Listening IP

[Dean, Barry]

Thanks for this, it will be helpful.

It does not solve the Nagios problem, because it causes "Option 54" to be set to the correct src IP, but the
"check_dhcp" program is not fooled that easily and sees the default IP of the host as the src IP, ignoring the Option 54.

We will just have to monitor using the primary IPs, not a show stopper by any means.

---------------
Barry Dean
Networks Team


-----Original Message-----
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On Behalf Of Glenn Satchell
Sent: 02 July 2008 14:41
To: dhcp-users at isc.org
Subject: Re: Listening IP


>Subject: Listening IP
>Date: Wed, 2 Jul 2008 14:26:00 +0100
>From: "Dean, Barry" <B.Dean at liverpool.ac.uk>
>To: <dhcp-users at isc.org>
>
>Hi, I have four DHCP servers, ISC DHCP 4.0.0 running on Solaris 10 x86 on Sun 
X4200 hardware.
>
>These machines have 4 network cards. The one I am using has id e1000g3.
>
>On this card there are two IPs, an example:
>
>e1000g3:	192.168.31.100	primary-name
>e1000g3:1	192.168.31.10	dhcp
>
>My config file says to listen on the 31 subnet thus:
>
>subnet 192.168.31.0 netmask 255.255.255.0 {
>  option subnet-mask 255.255.255.0 ;
>  option routers 192.168.31.1 ;
>  option domain-name-servers 192.168.120.100, 192.168.120.101 ;
>}
>
>Problem is clients report that the DHCP server that gave them an IP was 
192.168.31.100, the primary-name.
>
>I would like them to think it was the 192.168.31.10 IP, the dhcp name instead.
>
>With other daemons, like DNS, RADIUS, I am used to binding the service to an 
IP, with DHCP it looks like you bind to an entire subnet, can this be changed? I 
would only like the servers to listen on the "dhcp" interface and only reply 
from it.
>
>Thanks.

dhcp listens at the interface layer, rather than an IP address. This is
because  some of the packets it's looking for are broadcast to the
all-ones mac address (ff:ff:ff:ff:ff:ff) before an IP address is even
assigned.

You can use the server-identifier command to set the source address
used for replies, otherwise the default is to use the first IP address
assigned to the interface. See dhcpd.conf man page for more information
on this statement.

regards,
-glenn