Question about iptables and bridging

[Mike Wright]

Snyder, Brian wrote:
> Hi all,
>  
> I am currently using iptablesv1.3.5 on fedora core 5.  At this point I
> am just playing around and trying to learn the system, but my general
> idea is to setup my linux box as a bridge.  Ultimatly I then plan on
> using firewalling to redirect/drop whatever traffic is deemed worthy.
>  
> So I have linux setup correctly as a bridge as so:
>  
> br0    :   inet addr - 172.20.134.7
> eth0  and eth1 are unaddressed and part of the bridge.
>  
> I set this up as follows:
> sudo ifdown eth1
> sudo ifdown eth1
> sudo ifconfig eth0 0.0.0.0
> sudo ifconfig eth1 0.0.0.0
> sudo brctl addbr "br0"
> sudo brctl setfd br0 0
> sudo brctl addif br0 eth1
> sudo brctl addif br0 eth0
> sudo ifup br0
> sudo ifconfig  br0 172.20.134.7 netmask 255.255.0.0
> sudo ip route add default 172.20.255.254
>  
>  
>  
> Now my problem with iptable is the following:
>  
> I tried to setup a rule to look for ICMP from a particular interface as
> such.
>  
> iptables -t filter -A FORWARD -p icmp -i eth0
> iptables -t filter -A FORWARD -p icmp -i eth1
> iptables -t filter -A FORWARD -p icmp -i br0
>  
> Now, the only filter that ever matches any packets is br0.  So because
> of the way the bridge works, if I can't use eth0 or eth1 as input/output
> interfaces, then I can't really tell 'direction' of traffic between my
> lan and my wan.  
>  
> Is there something I am missing here? I would assume there is a way to
> do this (tell direction over bridge) but I don't know what it is.... so
> any help or pointers would be greatly appreciated.

Hi Brian,.

iptables -match physdev

 From the iptables manpage: This module matches on the bridge port input 
and output devices enslaved to a bridge device.

HTH