Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 2 ■ UDP
How do clients learn the IP addresses and ports to which they should connect? There are generally
three ways:
• Convention: Many port numbers have been designated as the official, well-known
ports for specific services by the IANA, the Internet Assigned Numbers Authority.
That is why we expected DNS to run at UDP port 53 in the foregoing example.
• Automatic configuration: Often the IP addresses of critical services like DNS are
learned when a computer first connects to a network, if a protocol like DHCP is
used. By combining these IP addresses with well-known port numbers, programs
can reach these essential services.
• Manual configuration: For all of the situations that are not covered by the
previous two cases, some other scheme will have to deliver an IP address or the
corresponding hostname.
There are all kinds of ways that IP addresses and port numbers can be provided manually: asking a
user to type a hostname; reading one from a configuration file; or learning the address from another
service. There was, once, even a movement afoot to popularize a portmap daemon on Unix machines
that would always live at port 2049 and answer questions about what ports other running programs were
listening on!
When making decisions about defining port numbers, like 53 for the DNS, the IANA thinks of them
as falling into three ranges—and this applies to both UDP and TCP port numbers:
• “Well-Known Ports” (0–1023) are for the most important and widely-used
protocols. On many Unix-like operating systems, normal user programs cannot
use these ports, which prevented troublesome undergraduates on multi-user
machines from running programs to masquerade as important system services.
Today the same protections apply when hosting companies hand out commandline
Linux accounts.
• “Registered Ports” (1024–49151) are not usually treated as special by operating
systems—any user can write a program that grabs port 5432 and pretends to be a
PostgreSQL database, for example—but they can be registered by the IANA for
specific protocols, and the IANA recommends that you avoid using them for
anything but their assigned protocol.
• The remaining port numbers (49152–65535) are free for any use. They, as we shall
see, are the pool on which modern operating systems draw in order to generate
random port numbers when a client does not care what port it is assigned.
When you craft programs that accept port numbers from user input like the command line or
configuration files, it is friendly to allow not just numeric port numbers but to let users type humanreadable
names for well-known ports. These names are standard, and are available through the
getservbyname() call supported by Python’s standard socket module. If we want to ask where the
Domain Name Service lives, we could have found out this way:
>>> import socket
>>> socket.getservbyname('domain')
53
As we will see in Chapter 4, port names can also be decoded by the more complicated getaddrinfo()
function, which also lives in the socket module.
The database of well-known service names is usually kept in the file /etc/services on Unix
machines, which you can peruse at your leisure. The lower end of the file, in particular, is littered with
ancient protocols that still have reserved numbers despite not having had an actual packet addressed to
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