Subject: DHCP problem

From: Umit Dericioglu <[email-masked]>
Date: Tue, 22 Jun 1999 07:05:31 -0400 (EDT)

Hello,

We have a big flat university LAN here. Most of the clients are
running Win 95 or Win NT. Our DHCP server is a Win NT 4.0 with
service pack 3. Last few days, the clients have been 
failing to get the IP addr, etc from the DHCP server. 
Everytime we try to renew the lease on a Win95 (or NT client), 
"Access denied" message is displayed. As for the DHCP server, 
we believe it is working fine as there are no error messages and it 
seems to serve fine as the "Active leases" display shows
around 2000 addresses leased with more than 25% of address 
space still unused.

Besides, we've set up another NT box
as the DHCP server, after stopping the first one. The 
problem is the same. Then, we stopped that one too, to see
if there is another unauthorized DHCP server somewhere
in the network, causing the trouble. There is none.
The clients display "DHCP server unavailable" during the 
renewal of lease when all DHCP servers are down.

We've also watched the packets. We could not find any 
unusual activity. 

Has anybody faced a similar problem?
Or any suggestion where to look as a possible cause?

Any hint will be greatly appreciated. This problem is
driving us crazy..

Umit Dericioglu
King Saud University
Riyadh

---

From: Ted Lemon <[email-masked]>
Date: Tue, 22 Jun 1999 08:17:35 -0400 (EDT)

Setting up a second DHCP server may not have been a good idea, unless
you were careful to assign each DHCP server seperate address ranges
from which to allocate IP addresses.  The symptoms you describe
actually suggest that somebody has set up a DHCP server on your
network without your knowledge, and that that DHCP server is sending
DHCPNAK messages to DHCP clients when they receive IP addresses from
the DHCP server.

If there is a rogue DHCP server, you can probably find it using a
network sniffer (e.g., tcpdump).   The ISC DHCP client can also be
used, since it prints meaningful log messages describing what's
happening, rather than sanitized dialog boxes that just tell you
something went wrong.   You'll need a Unix, BSD or Linux system on
which to run the ISC DHCP client or tcpdump.

			       _MelloN_