Two different IP addresses leased out to the same device

Simon Hobson dhcp1 at
Wed Sep 9 15:45:12 UTC 2009

Markus Zuercher wrote:

>I have one device which constantly receives different IP addresses
>given out from a single DHCPD 4.1.0 server.
>For a better understanding, this is the lease database of the server:
>lease {
>   starts 3 2009/09/09 14:24:53;
>   ends 4 2009/09/10 14:24:53;
>   tstp 4 2009/09/10 14:24:53;
>   cltt 3 2009/09/09 14:24:53;
>   binding state active;
>   next binding state free;
>   hardware ethernet xx:xx:xx:xx:xx:xx;
>   uid "\001\000\011k\007\346z";
>   option agent.circuit-id 0:96:0:41;
>   option agent.remote-id 0:1e:69:xx:xx:xx;
>   client-hostname "privat";
>lease {
>   starts 3 2009/09/09 14:28:14;
>   ends 4 2009/09/10 14:28:14;
>   tstp 4 2009/09/10 14:28:14;
>   cltt 3 2009/09/09 14:28:14;
>   binding state active;
>   next binding state free;
>   hardware ethernet xx:xx:xx:xx:xx:xx;
>   option agent.circuit-id 0:8a:0:41;
>   option agent.remote-id 0:18:9b:xx:xx:xx;
>   client-hostname "support-laptop";

Frank has hinted at this, I'll expand and explain.

As far as the server is concerned, they are different clients as 
required by the RFC.

One lease has a UID (aka Client-ID) which therefore forms the primary 
key in the database - the other has no UID and so the server falls 
back to using the MAC address.

This is a longstanding issue when a machine boots with multiple DHCP 
clients - and it's mostly down to Microsoft that the problem exists. 
The RFC doesn't specify what should be in the Client-ID, and 
Microsoft decided that putting the MAC address in there made sense. 
This isn't wrong, just different to everyone else who default to not 
supplying a Client-ID at all.

It first started popping up when people were multi-booting into 
Windows and other OS's like Linux. Then PXE came along and added to 
it. There have been patches to fudge the request packets (either 
remove the Client-ID or add it).

A proposed feature that I believe never made it was to allow the 
administrator to configure the database key - it currently defaults 
to "pick-first(client-id, hardware)". If changed to just plain 
"hardware" then Client-ID would be ignored.

Simon Hobson

Visit for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

More information about the dhcp-users mailing list