DHCP Server May Give Out Duplicate IP Addresses

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:

#     Source Addr.   Dest. Addr.    Protocol  Command
======================================================================
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.