@mwl For me there's also the option missing "I don't listen to audio, but would buy and give it a try"

But as the first book that came to my mind for this was the Networknomicon, I also further reduced my assessment of MY sanity...

@dvl usually you don't (and shouldn't) touch /sbin/dhclient-script. That script is invoked by dhclient and it sources /etc/dhclient-enter-hooks and /etc/dhclient-exit-hooks - those are the files you want to use to add custom behavior and e.g. run your own scripts to which you can pass the variables set by dhclient (e.g. $new_ip_address).

e.g. I'm calling my ddns script from /etc/dhclient-exit-hooks as follows:

[ ."$reason" = .BOUND ] && /usr/home/sko/bunny_ddns.sh -n <recordID> -i $new_ip_address -k <APIkey> -z <zoneID>

that script then basically just mangles the input data into a curl POST request for the API.

I always wanted to look into using the "EXPIRE", "FAIL" and "TIMEOUT" reasons to switch over to a fallback uplink...

@dvl I've been using /etc/dhclient-exit-hooks for that at home.

[ ."$reason" = .BOUND ] and [ ."$reason" = .RENEW ] will pass the new IP to a shellscript that updates my records at bunny.net DNS via their REST API.