IP destination address for DHCPOFFER?

Glenn Satchell Glenn.Satchell at uniq.com.au
Tue Jun 26 13:22:39 UTC 2007

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,


>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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