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Addressing, Protocol Families and Socket Types

A socket is one endpoint of a communication channel used by programs to pass data back and forth locally or across the Internet. Sockets have two primary properties controlling the way they send data: the address family controls the OSI network layer protocol used and the socket type controls the transport layer protocol.

Python supports three address families. The most common, AF_INET, is used for IPv4 Internet addressing. IPv4 addresses are made up of four octal values separated by dots (e.g., 10.1.1.5 and 127.0.0.1). These values are more commonly referred to as “IP addresses.” Almost all Internet networking is done using IP version 4 at this time.

AF_INET6 is used for IPv6 Internet addressing. IPv6 is the “next generation” version of the Internet protocol, and supports 128-bit addresses, traffic shaping, and routing features not available under IPv4. Adoption of IPv6 is still limited, but continues to grow.

AF_UNIX is the address family for Unix Domain Sockets (UDS), an interprocess communication protocol available on POSIX-compliant systems. The implementation of UDS typically allows the operating system to pass data directly from process to process, without going through the network stack. This is more efficient than using AF_INET, but because the filesystem is used as the namespace for addressing, UDS is restricted to processes on the same system. The appeal of using UDS over other IPC mechanisms such as named pipes or shared memory is that the programming interface is the same as for IP networking, so the application can take advantage of efficient communication when running on a single host, but use the same code when sending data across the network.

Note

The AF_UNIX constant is only defined on systems where UDS is supported.

The socket type is usually either SOCK_DGRAM for user datagram protocol (UDP) or SOCK_STREAM for transmission control protocol (TCP). UDP does not require transmission handshaking or other setup, but offers lower reliability of delivery. UDP messages may be delivered out of order, more than once, or not at all. TCP, by contrast, ensures that each message is delivered exactly once, and in the correct order. Most application protocols that deliver a large amount of data, such as HTTP, are built on top of TCP. UDP is commonly used for protocols where order is less important (since the message fits in a single packet, i.e., DNS), or for multicasting (sending the same data to several hosts).

Note

Python’s socket module supports other socket types but they are less commonly used, so are not covered here. Refer to the standard library documentation for more details.

Looking up Hosts on the Network

socket includes functions to interface with the domain name services on the network, to convert the host name of a server into its numerical network address. Applications do not need to convert addresses explicitly before using them to connect to a server, but it can be useful when reporting errors to include the numerical address as well as the name value being used.

To find the official name of the current host, use gethostname().

import socket

print socket.gethostname()

The name returned will depend on the network settings for the current system, and may change if it is on a different network (such as a laptop attached to a wireless LAN).

$ python socket_gethostname.py

farnsworth.hellfly.net

Use gethostbyname() to convert the name of a server to its numerical address:

import socket

for host in [ 'homer', 'www', 'www.python.org', 'nosuchname' ]:
    try:
        print '%15s : %s' % (host, socket.gethostbyname(host))
    except socket.error, msg:
        print '%15s : ERROR: %s' % (host, msg)

The name argument does not need to be a fully qualified name (i.e., it does not need to include the domain name as well as the base hostname). If the name cannot be found, an exception of type socket.error is raised.

$ python socket_gethostbyname.py

          homer : ERROR: [Errno 8] nodename nor servname provided, or not known
            www : ERROR: [Errno 8] nodename nor servname provided, or not known
 www.python.org : 82.94.164.162
     nosuchname : ERROR: [Errno 8] nodename nor servname provided, or not known

For access to more naming information about a server, use gethostbyname_ex(). It returns the canonical hostname of the server, any aliases, and all of the available IP addresses that can be used to reach it.

import socket

for host in [ 'homer', 'www', 'www.python.org', 'nosuchname' ]:
    print host
    try:
        hostname, aliases, addresses = socket.gethostbyname_ex(host)
        print '  Hostname:', hostname
        print '  Aliases :', aliases
        print ' Addresses:', addresses
    except socket.error, msg:
        print '%15s : ERROR: %s' % (host, msg)
    print

Having all known IP addresses for a server lets a client implement its own load balancing or fail-over algorithms.

$ python socket_gethostbyname_ex.py

homer
          homer : ERROR: [Errno 8] nodename nor servname provided, or not known

www
            www : ERROR: [Errno 8] nodename nor servname provided, or not known

www.python.org
  Hostname: www.python.org
  Aliases : []
 Addresses: ['82.94.164.162']

nosuchname
     nosuchname : ERROR: [Errno 8] nodename nor servname provided, or not known

Use getfqdn() to convert a partial name to a fully qualified domain name.

import socket

for host in [ 'homer', 'www' ]:
    print '%6s : %s' % (host, socket.getfqdn(host))

The name returned will not necessarily match the input argument in any way if the input is an alias, such as www is here.

$ python socket_getfqdn.py

 homer : homer
   www : www

When the address of a server is available, use gethostbyaddr() to do a “reverse” lookup for the name.

import socket

hostname, aliases, addresses = socket.gethostbyaddr('192.168.1.8')

print 'Hostname :', hostname
print 'Aliases  :', aliases
print 'Addresses:', addresses

The return value is a tuple containing the full hostname, any aliases, and all IP addresses associated with the name.

$ python socket_gethostbyaddr.py

Hostname : homer.hellfly.net
Aliases  : ['8.1.168.192.in-addr.arpa']
Addresses: ['192.168.1.8']

Finding Service Information

In addition to an IP address, each socket address includes an integer port number. Many applications can run on the same host, listening on a single IP address, but only one socket at a time can use a port at that address. The combination of IP address, protocol, and port number uniquely identify a communication channel and ensure that messages sent through a socket arrive at the correct destination.

Some of the port numbers are pre-allocated for a specific protocol. For example, communication between email servers using SMTP occurs over port number 25 using TCP, and web clients and servers use port 80 for HTTP. The port numbers for network services with standardized names can be looked up with getservbyname().

import socket
from urlparse import urlparse

for url in [ 'http://www.python.org',
             'https://www.mybank.com',
             'ftp://prep.ai.mit.edu',
             'gopher://gopher.micro.umn.edu',
             'smtp://mail.example.com',
             'imap://mail.example.com',
             'imaps://mail.example.com',
             'pop3://pop.example.com',
             'pop3s://pop.example.com',
             ]:
    parsed_url = urlparse(url)
    port = socket.getservbyname(parsed_url.scheme)
    print '%6s : %s' % (parsed_url.scheme, port)

Although a standardized service is unlikely to change ports, looking up the value with a system call instead of hard-coding it is more flexible when new services are added in the future.

$ python socket_getservbyname.py

  http : 80
 https : 443
   ftp : 21
gopher : 70
  smtp : 25
  imap : 143
 imaps : 993
  pop3 : 110
 pop3s : 995

To reverse the service port lookup, use getservbyport().

import socket
import urlparse

for port in [ 80, 443, 21, 70, 25, 143, 993, 110, 995 ]:
    print urlparse.urlunparse(
        (socket.getservbyport(port), 'example.com', '/', '', '', '')
        )

The reverse lookup is useful for constructing URLs to services from arbitrary addresses.

$ python socket_getservbyport.py

http://example.com/
https://example.com/
ftp://example.com/
gopher://example.com/
smtp://example.com/
imap://example.com/
imaps://example.com/
pop3://example.com/
pop3s://example.com/

The number assigned to a transport protocol can be retrieved with getprotobyname().

import socket

def get_constants(prefix):
    """Create a dictionary mapping socket module constants to their names."""
    return dict( (getattr(socket, n), n)
                 for n in dir(socket)
                 if n.startswith(prefix)
                 )

protocols = get_constants('IPPROTO_')

for name in [ 'icmp', 'udp', 'tcp' ]:
    proto_num = socket.getprotobyname(name)
    const_name = protocols[proto_num]
    print '%4s -> %2d (socket.%-12s = %2d)' % \
        (name, proto_num, const_name, getattr(socket, const_name))

The values for protocol numbers are standardized, and defined as constants in socket with the prefix IPPROTO_.

$ python socket_getprotobyname.py

icmp ->  1 (socket.IPPROTO_ICMP =  1)
 udp -> 17 (socket.IPPROTO_UDP  = 17)
 tcp ->  6 (socket.IPPROTO_TCP  =  6)

Looking Up Server Addresses

getaddrinfo() converts the basic address of a service into a list of tuples with all of the information necessary to make a connection. The contents of each tuple will vary, containing different network families or protocols.

import socket

def get_constants(prefix):
    """Create a dictionary mapping socket module constants to their names."""
    return dict( (getattr(socket, n), n)
                 for n in dir(socket)
                 if n.startswith(prefix)
                 )

families = get_constants('AF_')
types = get_constants('SOCK_')
protocols = get_constants('IPPROTO_')

for response in socket.getaddrinfo('www.python.org', 'http'):

    # Unpack the response tuple
    family, socktype, proto, canonname, sockaddr = response

    print 'Family        :', families[family]
    print 'Type          :', types[socktype]
    print 'Protocol      :', protocols[proto]
    print 'Canonical name:', canonname
    print 'Socket address:', sockaddr
    print

This program demonstrates how to look up the connection information for www.python.org.

$ python socket_getaddrinfo.py

Family        : AF_INET
Type          : SOCK_DGRAM
Protocol      : IPPROTO_UDP
Canonical name:
Socket address: ('82.94.164.162', 80)

Family        : AF_INET
Type          : SOCK_STREAM
Protocol      : IPPROTO_TCP
Canonical name:
Socket address: ('82.94.164.162', 80)

Family        : AF_INET6
Type          : SOCK_DGRAM
Protocol      : IPPROTO_UDP
Canonical name:
Socket address: ('2001:888:2000:d::a2', 80, 0, 0)

Family        : AF_INET6
Type          : SOCK_STREAM
Protocol      : IPPROTO_TCP
Canonical name:
Socket address: ('2001:888:2000:d::a2', 80, 0, 0)

getaddrinfo() takes several arguments to filter the result list. The host and port values given in the example are required arguments. The optional arguments are family, socktype, proto, and flags. The family, socktype, and proto values should be 0 or one of the constants defined by socket.

import socket

def get_constants(prefix):
    """Create a dictionary mapping socket module constants to their names."""
    return dict( (getattr(socket, n), n)
                 for n in dir(socket)
                 if n.startswith(prefix)
                 )

families = get_constants('AF_')
types = get_constants('SOCK_')
protocols = get_constants('IPPROTO_')

for response in socket.getaddrinfo('www.doughellmann.com', 'http',
                                   socket.AF_INET,      # family
                                   socket.SOCK_STREAM,  # socktype
                                   socket.IPPROTO_TCP,  # protocol
                                   socket.AI_CANONNAME, # flags
                                   ):
    
    # Unpack the response tuple
    family, socktype, proto, canonname, sockaddr = response

    print 'Family        :', families[family]
    print 'Type          :', types[socktype]
    print 'Protocol      :', protocols[proto]
    print 'Canonical name:', canonname
    print 'Socket address:', sockaddr
    print

Since flags includes AI_CANONNAME the canonical name of the server (different from the value used for the lookup) is included in the results this time. Without the flag, the canonical name value is left empty.

$ python socket_getaddrinfo_extra_args.py

Family        : AF_INET
Type          : SOCK_STREAM
Protocol      : IPPROTO_TCP
Canonical name: homer.doughellmann.com
Socket address: ('192.168.1.8', 80)

IP Address Representations

Network programs written in C use the data type struct sockaddr to represent IP addresses as binary values (instead of the string addresses usually found in Python programs). Convert IPv4 addresses between the Python representation and the C representation with inet_aton() and inet_ntoa().

import binascii
import socket
import struct
import sys

string_address = sys.argv[1]
packed = socket.inet_aton(string_address)

print 'Original:', string_address
print 'Packed  :', binascii.hexlify(packed)
print 'Unpacked:', socket.inet_ntoa(packed)

The four bytes in the packed format can be passed to C libraries, transmitted safely over the network, or saved to a database compactly.

$ python socket_address_packing.py 192.168.1.1

Original: 192.168.1.1
Packed  : c0a80101
Unpacked: 192.168.1.1

$ python socket_address_packing.py 127.0.0.1

Original: 127.0.0.1
Packed  : 7f000001
Unpacked: 127.0.0.1

The related functions inet_pton() and inet_ntop() work with both IPv4 and IPv6 addresses, producing the appropriate format based on the address family parameter passed in.

import binascii
import socket
import struct
import sys

string_address = sys.argv[1]
packed = socket.inet_pton(socket.AF_INET6, string_address)

print 'Original:', string_address
print 'Packed  :', binascii.hexlify(packed)
print 'Unpacked:', socket.inet_ntop(socket.AF_INET6, packed)

An IPv6 address is already a hexadecimal value, so converting the packed version to a series of hex digits produces a string similar to the original value.

$ python socket_ipv6_address_packing.py 2002:ac10:10a:1234:21e:52ff:fe74\
:40e

Original: 2002:ac10:10a:1234:21e:52ff:fe74:40e
Packed  : 2002ac10010a1234021e52fffe74040e
Unpacked: 2002:ac10:10a:1234:21e:52ff:fe74:40e

See also

Wikipedia: IPv6
Article discussing Internet Protocol Version 6 (IPv6).
Wikipedia: OSI Networking Model
Article describing the seven layer model of networking implementation.
Assigned Internet Protocol Numbers
List of standard protocol names and numbers.

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