[DHCPDISCOVER]unknown network segment.

Simon Hobson dhcp1 at thehobsons.co.uk
Thu Oct 16 11:12:07 UTC 2014

Glenn Satchell <glenn.satchell at uniq.com.au> wrote:

> The 'via' part of the log message indicates these two IPs are acting as
> DHCP relay agents. My guess is that they are two routers with vrrp (or
> other virtual address) advertising x.x.x.161 as the gateway.
> What this means is that there is a subnet somewhere on the *other* side pf
> these routers that is where the client is. It is this other subnet that is
> not defined so the dhcp server doesn't know how to assign an address. You
> can check this by using tcpdump to look at the packets - it will have a
> field with the address of the gateway on that subnet.

That's not right is it ?
The address given in the "via ..." part should be the GI-Addr field in the relayed packet. By definition, that *must* be an IP address configured on a relay agent interface on the network to which the client is directly attached. If there are any other upstream relay agents, then they would have filled in their own GI-Addr field and either :
1) unicasted the packets to the server.
2) broadcast, but the downstream relay agent would not further mangle the GI-Addr field.

A dump & decode of the packets might reveal something.

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