dhcrelay does not relay answers back to client

[Peter Daum]

I spent many hours trying to get dhcp working over a VPN setup.
It is "almost" working, but for some (at least to me) mysterious
reason, dhcrelay does not send the dhcp server answers back to
the original client.

The setup looks about like this:

             
DHCP          Gate    (vpn)  Gate   [eth0]
Server <----> way1  <------> way2 <---> Client
              (NAT)                           

On Gateway2, there is a dhcrelay running:
  dhcrelay [-i eth0] <dhcp server address)
(I tried various additional options which did not seem to affect the result)

The setup looks about like this:

      	     
DHCP  	      Gate    (vpn)  Gate   [eth0]
Server <----> way1  <------> way2 <---> Client
       	      (NAT)	   	 	      

On Gateway2, there is a dhcrelay running:
  dhcrelay [-i eth0] <dhcp server address)
(I tried various additional options which did not seem to affect the result)

The DHCP queries are successfully forwarded to the dhcpserver, which
sends it back to the address from the IP address of the relay agent.
With tcpdump I can verify that the answer also arrives on Gateway2
(via the interface "tun0", the VPN endpoint).
Despite some address translations on the way, all fields in the answer
have reasonable IP addresses, the destination address is the address
of eth0 on GW2.

No matter what interfaces I tell dhcrelay to listen on, it does not
seem to get the answer (or silently discards it).

I would be very grateful for any help because I am running out of
ideas ...

Regards,
                Peter