Two consecutive lease entries for same MAC address

Simon Hobson dhcp1 at
Tue Sep 22 12:55:04 UTC 2009

andreas.moroder at wrote:

>in our lease file I found consecutive lease entries for the same MAC 
>address with two different IP adresses. The first one has no 
>instructions for the dns registration and one vendor-string. The 
>second has the dns information and a completely different 
>The machines are thin clients ( old PCs ) that start from a intel 
>bootable bios and then load thinstation.
>Can anyone please tell me why dhcpd ( ancient V3.0rc4 ) give two 
>different IP addresses even if the MAC is the same ?

It's a classic "dual boot" problem where one client does not provide 
a UID (aka Client-ID) and another does. The standard *requires* that 
the DHCP server use the supplied Client-ID as the primary key, and 
the MAC address only if no Client-ID is supplied.

Thus the two clients ARE DIFFERNET devices as far as the DHCP server 
is concerned, and so they get different addresses.

>lease {
>   hardware ethernet 00:d0:b7:40:9d:aa;
>   set vendor-string = "PXEClient:Arch:00000:UNDI:002001";
>lease {
>   hardware ethernet 00:d0:b7:40:9d:aa;
>   uid "\001\000\320\267@\235\252";
>   set vendor-string = "udhcp 0.9.9-pre";
>   set ddns-fwd-name = "";
>   set ddns-rev-name = "";
>   client-hostname "derma-vormerk";

Simon Hobson

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