No Offer Sent by DHCP Relay

[Simon Hobson]

Josh wrote:

>  > This came up a little while ago, except I think in that case it was a
>>  PPP interface. In it's current design, the relay agent does not work
>>  with interfaces that don't support a broadcast mode. Ethernet is
>>  fine, ppp (and I assume tun) interfaces aren't.
>>
>>  Do you have any other machine on the client network you could run the
>>  relay agent on ? It can be on any machine, not just a router.
>>  
>At this point we have only the router (also the VPN machine) and clients
>on the remote subnet.  We could deploy a separate workstation to act as
>the DHCP relay, but would really rather not do that if there is any
>other option.
>
>If possible, can I get clarification on what problem the point-to-point
>tun adapter poses?  From what I see, the tun adapter is both passing the
>DHCP discover to the real DHCP server and receiving it back.
<snip>
>As I'm reading that, the tun interface is passing the DHCP request to
>the DHCP server and receiving a response as expected.  I'm just confused
>as to how the p-t-p link is the cause of this problem.  Since the return
>DHCP offer comes to the 192.168.101.1 address, what's preventing
>dhcrelay from sending it on back to the client from that same address?

This is only from memory, but AIUI the basic issue is that the ISC 
software was written on the assumption that all interfaces it listens 
on are broadcast. I guess that when it was written quite some time 
ago that P2P serial, PPP, and TUN interfaces weren't all that common 
as direct attachments to systems.

The consequence is that whilst the replay packets are received on the 
TUN interface, the relay agent is not actually listening and so the 
packets will be discarded.

It's not a fundamental limitation, just an artifact of assumptions 
made many years ago.

>If there is no good way around this issue, are there any other DHCP
>relay options available for Linux that allow relaying over
>point-to-point IP-level links?  Something along the lines of the Cisco
>"ip-helper" option is exactly what we're looking for.

Pass