In dhcpd.conf:
# defines the type of data used to send the routing informations
option classless-routes code 121 = array of unsigned integer 8;
# defines 2 gateways for 2 subnets:
# 192.168.100.0/24, gateway 192.168.1.2
# and 10.4.0.0/24, gateway 192.168.1.2
option classless-routes 24, 192,168,100, 192,168,1,2,
24, 10,4,0, 192,168,1,2;
dhclient on Debian/Ubuntu is able to handle the data out of the box.
To make the configuration work on Windows clients, duplicate the configuration with code 249:
option classless-routes code 121 = array of unsigned integer 8;
option classless-routes 24, 192,168,100, 192,168,1,2,
24, 10,4,0, 192,168,1,2;
option classless-routes-win code 249 = array of unsigned integer 8;
option classless-routes-win 24, 192,168,100, 192,168,1,2,
24, 10,4,0, 192,168,1,2;