linux-kernel - Re: DHCP and iptables

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Date:	Mon, 12 Oct 2009 15:45:58 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	lkml@...ubi.at
CC:	arndbergmann@...glemail.com, linux-kernel@...r.kernel.org
Subject: Re: DHCP and iptables

On 10/12/2009 02:50 PM, lkml@...ubi.at wrote:
> Well, I just looked for "ethernet protocol" and read some things about  
> DHCP again.
> 
> What's an ethernet protocol?
> 
> I also read, that "DHCP is built directly on UDP and IP" (RFC 2131).
> 
> It uses Ports (UDP 67/68) and the source address of the DHCP server is  
> an IP address.
> 
> Could you answer me more in detail, why I get an IP, but block  
> everything with iptables?
> 

The reason is that the DHCP client bypasses the Linux IP stack
completely (because it has special requirements.)

> |   |   DHCP is an ethernet protocol, not an IP protocol, so you have to use
> |   |   ebtables instead of iptables to filter it.
> |   |
> |   |   	Arnd <><

This is actually incorrect -- DHCP is an IP (UDP, in fact) protocol.  It
just has very special requirements (such as being able to use
src=0.0.0.0 dst=255.255.255.255) that aren't needed in normal operation,
so rather than slowing down the in-kernel IP stack it synthesizes raw
packets.

	-hpa
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