simple rules and unexpected traffic

Hi,

I use a simple set of iptables rules for my laptop to reject everything from outside using ip_conntrack (from the howto) :

# Generated by iptables-save v1.2.6a on Thu Jul 4 09:54:11 2002
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [43965:4118502]
:block - [0:0]
-A INPUT -j block
-A FORWARD -j block
-A block -m state --state RELATED,ESTABLISHED -j ACCEPT
-A block -i ! eth0 -m state --state NEW -j ACCEPT
-A block -i eth0 -m limit --limit 3/hour -j LOG --log-prefix "Bad packet from eth0:" -A block -i ! eth0 -m limit --limit 3/hour -j LOG --log-prefix "Bad packet not from eth0:" -A block -j DROP
COMMIT
# Completed on Thu Jul 4 09:54:11 2002

I have a ADSL connection and only a hub between my laptop and the ADSL-modem. Recently something changed, I guess on the router from my provider and now I see unexpected traffic.

I see it with the eth0 monitor in gkrellm and with iftop but not with lsof -i.
I was not expecting this traffic and the pattern seems strange : a constant 20kB incoming traffic during a few seconds. So I started looking closer. With ethereal I saw that it was a kind of flooding most of the time a lot of SYN packet but also netbios .... Each time both IPs are not one of my computer. For example I see during one of this flooding with 'tcpdump -c 2 -e'

tcpdump: listening on eth0
10:00:39.946940 0:0:c:c3:a:88 ff:ff:ff:ff:0:30 ip 62: 216-203-233-196.customer.algx.net.3574 > adsl-216-158-52-76.cust.oldcity.dca.net.www: S 2011680397:2011680397(0) win 16384 <mss 1460,nop,nop,sackOK> (DF) 10:00:39.949401 0:0:c:c3:a:88 ff:ff:ff:ff:0:30 ip 62: 216-203-233-196.customer.algx.net.3574 > adsl-216-158-52-76.cust.oldcity.dca.net.www: S 2011680397:2011680397(0) win 16384 <mss 1460,nop,nop,sackOK> (DF)

I am not sure how to interpret 'ff:ff:ff:ff:0:30' is it a kind of broadcasting at the ethernet level ?

Why can I see these packets that are not for me ?

Why this traffic is not dropped by netfilter ?

It seems to be a miss-configuration of my ISP router, no ? I believe it's harmless (except for my bandwidth) but I don't understand why I see (with gkrellm) this traffic which seems to be rejected before netfilter. Is gkrellm using packets information before the iptable processing ?

I have tried to set /proc/.../eth0/rp_filter to 0 without any difference.

Thanks,
Christophe

Christophe Barbé <[email protected]>
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Dogs come when they're called;
cats take a message and get back to you later. --Mary Bly

I have found at http://www.cavebear.com/CaveBear/Ethernet/multicast.html
that ff:ff:ff:ff:0:30 could be a multicast ethernet address (03-00-FF-FF-FF-FF) for 'All Stations Address'.

Is it something commonly used by script kiddies ?

If I undersatnd correctly, nothing has changed at the router, but somebody connected at the same router is doing bad stuff. Is it right ?

What I still don't understand is why I can see this traffic with my iptables rules. Is the traffic exposed (to user-space tools) before entering the iptables processing ?

Christophe

Christophe Barbé <[email protected]>
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Dogs believe they are human. Cats believe they are God.