This module provides access to the BSD socket interface. It is available on all modern Unix systems, Windows, MacOS, OS/2, and probably additional platforms.
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.
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 '18.104.22.168', 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.
AF_NETLINK sockets are represented as pairs pid, groups.
Linux-only support for TIPC is also 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.
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:
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.
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.
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 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 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 the source for socket and other library modules for a typical usage of the function.
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).
Convert an IPv4 address from dotted-quad string format (for example, ‘22.214.171.124’) 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.
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.
Convert a 32-bit packed IPv4 address (a bytes object four characters in length) to its standard dotted-quad string representation (for example, ‘126.96.36.199’). 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, socket.error will be raised. inet_ntoa() does not support IPv6, and getnameinfo() should be used instead for IPv4/v6 dual stack support.
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).
Convert a packed IP address (a bytes object of some number of characters) to its standard, family-specific string representation (for example, '188.8.131.52' 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).
Socket objects have the following methods. Except for makefile() these correspond to Unix system calls applicable to sockets.
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.
The ioctl() method is a limited interface to the WSAIoctl system interface. Please refer to the MSDN documentation for more information.
Return a file object associated with the socket. (File objects are described in File Objects.) The file object references a dupped version of the socket file descriptor, so the file object and socket object may be closed or garbage-collected independently. 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.
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.
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. 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. 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().
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 bytes object representing a buffer. In the latter 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).
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.
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 send()/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 True: data = conn.recv(1024) if not data: break conn.send(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.send(b'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 True: 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.send(b'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)