IPTables Masquerading Issue

[johncongdon]

I have setup masquerading dozens of times with no issues.

I have 2 linux boxes (A=Private, B=Masquerader)

Here are the checks I have done
A - Default gateway is B
B - iptables is wide open with 1 postrouting statement
iptables -t nat -A POSTROUTING -s 10.0.73.11 -j SNAT --to-source PUBLIC_IP
B - IP Forwarding is enabled.

I can ping from A to B's private address. Cannot go past that.
If I run iptraf on B, I can see the ping req/reply from A to another IP.

If I ssh from A to another machine outside the firewall, I can see the connection attempt with netstat -an | grep :22 on the remote machine.

So the connection are being transmitted out correctly, but not getting returned correctly through SNAT. Any ideas?

[Mark_NL]

why do you SNAT?

for a simple and fast gateway you should do this:

set net.ipv4.ip_forward to 1 (/etc/sysctl.conf, then run: sysctl -p)
iptables -t nat -I POSTROUTING -o eth0 -j MASQUERADE

eth0 being your external interface (wan connection).

That's it.

[johncongdon]

I have done that as well. Both give me the same results. The SNAT was a last attempt and something that I read was "more secure" than just masquerade.

Either way, the end result was the same

[Mark_NL]

If my solution doesn't work, then there is something you haven't told us about your setup.

A -> B -> C

you're able to ping C unless "C" is dropping stuff

A: ping c
B: tcpdump -i eth0 icmp
C: tcpdump -i eth0 icmp

now see what packets go back and forth on B and C

[johncongdon]

I wish I was not telling you something. That's why I tried to be very specific in my setup and testing that I have done. I have setup simple masquerading before, should not be this difficult. I also made sure selinux was off, in case that was the issue. I can ping from A to the private side of B, so ping is not being blocked on A.

I also went back to your suggestion of iptables -t nat -I POSTROUTING -o eth1 -j MASQUERADE, and I get the same results.

I see the ping request/reply on C (in your example).
I see the ping request/reply on B ( on both eth0 and eth1 )

The firewall on A is default open

root@PSWEBNODE1 [~]# iptables -L -n
Chain INPUT (policy ACCEPT)
target prot opt source destination

Chain FORWARD (policy ACCEPT)
target prot opt source destination

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

Chain acctboth (0 references)
target prot opt source destination
root@PSWEBNODE1 [~]# cat /etc/redhat-release
CentOS release 5.6 (Final)

Output from B

[root@psfw1 ~]# tcpdump -i eth0 icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
11:46:49.508565 IP 10.0.73.11 > MachineC: ICMP echo request, id 36931, seq 1, length 64
11:46:49.528951 IP MachineC > 10.0.73.11: ICMP echo reply, id 36931, seq 1, length 64
11:46:50.508192 IP 10.0.73.11 > MachineC: ICMP echo request, id 36931, seq 2, length 64
11:46:50.529028 IP MachineC > 10.0.73.11: ICMP echo reply, id 36931, seq 2, length 64

4 packets captured
4 packets received by filter
0 packets dropped by kernel
[root@psfw1 ~]# tcpdump -i eth1 icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth1, link-type EN10MB (Ethernet), capture size 96 bytes
11:46:53.507654 IP MachineB_PublicIP > MachineC: ICMP echo request, id 36931, seq 5, length 64
11:46:53.527257 IP MachineC > MachineB_PublicIP: ICMP echo reply, id 36931, seq 5, length 64

2 packets captured
2 packets received by filter
0 packets dropped by kernel

Output from C

[root@squishy scanner]# tcpdump -i eth0 icmp
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 96 bytes
11:47:18.171359 IP MachineB_PublicIP > MachineC: icmp 64: echo request seq 74
11:47:18.250561 IP MachineC > MachineB_PublicIP: icmp 64: echo reply seq 74

2 packets captured
2 packets received by filter
0 packets dropped by kernel

[Franz]

did you configure this on machine B?

 echo 1 > /proc/sys/net/ipv4/ip_forward