Dhclient does not support supersede statement for option 54
The German cable internet provider Unitymedia uses DHCP relays which only answer to broadcasts. Dhclient renews WAN leases by sending unicasts to the relay, which doesn't forward them to the DHCP server. If, in the rebind phase, the broadcast packet is lost, the WAN IP expires, all connections are dropped and Dhclient needs to aquire a new lease. This is discussed in [[https://forum.pfsense.org/index.php?topic=127023.0]].
The problem can be avoided by setting DHCP option 54 (dhcp-server-identifier) to 255.255.255.255 via Interfaces->WAN->Advanced configuration->Option Modifiers. However, the required supersede statement for this option is not implemented in dhclient.c. Thus the DHCP client of Pfsense does not use the option and the value given by the DHCP server remains in effect (which points to the IP of the relay), which then results in the observed problem.
To solve this a section which checks if "supersede dhcp-server-identifier" is set has been added to dhclient.c (see "Start of the updated section" at line 855 in the attached file).