Regarding option routers

Vithal Shirodkar vithals at
Thu Jul 13 14:52:28 UTC 2006

Hi Simon,

Thanks for the detailed response and the note of caution.  I am running some 
experiments with provisioning of multiple networks on the same set of 
hardware (routers, server, etc) and so client reconfiguration is not an 
issue, since when router gets a new config, the servers reboot too.

Can you explain why you said the ISC server won't work if there are no 
shared subnets ?  Can't I have the dhcp server say on subnet 
connected to a router which also is connected to 2 other subnets 
and and a helper configured in each of those subnets pointing to 
the dhcp server on  Or have I just described a shared subnet ?


>From: Simon Hobson <dhcp1 at>
>Reply-To: dhcp-users at
>To: dhcp-users at
>Subject: Re: Regarding option routers
>Date: Thu, 13 Jul 2006 08:04:05 +0100
>Vithal Shirodkar wrote:
> >Is there a way of configuring the ISC DHCP server, so that when "option
> >routers" is sent back as part of dhcpack it contains the address of the 
> >helper address that came with the dhcp request.
> >
> >My DHCP server is serving multiple L3 subnets and a router connected to 
> >of these also acts as the dhcphelper.  I want to keep my dhcp config 
> >static, but the helper address might change.  Hence, I need the dhcp 
> >to dynamically figure out the helper address and send it back as option
> >routers.
>Let me clarify this, several separate networks, each with it's own
>router - so no shared subnets or anything like that ?
>The ISC server won't do what you want, unless you go in and hack the code.
>However, leaving aside DHCP, I would suggest that having your router
>addresses liable to change so often that you want to do this suggests
>a worryingly unstable network - IMHO this is storing up problems that
>you don't need as an admin ! I am curious to know why you believe you
>need this.
>Bear in mind that every time you change the router address, you will
>have to reconfigure every device. Some you can do by DHCP (but you
>will still need to force a release/renew on some clients*), but I'm
>sure you'll have some statically configured stuff to walk round and
>fix manually. Changing one setting in the DHCP server seems fairly
>trivial by comparison.
>* IIRC, some clients will not pick up changed options in a renewal,
>therefore they continue using old settings (eg DNS servers) until
>they have their lease released and then renewed - something you
>cannot force from the server.
>If you are doing this because you have a dynamic network, and have to
>add/remove routers as things change, then I'd consider adding a
>secondary address to one router (such as n.n.n.1) and use that as the
>default route for the network. If you have to change the router(s),
>then remove that secondary address and add it to whichever router now
>does the default route. It's a simple step and will avoid the
>heartache of reconfiguring the clients and then fielding the support
>calls from people with broken networking.
>Finally, as long as all routers are correctly configured (ie all know
>the same routing table), then it doesn't matter which router a client
>has as it's default - if it uses the wrong one, it will simply be
>redirected invisibly to the user.

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