Re: problems filtering ms file shares

[Matthew Pocock <matthew.pocock@ncl.ac.uk>]

Gandalf in #nefilter on irc.freenode.net sorted me out. I thought I 
should post the solution - somebody else may be caring.

Apparently, ms file shares fall back on IPX & netbui when IP isn't 
available. The linux bridge code was bridging all ethernet traffic, and 
of course iptables only sees IP. So, I added rules to ebtables to block 
everything but ARP and IP, and that seems to have stopped the ms file 
sharing tunneling through. The extra commands on the end of my bridge 
script are:

/sbin/ebtables --policy DROP
/sbin/ebtables --append FORWARD -p ARP -j ACCEPT
/sbin/ebtables --append FORWARD -p IP -j ACCEPT

Sorry for the noise.

M

Matthew Pocock wrote:

> Hi,
>
> I'm having trouble setting fire wall rules on a bridge. I'm running 
> rh9, and
> have patched the kernel with bridge-nf-0.0.10-against-2.4.20.diff and
> ebtables-v2.0.003_vs_2.4.20.diff as well as installing the utilities 
> brctl and
> ebtables.
>
> Using iptables (editing my uni2lan & lan2uni chains), I seem to be 
> able to
> control what goes through the firewall well enough to convince ping, 
> http,
> nmap and traceroute (with and without the -I option) that protocols & 
> ports
> are reachable or not.
>
> However, windows file sharing seems go go right through. If I pull either
> cable out of the bridge, then windows file sharing across the bridge 
> stops (no
> supprise there), but if the cable is in, file sharing is always 
> functional,
> even if I set all chains to deny everything (by uncommenting the 
> debugging
> rules below).
>
> Down at the bottom of the netfilter/iptables docs, it says I should be 
> using
> the physdev module to match bridge ports, but it seems that this 
> module is not
> present on my system. Other docs say that it is only needed for 
> kernels 2.5.44
> and greater...
>
> iptables-restore v1.2.7a: Couldn't load match
> `physdev':/lib/iptables/libipt_physdev.so: cannot open shared object 
> file: No
> such file or directory
>
> tcpdump (run seperately on eth0 and eth1) seems to be showing exactly 
> what you
> would expect given the firewall rules & doesn't show the packets 
> transmitting
> information between the two PCs, but the two PCs still are doing file 
> sharing.
>
> I'm feeling bewildered.
>
> I use this script to set up the bridge:
>
> /usr/sbin/brctl addbr br0
> /usr/sbin/brctl stp br0 off
> /usr/sbin/brctl addif br0 eth0
> /usr/sbin/brctl addif br0 eth1
> /sbin/ifconfig eth0 down
> /sbin/ifconfig eth1 down
> /sbin/ifconfig eth0 0.0.0.0 up
> /sbin/ifconfig eth1 0.0.0.0 up
> /sbin/ifconfig br0 128.240.227.17
> echo "1" > /proc/sys/net/ipv4/ip_forward
> /sbin/route add default gw 128.240.227.251
>
> My iptables config looks like this:
>
> ##############################################################################
> #
> # iptables config file written by Matthew Pocock 
> (matthew.pocock@ncl.ac.uk)
> #
>
> ## main chains
> *filter
> :INPUT ACCEPT [0:0]
> :FORWARD DROP [0:0]
> :OUTPUT ACCEPT [0:0]
> :lan2uni - [0:0]
> :uni2lan - [0:0]
>
> ##############################################################################
> #
> ## debugging rules
> #-A FORWARD -j DROP
> #-A INPUT -j DROP
> #-A OUTPUT -j DROP
>
> ##############################################################################
> #
> ## drop all invalid packets, iregardless
> -A FORWARD -m state --state INVALID -j DROP
>
> ## split traffic depending upon direction
> -A FORWARD -i eth0 -o eth1 -j uni2lan
> -A FORWARD -i eth1 -o eth0 -j lan2uni
>
> ##############################################################################
> #
> ## let everything out - is this a good plan?
> ## we should realy be a bit more careful here, but hey-ho
> -A lan2uni -j ACCEPT
>
> ##############################################################################
> #
> ## let only specific things in
>
> ## stuff we've seen before should get through
> -A uni2lan -m state --state RELATED,ESTABLISHED -j ACCEPT
>
> ## accept some new connections for 'nice' protocols
> ## we know they are new, as we've dropped all invalid things earlier, 
> and just
> ## now we let related & established through.
>
> ## ping, ssh
> -A uni2lan -p icmp -j ACCEPT
> -A uni2lan -p tcp --dport 22 -j ACCEPT
>
> ## drop everything else comming in
> -A uni2lan -j REJECT
>
>
>
> COMMIT
>
>
> Thanks,
>
> Matthew
>
>
>
>