:mod:`socket` --- Low-level networking interface

================================================

.. module:: socket

   :synopsis: 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.

.. note::

   Some behavior may be platform dependent, since calls are made to the operating

   system socket APIs.

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.

.. index:: object: socket

The Python interface is a straightforward transliteration of the Unix system

call and library interface for sockets to Python's object-oriented style: the

:func:`socket` function returns a :dfn:`socket object` whose methods implement

the various socket system calls.  Parameter types are somewhat higher-level than

in the C interface: as with :meth:`read` and :meth:`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

:const:`AF_UNIX` address family. A pair ``(host, port)`` is used for the

:const:`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 integral port number. For

:const:`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 :const:`struct sockaddr_in6` in C. For

:mod:`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 :const:`INADDR_ANY`, and the string

``'<broadcast>'`` represents :const:`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.

.. versionadded:: 2.5

   AF_NETLINK sockets are represented as  pairs ``pid, groups``.

.. versionadded:: 2.6

   Linux-only support for TIPC is also available using the :const:`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.

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 :exc:`socket.error`.

Non-blocking mode is supported through :meth:`setblocking`.  A generalization of

this based on timeouts is supported through :meth:`settimeout`.

The module :mod:`socket` exports the following constants and functions:

.. exception:: error

   .. index:: module: errno

   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 :exc:`os.error`. See the module :mod:`errno`, which contains names

   for the error codes defined by the underlying operating system.

   .. versionchanged:: 2.6

      :exc:`socket.error` is now a child class of :exc:`IOError`.

.. exception:: herror

   This exception is raised for address-related errors, i.e. for functions that use

   *h_errno* in the C API, including :func:`gethostbyname_ex` and

   :func:`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 :cfunc:`hstrerror` C function.

.. exception:: gaierror

   This exception is raised for address-related errors, for :func:`getaddrinfo` and

   :func:`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 :cfunc:`gai_strerror` C function. The

   *error* value will match one of the :const:`EAI_\*` constants defined in this

   module.

.. exception:: timeout

   This exception is raised when a timeout occurs on a socket which has had

   timeouts enabled via a prior call to :meth:`settimeout`.  The accompanying value

   is a string whose value is currently always "timed out".

   .. versionadded:: 2.3

.. data:: AF_UNIX

          AF_INET

          AF_INET6

   These constants represent the address (and protocol) families, used for the

   first argument to :func:`socket`.  If the :const:`AF_UNIX` constant is not

   defined then this protocol is unsupported.

.. data:: SOCK_STREAM

          SOCK_DGRAM

          SOCK_RAW

          SOCK_RDM

          SOCK_SEQPACKET

   These constants represent the socket types, used for the second argument to

   :func:`socket`. (Only :const:`SOCK_STREAM` and :const:`SOCK_DGRAM` appear to be

   generally useful.)

.. data:: SO_*

          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 :meth:`setsockopt` and :meth:`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.

.. data:: SIO_*

          RCVALL_*

   Constants for Windows' WSAIoctl(). The constants are used as arguments to the

   :meth:`ioctl` method of socket objects.

   .. versionadded:: 2.6

.. data:: TIPC_*

   TIPC related constants, matching the ones exported by the C socket API. See

   the TIPC documentation for more information.

   .. versionadded:: 2.6

.. data:: has_ipv6

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

   this platform.

   .. versionadded:: 2.3

.. function:: create_connection(address[, timeout])

   Convenience function.  Connect to *address* (a 2-tuple ``(host, port)``),

   and return the socket object.  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

   :func:`getdefaulttimeout` is used.

   .. versionadded:: 2.6

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

   Resolves the *host*/*port* argument, into a sequence of 5-tuples that contain

   all the necessary arguments for creating the corresponding socket. *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``.

   The rest of the arguments are optional and must be numeric if specified.  

   By passing ``None`` as the value of *host* and *port*, , you can pass ``NULL`` to the C API.

   The :func:`getaddrinfo` function returns a list of 5-tuples with the following

   structure:

   ``(family, socktype, proto, canonname, sockaddr)``

   *family*, *socktype*, *proto* are all integers and are meant to be passed to the

   :func:`socket` function. *canonname* is a string representing the canonical name

   of the *host*. It can be a numeric IPv4/v6 address when :const:`AI_CANONNAME` is

   specified for a numeric *host*. *sockaddr* is a tuple describing a socket

   address, as described above. See the source for :mod:`socket` and other

   library modules for a typical usage of the function.

   .. versionadded:: 2.2

.. function:: 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 :func:`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

   :func:`gethostname` is returned.

   .. versionadded:: 2.0

.. function:: 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 :func:`gethostbyname_ex` for a more complete

   interface. :func:`gethostbyname` does not support IPv6 name resolution, and

   :func:`getaddrinfo` should be used instead for IPv4/v6 dual stack support.

.. function:: 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). :func:`gethostbyname_ex` does not support IPv6 name

   resolution, and :func:`getaddrinfo` should be used instead for IPv4/v6 dual

   stack support.

.. function:: 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: :func:`gethostname` doesn't always return the fully qualified domain

   name; use ``getfqdn()`` (see above).

.. function:: 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 :func:`getfqdn`. :func:`gethostbyaddr` supports

   both IPv4 and IPv6.

.. function:: 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.

   .. versionadded:: 2.2

.. function:: getprotobyname(protocolname)

   Translate an Internet protocol name (for example, ``'icmp'``) to a constant

   suitable for passing as the (optional) third argument to the :func:`socket`

   function.  This is usually only needed for sockets opened in "raw" mode

   (:const:`SOCK_RAW`); for the normal socket modes, the correct protocol is chosen

   automatically if the protocol is omitted or zero.

.. function:: 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.

.. function:: 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.

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

   Create a new socket using the given address family, socket type and protocol

   number.  The address family should be :const:`AF_INET` (the default),

   :const:`AF_INET6` or :const:`AF_UNIX`.  The socket type should be

   :const:`SOCK_STREAM` (the default), :const:`SOCK_DGRAM` or perhaps one of the

   other ``SOCK_`` constants.  The protocol number is usually zero and may be

   omitted in that case.

.. function:: 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 :func:`socket` function above. The default family is :const:`AF_UNIX`

   if defined on the platform; otherwise, the default is :const:`AF_INET`.

   Availability: Unix.

   .. versionadded:: 2.4

.. function:: fromfd(fd, family, type[, proto])

   Duplicate the file descriptor *fd* (an integer as returned by a file object's

   :meth:`fileno` method) and build a socket object from the result.  Address

   family, socket type and protocol number are as for the :func:`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.

.. function:: 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.

.. function:: 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.

.. function:: 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.

.. function:: 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.

.. function:: 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 :ctype:`struct in_addr`, which is the C type

   for the 32-bit packed binary this function returns.

   If the IPv4 address string passed to this function is invalid,

   :exc:`socket.error` will be raised. Note that exactly what is valid depends on

   the underlying C implementation of :cfunc:`inet_aton`.

   :func:`inet_aton` does not support IPv6, and :func:`getnameinfo` should be used

   instead for IPv4/v6 dual stack support.

.. function:: 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 :ctype:`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,

   :exc:`socket.error` will be raised. :func:`inet_ntoa` does not support IPv6, and

   :func:`getnameinfo` should be used instead for IPv4/v6 dual stack support.

.. function:: inet_pton(address_family, ip_string)

   Convert an IP address from its family-specific string format to a packed, binary

   format. :func:`inet_pton` is useful when a library or network protocol calls for

   an object of type :ctype:`struct in_addr` (similar to :func:`inet_aton`) or

   :ctype:`struct in6_addr`.

   Supported values for *address_family* are currently :const:`AF_INET` and

   :const:`AF_INET6`. If the IP address string *ip_string* is invalid,

   :exc:`socket.error` will be raised. Note that exactly what is valid depends on

   both the value of *address_family* and the underlying implementation of

   :cfunc:`inet_pton`.

   Availability: Unix (maybe not all platforms).

   .. versionadded:: 2.3

.. function:: 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'``) :func:`inet_ntop` is useful when a library or network protocol

   returns an object of type :ctype:`struct in_addr` (similar to :func:`inet_ntoa`)

   or :ctype:`struct in6_addr`.

   Supported values for *address_family* are currently :const:`AF_INET` and

   :const:`AF_INET6`. If the string *packed_ip* is not the correct length for the

   specified address family, :exc:`ValueError` will be raised.  A

   :exc:`socket.error` is raised for errors from the call to :func:`inet_ntop`.

   Availability: Unix (maybe not all platforms).

   .. versionadded:: 2.3

.. function:: getdefaulttimeout()

   Return the default timeout in floating seconds 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``.

   .. versionadded:: 2.3

.. function:: setdefaulttimeout(timeout)

   Set the default timeout in floating seconds 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``.

   .. versionadded:: 2.3

.. data:: SocketType

   This is a Python type object that represents the socket object type. It is the

   same as ``type(socket(...))``.

.. seealso::

   Module :mod:`SocketServer`

      Classes that simplify writing network servers.

.. _socket-objects:

Socket Objects

--------------

Socket objects have the following methods.  Except for :meth:`makefile` these

correspond to Unix system calls applicable to sockets.

.. method:: 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.

.. method:: 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 :const:`AF_INET`

      addresses instead of only a tuple.  This was never intentional and is no longer

      available in Python 2.0 and later.

.. method:: 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.

.. method:: socket.connect(address)