XP clients sometimes ignore DHCPOFFERs
Hagedorn at uni-koeln.de
Wed Mar 15 16:09:51 UTC 2006
We have noticed problems with at least some Windows XP SP2 clients. Users
report that they were unable to get leases from our dhcpd 3.0.3 server. In
our logs we can see DHCPDISCOVER and DHCPOFFER lines for those clients, but
the clients never send a DHCPREQUEST. Most of the time this situation can
be remedied by rebooting the client, but apparently not always. Repairing
the network connection doesn't seem to do any good, whereas a cycle of
"ipconfig release / ipconfig renew" in cmd.exe seems to be successful.
This issue seems to be related to previous leases that have been acquired
in other networks (i.e. home networks). Our server is configured as
authoritative and sends DHCPNAK messages when a foreign address is
requested. Apparently the clients accept that, because they start sending
DHCPDISCOVER messages afterwards. They keep on requesting their old address
in these messages, which is normal according to the Microsoft
Knowledgebase. It's also normal that they send three DISCOVER messages, as
However, in contrast to the server in the article, the ISC server responds
to all three of them with the same OFFER. When things go wrong the client
just gives up after the third DHCPDISCOVER and assigns a link-local IP
We are told that this problem never occurs with other servers, e.g. those
built into home DSL routers. All things being equal one would expect the
*same* problem when those users return from work and plug their computers
into their routers at home.
We have sniffed the entire "conversation" between client and server using
Ethereal on *both* the client and the server. To my eyes everything looks
the way it should look. We would like to just blame Microsoft, but I can't
find anything regarding this in the KnowledgeBase and, more importantly,
something must be different if this never happens with those routers at
We have pcap files of the sniffed sessions if that would help. I'd be
grateful for any suggestions.
Cheers, Sebastian Hagedorn
.:.Sebastian Hagedorn - RZKR-R1 (Gebäude 52), Zimmer 18.:.
Zentrum für angewandte Informatik - Universitätsweiter Service RRZK
.:.Universität zu Köln / Cologne University - Tel. +49-221-478-5587.:.
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