DHCP RENEW on Multi homed client

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Nov 25 07:37:37 UTC 2009

Jolynn Schmidt wrote:

>I am trying to understand a problem we are seeing and understand 
>what options are available to me.
>We have servers that are dual homed
>eth0 - 10.114.10
>         default gateway on host is out eth0 to
>eth1 - 10.114.12
>DHCP Server on 10.114.100
>We have DHCP helpers in place for both 10.114.10 and 10.114.12 to 
>point to our DHCP server on 10.114.100.
>Eth0 and Eth1 have no problem getting their initial dhcp request, 
>but we are having problems when dhclient for eth1 renews it's lease. 
>The default gateway points out eth0 so the renew for eth1 is being 
>sent out eth0, I see the request and ack transaction occur between 
>the server and the client on eth0 but dhclient for eth1 never sees 
>it, and just keeps send out a request over and over again.
>Is this just not going to work, do I need to static address my eth1 
>interface or should this be working?

I'd guess that the router at doesn't know how to route 
packets to the DHCP server at 10.114.100.n. Or the server doesn't 
know how to route packets back to the client.

Renewals are done by unicast - ie they are directly addressed from 
client to server, and the replies directly from server to client. 
They don't use any helpers*, and they rely on normal IP packet 

* An exception is if you have switches configured to intercept 
packets and insert option82 information.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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