Some DHCP clients do not use Client identifier option

A while ago I've documented how you can cope with DHCP clients that do not send Client identifier (DHCP option 61) in their DHCP Discover/Request messages, but some people are still trying to persuade me that the client-identifier pool configuration command should work. I really wanted to be sure I hadn't missed something, so I started Wireshark and captured the actual DHCP Discover packet generated by a Linux host:

 
As you can see, the DHCP packet does not contain the Client identifier option, so the DHCP server (the router) has nothing to compare the value of the client-identifier parameter with. The only parameter the DHCP server can use is the Client MAC address field in the DHCP Discover message, which is matched with the mac-address pool configuration command.
In contrast to the default Linux behavior, DHCP Discover messages generated by other platforms (for example, Windows or a Cisco router) include the Client identifier option:
 

1 comment:

  1. Yup, I believe that was the problem I was having using the Untangle Linux distribution. I will test it today and see what happens.

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Ivan Pepelnjak, CCIE#1354, is the chief technology advisor for NIL Data Communications. He has been designing and implementing large-scale data communications networks as well as teaching and writing books about advanced technologies since 1990. See his full profile, contact him or follow @ioshints on Twitter.