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

CHAPTER 4 ■ SOCKET NAMES AND DNS
impossible for your computer to reach. For example, an organization might have both an IPv4 and an
IPv6 range of IP addresses; but if your particular host supports only IPv4, then you will want the results
filtered to include only addresses in that family. In case the local machine has only an IPv6 network
interface but the service you are connecting to is supporting only IPv4, the AI_V4MAPPED will return you
those IPv4 addresses re-encoded as IPv6 addresses that you can actually use.
So you will usually use getaddrinfo() this way when connecting:
>>> getaddrinfo('ftp.kernel.org', 'ftp', 0, socket.SOCK_STREAM, 0,
... socket.AI_ADDRCONFIG | socket.AI_V4MAPPED)
[(2, 1, 6, '', ('204.152.191.37', 21)),
(2, 1, 6, '', ('149.20.20.133', 21))]
And we have gotten exactly what we wanted: every way to connect to a host named ftp.kernel.org
through a TCP connection to its FTP port. Note that several IP addresses were returned because this
service, to spread load, is located at several different machines on the Internet. You should generally
always use the first address returned, and if a connection fails, then try the remaining ones, because
there is intelligence built into the name-resolution system to properly randomize the order in which you
receive them. By always trying the first server IP address first, you will offer the various servers a
workload that is in the proportion that the machine administrators intend.
Here is another query, which describes how I can connect from my laptop to the HTTP interface of
the IANA that assigns port numbers in the first place:
>>> getaddrinfo('iana.org', 'www', 0, socket.SOCK_STREAM, 0,
... socket.AI_ADDRCONFIG | socket.AI_V4MAPPED)
[(2, 1, 6, '', ('192.0.43.8', 80))]
The IANA web site is actually a good one for demonstrating the utility of the AI_ADDRCONFIG flag,
because—like any other good Internet standards organization—their web site already supports IPv6. It
just so happens that my laptop can speak only IPv4 on the wireless network to which it is currently
connected, so the foregoing call was careful to return only an IPv4 address. But if we take away our
carefully chosen flags in the sixth parameter, then we will also be able to see their IPv6 address:
>>> getaddrinfo('iana.org', 'www', 0, socket.SOCK_STREAM, 0)
[(2, 1, 6, '', ('192.0.43.8', 80)),
(10, 1, 6, '', ('2001:500:88:200::8', 80, 0, 0))]
This can be useful if you are not going to try to use the addresses yourself, but if you are providing
some sort of directory information to other hosts or programs.
Asking getaddrinfo() for Pretty Hostnames
One last circumstance that you will commonly encounter is where you either are making a new
connection, or maybe have just received a connection to one of your own sockets, and you want an
attractive hostname to display to the user or record in a log file. This is slightly dangerous because a
hostname lookup can take quite a bit of time, even on the modern Internet, and might return a
hostname that no longer works by the time you go and check your logs—so for log files, try to record
both the hostname and raw IP address!
But if you have a good use for the “canonical name” of a host, then try running getaddrinfo() with
the AI_CANONNAME flag turned on, and the fourth item of any of the tuples that it returns—that were
always empty strings in the foregoing examples, you will note—will contain the canonical name:
>>> getaddrinfo('iana.org', 'www', 0, socket.SOCK_STREAM, 0,
... socket.AI_ADDRCONFIG | socket.AI_V4MAPPED | socket.AI_CANONNAME)
[(2, 1, 6, '43-8.any.icann.org', ('192.0.43.8', 80))]
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