dhclient ddns update script

[dave]

Hello,
    Does anyone have a dhclient updater script that calls nsupdate whenever
the external IP changes? I've got an external IP, which changes
occationally, usually at an inopportune time, i want a dhclient-hook, can't
remember the fullname of that, that will notify the ddns server and update
an A record.
Thanks.
Dave.