dhcrelay over wireless mesh

Randall C Grimshaw rgrimsha at syr.edu
Wed Jul 11 12:46:51 UTC 2012

The mesh network should not be introducing any routing, so putting a relay on each of the nodes is going to introduce a magnifying effect that will eventually contribute to a DOS. You only need one relay per layer-2 network and that is usually done on the router with the layer-3 interface.

Randall Grimshaw rgrimsha at syr.edu
From: dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org [dhcp-users-bounces+rgrimsha=syr.edu at lists.isc.org] on behalf of Simon Hobson [dhcp1 at thehobsons.co.uk]
Sent: Wednesday, July 11, 2012 7:29 AM
To: Users of ISC DHCP
Subject: Re: dhcrelay over wireless mesh

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

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