linux-kernel - Re: DHCP and iptables

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Date:	Tue, 13 Oct 2009 00:08:57 +0200
From:	Arnd Bergmann <arndbergmann@...glemail.com>
To:	lkml@...ubi.at
Cc:	arndbergmann@...glemail.com, linux-kernel@...r.kernel.org
Subject: Re: DHCP and iptables

On Monday 12 October 2009, 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.

Ah, right. I confused it with the way that the ancient RARP protocol did
this before DHCP.

Sorry for the confusion.
 
> Could you answer me more in detail, why I get an IP, but block  
> everything with iptables?

The dhcp client opens a raw packet socket at the ethernet device
(if I'm not mistaken again), because there is no IP address assigned
with the netdev at that time. This allows it to send and receive
ethernet frames, bypassing the TCP/IP stack.

	Arnd <><
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