dhcpd sending on the same IP it receives on

Frank Bulk frnkblk at iname.com
Wed Dec 12 17:55:09 UTC 2007

One solution I've seen is iptables.


-----Original Message-----
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On Behalf
Of Glenn Satchell
Sent: Wednesday, December 12, 2007 6:44 AM
To: dhcp-users at isc.org
Subject: Re: dhcpd sending on the same IP it receives on

>Date: Tue, 11 Dec 2007 12:04:29 -0800
>From: Scott Baker <bakers at web-ster.com>
>To: dhcp-users at isc.org
>Subject: dhcpd sending on the same IP it receives on
>My DHCP server has multiple ips on its primary NIC. For example:
>eth0   =
>eth0:0 =
>I have my access equipment set to send DHCP relay to, but
>when the DHCP server responds it sends the response packet out via
>the address since that is the primary address on that NIC.
>This causes my access equipment to drop the packet because the IP it
>sent to, and got the response from do not match.
>Is there way to tell the dhcpd daemon to send the packets outbound
>using the same IP (interface) that it received the packet on?
>The simple answer is to reverse the IPs so that is the
>primary, but that's not really feasible since I have other services
>that require that be the primary IP.
>Scott Baker - Canby Telcom
>RHCE - System Administrator - 503.266.8253
Have a look at the server-identifier statement, and maybe the
local-address statement. Between them they may do what you want.

In general though, the source address on an IP packet is a function of
the IP kernel driver and not the application. Virtual interfaces are
not like separate physical interfaces, the packets always go in and out
via eth0, it's just that it will accept packets with either IP


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