Received b'Hello, world'

$ python echo-server.py 
Connected by ('127.0.0.1', 64623)

$ netstat -an
Active Internet connections (including servers)
Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4       0      0  127.0.0.1.65432        *.*                    LISTEN

$ netstat -an
Active Internet connections (including servers)
Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4       0      0  *.65432                *.*                    LISTEN

$ lsof -i -n
COMMAND     PID   USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
Python    67982 nathan    3u  IPv4 0xecf272      0t0  TCP *:65432 (LISTEN)

$ python echo-client.py 
Traceback (most recent call last):
  File "./echo-client.py", line 9, in <module>
    s.connect((HOST, PORT))
ConnectionRefusedError: [Errno 61] Connection refused

# echo-client.py

# ...

with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.connect((HOST, PORT))
    s.sendall(b"Hello, world")
    data = s.recv(1024)

print(f"Received {data!r}")

# multiconn-server.py

import sys
import socket
import selectors
import types

sel = selectors.DefaultSelector()

# ...

host, port = sys.argv[1], int(sys.argv[2])
lsock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
lsock.bind((host, port))
lsock.listen()
print(f"Listening on {(host, port)}")
lsock.setblocking(False)
sel.register(lsock, selectors.EVENT_READ, data=None)

# multiconn-server.py

# ...

try:
    while True:
        events = sel.select(timeout=None)
        for key, mask in events:
            if key.data is None:
                accept_wrapper(key.fileobj)
            else:
                service_connection(key, mask)
except KeyboardInterrupt:
    print("Caught keyboard interrupt, exiting")
finally:
    sel.close()

# multiconn-server.py

# ...

def accept_wrapper(sock):
    conn, addr = sock.accept()  # Should be ready to read
    print(f"Accepted connection from {addr}")
    conn.setblocking(False)
    data = types.SimpleNamespace(addr=addr, inb=b"", outb=b"")
    events = selectors.EVENT_READ | selectors.EVENT_WRITE
    sel.register(conn, events, data=data)

# ...

# multiconn-server.py

# ...

def accept_wrapper(sock):
    conn, addr = sock.accept()  # Should be ready to read
    print(f"Accepted connection from {addr}")
    conn.setblocking(False)
    data = types.SimpleNamespace(addr=addr, inb=b"", outb=b"")
    events = selectors.EVENT_READ | selectors.EVENT_WRITE
    sel.register(conn, events, data=data)

# ...

# multiconn-server.py

# ...

def service_connection(key, mask):
    sock = key.fileobj
    data = key.data
    if mask & selectors.EVENT_READ:
        recv_data = sock.recv(1024)  # Should be ready to read
        if recv_data:
            data.outb += recv_data
        else:
            print(f"Closing connection to {data.addr}")
            sel.unregister(sock)
            sock.close()
    if mask & selectors.EVENT_WRITE:
        if data.outb:
            print(f"Echoing {data.outb!r} to {data.addr}")
            sent = sock.send(data.outb)  # Should be ready to write
            data.outb = data.outb[sent:]

# ...

# multiconn-server.py

# ...

def service_connection(key, mask):
    sock = key.fileobj
    data = key.data
    if mask & selectors.EVENT_READ:
        recv_data = sock.recv(1024)  # Should be ready to read
        if recv_data:
            data.outb += recv_data
        else:
            print(f"Closing connection to {data.addr}")
            sel.unregister(sock)
            sock.close()
    if mask & selectors.EVENT_WRITE:
        if data.outb:
            print(f"Echoing {data.outb!r} to {data.addr}")
            sent = sock.send(data.outb)  # Should be ready to write
            data.outb = data.outb[sent:]

# ...

# multiconn-server.py

# ...

def service_connection(key, mask):

    # ...

    if mask & selectors.EVENT_WRITE:
        if data.outb:
            print(f"Echoing {data.outb!r} to {data.addr}")
            sent = sock.send(data.outb)  # Should be ready to write
            data.outb = data.outb[sent:]

# ...

# multiconn-client.py

import sys
import socket
import selectors
import types

sel = selectors.DefaultSelector()
messages = [b"Message 1 from client.", b"Message 2 from client."]

def start_connections(host, port, num_conns):
    server_addr = (host, port)
    for i in range(0, num_conns):
        connid = i + 1
        print(f"Starting connection {connid} to {server_addr}")
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        sock.setblocking(False)
        sock.connect_ex(server_addr)
        events = selectors.EVENT_READ | selectors.EVENT_WRITE
        data = types.SimpleNamespace(
            connid=connid,
            msg_total=sum(len(m) for m in messages),
            recv_total=0,
            messages=messages.copy(),
            outb=b"",
        )
        sel.register(sock, events, data=data)

# ...

 def service_connection(key, mask):
     sock = key.fileobj
     data = key.data
     if mask & selectors.EVENT_READ:
         recv_data = sock.recv(1024)  # Should be ready to read
         if recv_data:
-            data.outb += recv_data
+            print(f"Received {recv_data!r} from connection {data.connid}")
+            data.recv_total += len(recv_data)
-        else:
-            print(f"Closing connection {data.connid}")
+        if not recv_data or data.recv_total == data.msg_total:
+            print(f"Closing connection {data.connid}")
             sel.unregister(sock)
             sock.close()
     if mask & selectors.EVENT_WRITE:
+        if not data.outb and data.messages:
+            data.outb = data.messages.pop(0)
         if data.outb:
-            print(f"Echoing {data.outb!r} to {data.addr}")
+            print(f"Sending {data.outb!r} to connection {data.connid}")
             sent = sock.send(data.outb)  # Should be ready to write
             data.outb = data.outb[sent:]

$ python multiconn-server.py
Usage: multiconn-server.py <host> <port>

$ python multiconn-client.py
Usage: multiconn-client.py <host> <port> <num_connections>

$ python multiconn-server.py 127.0.0.1 65432
Listening on ('127.0.0.1', 65432)
Accepted connection from ('127.0.0.1', 61354)
Accepted connection from ('127.0.0.1', 61355)
Echoing b'Message 1 from client.Message 2 from client.' to ('127.0.0.1', 61354)
Echoing b'Message 1 from client.Message 2 from client.' to ('127.0.0.1', 61355)
Closing connection to ('127.0.0.1', 61354)
Closing connection to ('127.0.0.1', 61355)

$ python multiconn-client.py 127.0.0.1 65432 2
Starting connection 1 to ('127.0.0.1', 65432)
Starting connection 2 to ('127.0.0.1', 65432)
Sending b'Message 1 from client.' to connection 1
Sending b'Message 2 from client.' to connection 1
Sending b'Message 1 from client.' to connection 2
Sending b'Message 2 from client.' to connection 2
Received b'Message 1 from client.Message 2 from client.' from connection 1
Closing connection 1
Received b'Message 1 from client.Message 2 from client.' from connection 2
Closing connection 2

$ python -c 'import sys; print(repr(sys.byteorder))'
'little'

$ python -c 'import sys; print(repr(sys.byteorder))'
'big'

# app-server.py

# ...

def accept_wrapper(sock):
    conn, addr = sock.accept()  # Should be ready to read
    print(f"Accepted connection from {addr}")
    conn.setblocking(False)
    message = libserver.Message(sel, conn, addr)
    sel.register(conn, selectors.EVENT_READ, data=message)

# ...

# app-server.py

# ...

try:
    while True:
        events = sel.select(timeout=None)
        for key, mask in events:
            if key.data is None:
                accept_wrapper(key.fileobj)
            else:
                message = key.data
                try:
                    message.process_events(mask)
                # ...

# ...

# libserver.py

# ...

class Message:
    def __init__(self, selector, sock, addr):
        # ...

    # ...

    def process_events(self, mask):
        if mask & selectors.EVENT_READ:
            self.read()
        if mask & selectors.EVENT_WRITE:
            self.write()

    # ...

# libserver.py

# ...

class Message:

    # ...

    def read(self):
        self._read()

        if self._jsonheader_len is None:
            self.process_protoheader()

        if self._jsonheader_len is not None:
            if self.jsonheader is None:
                self.process_jsonheader()

        if self.jsonheader:
            if self.request is None:
                self.process_request()

    # ...

# libserver.py

# ...

class Message:

    # ...

    def write(self):
        if self.request:
            if not self.response_created:
                self.create_response()

        self._write()

    # ...

# libclient.py

# ...

class Message:
    def __init__(self, selector, sock, addr, request):
        # ...

    def write(self):
        if not self._request_queued:
            self.queue_request()

        self._write()

        if self._request_queued:
            if not self._send_buffer:
                # Set selector to listen for read events, we're done writing.
                self._set_selector_events_mask("r")

    # ...

$ python app-server.py
Usage: app-server.py <host> <port>

$ python app-server.py 127.0.0.1 65432
Listening on ('127.0.0.1', 65432)

# app-server.py

# ...

host, port = sys.argv[1], int(sys.argv[2])
lsock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
# Avoid bind() exception: OSError: [Errno 48] Address already in use
lsock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
lsock.bind((host, port))
lsock.listen()
print(f"Listening on {(host, port)}")
lsock.setblocking(False)
sel.register(lsock, selectors.EVENT_READ, data=None)

# ...

# app-server.py

# ...

try:
    while True:
        events = sel.select(timeout=None)
        for key, mask in events:
            if key.data is None:
                accept_wrapper(key.fileobj)
            else:
                message = key.data
                try:
                    message.process_events(mask)
                except Exception:
                    print(
                        f"Main: Error: Exception for {message.addr}:n"
                        f"{traceback.format_exc()}"
                    )
                    message.close()
except KeyboardInterrupt:
    print("Caught keyboard interrupt, exiting")
finally:
    sel.close()

# app-server.py

# ...

def accept_wrapper(sock):
    conn, addr = sock.accept()  # Should be ready to read
    print(f"Accepted connection from {addr}")
    conn.setblocking(False)
    message = libserver.Message(sel, conn, addr)
    sel.register(conn, selectors.EVENT_READ, data=message)

# ...

# libserver.py

# ...

class Message:
    def __init__(self, selector, sock, addr):
        # ...

    # ...

    def process_protoheader(self):
        hdrlen = 2
        if len(self._recv_buffer) >= hdrlen:
            self._jsonheader_len = struct.unpack(
                ">H", self._recv_buffer[:hdrlen]
            )[0]
            self._recv_buffer = self._recv_buffer[hdrlen:]

    # ...

# libserver.py

# ...

class Message:

    # ...

    def process_jsonheader(self):
        hdrlen = self._jsonheader_len
        if len(self._recv_buffer) >= hdrlen:
            self.jsonheader = self._json_decode(
                self._recv_buffer[:hdrlen], "utf-8"
            )
            self._recv_buffer = self._recv_buffer[hdrlen:]
            for reqhdr in (
                "byteorder",
                "content-length",
                "content-type",
                "content-encoding",
            ):
                if reqhdr not in self.jsonheader:
                    raise ValueError(f"Missing required header '{reqhdr}'.")

    # ...

# libserver.py

# ...

class Message:

    # ...

    def process_request(self):
        content_len = self.jsonheader["content-length"]
        if not len(self._recv_buffer) >= content_len:
            return
        data = self._recv_buffer[:content_len]
        self._recv_buffer = self._recv_buffer[content_len:]
        if self.jsonheader["content-type"] == "text/json":
            encoding = self.jsonheader["content-encoding"]
            self.request = self._json_decode(data, encoding)
            print(f"Received request {self.request!r} from {self.addr}")
        else:
            # Binary or unknown content-type
            self.request = data
            print(
                f"Received {self.jsonheader['content-type']} "
                f"request from {self.addr}"
            )
        # Set selector to listen for write events, we're done reading.
        self._set_selector_events_mask("w")

    # ...

# libserver.py

# ...

class Message:

    # ...

    def create_response(self):
        if self.jsonheader["content-type"] == "text/json":
            response = self._create_response_json_content()
        else:
            # Binary or unknown content-type
            response = self._create_response_binary_content()
        message = self._create_message(**response)
        self.response_created = True
        self._send_buffer += message

# libserver.py

# ...

class Message:

    # ...

    def _write(self):
        if self._send_buffer:
            print(f"Sending {self._send_buffer!r} to {self.addr}")
            try:
                # Should be ready to write
                sent = self.sock.send(self._send_buffer)
            except BlockingIOError:
                # Resource temporarily unavailable (errno EWOULDBLOCK)
                pass
            else:
                self._send_buffer = self._send_buffer[sent:]
                # Close when the buffer is drained. The response has been sent.
                if sent and not self._send_buffer:
                    self.close()

    # ...

$ python app-client.py
Usage: app-client.py <host> <port> <action> <value>

$ python app-client.py 127.0.0.1 65432 search needle

# app-client.py

# ...

def start_connection(host, port, request):
    addr = (host, port)
    print(f"Starting connection to {addr}")
    sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    sock.setblocking(False)
    sock.connect_ex(addr)
    events = selectors.EVENT_READ | selectors.EVENT_WRITE
    message = libclient.Message(sel, sock, addr, request)
    sel.register(sock, events, data=message)

# ...

# libclient.py

# ...

class Message:

    # ...

    def queue_request(self):
        content = self.request["content"]
        content_type = self.request["type"]
        content_encoding = self.request["encoding"]
        if content_type == "text/json":
            req = {
                "content_bytes": self._json_encode(content, content_encoding),
                "content_type": content_type,
                "content_encoding": content_encoding,
            }
        else:
            req = {
                "content_bytes": content,
                "content_type": content_type,
                "content_encoding": content_encoding,
            }
        message = self._create_message(**req)
        self._send_buffer += message
        self._request_queued = True

    # ...

# libclient.py

# ...

class Message:

    # ...

    def process_response(self):

        # ...

        # Close when response has been processed
        self.close()

    # ...

# app-client.py

# ...

try:
    while True:
        events = sel.select(timeout=1)
        for key, mask in events:
            message = key.data
            try:
                message.process_events(mask)
            except Exception:
                print(
                    f"Main: Error: Exception for {message.addr}:n"
                    f"{traceback.format_exc()}"
                )
                message.close()
        # Check for a socket being monitored to continue.
        if not sel.get_map():
            break
except KeyboardInterrupt:
    print("Caught keyboard interrupt, exiting")
finally:
    sel.close()

# libclient.py

# ...

class Message:

    # ...

    def _read(self):
        try:
            # Should be ready to read
            data = self.sock.recv(4096)
        except BlockingIOError:
            # Resource temporarily unavailable (errno EWOULDBLOCK)
            pass
        else:
            if data:
                self._recv_buffer += data
            else:
                raise RuntimeError("Peer closed.")

    # ...

$ python app-server.py '' 65432
Listening on ('', 65432)

$ python app-client.py 10.0.1.1 65432 search morpheus
Starting connection to ('10.0.1.1', 65432)
Sending b'x00d{"byteorder": "big", "content-type": "text/json", "content-encoding": "utf-8", "content-length": 41}{"action": "search", "value": "morpheus"}' to ('10.0.1.1', 65432)
Received response {'result': 'Follow the white rabbit. 🐰'} from ('10.0.1.1', 65432)
Got result: Follow the white rabbit. 🐰
Closing connection to ('10.0.1.1', 65432)

$ python app-client.py 10.0.1.1 65432 search 🐶
Starting connection to ('10.0.1.1', 65432)
Sending b'x00d{"byteorder": "big", "content-type": "text/json", "content-encoding": "utf-8", "content-length": 37}{"action": "search", "value": "xf0x9fx90xb6"}' to ('10.0.1.1', 65432)
Received response {'result': '🐾 Playing ball! 🏐'} from ('10.0.1.1', 65432)
Got result: 🐾 Playing ball! 🏐
Closing connection to ('10.0.1.1', 65432)

Accepted connection from ('10.0.2.2', 55340)
Received request {'action': 'search', 'value': 'morpheus'} from ('10.0.2.2', 55340)
Sending b'x00g{"byteorder": "little", "content-type": "text/json", "content-encoding": "utf-8", "content-length": 43}{"result": "Follow the white rabbit. xf0x9fx90xb0"}' to ('10.0.2.2', 55340)
Closing connection to ('10.0.2.2', 55340)

Accepted connection from ('10.0.2.2', 55338)
Received request {'action': 'search', 'value': '🐶'} from ('10.0.2.2', 55338)
Sending b'x00g{"byteorder": "little", "content-type": "text/json", "content-encoding": "utf-8", "content-length": 37}{"result": "xf0x9fx90xbe Playing ball! xf0x9fx8fx90"}' to ('10.0.2.2', 55338)
Closing connection to ('10.0.2.2', 55338)

$ python app-client.py 10.0.1.1 65432 binary 😃
Starting connection to ('10.0.1.1', 65432)
Sending b'x00|{"byteorder": "big", "content-type": "binary/custom-client-binary-type", "content-encoding": "binary", "content-length": 10}binaryxf0x9fx98x83' to ('10.0.1.1', 65432)
Received binary/custom-server-binary-type response from ('10.0.1.1', 65432)
Got response: b'First 10 bytes of request: binaryxf0x9fx98x83'
Closing connection to ('10.0.1.1', 65432)

$ python app-server.py '' 65432
Listening on ('', 65432)
Accepted connection from ('10.0.2.2', 55320)
Received binary/custom-client-binary-type request from ('10.0.2.2', 55320)
Sending b'x00x7f{"byteorder": "little", "content-type": "binary/custom-server-binary-type", "content-encoding": "binary", "content-length": 37}First 10 bytes of request: binaryxf0x9fx98x83' to ('10.0.2.2', 55320)
Closing connection to ('10.0.2.2', 55320)

$ ping -c 3 127.0.0.1
PING 127.0.0.1 (127.0.0.1): 56 data bytes
64 bytes from 127.0.0.1: icmp_seq=0 ttl=64 time=0.058 ms
64 bytes from 127.0.0.1: icmp_seq=1 ttl=64 time=0.165 ms
64 bytes from 127.0.0.1: icmp_seq=2 ttl=64 time=0.164 ms

--- 127.0.0.1 ping statistics ---
3 packets transmitted, 3 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 0.058/0.129/0.165/0.050 ms

$ python app-server-test.py 127.0.0.1 65432
Listening on ('127.0.0.1', 65432)

$ python app-client-test.py 127.0.0.1 65432 binary test
Error: socket.send() blocking io exception for ('127.0.0.1', 65432):
BlockingIOError(35, 'Resource temporarily unavailable')

$ netstat -an | grep 65432
Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4  408300      0  127.0.0.1.65432        127.0.0.1.53225        ESTABLISHED
tcp4       0 269868  127.0.0.1.53225        127.0.0.1.65432        ESTABLISHED
tcp4       0      0  127.0.0.1.65432        *.*                    LISTEN

Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4  408300      0  127.0.0.1.65432        127.0.0.1.53225        ESTABLISHED

Proto Recv-Q Send-Q  Local Address          Foreign Address        (state)
tcp4       0 269868  127.0.0.1.53225        127.0.0.1.65432        ESTABLISHED

$ tshark -i lo0 'tcp port 65432'
Capturing on 'Loopback'
    1   0.000000    127.0.0.1 → 127.0.0.1    TCP 68 53942 → 65432 [SYN] Seq=0 Win=65535 Len=0 MSS=16344 WS=32 TSval=940533635 TSecr=0 SACK_PERM=1
    2   0.000057    127.0.0.1 → 127.0.0.1    TCP 68 65432 → 53942 [SYN, ACK] Seq=0 Ack=1 Win=65535 Len=0 MSS=16344 WS=32 TSval=940533635 TSecr=940533635 SACK_PERM=1
    3   0.000068    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [ACK] Seq=1 Ack=1 Win=408288 Len=0 TSval=940533635 TSecr=940533635
    4   0.000075    127.0.0.1 → 127.0.0.1    TCP 56 [TCP Window Update] 65432 → 53942 [ACK] Seq=1 Ack=1 Win=408288 Len=0 TSval=940533635 TSecr=940533635
    5   0.000216    127.0.0.1 → 127.0.0.1    TCP 202 53942 → 65432 [PSH, ACK] Seq=1 Ack=1 Win=408288 Len=146 TSval=940533635 TSecr=940533635
    6   0.000234    127.0.0.1 → 127.0.0.1    TCP 56 65432 → 53942 [ACK] Seq=1 Ack=147 Win=408128 Len=0 TSval=940533635 TSecr=940533635
    7   0.000627    127.0.0.1 → 127.0.0.1    TCP 204 65432 → 53942 [PSH, ACK] Seq=1 Ack=147 Win=408128 Len=148 TSval=940533635 TSecr=940533635
    8   0.000649    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [ACK] Seq=147 Ack=149 Win=408128 Len=0 TSval=940533635 TSecr=940533635
    9   0.000668    127.0.0.1 → 127.0.0.1    TCP 56 65432 → 53942 [FIN, ACK] Seq=149 Ack=147 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   10   0.000682    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [ACK] Seq=147 Ack=150 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   11   0.000687    127.0.0.1 → 127.0.0.1    TCP 56 [TCP Dup ACK 6#1] 65432 → 53942 [ACK] Seq=150 Ack=147 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   12   0.000848    127.0.0.1 → 127.0.0.1    TCP 56 53942 → 65432 [FIN, ACK] Seq=147 Ack=150 Win=408128 Len=0 TSval=940533635 TSecr=940533635
   13   0.001004    127.0.0.1 → 127.0.0.1    TCP 56 65432 → 53942 [ACK] Seq=150 Ack=148 Win=408128 Len=0 TSval=940533635 TSecr=940533635
^C13 packets captured

>>> socket.getaddrinfo("example.org", 80, proto=socket.IPPROTO_TCP)
[(<AddressFamily.AF_INET6: 10>, <SocketType.SOCK_STREAM: 1>,
 6, '', ('2606:2800:220:1:248:1893:25c8:1946', 80, 0, 0)),
 (<AddressFamily.AF_INET: 2>, <SocketType.SOCK_STREAM: 1>,
 6, '', ('93.184.216.34', 80))]

import struct
network_byteorder_int = struct.pack('>H', 256)
python_int = struct.unpack('>H', network_byteorder_int)[0]

socket — Low-level networking interface

This module provides access to the BSD socket interface. It is available on all modern Unix systems, Windows, Mac OS X, BeOS, OS/2, and probably additional platforms.

For an introduction to socket programming (in C), see the following papers: An Introductory 4.3BSD Interprocess Communication Tutorial, by Stuart Sechrest and An Advanced 4.3BSD Interprocess Communication Tutorial, by Samuel J. Leffler et al, both in the UNIX Programmer’s Manual, Supplementary Documents 1 (sections PS1:7 and PS1:8). The platform-specific reference material for the various socket-related system calls are also a valuable source of information on the details of socket semantics. For Unix, refer to the manual pages; for Windows, see the WinSock (or Winsock 2) specification. For IPv6-ready APIs, readers may want to refer to RFC 3493 titled Basic Socket Interface Extensions for IPv6.

The Python interface is a straightforward transliteration of the Unix system call and library interface for sockets to Python’s object-oriented style: the socket() function returns a socket object whose methods implement the various socket system calls. Parameter types are somewhat higher-level than in the C interface: as with read() and write() operations on Python files, buffer allocation on receive operations is automatic, and buffer length is implicit on send operations.

Socket addresses are represented as follows: A single string is used for the AF_UNIX address family. A pair (host, port) is used for the AF_INET address family, where host is a string representing either a hostname in Internet domain notation like 'daring.cwi.nl' or an IPv4 address like '100.50.200.5', and port is an integer. For AF_INET6 address family, a four-tuple (host, port, flowinfo,
scopeid) is used, where flowinfo and scopeid represents sin6_flowinfo and sin6_scope_id member in struct sockaddr_in6 in C. For socket module methods, flowinfo and scopeid can be omitted just for backward compatibility. Note, however, omission of scopeid can cause problems in manipulating scoped IPv6 addresses. Other address families are currently not supported. The address format required by a particular socket object is automatically selected based on the address family specified when the socket object was created.

For IPv4 addresses, two special forms are accepted instead of a host address: the empty string represents INADDR_ANY, and the string '<broadcast>' represents INADDR_BROADCAST. The behavior is not available for IPv6 for backward compatibility, therefore, you may want to avoid these if you intend to support IPv6 with your Python programs.

If you use a hostname in the host portion of IPv4/v6 socket address, the program may show a nondeterministic behavior, as Python uses the first address returned from the DNS resolution. The socket address will be resolved differently into an actual IPv4/v6 address, depending on the results from DNS resolution and/or the host configuration. For deterministic behavior use a numeric address in host portion.

New in version 2.5: AF_NETLINK sockets are represented as pairs pid, groups.

All errors raise exceptions. The normal exceptions for invalid argument types and out-of-memory conditions can be raised; errors related to socket or address semantics raise the error socket.error.

Non-blocking mode is supported through setblocking(). A generalization of this based on timeouts is supported through settimeout().

The module socket exports the following constants and functions:

exception socket.error

This exception is raised for socket-related errors. The accompanying value is either a string telling what went wrong or a pair (errno, string) representing an error returned by a system call, similar to the value accompanying os.error. See the module errno, which contains names for the error codes defined by the underlying operating system.

Changed in version 2.6: socket.error is now a child class of IOError.

exception socket.herror

This exception is raised for address-related errors, i.e. for functions that use h_errno in the C API, including gethostbyname_ex() and gethostbyaddr().

The accompanying value is a pair (h_errno, string) representing an error returned by a library call. string represents the description of h_errno, as returned by the hstrerror() C function.

exception socket.gaierror

This exception is raised for address-related errors, for getaddrinfo() and getnameinfo(). The accompanying value is a pair (error, string) representing an error returned by a library call. string represents the description of error, as returned by the gai_strerror() C function. The error value will match one of the EAI_* constants defined in this module.

exception socket.timeout

This exception is raised when a timeout occurs on a socket which has had timeouts enabled via a prior call to settimeout(). The accompanying value is a string whose value is currently always “timed out”.

New in version 2.3.

socket.AF_UNIX

socket.AF_INET

socket.AF_INET6

These constants represent the address (and protocol) families, used for the first argument to socket(). If the AF_UNIX constant is not defined then this protocol is unsupported.

socket.SOCK_STREAM

socket.SOCK_DGRAM

socket.SOCK_RAW

socket.SOCK_RDM

socket.SOCK_SEQPACKET

These constants represent the socket types, used for the second argument to socket(). (Only SOCK_STREAM and SOCK_DGRAM appear to be generally useful.)

SO_*

socket.SOMAXCONN

MSG_*

SOL_*

IPPROTO_*

IPPORT_*

INADDR_*

IP_*

IPV6_*

EAI_*

AI_*

NI_*

TCP_*

Many constants of these forms, documented in the Unix documentation on sockets and/or the IP protocol, are also defined in the socket module. They are generally used in arguments to the setsockopt() and getsockopt() methods of socket objects. In most cases, only those symbols that are defined in the Unix header files are defined; for a few symbols, default values are provided.

SIO_*

RCVALL_*

Constants for Windows’ WSAIoctl(). The constants are used as arguments to the ioctl() method of socket objects.

New in version 2.6.

TIPC_*

TIPC related constants, matching the ones exported by the C socket API. See the TIPC documentation for more information.

New in version 2.6.

socket.has_ipv6

This constant contains a boolean value which indicates if IPv6 is supported on this platform.

New in version 2.3.

socket.create_connection(address[, timeout[, source_address]])

Connect to a TCP service listening on the Internet address (a 2-tuple (host, port)), and return the socket object. This is a higher-level function than socket.connect(): if host is a non-numeric hostname, it will try to resolve it for both AF_INET and AF_INET6, and then try to connect to all possible addresses in turn until a connection succeeds. This makes it easy to write clients that are compatible to both IPv4 and IPv6.

Passing the optional timeout parameter will set the timeout on the socket instance before attempting to connect. If no timeout is supplied, the global default timeout setting returned by getdefaulttimeout() is used.

If supplied, source_address must be a 2-tuple (host, port) for the socket to bind to as its source address before connecting. If host or port are ‘’ or 0 respectively the OS default behavior will be used.

New in version 2.6.

Changed in version 2.7: source_address was added.

socket.getaddrinfo(host, port[, family[, socktype[, proto[, flags]]]])

Translate the host/port argument into a sequence of 5-tuples that contain all the necessary arguments for creating a socket connected to that service. host is a domain name, a string representation of an IPv4/v6 address or None. port is a string service name such as 'http', a numeric port number or None. By passing None as the value of host and port, you can pass NULL to the underlying C API.

The family, socktype and proto arguments can be optionally specified in order to narrow the list of addresses returned. By default, their value is 0, meaning that the full range of results is selected. The flags argument can be one or several of the AI_* constants, and will influence how results are computed and returned. Its default value is 0. For example, AI_NUMERICHOST will disable domain name resolution and will raise an error if host is a domain name.

The function returns a list of 5-tuples with the following structure:

(family, socktype, proto, canonname, sockaddr)

In these tuples, family, socktype, proto are all integers and are meant to be passed to the socket() function. canonname will be a string representing the canonical name of the host if AI_CANONNAME is part of the flags argument; else canonname will be empty. sockaddr is a tuple describing a socket address, whose format depends on the returned family (a (address, port) 2-tuple for AF_INET, a (address, port, flow info, scope id) 4-tuple for AF_INET6), and is meant to be passed to the socket.connect() method.

The following example fetches address information for a hypothetical TCP connection to example.org on port 80 (results may differ on your system if IPv6 isn’t enabled):

>>> socket.getaddrinfo("example.org", 80, 0, 0, socket.IPPROTO_TCP)
[(10, 1, 6, '', ('2606:2800:220:1:248:1893:25c8:1946', 80, 0, 0)),
 (2, 1, 6, '', ('93.184.216.34', 80))]

New in version 2.2.

socket.getfqdn([name])

Return a fully qualified domain name for name. If name is omitted or empty, it is interpreted as the local host. To find the fully qualified name, the hostname returned by gethostbyaddr() is checked, followed by aliases for the host, if available. The first name which includes a period is selected. In case no fully qualified domain name is available, the hostname as returned by gethostname() is returned.

New in version 2.0.

socket.gethostbyname(hostname)

Translate a host name to IPv4 address format. The IPv4 address is returned as a string, such as '100.50.200.5'. If the host name is an IPv4 address itself it is returned unchanged. See gethostbyname_ex() for a more complete interface. gethostbyname() does not support IPv6 name resolution, and getaddrinfo() should be used instead for IPv4/v6 dual stack support.

socket.gethostbyname_ex(hostname)

Translate a host name to IPv4 address format, extended interface. Return a triple (hostname, aliaslist, ipaddrlist) where hostname is the primary host name responding to the given ip_address, aliaslist is a (possibly empty) list of alternative host names for the same address, and ipaddrlist is a list of IPv4 addresses for the same interface on the same host (often but not always a single address). gethostbyname_ex() does not support IPv6 name resolution, and getaddrinfo() should be used instead for IPv4/v6 dual stack support.

socket.gethostname()

Return a string containing the hostname of the machine where the Python interpreter is currently executing.

If you want to know the current machine’s IP address, you may want to use gethostbyname(gethostname()). This operation assumes that there is a valid address-to-host mapping for the host, and the assumption does not always hold.

Note: gethostname() doesn’t always return the fully qualified domain name; use getfqdn() (see above).

socket.gethostbyaddr(ip_address)

Return a triple (hostname, aliaslist, ipaddrlist) where hostname is the primary host name responding to the given ip_address, aliaslist is a (possibly empty) list of alternative host names for the same address, and ipaddrlist is a list of IPv4/v6 addresses for the same interface on the same host (most likely containing only a single address). To find the fully qualified domain name, use the function getfqdn(). gethostbyaddr() supports both IPv4 and IPv6.

socket.getnameinfo(sockaddr, flags)

Translate a socket address sockaddr into a 2-tuple (host, port). Depending on the settings of flags, the result can contain a fully-qualified domain name or numeric address representation in host. Similarly, port can contain a string port name or a numeric port number.

New in version 2.2.

socket.getprotobyname(protocolname)

Translate an Internet protocol name (for example, 'icmp') to a constant suitable for passing as the (optional) third argument to the socket() function. This is usually only needed for sockets opened in “raw” mode (SOCK_RAW); for the normal socket modes, the correct protocol is chosen automatically if the protocol is omitted or zero.

socket.getservbyname(servicename[, protocolname])

Translate an Internet service name and protocol name to a port number for that service. The optional protocol name, if given, should be 'tcp' or 'udp', otherwise any protocol will match.

socket.getservbyport(port[, protocolname])

Translate an Internet port number and protocol name to a service name for that service. The optional protocol name, if given, should be 'tcp' or 'udp', otherwise any protocol will match.

socket.socket([family[, type[, proto]]])

Create a new socket using the given address family, socket type and protocol number. The address family should be AF_INET (the default), AF_INET6 or AF_UNIX. The socket type should be SOCK_STREAM (the default), SOCK_DGRAM or perhaps one of the other SOCK_ constants. The protocol number is usually zero and may be omitted in that case.

socket.socketpair([family[, type[, proto]]])

Build a pair of connected socket objects using the given address family, socket type, and protocol number. Address family, socket type, and protocol number are as for the socket() function above. The default family is AF_UNIX if defined on the platform; otherwise, the default is AF_INET. Availability: Unix.

New in version 2.4.

socket.fromfd(fd, family, type[, proto])

Duplicate the file descriptor fd (an integer as returned by a file object’s fileno() method) and build a socket object from the result. Address family, socket type and protocol number are as for the socket() function above. The file descriptor should refer to a socket, but this is not checked — subsequent operations on the object may fail if the file descriptor is invalid. This function is rarely needed, but can be used to get or set socket options on a socket passed to a program as standard input or output (such as a server started by the Unix inet daemon). The socket is assumed to be in blocking mode. Availability: Unix.

socket.ntohl(x)

Convert 32-bit positive integers from network to host byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 4-byte swap operation.

socket.ntohs(x)

Convert 16-bit positive integers from network to host byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 2-byte swap operation.

socket.htonl(x)

Convert 32-bit positive integers from host to network byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 4-byte swap operation.

socket.htons(x)

Convert 16-bit positive integers from host to network byte order. On machines where the host byte order is the same as network byte order, this is a no-op; otherwise, it performs a 2-byte swap operation.

socket.inet_aton(ip_string)

Convert an IPv4 address from dotted-quad string format (for example, ‘123.45.67.89’) to 32-bit packed binary format, as a string four characters in length. This is useful when conversing with a program that uses the standard C library and needs objects of type struct in_addr, which is the C type for the 32-bit packed binary this function returns.

inet_aton() also accepts strings with less than three dots; see the Unix manual page inet(3) for details.

If the IPv4 address string passed to this function is invalid, socket.error will be raised. Note that exactly what is valid depends on the underlying C implementation of inet_aton().

inet_aton() does not support IPv6, and inet_pton() should be used instead for IPv4/v6 dual stack support.

socket.inet_ntoa(packed_ip)

Convert a 32-bit packed IPv4 address (a string four characters in length) to its standard dotted-quad string representation (for example, ‘123.45.67.89’). This is useful when conversing with a program that uses the standard C library and needs objects of type struct in_addr, which is the C type for the 32-bit packed binary data this function takes as an argument.

If the string passed to this function is not exactly 4 bytes in length, socket.error will be raised. inet_ntoa() does not support IPv6, and inet_ntop() should be used instead for IPv4/v6 dual stack support.

socket.inet_pton(address_family, ip_string)

Convert an IP address from its family-specific string format to a packed, binary format. inet_pton() is useful when a library or network protocol calls for an object of type struct in_addr (similar to inet_aton()) or struct in6_addr.

Supported values for address_family are currently AF_INET and AF_INET6. If the IP address string ip_string is invalid, socket.error will be raised. Note that exactly what is valid depends on both the value of address_family and the underlying implementation of inet_pton().

Availability: Unix (maybe not all platforms).

New in version 2.3.

socket.inet_ntop(address_family, packed_ip)

Convert a packed IP address (a string of some number of characters) to its standard, family-specific string representation (for example, '7.10.0.5' or '5aef:2b::8') inet_ntop() is useful when a library or network protocol returns an object of type struct in_addr (similar to inet_ntoa()) or struct in6_addr.

Supported values for address_family are currently AF_INET and AF_INET6. If the string packed_ip is not the correct length for the specified address family, ValueError will be raised. A socket.error is raised for errors from the call to inet_ntop().

Availability: Unix (maybe not all platforms).

New in version 2.3.

socket.getdefaulttimeout()

Return the default timeout in seconds (float) for new socket objects. A value of None indicates that new socket objects have no timeout. When the socket module is first imported, the default is None.

New in version 2.3.

socket.setdefaulttimeout(timeout)

Set the default timeout in seconds (float) for new socket objects. A value of None indicates that new socket objects have no timeout. When the socket module is first imported, the default is None.

New in version 2.3.

socket.SocketType

This is a Python type object that represents the socket object type. It is the same as type(socket(...)).

1. Socket Objects

Socket objects have the following methods. Except for makefile() these correspond to Unix system calls applicable to sockets.

socket.accept()

Accept a connection. The socket must be bound to an address and listening for connections. The return value is a pair (conn, address) where conn is a new socket object usable to send and receive data on the connection, and address is the address bound to the socket on the other end of the connection.

socket.bind(address)

Bind the socket to address. The socket must not already be bound. (The format of address depends on the address family — see above.)

Note

This method has historically accepted a pair of parameters for AF_INET addresses instead of only a tuple. This was never intentional and is no longer available in Python 2.0 and later.

socket.close()

Close the socket. All future operations on the socket object will fail. The remote end will receive no more data (after queued data is flushed). Sockets are automatically closed when they are garbage-collected.

Note

close() releases the resource associated with a connection but does not necessarily close the connection immediately. If you want to close the connection in a timely fashion, call shutdown() before close().

socket.connect(address)

Connect to a remote socket at address. (The format of address depends on the address family — see above.)

Note

This method has historically accepted a pair of parameters for AF_INET addresses instead of only a tuple. This was never intentional and is no longer available in Python 2.0 and later.

socket.connect_ex(address)

Like connect(address), but return an error indicator instead of raising an exception for errors returned by the C-level connect() call (other problems, such as “host not found,” can still raise exceptions). The error indicator is 0 if the operation succeeded, otherwise the value of the errno variable. This is useful to support, for example, asynchronous connects.

Note

This method has historically accepted a pair of parameters for AF_INET addresses instead of only a tuple. This was never intentional and is no longer available in Python 2.0 and later.

socket.fileno()

Return the socket’s file descriptor (a small integer). This is useful with select.select().

Under Windows the small integer returned by this method cannot be used where a file descriptor can be used (such as os.fdopen()). Unix does not have this limitation.

socket.getpeername()

Return the remote address to which the socket is connected. This is useful to find out the port number of a remote IPv4/v6 socket, for instance. (The format of the address returned depends on the address family — see above.) On some systems this function is not supported.

socket.getsockname()

Return the socket’s own address. This is useful to find out the port number of an IPv4/v6 socket, for instance. (The format of the address returned depends on the address family — see above.)

socket.getsockopt(level, optname[, buflen])

Return the value of the given socket option (see the Unix man page getsockopt(2)). The needed symbolic constants (SO_* etc.) are defined in this module. If buflen is absent, an integer option is assumed and its integer value is returned by the function. If buflen is present, it specifies the maximum length of the buffer used to receive the option in, and this buffer is returned as a string. It is up to the caller to decode the contents of the buffer (see the optional built-in module struct for a way to decode C structures encoded as strings).

socket.ioctl(control, option)

Platform

Windows

The ioctl() method is a limited interface to the WSAIoctl system interface. Please refer to the Win32 documentation for more information.

On other platforms, the generic fcntl.fcntl() and fcntl.ioctl() functions may be used; they accept a socket object as their first argument.

New in version 2.6.

socket.listen(backlog)

Listen for connections made to the socket. The backlog argument specifies the maximum number of queued connections and should be at least 0; the maximum value is system-dependent (usually 5), the minimum value is forced to 0.

socket.makefile([mode[, bufsize]])

Return a file object associated with the socket. (File objects are described in File Objects.) The file object does not close the socket explicitly when its close() method is called, but only removes its reference to the socket object, so that the socket will be closed if it is not referenced from anywhere else.

The socket must be in blocking mode (it can not have a timeout). The optional mode and bufsize arguments are interpreted the same way as by the built-in file() function.

Note

On Windows, the file-like object created by makefile() cannot be used where a file object with a file descriptor is expected, such as the stream arguments of subprocess.Popen().

socket.recv(bufsize[, flags])

Receive data from the socket. The return value is a string representing the data received. The maximum amount of data to be received at once is specified by bufsize. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero.

Note

For best match with hardware and network realities, the value of bufsize should be a relatively small power of 2, for example, 4096.

socket.recvfrom(bufsize[, flags])

Receive data from the socket. The return value is a pair (string, address) where string is a string representing the data received and address is the address of the socket sending the data. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero. (The format of address depends on the address family — see above.)

socket.recvfrom_into(buffer[, nbytes[, flags]])

Receive data from the socket, writing it into buffer instead of creating a new string. The return value is a pair (nbytes, address) where nbytes is the number of bytes received and address is the address of the socket sending the data. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero. (The format of address depends on the address family — see above.)

New in version 2.5.

socket.recv_into(buffer[, nbytes[, flags]])

Receive up to nbytes bytes from the socket, storing the data into a buffer rather than creating a new string. If nbytes is not specified (or 0), receive up to the size available in the given buffer. Returns the number of bytes received. See the Unix manual page recv(2) for the meaning of the optional argument flags; it defaults to zero.

New in version 2.5.

socket.send(string[, flags])

Send data to the socket. The socket must be connected to a remote socket. The optional flags argument has the same meaning as for recv() above. Returns the number of bytes sent. Applications are responsible for checking that all data has been sent; if only some of the data was transmitted, the application needs to attempt delivery of the remaining data. For further information on this concept, consult the Socket Programming HOWTO.

socket.sendall(string[, flags])

Send data to the socket. The socket must be connected to a remote socket. The optional flags argument has the same meaning as for recv() above. Unlike send(), this method continues to send data from string until either all data has been sent or an error occurs. None is returned on success. On error, an exception is raised, and there is no way to determine how much data, if any, was successfully sent.

socket.sendto(string, address)

socket.sendto(string, flags, address)

Send data to the socket. The socket should not be connected to a remote socket, since the destination socket is specified by address. The optional flags argument has the same meaning as for recv() above. Return the number of bytes sent. (The format of address depends on the address family — see above.)

socket.setblocking(flag)

Set blocking or non-blocking mode of the socket: if flag is 0, the socket is set to non-blocking, else to blocking mode. Initially all sockets are in blocking mode. In non-blocking mode, if a recv() call doesn’t find any data, or if a send() call can’t immediately dispose of the data, an error exception is raised; in blocking mode, the calls block until they can proceed. s.setblocking(0) is equivalent to s.settimeout(0.0); s.setblocking(1) is equivalent to s.settimeout(None).

socket.settimeout(value)

Set a timeout on blocking socket operations. The value argument can be a nonnegative float expressing seconds, or None. If a float is given, subsequent socket operations will raise a timeout exception if the timeout period value has elapsed before the operation has completed. Setting a timeout of None disables timeouts on socket operations. s.settimeout(0.0) is equivalent to s.setblocking(0); s.settimeout(None) is equivalent to s.setblocking(1).

New in version 2.3.

socket.gettimeout()

Return the timeout in seconds (float) associated with socket operations, or None if no timeout is set. This reflects the last call to setblocking() or settimeout().

New in version 2.3.

Some notes on socket blocking and timeouts: A socket object can be in one of three modes: blocking, non-blocking, or timeout. Sockets are always created in blocking mode. In blocking mode, operations block until complete or the system returns an error (such as connection timed out). In non-blocking mode, operations fail (with an error that is unfortunately system-dependent) if they cannot be completed immediately. In timeout mode, operations fail if they cannot be completed within the timeout specified for the socket or if the system returns an error. The setblocking() method is simply a shorthand for certain settimeout() calls.

Timeout mode internally sets the socket in non-blocking mode. The blocking and timeout modes are shared between file descriptors and socket objects that refer to the same network endpoint. A consequence of this is that file objects returned by the makefile() method must only be used when the socket is in blocking mode; in timeout or non-blocking mode file operations that cannot be completed immediately will fail.

Note that the connect() operation is subject to the timeout setting, and in general it is recommended to call settimeout() before calling connect() or pass a timeout parameter to create_connection(). The system network stack may return a connection timeout error of its own regardless of any Python socket timeout setting.

socket.setsockopt(level, optname, value)

Set the value of the given socket option (see the Unix manual page setsockopt(2)). The needed symbolic constants are defined in the socket module (SO_* etc.). The value can be an integer or a string representing a buffer. In the latter case it is up to the caller to ensure that the string contains the proper bits (see the optional built-in module struct for a way to encode C structures as strings).

socket.shutdown(how)

Shut down one or both halves of the connection. If how is SHUT_RD, further receives are disallowed. If how is SHUT_WR, further sends are disallowed. If how is SHUT_RDWR, further sends and receives are disallowed. Depending on the platform, shutting down one half of the connection can also close the opposite half (e.g. on Mac OS X, shutdown(SHUT_WR) does not allow further reads on the other end of the connection).

Note that there are no methods read() or write(); use recv() and send() without flags argument instead.

Socket objects also have these (read-only) attributes that correspond to the values given to the socket constructor.

socket.family

The socket family.

New in version 2.5.

socket.type

The socket type.

New in version 2.5.

socket.proto

The socket protocol.

New in version 2.5.

2. Example

Here are four minimal example programs using the TCP/IP protocol: a server that echoes all data that it receives back (servicing only one client), and a client using it. Note that a server must perform the sequence socket(), bind(), listen(), accept() (possibly repeating the accept() to service more than one client), while a client only needs the sequence socket(), connect(). Also note that the server does not sendall()/recv() on the socket it is listening on but on the new socket returned by accept().

The first two examples support IPv4 only.

# Echo server program
import socket

HOST = ''                 # Symbolic name meaning all available interfaces
PORT = 50007              # Arbitrary non-privileged port
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.bind((HOST, PORT))
s.listen(1)
conn, addr = s.accept()
print 'Connected by', addr
while 1:
    data = conn.recv(1024)
    if not data: break
    conn.sendall(data)
conn.close()

# Echo client program
import socket

HOST = 'daring.cwi.nl'    # The remote host
PORT = 50007              # The same port as used by the server
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
s.sendall('Hello, world')
data = s.recv(1024)
s.close()
print 'Received', repr(data)

The next two examples are identical to the above two, but support both IPv4 and IPv6. The server side will listen to the first address family available (it should listen to both instead). On most of IPv6-ready systems, IPv6 will take precedence and the server may not accept IPv4 traffic. The client side will try to connect to the all addresses returned as a result of the name resolution, and sends traffic to the first one connected successfully.

# Echo server program
import socket
import sys

HOST = None               # Symbolic name meaning all available interfaces
PORT = 50007              # Arbitrary non-privileged port
s = None
for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC,
                              socket.SOCK_STREAM, 0, socket.AI_PASSIVE):
    af, socktype, proto, canonname, sa = res
    try:
        s = socket.socket(af, socktype, proto)
    except socket.error as msg:
        s = None
        continue
    try:
        s.bind(sa)
        s.listen(1)
    except socket.error as msg:
        s.close()
        s = None
        continue
    break
if s is None:
    print 'could not open socket'
    sys.exit(1)
conn, addr = s.accept()
print 'Connected by', addr
while 1:
    data = conn.recv(1024)
    if not data: break
    conn.send(data)
conn.close()

# Echo client program
import socket
import sys

HOST = 'daring.cwi.nl'    # The remote host
PORT = 50007              # The same port as used by the server
s = None
for res in socket.getaddrinfo(HOST, PORT, socket.AF_UNSPEC, socket.SOCK_STREAM):
    af, socktype, proto, canonname, sa = res
    try:
        s = socket.socket(af, socktype, proto)
    except socket.error as msg:
        s = None
        continue
    try:
        s.connect(sa)
    except socket.error as msg:
        s.close()
        s = None
        continue
    break
if s is None:
    print 'could not open socket'
    sys.exit(1)
s.sendall('Hello, world')
data = s.recv(1024)
s.close()
print 'Received', repr(data)

The last example shows how to write a very simple network sniffer with raw sockets on Windows. The example requires administrator privileges to modify the interface:

import socket

# the public network interface
HOST = socket.gethostbyname(socket.gethostname())

# create a raw socket and bind it to the public interface
s = socket.socket(socket.AF_INET, socket.SOCK_RAW, socket.IPPROTO_IP)
s.bind((HOST, 0))

# Include IP headers
s.setsockopt(socket.IPPROTO_IP, socket.IP_HDRINCL, 1)

# receive all packages
s.ioctl(socket.SIO_RCVALL, socket.RCVALL_ON)

# receive a package
print s.recvfrom(65565)

# disabled promiscuous mode
s.ioctl(socket.SIO_RCVALL, socket.RCVALL_OFF)

Running an example several times with too small delay between executions, could lead to this error:

socket.error: [Errno 98] Address already in use

This is because the previous execution has left the socket in a TIME_WAIT state, and can’t be immediately reused.

There is a socket flag to set, in order to prevent this, socket.SO_REUSEADDR:

s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
s.bind((HOST, PORT))

the SO_REUSEADDR flag tells the kernel to reuse a local socket in TIME_WAIT state, without waiting for its natural timeout to expire.