Copyright © 2001, 2002, 2004 Thomas M. Eastep
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
2004-05-24
Table of Contents
For most operations, DHCP software interfaces to the Linux IP stack at a level below Netfilter. Hence, Netfilter (and therefore Shorewall) cannot be used effectively to police DHCP. The “dhcp” interface option described in this article allows for Netfilter to stay out of DHCP's way for those operations that can be controlled by Netfilter and prevents unwanted logging of DHCP-related traffic by Shorewall-generated Netfilter logging rules.
Specify the “dhcp” option on each interface to be
served by your server in the /etc/shorewall/interfaces
file. This will generate rules that will allow DHCP to and from your
firewall system.
When starting “dhcpd”, you need to list those
interfaces on the run line. On a RedHat system, this is done by
modifying /etc/sysconfig/dhcpd.
Specify the “dhcp” option for this interface in the
/etc/shorewall/interfaces
file. This will generate rules that will allow DHCP to and from
your firewall system.
If you know that the dynamic address is always going to be in
the same subnet, you can specify the subnet address in the
interface's entry in the /etc/shorewall/interfaces
file.
If you don't know the subnet address in advance, you should
specify “detect” for the interface's subnet address
in the /etc/shorewall/interfaces
file and start Shorewall after the interface has started.
In the event that the subnet address might change while Shorewall is started, you need to arrange for a “shorewall refresh” command to be executed when a new dynamic IP address gets assigned to the interface. Check your DHCP client's documentation.