dhcpd sending on the same IP it receives on

Scott Baker bakers at web-ster.com
Tue Feb 5 18:37:33 UTC 2008

Jared Gillis wrote:
> Hello,
> I am having a similar problem as initially discussed in this thread:
> http://marc.info/?l=dhcp-users&m=119740526823069&w=2
> I have a linux DHCP server that has two uplinks, each of which goes to a 
> router that is relaying DHCP requests to the DHCP server. Router A is on 
> eth0, Router B is on eth1. The DHCP server's default gateway points at 
> Router A. There is a very small corner case in which this causes a problem:
> Router B gets a broadcast DHCPDISCOVER, and relays the request to the 
> DHCP server, then takes the reply and hands it to the client. The 
> client's DHCP implementation then switches to unicast for further DHCP 
> traffic, sending to the server's eth1 address. The client sends a 
> DHCPREQUEST packet to the server to renew the lease, and the server 
> replies, but from eth0's IP address.
> This *is* broken, as we are talking about unicast IP traffic, and if the 
> DHCP client happens to be behind a stateful packet inspection firewall, 
> the DHCP reply gets dropped, and the client never receives the response.
> I looked into the local-address configuration option, but since we are 
> answering DHCP requests on both interfaces, that would mean we'd have to 
> run two daemons, which is not a good solution for us.
> Does anyone know of a workaround/fix for this, and if not, where can I 
> submit a bug report? =)

That seems like a semi-complex setup for just one server. Wouldn't 
it be easier to just run two different (physical) servers for that 
odd corner case you're running in to?

Scott Baker - Canby Telcom
RHCE - System Administrator - 503.266.8253

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