Having both dhcp3 server and dhcp3 relay on the same host

[Simon Hobson]

Laurent CARON wrote:

>I'm considering installing dhcp3 relay on my linux boxes.
>
>I already have dhcp3 server running.
>
>Wouldn't those packages conflict ?

Yes, they both listen on the same port - and because they deal with 
broadcast packets you cannot get around this by adding a second IP 
address.

>I mean:
>Is it a correct behavior to run dhcp3 relay as a fallback if the dhcp3
>server is unavailable (misconfigured, crashed...)?

You can run a relay agent and a server on the same subnet, in the 
same way as you can run two servers on the same subnet*. However, you 
MUST take steps to avoid conflicts and it is generally inadvisable 
unless the two servers are running as a failover pair.

* From the clients point of view, dealing via a relay agent is very, 
very similar to dealing with a local dhcp server.

Don't forget that a relay agent does not have to run on a router, it 
can run on any machine on the subnet.

As for availability, don't forget that the DHCP server dying will not 
make your network crash - it will only prevent clients 
obtaining/renewing leases. For Windows clients, they will continue to 
use their existing lease even after a shutdown as long as it is still 
valid. Not having a dhcp server will have most impact on Macs 
(release their lease on shutdown) and on laptops moved from another 
network. If your lease times are measured in weeks, then loss of a 
dhcp server may go unnoticed for a few days !