Q152273: DHCP Server May Give Out Duplicate IP Addresses
Article: Q152273
Product(s): Microsoft Windows NT
Version(s): winnt:3.5,3.51,4.0
Operating System(s):
Keyword(s): kbnetwork
Last Modified: 09-AUG-2001
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The information in this article applies to:
- Microsoft Windows NT Workstation versions 3.5, 3.51, 4.0
- Microsoft Windows NT Server versions 3.5, 3.51, 4.0
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SYMPTOMS
========
In network environments that allow duplicate routes between DHCP clients and a
DHCP server, a DHCP server gives out one IP address to two different clients.
This can occur in several different network environments, including Token Ring
environments with large numbers of source routing bridges (where the bridges
would be set up in a way that would provide multiple, unique routes between the
server and client) and Ethernet environments with multiple routers enabled with
BOOTP relay agents or IP helper addressing.
RESOLUTION
==========
Obtain the hotfix mentioned below for your version of Windows NT Server.
STATUS
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Microsoft has confirmed this to be a problem in Windows NT version 4.0.
A supported fix is now available, but has not been fully regression tested and
should be applied only to systems experiencing this specific problem. Unless you
are severely impacted by this specific problem, Microsoft recommends that you
wait for the next Service Pack that contains this fix. Contact Microsoft
Technical Support for more information.
Microsoft has confirmed this to be a problem in Windows NT versions 3.5 and 3.51.
A supported fix is now available, but has not been fully regression tested and
should be applied only to systems experiencing this specific problem. Unless you
are severely impacted by this specific problem, Microsoft recommends that you
wait for the next Service Pack that contains this fix. Contact Microsoft
Technical Support for more information.
MORE INFORMATION
================
A network trace of the problem shows multiple DISCOVER packets arriving at the
DHCP server with the same XID. Only one DISCOVER was sent by the client, but
because of the multiple routes the packet can take through the source routing
bridges, more than one of the same packet arrives at the DHCP server. The DHCP
server responds to each of the DISCOVER packets with an OFFER. Each time an
OFFER is sent, the DHCP server resets the lease on the IP address to 15 minutes.
When a REQUEST is received by the DHCP server, it sets the lease to the lease
period that was designated by the administrator of the DHCP server. It then
sends an ACK to the client, and the client then completes the initialization of
TCP/IP.
Since multiple DISCOVER packets with the same XID and client MAC address are
being processed, a REQUEST from the client might be received while the server is
sending OFFERs. If this happens and the DHCP server ACKs the client and then
sends OFFERs after ACKing, the IP address is set to the temporary lease time of
15 minutes. Since the client already has bound to the IP address that it
received and ACKed for, it will ignore subsequent OFFER packets from the DHCP
server. The temporary lease on the IP address will expire, since the client will
not respond to the OFFER packets that are sent after the first OFFER packet. The
IP address will then be marked as available for leasing by the next client that
requests an IP address. The example below illustrates the process in which this
could happen.
NOTE: This behavior may also happen in Ethernet configurations that are routed
with slower or multiple BOOTP relay agents.
Example of a Network Monitor trace:
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# Source Addr. Dest. Addr. Protocol Command
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20 Client *BROADCAST DHCP Discover (xid=35663566)
Client sends Discover; there are 4 because the packet takes 4 routes
to the server.
21 Client *BROADCAST DHCP Discover (xid=35663566)
22 Client *BROADCAST DHCP Discover (xid=35663566)
24 DHCPServer *BROADCAST DHCP Offer (xid=35663566)
DHCP Server sends Offer.
25 Client *BROADCAST DHCP Request (xid=36773677)
Client sends Request; again 4 because of 4 routes to the server.
26 Client *BROADCAST DHCP Request (xid=36773677)
27 Client *BROADCAST DHCP Request (xid=36773677)
28 Client *BROADCAST DHCP Request (xid=36773677)
29 DHCPServer *BROADCAST DHCP Offer (xid=35663566)
Server sends another Offer. Notice XID is the same as the first Offer.
30 DHCPServer *BROADCAST DHCP ACK (xid=36773677)
31 DHCPServer *BROADCAST DHCP ACK (xid=36773677)
32 DHCPServer *BROADCAST DHCP ACK (xid=36773677)
33 DHCPServer *BROADCAST DHCP ACK (xid=36773677)
Server ACKs the 4 Requests it receives from the client; at this time
server removes IP address from temporary lease.
34 DHCPServer *BROADCAST DHCP Offer (xid=35663566)
35 DHCPServer *BROADCAST DHCP Offer (xid=35663566)
Server sends 2 more Offers. XID is the same as previous 2 Offers and puts the IP
address in temporary 15 minute lease. This lease will expire, since the client
has an IP address per the ACK from the server. When this 15 minute lease
expires, the server will give this IP address to the next client that renews and
doesn't have a reserved IP address.
Additional query words: prodnt
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Keywords : kbnetwork
Technology : kbWinNTsearch kbWinNTWsearch kbWinNTW400 kbWinNTW400search kbWinNT351search kbWinNT350search kbWinNT400search kbWinNTW350 kbWinNTW350search kbWinNTW351search kbWinNTW351 kbWinNTSsearch kbWinNTS400search kbWinNTS400 kbWinNTS351 kbWinNTS350 kbWinNTS351search kbWinNTS350search
Version : winnt:3.5,3.51,4.0
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