IP destination address for DHCPOFFER?

Dan Sahlin dan.sahlin at raycore-fos.com
Wed Jun 27 08:10:00 UTC 2007

I find it quite a bit strange that DHCP clients are supposed to listen to an
IP adress in the DHCPOFFER message that they have not requested or has been
I really had expected that RFC to be more clear that this.
Anyway, the advice I got from Glenn solved my problem.


Dan Sahlin

2007/6/26, Glenn Satchell <Glenn.Satchell at uniq.com.au >:
> You could try this option. It's a grey area, the client is supposed to
> request that responses be broadcast but not all do, so the dhcp server
> gives you the each way option.
> man dhcpd.conf
> ...
>      The always-broadcast statement
>        always-broadcast flag;
>        The DHCP and BOOTP protocols both require DHCP  and  BOOTP
>        clients to set the broadcast bit in the flags field of the
>        BOOTP message header.  Unfortunately, some DHCP and  BOOTP
>        clients  do  not  do  this,  and therefore may not receive
>        responses from the DHCP server.   The DHCP server  can  be
>        made  to always broadcast its responses to clients by set-
>        ting this flag to 'on' for the  relevant  scope;  relevant
>        scopes  would  be  inside  a  conditional  statement, as a
>        parameter for a class,  or  as  a  parameter  for  a  host
>        declaration.    To avoid creating excess broadcast traffic
>        on your network, we recommend that you restrict the use of
>        this  option to as few clients as possible.   For example,
> regards,
> -glenn
> >Date: Tue, 26 Jun 2007 14:07:37 +0200
> >From: "Dan Sahlin" <dan.sahlin at raycore-fos.com>
> >
> >What is the proper IP destination address for a DHCPOFFER message?
> >
> >ISC DHCP v3.0.4 uses the new proposed IP address as the destination
> >address in the DHCPOFFER message, i.e. the same as the yiaddr.
> >
> >One DHCP client I have (a managed fiber converter) ignores this
> >DHCPOFFER message as it expects the broadcast IP adress (
> > instead.
> >
> >In "The DHCP Handbook" on top of page 107 the DHCPOFFER destination
> >address is shown as the broadcast address. However, the text on the
> >same page says:
> >"This message is sent to the Ethernet broadcast address
> >ff:ff:ff:ff:ff:ff and to the new assigned IP address"
> >
> >I do not find the RFC2131 very clear either regarding the proper
> >addressing of DHCPOFFER. In section  3.1.2 it simply says
> >"Each server may respond with a DHCPOFFER message that includes an
> >available network address in the 'yiaddr' field..."
> >
> >So how should the standards properly be interpreted regarding the IP
> >and MAC adresses of the DHCPOFFER message?
> >
> >  /Dan Sahlin
> >
> >email:  dan.sahlin at raycore-fos.com
> >phone: +46 70 5198187
> >

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