DHCPD Stopping

Dean, Barry B.Dean at liverpool.ac.uk
Fri Mar 20 10:21:49 UTC 2009


I am still having the occasional problem with DHCP ceasing to answer requests from clients.

I have traced it to a reproducible situation.

Client (only seen with Linux and HP Printers at the moment) is sitting on a dynamically allocated IP from the pool and all is happy.

We allocate a fixed IP to the MAC address by editing in a "host {}" entry into the config and restart the master and slave server in the failover pair.

Client does a DHCPREQUEST for the dynamic IP, the server issues a DHCPNAK.

The client and server are on separate subnets with Cisco 6905E routers doing the dhcp relaying.

The interesting thing is that I see lots of DHCPNAKs, most do not cause a problem. Looking in the logs the "safe" NAKS look like:

DHCPNAK on <client IP> to <client MAC> via <router IP>

But the server stops serving clients as soon as it issues a NAK that is logged as:

DHCPNAK on <client IP> to <client MAC> via e1000g0

e1000g0 being the interface on the DHCP server (a Sun X4200).

This looks like DHCPD had broadcast the NAK instead of unicasting it to the router for relaying. The upshot is that the client does not see the NAK and keeps on using the dynamic IP it has.

This is causing us a bit of trouble at the moment as we have printers and PCs being moved all over the shop and we are losing the DHCP service several times a day.

What makes the server broadcast the NAK on its own local subnet? How can I stop it? I looks like a client issue, but the server's way of dealing with it is a cause for concern...

As the leases on our dynamic pools are 8 days, it can be a while after you issue the fixed address that the server dies. If it happens over a weekend we lose services. Our wireless access points seem particularly sensitive and we have lost wireless everywhere on one occasion. 

Any ideas, I am getting hassle over this! Any help much appreciated.

Thanks.
---------------
Barry Dean
Networks Team
Computing Services Department
Tel: 0151 794 5641 (x45641), Web: http://pcwww.liv.ac.uk/~bvd/
---
Nice boy, but about as sharp as a sack of wet mice.
                -- Foghorn Leghorn





More information about the dhcp-users mailing list