I know these howtos are all over the place but I wanted to keep one handy. We lost one of our dyndns domains today due to inactivity. So I setup this little script to help.

#!/bin/sh

USERNAME=yourusername
PASSWORD=yourpassword
HOSTNAME=your.domain.name

cd ~/.dyndns
if [ -f ~/.dyndns/ipcheck.dat ]; then
  /usr/sbin/ipcheck -r checkip.dyndns.org:8245 \
 $USERNAME $PASSWORD $HOSTNAME
else
  /usr/sbin/ipcheck --makedat -r checkip.dyndns.org:8245 \
 $USERNAME $PASSWORD $HOSTNAME
fi

Place that in a path hidden to your users (it stores a password in text) and then add a cron to run it every month (Dyndns will expire free accounts after 30 days).