- all the patches use temporary option code for v4 and v6
  until IANA assigned numbers for draft-ietf-dhc-relay-port.
  https://tools.ietf.org/html/draft-ietf-dhc-relay-port-05

- for relay patches, the patch allows user to define a non-DHCP port
  (non-67 for v4, non-547 for v6) in command line with option '-rp'.
  this is similar to the current option '-p'. the global variable
  'uint16_t relay_port' is configured with this '-rp' option.

  the reasons not using interface based port definition include:
   - the user defined udp source port is only for upstream interfaces to
     the servers, but still uses standard dhcp port for downstreams
   - the relay-agent options are formatted when receiving from a downstream
     interface, and once formatted it can be sent to multiple upstream
     servers. thus if we define the relay-port on upstream interfaces,
     the current code flow does not fit when adding this relay options, and
     if we define the 'relay-port' on downstream interfaces, it does not
     make much sense, and it can also be too many interfaces to configure.
   - this feature mainly is for multiple-instances sharing the same
     ip address, but have to use relay-port to distinguish the incoming
     message from upstream server, thus it won't have the case for
     some server may use port 67/547 but other servers use non-dhcp udp port.

- 5 patches, as Thomas suggested in our email exchange.
  v4 server, v4 relay, v6 server, v6 relay and 4o6 support.


(1) patch v4 relay

 - if '-rp' source port is used for communicating to server, a new
   bpf filter is defined to accept either port 67 and a user defined
   udp port. this is to allow DHCP client sending to us using port 67,
   but the DHCP server sending to us using a non-67 udp port.

(2) patch v4 server

 - if the relay forward message has the relay-agent option of RELAY-PORT,
   then use the udp port from the packet->client_port when sending
   relay reply message to the relay agent

(3) patch v6 relay

 - same as in v4 relay, the parameter '-rp' is used for user to define
   an non-547 UDP source port.

 - in normal v6 relay, it only needs to setup one v6 socket on the relay,
   which defines the source udp port of 547.

   in this relay-port feature, if the relay-port is defined, the relay
   needs to setup two sockets, one for the udp port of 547 in order to
   continue receiving messages from dhcp client or downstream relay;
   the other for the non-dhcp udp port for dhcp server to send to us,
   or for the upstream relay agent when sending relay-reply message
   to us.

 - in this patch, in 'discover_interfaces', instead of finding the
   first interface and breaking out from the loop, it looks for
   an upstream interface and a downstream interface. we break out
   of the loop if we have found one upstream and one downstream
   interfaces.

 - in if_regster6, it modifies the code, such that if the 'relay-port'
   is defined, as described above finding the 'upstream' and 'downstream'
   two interfaces, we need to assign the two interfaces using different
   socket functions. The 'downstream' interface section is the same
   as the normal code, but the upstream interface will have the
   'relay_port_v6_socket' function assigned for the socket.

 - the v6 options has a 'downstream source port' field, which is used
   to record a downstream relay-agent if using a non-547 UDP port.

 - If the local relay (this process) uses a non-547 udp port, or our
   'downstream' relay-agent uses a non-547 udp port, this relay
   when sending the relay-forw message to upstream, will include the
   Relay Source Port option in the message.

 - when receiving from server or an upstream relay agent the relay-reply
   message, and if there is the Relay Source Port option in the
   message, and also if the 'downstream source port' value is non-zero,
   then this process needs to use this value as the udp destination
   for the downstream relay-agent when sending the relay-reply message.

(4) patch server v6

 - server has to copy back the 'Relay Source Port' option when sending
   back the relay-reply message. and If the relay options has this
   'Relay Source Port' option, it uses the packet->client_port when
   sending back the relay-reply message to the relay agent.