linux-kernel - Re: DHCP and iptables

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <4AD3B1A6.10508@zytor.com>
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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/