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

CHAPTER 4 ■ SOCKET NAMES AND DNS
• The system on which your Python script is running already knows about the local
domain nameservers, thanks either to manual intervention by your system
administrator or a network configuration protocol like DHCP in your office, home,
or coffee shop. To crank up DNS right inside your Python program, you will have
to learn how to query your particular operating system for this information—an
operating-system-specific action that we will not be covering in this book.
• If you do not use the local DNS server, then you will not be able to benefit from its
own cache that would prevent your application and other applications running on
the same network from repeating requests about a hostname that is in frequent
use at your location.
• From time to time, adjustments are made to the world DNS infrastructure, and
operating system libraries and daemons are gradually updated to accommodate
this. If your program makes raw DNS calls of its own, then you will have to follow
these changes yourself and make sure that your code stays up-to-date with the
latest changes in TLD server IP addresses, conventions involving
internationalization, and tweaks to the DNS protocol itself.
Finally, note that Python does not come with any DNS facilities built into the Standard Library. If
you are going to talk DNS using Python, then you must choose and learn a third-party library for doing
so.
Why to Use DNS
There is, however, a solid and legitimate reason to make a DNS call from Python: because you are a mail
server, or at the very least a client trying to send mail directly to your recipients without needing to run a
local mail relay, and you want to look up the MX records associated with a domain so that you can find
the correct mail server for your friends at @example.com.
So we are going to go ahead and take a look at one of the third-party DNS libraries for Python as we
bring this chapter to its close. There are at least two good ones available for Python at the moment. They
are available for quick installation into a virtual environment if you want to try them out. (See Chapter 1
to remember how to use virtualenv and pip.)
We will focus on the slightly more popular distribution, pydns, which descends from a DNS module
first written by Guido van Rossum, which at least gives it a glow of historical legitimacy. It makes a DNS
package available for you to import. Its competitor, the dnspython distribution, creates a lower-case dns
package instead, just so you can keep things straight! Both distributions have seen updates within the
past year, as of this writing—in fact, as I type this in September 2010, I can see that both packages were
updated within a few days of each other back in January 2010.
Note that neither project provides code that knows how to “start from scratch” and begin a query
with a search of the Internet root domain nameservers! Instead, each library uses its own tricks to find
out what domain nameservers your Windows or POSIX operating system is currently using, and then
asks those servers to go do recursive queries on its behalf. So not a single piece of code in this chapter
avoids needing to have a correctly configured host which an administrator or network configuration
service has already configured with working nameservers.
Since both are on the Python Package Index, you can install and try one of them out like this:
$ pip install pydns
Your Python interpreter will then gain the ability to run our first DNS program listing, shown in
Listing 4–3. Neither package seems to have any real documentation, so you will have to start with what is
shown here and extrapolate by reading whatever example code you can find on the Web.
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