Allow adding a reject option to the dhclient config. This is useful if you have a failover route and a modem that assigns an IP to 192.168.100.x with a default route that makes your failover route no longer work.
From the dhclient.conf man page
reject cidr-ip-address [, ... cidr-ip-address ] ; The reject statement causes the DHCP client to reject offers from servers whose server identifier matches any of the specified hosts or subnets. This can be used to avoid being configured by rogue or misconfigured dhcp servers, although it should be a last resort - better to track down the bad DHCP server and fix it. The cidr-ip-address configuration type is of the form ip-address[/prefixlen], where ip-address is a dotted quad IP address, and prefixlen is the CIDR prefix length of the subnet, counting the number of significant bits in the netmask starting from the leftmost end. Example configuration syntax: reject 192.168.0.0/16, 10.0.0.5; The above example would cause offers from any server identifier in the entire RFC 1918 "Class C" network 192.168.0.0/16, or the specific single address 10.0.0.5, to be rejected.