# 7.2 `socket` -- Low-level networking interface

This module provides access to the BSD socket interface. It is available on all modern Unix systems, Windows, MacOS, 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 2553 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 integral port number. 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.

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 the `setblocking()` method.

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

exception `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.

exception `herror`
  This exception is raised for address-related errors, i.e. for functions that use h_errno in 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 `hstrerror` C API.

exception `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 `gai_strerror` C API.

`AF_UNIX`
`AF_INET`
`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.

`SOCK_STREAM`
`SOCK_DGRAM`
`SOCK_RAW`
`SOCK_RDM`
`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_*`
`SOMAXCONN`
`MSG_*`
`SOL_*`
`IPPROTO_*`
`IPPORT_*`
`INADDR_*`
`IP_*`
`IPV6_*`
`EAI_*`
`AI_*`
`NI_*`
  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.

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

  Resolves the host/port argument, into a sequence of 5-tuples that contain all the necessary argument for the sockets manipulation. host is a domain name, a string representation of IPv4/v6 address or `None`. port is a string service name (like ```http''`), a numeric port number or `None`.

  The rest of the arguments are optional and must be numeric if specified. For host and port, by passing either an empty string or `None`, you can pass `NULL` to the C API. The `getaddrinfo()` function returns a list of 5-tuples with the following structure:

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

  family, socktype, proto are all integer and are meant to be passed to the `socket()` function. canonname is a string representing the canonical name of the host. It can be a numeric IPv4/v6 address when `AI_CANONNAME` is specified for a numeric host. sockaddr is a tuple describing a socket address, as described above. See `Lib/httplib.py` and other library files for a typical usage of the function. New in version 2.2.

`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, then 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 is returned. New in version 2.0.

`gethostbyname`(hostname)
  Translate a host name to IPv4 address format. The IPv4 address is returned as a string, e.g., `'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.

`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.

`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 `gethostbyaddr(gethostname())` (see below).

`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.

`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.

`getprotobyname`(protocolname)
  Translate an Internet protocol name (e.g. `'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.

`getservbyname`(servicename, protocolname)
  Translate an Internet service name and protocol name to a port number for that service. The protocol name should be `'tcp'` or `'udp'`.

`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`, `AF_INET6` or `AF_UNIX`. The socket type should be `SOCK_STREAM`, `SOCK_DGRAM` or perhaps one of the other "`SOCK_`" constants. The protocol number is usually zero and may be omitted in that case.

`ssl`(sock[, keyfile, certfile])
  Initiate a SSL connection over the socket sock. keyfile is the name of a PEM formatted file that contains your private key. certfile is a PEM formatted certificate chain file. On success, a new `SSLObject` is returned.

  Warning: This does not do any certificate verification!

`fromfd`(fd, family, type[, proto])
  Build a socket object from an existing file descriptor (an integer as returned by a file object's `fileno()` method). 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 (e.g. a server started by the Unix inet daemon).

`ntohl`(x)
  Convert 32-bit 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.

`ntohs`(x)
  Convert 16-bit 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.

`htonl`(x)
  Convert 32-bit 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.

`htons`(x)
  Convert 16-bit 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.

`inet_aton`(ip_string)
  Convert an IPv4 address from dotted-quad string format (e.g. '123.45.67.89') to 32-bit packed binary format, as a string four characters in length.

  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.

  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 `getnameinfo()` should be used instead for IPv4/v6 dual stack support.

`inet_ntoa`(packed_ip)
  Convert a 32-bit packed IPv4 address (a string four characters in length) to its standard dotted-quad string representation (e.g. '123.45.67.89').

  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 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 `getnameinfo()` should be used instead for IPv4/v6 dual stack support.

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

See Also:

Module `SocketServer`:
  Classes that simplify writing network servers.

Subsections

- 7.2.1 Socket Objects
- 7.2.2 SSL Objects
- 7.2.3 Example