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

CHAPTER 4 ■ SOCKET NAMES AND DNS
Python code to make extra calls if the user might want to provide a symbolic port number like www or
smtp instead of 80 or 25.
Asking getaddrinfo() Where to Bind
Before tackling all of the options that getaddrinfo() supports, it will be more useful to see how it is used
to support three basic network operations. We will tackle them in the order that you might perform
operations on a socket: binding, connecting, and then identifying a remote host who has sent you
information.
If you want an address to provide to bind(), either because you are creating a server socket or
because you for some reason want your client to be connecting to someone else but from a predictable
address, then you will call getaddrinfo() with None as the hostname but with the port number and
socket type filled in. Note that here, as in the following getaddrinfo() calls, zeros serve as wildcards in
fields that are supposed to contain numbers:
>>> from socket import getaddrinfo
>>> getaddrinfo(None, 'smtp', 0, socket.SOCK_STREAM, 0, socket.AI_PASSIVE)
[(2, 1, 6, '', ('0.0.0.0', 25)), (10, 1, 6, '', ('::', 25, 0, 0))]
>>> getaddrinfo(None, 53, 0, socket.SOCK_DGRAM, 0, socket.AI_PASSIVE)
[(2, 2, 17, '', ('0.0.0.0', 53)), (10, 2, 17, '', ('::', 53, 0, 0))]
Here we asked about where we should bind() a socket if we want to serve SMTP traffic using TCP,
and if we want to serve DNS traffic using DCP, respectively. The answers we got back in each case are the
appropriate wildcard addresses that will let us bind to every IPv4 and every IPv6 interface on the local
machine with all of the right values for the socket family, socket type, and protocol in each case.
If you instead want to bind() to a particular IP address that you know that the local machine holds,
then omit the AI_PASSIVE flag and just specify the hostname. For example, here are two ways that you
might try binding to localhost:
>>> getaddrinfo('127.0.0.1', 'smtp', 0, socket.SOCK_STREAM, 0)
[(2, 1, 6, '', ('127.0.0.1', 25))]
>>> getaddrinfo('localhost', 'smtp', 0, socket.SOCK_STREAM, 0)
[(10, 1, 6, '', ('::1', 25, 0, 0)), (2, 1, 6, '', ('127.0.0.1', 25))]
You can see that supplying the IPv4 address for the localhost locks you down to receiving
connections only over IPv4, while using the symbolic name localhost (at least on my Linux laptop, with
a well-configured /etc/hosts file) makes available both the IPv4 and IPv6 local names for the machine.
One question that you might already be asking at this point, by the way, is what on earth you are
supposed to do when you assert that you want to supply a basic service and getaddrinfo() goes and
gives you several addresses to use—you certainly cannot create a single socket and bind() it to more
than one address! In Chapter 7, we will tackle the techniques that you can use if you are writing server
code and want to have several sockets going at once.
Asking getaddrinfo() About Services
Except for the use shown in the previous section, all other uses of getaddrinfo() are outward-looking,
and generate information suitable for connecting you to other applications. In all such cases, you can
either use an empty string to indicate that you want to connect back to the localhost using the loopback
interface, or provide a string giving an IPv4 address, IPv6 address, or hostname to name your
destination.
The usual use of getaddrinfo() in all other cases—which, basically, is when you are preparing to
connect() or sendto()—is to specify the AI_ADDRCONFIG flag, which filters out any addresses that are
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