Purpose

I like to always be able to get to my machines or at least know where they're at. Mostly, I want to be able to get into my home desktop which is DHCP'd via cable and the machine that runs the forums is also DHCP'd via DSL. It would be nice to not have to update DNS tables by hand.

Theory

An SSH public key for each DHCP host line is stored on the DNS host. This key contains a command line that runs a script on the DNS machine that updates the DNS tables.

Practical

Create a new DSA keypair without a password:
# ssh-keygen -t dsa -N "" -f dns.key
Not having a passphrase is a security risk, but it's not too bad assuming you effectively guard your private key and that the public key on the DNS machine is limited to the required commands.
Edit the public key to run the DNS updating script and do nothing else:

command="/usr/bin/perl /etc/tinydns/dynupdate",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty

[Just in case you're getting any ideas, this isn't a key that I use. I just made it for example of this page.]
Copy the public key to the DNS machine and the private key to the DHCP assigned machine.
Put a copy of this script into /etc/tinydns/. I can't stress this enough: READ IT FIRST. Basically what it does is:
- read the hostname from the command line
- append the domain name
- grab your ip from the ssh environment variable
- replace or append your hostname and ip in the dynamic.data file
- replace any other instances (aliases and the like) in dynamic.data of your old ip with the new one
- run the new Makefile
Replace the Makefile in /etc/tinydns/root/. The new one:
- checks and if necessary adjusts the data file permissions
- combines static.data with dynamic.data to make the data file tinydns needs
- compiles the data file as the old Makefile did
Setup the files on the DNS server:
# touch /etc/tinydns/root/dynamic.data
# cp /etc/tinydns/root/data /etc/tinydns/root/static.data
Tell the dynamically assigned machine to connect to the DNS server every time its address changes. Using dhcpcd under Debian / Testing on my VPR Matrix 220a5, I did the following:
- Added the following line to my dhcpcd.exe:
  /etc/dhcpc/dhcpc-$INTERFACE.exe
  which runs a new script every time the status of an interface changes.
- Created dhcpc-eth0.exe with very simple contents:
  ssh -i /root/.ssh/dns.key -n blessed.wolfteck.com
  You need to adjust it to connect to your DNS server. If you don't like the whole automation system interacting with DHCP, you can just run this command by hand every time you get a new IP address.