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Intended for dynamic ip OpenDNS users, this command will update your OpenDNS network IP.
For getting your IP, you can use one of the many one-liners here on commandlinefu.
I use this in a script which is run by kppp after it has successfully connected to my ISP:
IP="`curl -s http://checkip.dyndns.org/ | grep -o '[[:digit:].]\+'`"
if [ "$IP" == "" ] ; then echo 'Not online.' ; exit 1
wget -q --user=topsecret --password="`echo $PW | xxd -ps -r`" 'https://updates.opendns.com/nic/update?hostname=myhostname&myip='"$IP" -O -
/etc/init.d/ntp-client restart &
PS: DynDNS should use a similar method, if you know the URL, please post a comment. (Something with members.dyndns.org, if I recall correctly)