Messages in this thread | | | Date | Mon, 12 Oct 2009 15:45:58 -0700 | From | "H. Peter Anvin" <> | Subject | Re: DHCP and iptables |
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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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