Cisco phones use the proprietary option 150 to tell the phone what server to contact for configuration information.
How can I provide this option using the ISC DHCP server on Ubuntu?
In order to provide option 150 (or other custom options) two configuration entries are required, first, in the top level of the configuration file located at /etc/dhcp/dhcpd.conf you need to define your custom option:
option voip-tftp-server code 150 = { ip-address };
Then, in the subnet stanza for where you need to provide the information you'll configure the option with the appropriate value.
subnet 192.168.0.0 netmask 255.255.255.0 {
[...]
option voip-tftp-server 10.101.0.10;
}
If you place the option definition in the subnet stanza then dhcpd will not work.
