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SubjectRe: DHCP and iptables
On 10/12/2009 02:50 PM, lkml@makubi.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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