Python exception socket error

Note

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

See also

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.

• 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
'<broadcast>' 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.

18.1.3. 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().

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.

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).

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.)

socket.close()¶

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.

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.)

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).

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.

socket.detach()¶

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.

socket.dup()¶

Duplicate the socket.

The newly created socket is non-inheritable.

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

socket.fileno()¶

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.

socket.get_inheritable()¶

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.

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 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).

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().

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.

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.

socket.listen([backlog])¶

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.

socket.makefile(mode=’r’, buffering=None, *, encoding=None, errors=None, newline=None)¶

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.

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 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.

Note

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).

socket.recvfrom(bufsize[, flags])¶

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).

socket.recvmsg(bufsize[, ancbufsize[, flags]])¶

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().

import socket, array

def recv_fds(sock, msglen, maxfds):
    fds = array.array("i")   # Array of ints
    msg, ancdata, flags, addr = sock.recvmsg(msglen, socket.CMSG_LEN(maxfds * fds.itemsize))
    for cmsg_level, cmsg_type, cmsg_data in ancdata:
        if (cmsg_level == socket.SOL_SOCKET and cmsg_type == socket.SCM_RIGHTS):
            # Append data, ignoring any truncated integers at the end.
            fds.fromstring(cmsg_data[:len(cmsg_data) - (len(cmsg_data) % fds.itemsize)])
    return msg, list(fds)

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).

socket.recvmsg_into(buffers[, ancbufsize[, flags]])¶

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().

Example:

>>> import socket
>>> s1, s2 = socket.socketpair()
>>> b1 = bytearray(b'----')
>>> b2 = bytearray(b'0123456789')
>>> b3 = bytearray(b'--------------')
>>> s1.send(b'Mary had a little lamb')
22
>>> s2.recvmsg_into([b1, memoryview(b2)[2:9], b3])
(22, [], 0, None)
>>> [b1, b2, b3]
[bytearray(b'Mary'), bytearray(b'01 had a 9'), bytearray(b'little lamb---')]

Availability: most Unix platforms, possibly others.

New in version 3.3.

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

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).

socket.sendall(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.
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).

socket.sendto(bytes, address)¶

socket.sendto(bytes, 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.)

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).

socket.sendmsg(buffers[, ancdata[, flags[, address]]])¶

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().

import socket, array

def send_fds(sock, msg, fds):
    return sock.sendmsg([msg], [(socket.SOL_SOCKET, socket.SCM_RIGHTS, array.array("i", fds))])

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).

socket.sendmsg_afalg([msg, ]*, op[, iv[, assoclen[, flags]]])¶

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.

socket.sendfile(file, offset=0, count=None)¶

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.

socket.set_inheritable(inheritable)¶

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

New in version 3.4.

socket.setblocking(flag)¶

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

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.

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.

Availability: Windows.

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.

socket.family¶

The socket family.

socket.type¶

The socket type.

socket.proto¶

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.

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.

# Echo server program
import socket

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

# Echo client program
import socket

HOST = 'daring.cwi.nl'    # The remote host
PORT = 50007              # The same port as 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('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 OSError as msg:
        s = None
        continue
    try:
        s.bind(sa)
        s.listen(1)
    except OSError as msg:
        s.close()
        s = None
        continue
    break
if s is None:
    print('could not open socket')
    sys.exit(1)
conn, addr = s.accept()
with conn:
    print('Connected by', addr)
    while True:
        data = conn.recv(1024)
        if not data: break
        conn.send(data)

# 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 OSError as msg:
        s = None
        continue
    try:
        s.connect(sa)
    except OSError as msg:
        s.close()
        s = None
        continue
    break
if s is None:
    print('could not open socket')
    sys.exit(1)
with s:
    s.sendall(b'Hello, world')
    data = s.recv(1024)
print('Received', repr(data))

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:

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)

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:

socket.socket(socket.AF_CAN, socket.SOCK_DGRAM, socket.CAN_BCM)

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:

import socket
import struct


# CAN frame packing/unpacking (see 'struct can_frame' in <linux/can.h>)

can_frame_fmt = "=IB3x8s"
can_frame_size = struct.calcsize(can_frame_fmt)

def build_can_frame(can_id, data):
    can_dlc = len(data)
    data = data.ljust(8, b'x00')
    return struct.pack(can_frame_fmt, can_id, can_dlc, data)

def dissect_can_frame(frame):
    can_id, can_dlc, data = struct.unpack(can_frame_fmt, frame)
    return (can_id, can_dlc, data[:can_dlc])


# create a raw socket and bind it to the 'vcan0' interface
s = socket.socket(socket.AF_CAN, socket.SOCK_RAW, socket.CAN_RAW)
s.bind(('vcan0',))

while True:
    cf, addr = s.recvfrom(can_frame_size)

    print('Received: can_id=%x, can_dlc=%x, data=%s' % dissect_can_frame(cf))

    try:
        s.send(cf)
    except OSError:
        print('Error sending CAN frame')

    try:
        s.send(build_can_frame(0x01, b'x01x02x03'))
    except OSError:
        print('Error sending CAN frame')

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

OSError: [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.