dhcrelay and ipip tunnels

Simon Hobson dhcp1 at thehobsons.co.uk
Sat Sep 8 07:31:30 UTC 2012

Homer Wilson Smith wrote:

>      Apparently dhcrelay-3.03 will not run properly across an ipip or
>gre tunnel, because the tunnel interface will not properly pick up and
>send the return answer from dhcdp.
>      Notes on the net claim this is because dhcrelay is using LPF rather
>than sockets, but turning on sockets in site.h causes dhcrelay to send
>all answers as broadcasts which causes everyone to get all dhcp
>responses all the time, including ACKS.

I don't know the details, but the ISC code was written (or at least 
the design philosophy laid down) back in the days when networks were 
almost all either a) ethernet like, or b) SLIP or PPP like.
For what were good reasons then, the design was done such that it 
only works properly on networks that are ethernet like - in 
particular having ethernet like broadcast mechanisms. Thus PPP (and I 
assume GRE) interfaces don't work and aren't supported.

For the server, and simply going from previous threads (I'm not 
really a programmer), the server can be compiled to use sockets - at 
which point it can handle non-ethernet networks but cannot handle 
clients on the local network. I suspect the same applies to the relay 
agent - and making it not handle local clients would rather defeat 
it's purpose.

What I can suggest is :
Run the relay agent on any other host on the network(s) rather than 
the gateway running the tunnel interfaces. For the server(s), either 
do the same, or compile them to use sockets (assuming you don't need 
local client support. If your relay agent is on a different device to 
the gateway, then communications between it and the server are just 
unicast packets - and the tunnel is not visible to it.

It may even be possible to run the relay agent on the gateway if you 
can set it up in some sort of partitioned environment with it's own 
ethernet interface such that it uses one interface and it's server 
traffic goes via the local wire and back in on the main ethernet 
interface (and vice-versa for return packets).

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
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