dhcrelay over wireless mesh

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Jul 11 11:29:55 UTC 2012

Luca Tavanti wrote:

>I'm trying to set up a wireless mesh network in which the IP address 
>(Ipv4) is assigned to all nodes by a central DHCP server. All nodes 
>are  in the same IP subnet (say 192.168.10.x). To reach the nodes 
>that are multiple hops away I've arranged to use dhcrelay.
>The relay service is run on every node (except the server) with:
>root[/]# dhcrelay -4 -i wlan0 -m forward
>where wlan0 is the (unique) wireless interface of each node, and 
> is the dhcp server address.
>However, till now I've never succeeded in going beyond the second hop.

As I understand your setup, it isn't actually correct for use of a relay agent.

Some logs (from both relay agent and server) would probably help a 
lot, but here is what I think may be happening ...

Client broadcasts a request. What does your access node do with the 
packet ? The relay agent should detect it and forward it to the 
server, but in a flat layer 2 network, it should travel through the 
network on it's own and reach the server.

If the server gets a broadcast packet from the client, it will reply 
directly - and that broadcast replay should get back to the client. 
Hence things work.

But if the request is relayed via a relay agent, then the server may 
get confused since it has a local interface on the network. I don't 
know what the ISC server does in that case - whether it sends the 
reply back via the relay agent, or whether it replies directly.
If this is the problem, then using a server which is not on the local 
subnet may solve the problem.

Simon Hobson

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