linux: how to access DHCP options from client

As you may know, you can configure any DHCP server to send many options to the clients; for example to setup dns domains, http proxy (WPAD) and so on.

If you need to access these options from a linux client, you must configure the client to ASK the server for the new options, by editing /etc/dhcp/dhclient.conf, and add an entry like:

option WPAD code 252 = string;
also request WPAD;

done that, when you’ll ask for a dhcp, the dhclient process will invoke your hook scripts with two new environment variables, old_WPAD and new_WPAD, with the values before and after the renewal.

so you can put a script in the folder /etc/dhcp/dhclient-enter-hooks.d or /etc/dhcp/dhclient-exit-hooks.d to simply “use” the value, by writing it in a configuration file or somewhere else.

#!/bin/bash
echo "I got a new value of WPAD from DHCP : ${new_WPAD}" > /tmp/wpad.txt