SRV Lookups In Erlang

October 30, 2008

I’m working on a new bit of code in Erlang that needs to do SRV lookups. I figured since Erlang is famous for being great at building servers that it would have built in facilities for querying DNS in all kinds of ways. Imagine my surprise when I discovered that the only documented ability it has for DNS is the really simple inet:gethostbyname. Digging around in the Erlang/OTP source code and through ejabberd I was able to piece together the needed pieces to make an SRV module.

:EXTENDED:

What Happened To The Documentation?

A lot of Erlang is very well documented. There is a book, a design guide, and a bunch of well written man pages. Unfortunately the documentation stops abruptly at the inet module, even though there are lots of sub modules that contain very useful functions.

inet_res:getbyname/2 is what is needed to do SRV lookups. It’s not clear to me why all these different inet sub-modules do. For example, there are inet_db, inet_res, and inet_config modules. DNS lookup routines are spread over all of these, and often they have the same names. I’ve seen mention on the mailing lists that parts of these might be deprecated, but I don’t really understand why when they seem to contain useful functions that don’t exist elsewhere.

Resolver Configs And More Missing Documentation

One issue you might run into trying to use inet_res is that it always returns {error, timeout}. It turns out that through some idiosyncrasy of the virtual machine startup, the configuration for the resolver only gets set up if the virtual machine is used with the -name parameter. Without -name and even with -sname, the resolver just doesn’t get set up.

Digging through ejabberd’s source code, I found the inetrc file, and this is pointed to by the ERL_INETRC environment variable which exists in all the ejabberd scripts. It turns out this is yet another undocumented Erlang bit. If ERL_INETRC is defined and points to a valid inetrc file, all the DNS resolver functions will magically work, even without the -name parameter.

Oddly, inet:gethostbyname always works, even without special configuration or parameters. Too bad it can’t do SRV lookups.

The Result

The SRV lookup module I created is available here. I’ll go over the code and how to use it in my next post. It’s nothing earth-shattering, but if any of you are budding Erlang programmers, you might find it helpful.