Bug? INFORM and REQUEST getting ACK'ed with different DNS server lists (3.0.4, 3.1.1, 3.1.2)

Axel Beckert beckert at phys.ethz.ch
Tue Feb 3 17:35:58 UTC 2009


since a few months we have one group in our dhcpd.conf which has
different DNS servers than all other hosts:

# ----------------
# Network defaults
# ----------------
option domain-name-servers,;


# --------------
# PXE Clients AD
# --------------
group {
  option domain-name-servers,;

Nonetheless, under some circumstances the clients in this group (only
Windows Vista and Windows Server 2008 machines) get the globally
defined DNS servers instead of the AD servers.

Through sniffing with tcpdump and wireshark, we found out that this
only happens if the client sent a DHCP INFORM packet -- which happens
about once per day and machine.

Today I found http://c3rb3r.openwall.net/dhcping/ (which is not the C
written dhcping as available in e.g. the Debian GNU/Linux
distribution). With this tool I can reliably reproduce this unexpected
behaviour on demand (on a Linux box), too:

acromantula:~/DHCPing-0.91/sbin# perl -I../lib ./dhcping -m 00:1c:c0:3f:af:3f -M 00:0a:5e:54:6b:2f -i -I -opttype REQUEST | fgrep DNS
<< BOOTP option  (006 [0x06]) : DNS_SERVER :
acromantula:~/DHCPing-0.91/sbin# perl -I../lib ./dhcping -m 00:1c:c0:3f:af:3f -M 00:0a:5e:54:6b:2f -i -I -opttype INFORM | fgrep DNS
<< BOOTP option  (006 [0x06]) : DNS_SERVER :

(The machine with the MAC address 00:1c:c0:3f:af:3f and IP address is in the above mentioned group. the machine with MAC
address  00:0a:5e:54:6b:2f and IP address is one of our
DHCP servers.)

As far as I understand DHCP there should be no difference in any of
the server lists equal if the query was a DHCP REQUEST or a DHCP
INFORM packet.

So I'm considering this being a bug in ISC DHCP.

We had this behaviour with the following versions of the ISC DHCPd:

+ 3.0.4 (current Debian 4.0)
+ 3.1.1 (upcoming Debian 5.0)
+ 3.1.2 (self built Debian package based on the Debian 3.1.1 package)

Both our DHCP servers are two more or less equal 32-bit x86 Linux
boxes running either 2.6.18 from Debian 4.0 or 2.6.26 from Debian 5.0.

I haven't found any bug tracking system or similar mentioned on
http://www.isc.org/, so I'm sending this issue to the dhcp-users
mailing list to which I'm subscribed since a few days ago.

		Kind regards, Axel Beckert
Axel Beckert <beckert at phys.ethz.ch>       support: +41 44 633 26 68
IT Services Group, HPT D 17                 voice: +41 44 633 41 89
Departement of Physics, ETH Zurich
CH-8093 Zurich, Switzerland		   http://nic.phys.ethz.ch/

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