Regarding option routers
dhcp1 at thehobsons.co.uk
Thu Jul 13 15:44:08 UTC 2006
Vithal Shirodkar wrote:
>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 ?
I didn't say that ! I said that it won't do what you want - ie take
the relay agent address and use that as the router address in any
>Can't I have the dhcp server say on subnet 22.214.171.124/24
>connected to a router which also is connected to 2 other subnets 126.96.36.199/24
>and 188.8.131.52/24 and a helper configured in each of those subnets pointing to
>the dhcp server on 184.108.40.206/24.
Yes you can.
>Or have I just described a shared subnet ?
It depends - your description isn't clear enough to say ! I'm not
clear whether you are talking networks or subnets above - you only
need one relay agent per network because when a client does not yet
have an address to use, it broadcasts to the all ones broadcast
address which doesn't care about what subnets are defined.
In a basic network without shared subnets, it would be possible to
use the relay agent address as a gateway address no server I am aware
of will do it automatically for you.
Thinking again, I could have been wrong in my earlier message, it may
be possible to do it - I'm not familiar enough with the evaluation
capabilities to say for sure.
I would look at 'man dhcp-eval', and perhaps try something along the lines of :
option routers packet (24,4) ;
If that doesn't work, if you look back to earlier today, there's a
thread "Identifying DHCP Relay Agent" which identifies another
approach you CAN use if your routers are of a limited range of
addresses (ie you can write a class statement or an if statement for
each distinct address a router may be at).
The reason for checking that you aren't using a shared network is
that in a shared network (which is where you have more than one
subnet on a physical network), the ip address given by the relay
agent may not be in the same subnet as the client will be getting a
lease in. For example, if 220.127.116.11 and 18.104.22.168 were a shared network,
then the relay agent could be 22.214.171.124 whilst the client gets
126.96.36.199 as an address - clearly 188.8.131.52 would not be a suitable
address for the client to try and use as it's default router.
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