DHCPNAK reply using wrong server identifier IP

Glenn Satchell glenn.satchell at uniq.com.au
Thu May 26 07:51:49 UTC 2011

dhcp-server-identifier sets the IP inside the internal dhcp option space,
not the source IP address on the packet.


> Hi ISC-DHCP-Users,
> I have configured dhcpd.conf to use an global 'option
> dhcp-server-identifier
> xxx.xxx.xxx.xxx' directive since my server box has more than one IP
> (raised
> on an alias device). This 'dhcp-server-identifier IP' is actually an alias
> IP via eth0:0.
> Now, after playing with Wire Shark, I noticed that each time the server
> issues a DHCPNAK reply (due to a client DHCPREQUEST for a wrong IP), the
> DHCPNAK is being sent thru the real IP in eth0, and not thru the
> configured
> 'option dhcp-server-identifier IP' eth0:0 IP.
> My questions now are:
> 1. Is there a specific reason why DHCPNAK replies do not follow the global
> 'option dhcp-server-identifier xxx.xxx.xxx.xxx' directive?
> 2. Should I use a 'server-identifier IP' sub-directive ('tho I haven't
> tried
> yet) instead of a global one?
> Any tips/explanations are very much welcome.
> Best regards,
> Vic

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