Simple IPTables Question.

Curtix · 04-11-2008, 09:23 AM

Take a look at this rule:

Code:

iptables -t nat -A PREROUTING -p tcp -i eth0 -d 123.123.123.187 --dport 23 -j DNAT --to 192.168.1.202:23

Should that not forward any telnet request to 123.123.123.187 to 192.168.1.202?
It wont work for me.

Curtix · 04-11-2008, 12:28 PM

Really i just need my syntax checked.
because based on what i know about iptables that should work.

bzyk · 04-11-2008, 02:21 PM

Maybe you have another rule before this one? And of course, you must hava SNAT rule... Maybe you have FORWARD closed? show us iptables -L and iptables -L -t nat

Curtix · 04-14-2008, 11:24 AM

I found some garbage in my tables.
As you can see I am trying to do a simple port forward.
I flushed all my other junk rules and have forward set to accept.
So is this correct.

Code:

iptables -t nat -A PREROUTING -p tcp -i eth0 -d 123.123.123.187 --dport 23 -j DNAT --to 192.168.1.202:23

Could it be simplified even more.
I just want all port 23 requests that come to 123.123.123.187 to forward to the internal 192.168.1.202 port 23.

bzyk · 04-15-2008, 02:20 AM

Yes, this is correct, but remember about FORWARD chain (you must allow for this traffic there if you have DROP policy on it), and of course you must have SNAT rule in order to make any usefull efect. something like this;
iptables -A POSTROUTING -t nat -o eth0 -j SNAT --to-source 123.123.123.123 -s 192.168.1.202

mtimbro · 04-15-2008, 08:13 AM

Quote:

Originally Posted by Curtix

I found some garbage in my tables.
As you can see I am trying to do a simple port forward.
I flushed all my other junk rules and have forward set to accept.
So is this correct.

Code:

iptables -t nat -A PREROUTING -p tcp -i eth0 -d 123.123.123.187 --dport 23 -j DNAT --to 192.168.1.202:23

Could it be simplified even more.
I just want all port 23 requests that come to 123.123.123.187 to forward to the internal 192.168.1.202 port 23.

You'll need another rule as follows:

/sbin/iptables -A FORWARD -p tcp -i eth0 -d 192.168.1.202 --dport 23 -j ACCEPT

Hope this helps.

Curtix · 04-15-2008, 09:00 AM

Do I not need to state to state a source IP because this system may host multiple IPs on one nic (eth0), Rather can I add -i eth0 to the table.

Curtix · 04-15-2008, 09:49 AM

This just wont work.
Here is my nat table ATM:

Code:

[root@gabriel ~]# iptables -L -t nat
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
SNAT       all  --  192.168.0.0/16       anywhere            to:123.123.123.187

Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
DNAT       tcp  --  anywhere             123-123-123-187.isp.net tcp multiport ports telnet to:192.168.1.202:23
[root@gabriel ~]#

Code:

[root@gabriel ~]# iptables -L
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:telnet

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

What am I missing?

bzyk · 04-16-2008, 01:44 AM

Of course you have 123.123.123.187 ip addres on eth0, right?
This rules should work. Hmm... Try to iptables -L -t nat -v to look if any packets coming in this dnat chain. Try to use tcpdump on your LAN card (eth1?), to watch if any packets to telnet service on that lan host coming. something like;
tcpump -i eth1 port 23 and host 192.168.1.202 -n
And watching on tcpdump try to login on telnet (from any remote machine out of your network). Tell us if any packets going out through your router to that telnet machine (tcpudmp show it to you).

Curtix · 04-16-2008, 07:33 AM

Code:

[root@gabriel ~]# iptables -L -t nat -v
Chain OUTPUT (policy ACCEPT 7 packets, 545 bytes)
 pkts bytes target     prot opt in     out     source               destination                       

Chain POSTROUTING (policy ACCEPT 116 packets, 31996 bytes)
 pkts bytes target     prot opt in     out     source               destination                       
67788 3446K SNAT       all  --  any    eth0    192.168.0.0/16       anywhere                                  to:123.123.123.187

Chain PREROUTING (policy ACCEPT 82987 packets, 5701K bytes)
 pkts bytes target     prot opt in     out     source               destination                       
    2   100 DNAT       tcp  --  eth0   any     anywhere             123-123-123-187.i                      sp.net tcp multiport ports telnet to:192.168.1.202:23
[root@gabriel ~]#

See anything thing?
and when I try to telnet to the 123.123.123.187 I just get:

Code:

Trying 123.123.123.187...

Forever.

trryhend · 04-16-2008, 08:27 AM

Quote:

Originally Posted by Curtix

Take a look at this rule:

Code:

iptables -t nat -A PREROUTING -p tcp -i eth0 -d 123.123.123.187 --dport 23 -j DNAT --to 192.168.1.202:23

Should that not forward any telnet request to 123.123.123.187 to 192.168.1.202?
It wont work for me.

Not really an iptables expert but...

If it's ok to just redirect all port 23 connections:

Code:

iptables -t nat -A PREROUTING -p tcp -m tcp --dport 23 -j DNAT --to 192.168.1.202

You may need to try it, get someone to initiate a telnet session while looking at tcpdump to see what's happening, and what port is actually being used, (because what you have looks ok to me, but wouldn't know for sure unless tested).

You might also need tell the client to specify port 23

Curtix · 04-16-2008, 04:39 PM

Seeing as how i was LOST. I opted to strip out any garbage and start from scratch.
My Firewall is set to simple NAT outbound connections to share the internet.
All default chains are set to allow.
________________

So NOW I want to just forward inbound ( from the internet ) telnet ( port 23) connections to the WAN ip of 123.123.123.187 (eth0) through 192.168.1.1 (eth1) to to host server 192.168.1.202 on port 23.
Please advise accordingly.
Thanks a lot.

bzyk · 04-17-2008, 02:04 AM

Code:

Chain PREROUTING (policy ACCEPT 82987 packets, 5701K bytes)
 pkts bytes target     prot opt in     out     source               destination                       
    2   100 DNAT       tcp  --  eth0   any     anywhere             123-123-123-187.i                      sp.net tcp multiport ports telnet to:192.168.1.202:23

It looks like packets to telnet machine going out from router (2 pkts). The question is; is 192.168.1.203 answering fot this packets? Is there a telnet service? check it by running tcpdump with option; tcpdump -i eth1 host 192.168.1.203 and port 23 -n
And watch if telnet machine answering for syn packet (try to login from "somewhere in world" to telnet, and in this time look on tcpdump screen)
If you can, show here some log of this tcpdump session.

Curtix · 04-17-2008, 07:16 AM

I can telnet on port 23 to the internal machine from my firewall so I know that much works. As well as a ton of other hosts. Its an application server and everyone telnets into it. Could you suggest some rules again; just the simplest port forward rule, minimal stuff needed to get the port forwarded.

Curtix · 04-17-2008, 07:42 AM

This is what I have ATM and it does not work.
could there be something besides the firewall that would block all inbound telnet connections on the firewall computer?

Code:

[root@gabriel sysconfig]# iptables -L
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             anywhere            tcp spt:telnet dpt:telnet

Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
[root@gabriel sysconfig]#

Code:

[root@gabriel sysconfig]# iptables -L -t nat
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination
SNAT       all  --  192.168.0.0/16       anywhere            to:123.123.123.187

Chain PREROUTING (policy ACCEPT)
target     prot opt source               destination
DNAT       tcp  --  anywhere             anywhere            tcp dpt:telnet to:192.168.1.202:23
[root@gabriel sysconfig]#