This is somewhat obsolete now; instead of using this, as of August, I'm using the dynamic-domain service at Monolith. So the development of this software is on hold, since no one is using it; if anyone would like to, I'll be happy to help.
I wrote a simple DNS-update server in the first week of May 1997; it's currently locally tested and working OK. It's able to run as part of a multi-zone production nameserver, but it does need to be able to send signals to the nameserver, and it signals the nameserver to reload its database whenever there are any updates.
The code is available in dynamic-dns-toy-3.tar.gz, which is less than 10K. It's mostly written in Perl, except for the start-server shell script and the sighup-named.c program, which runs as root, and is written in C. A Linux-2.0.0-ELF binary is included.
If you decide to use or test it, please keep in mind that it's pre-alpha code; let me know you're using it, so I can incorporate fixes for the bugs you discover back into the main source.