[Bug 217978] Dhclient does not support supersede statement for option 54

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=217978

            Bug ID: 217978
           Summary: Dhclient does not support supersede statement for
                    option 54
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: fabian.kurtz at udo.edu

Created attachment 181028
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=181028&action=edit
Changes to enable supersede statement for option 54

The German cable internet provider Unitymedia (and possibily other ISPs from
the Liberty Global group) 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.

The problem can be avoided by setting DHCP option 54 (dhcp-server-identifier)
to 255.255.255.255. However, the required supersede statement for this option
is not implemented in dhclient.c. Thus the DHCP client 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).

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