Iptables rules, forward between two interfaces

forwardingiptables

i have a some difficulties in configuring my ubuntu server firewall …
my situation is this:

eth0 -> internet

eth1 -> lan1

eth2 -> lan2

I want that clients from lan1 can't communicate with clients from lan2, except for some specific services. E.g. i want that clients in lan1 can ssh into client in lan2, but only that. Any other comunication is forbidden.

So, i add this rules to iptables:

#Block all traffic between lan, but permit traffic to internet
iptables -I FORWARD -i eth1 -o ! eth0 -j DROP
iptables -I FORWARD -i eth2 -o ! eth0 -j DROP
# Accept ssh traffic from lan1 to client 192.168.20.2 in lan2
iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 22 -d 192.168.20.2 -j ACCEPT

This didn't works. Doing iptables -L FORWARD -v i see:

    Chain FORWARD (policy DROP 0 packets, 0 bytes)
 pkts bytes target     prot opt in     out     source               destination
    33   144 DROP       all  --  eth1 !eth0   anywhere             anywhere
    0     0 DROP       all  --  eth2 !eth0   anywhere             anywhere
23630   20M ACCEPT     all  --  any    any     anywhere             anywhere            state RELATED,ESTABLISHED
    0     0 ACCEPT     all  --  eth1   any     anywhere             anywhere
  175  9957 ACCEPT     all  --  eth1 any     anywhere             anywhere
  107  6420 ACCEPT     all  --  eth2 any     anywhere             anywhere
    0     0 ACCEPT     all  --  pptp+  any     anywhere             anywhere
    0     0 ACCEPT     all  --  tun+   any     anywhere             anywhere
    0     0 ACCEPT     tcp  --  eth1 eth2  anywhere             server2.lan tcp dpt:ssh

All packets are dropped, and the count of packets for the last rule is 0 …

How i have to modify my configuration? Thank you.

Regards
Marco

Best Answer

Your DROPs are at the front of the chain (and will be processed first). You want to put the DROPs after the rule that permits SSH access for the SSH rule to be used.

Something like this should work:

#Accept ssh traffic from lan1 to client 192.168.20.2 in lan2
iptables -A FORWARD -i eth1 -o eth2 -p tcp --dport 22 -d 192.168.20.2 -j ACCEPT

#Block all traffic between lan, but permit traffic to internet
iptables -A FORWARD -i eth1 -o ! eth0 -j DROP
iptables -A FORWARD -i eth2 -o ! eth0 -j DROP

(use the -A to append these rules to the end of the chain)

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