Question

How to Port Forward Using IPtables - my first try

Asked by: Rigged

Hi

I am playing around with IPtables for the first time (on a Debian Lenny box) and I am having some trouble - It is a bit of a learning curb for me :P

What I want to achieve is making an internal application (on a local/internal Windows server) that has a web front-end externally available, so that my users can access it from anywhere with an internet connection.

The Debian box has two network interfaces - the one has a static IP that is externally accessible (tested and confirmed that the server is accessible on the IP) - eth1 with 196.211.X.X and the other interface is for my internal network eth0 on IP 10.47.134.11. The server who is running the web front-end I want made available is running on 10.47.134.36.

My IPTABLES doesn't have any rules set and it accepts all connections (for now while I am testing - I will be restricting access as soon as I got the port forwarding working).

I add the port FOWARD on IPtables as follows (I tried 2 different IPtables setups for the port fowards):
______________________________
iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 80 -j DNAT --to 10.47.134.36:80
iptables -A INPUT -p tcp -m state --state NEW --dport 80 -i eth1 -j ACCEPT
______________________________
iptables -A PREROUTING -t nat -i eth1 -p tcp --dport 80 -j DNAT --to 10.47.134.36:80
iptables -A FORWARD -p tcp -m state --state NEW -d 10.47.134.36 --dport 80 -j ACCEPT
______________________________

With both the setups above the results were the same when trying to access the the static IP on port 80 from a web browser --- "Network Timeout - The server at 196.211.X.X is taking too long to respond."

Is one of the port forward setups I used correct, or should I be setting the rules differently - Is there log files maybe that can give more info on the IPtables traffic - or is there something else wrong that I should be looking at?

THX in advance
Hendrik

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Asked On
2009-09-01 at 01:33:30ID24697258
Tags

port forward

,

iptables

,

debian

Topics

IP Tables/IP Chains

,

Linux Networking

,

Linux Network Security

Participating Experts
2
Points
500
Comments
8

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Answers

 

by: eokePosted on 2009-09-01 at 02:58:59ID: 25229869

Hi, have you enabled ip forwarding in /etc/sysctl.conf?

#net.ipv4.ip_forward=1
#net.ipv6.conf.all.forwarding=1

As I'm sure you know the # needs removing to uncomment the line.

 

by: RiggedPosted on 2009-09-01 at 04:57:19ID: 25230549

Hey

I uncommented the lines for forwarding in the sysctl.conf and tested it with the rules, but it didn't work, the results stayed the same. The applications Webfrontend is hosted by IIS - should I be changing something in there as well?

Below is some output from iptables -S and L - maybe this helps to spot a problem (the destination it shows at the forwarder is the actual server name for the IP I am trying to forward to - quite cool that it could resolve it)

dmzgw:~# iptables -S
-P INPUT ACCEPT
-P FORWARD ACCEPT
-P OUTPUT ACCEPT
-A FORWARD -d 10.47.134.36/32 -p tcp -m state --state NEW -m tcp --dport 80 -j ACCEPT

dmzgw:~# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
ACCEPT     tcp  --  anywhere             truxton.javadev.net state NEW tcp dpt:www

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

 

by: BlazPosted on 2009-09-01 at 05:29:41ID: 25230825

Later, when you will try to block some traffic it is important to add rule:
iptables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT

But this does not matter since you accept all packets currently.

Have you rebooted the server after you set /etc/sysctl.conf file? What is the output of:
cat /proc/sys/net/ipv4/ip_forward

If it is 0 write:
echo 1 > /proc/sys/net/ipv4/ip_forward

 

by: RiggedPosted on 2009-09-01 at 06:06:37ID: 25231127

Thanks for the tip about the rule when i implement the security.

Yeah I have rebooted the server after changing sysctl.conf and did it another time just for good measure :P
The /proc/sys/net/ipv4/ip_forward was already at 1 so I left it as is.

 

by: eokePosted on 2009-09-01 at 06:18:15ID: 25231225

Hi,

I think your problem may be that the traffic from your web server can't traverse the firewall back out to the client on the internet.

I've attached a base bash file for you to try and if it works you can add to it as you need to.  Make sure you change LAN_NET= value for your subnet.  It's probably also worth checking the other addresses, adapters as well as it's tricky to write these things when you can't test them.

#!/bin/sh
 
# forwarding off
echo 0 > /proc/sys/net/ipv4/ip_forward
 
LAN_NET='10.47.134.11/?'
LAN_NIC='eth0'
WAN_NIC='eth1'
 
# flush
iptables -t nat -F POSTROUTING
iptables -t nat -F PREROUTING
iptables -t nat -F OUTPUT
iptables -F
 
# policy
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT
 
# masquerade / forwarding for internal hosts (allow them to access web)
iptables -t nat -A POSTROUTING -s $LAN_NET -j MASQUERADE
iptables -A FORWARD -j ACCEPT -i $LAN_NIC -s $LAN_NET
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
 
# state related for me
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
 
# forward for lan server
iptables -A FORWARD -j ACCEPT -p tcp --dport 80
iptables -t nat -A PREROUTING -i $WAN_NIC -p tcp --dport 80 -j DNAT --to 10.47.134.36:80
 
# forwarding on
echo 1 > /proc/sys/net/ipv4/ip_forward
                                              
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by: RiggedPosted on 2009-09-02 at 00:16:51ID: 25238587

WOW, thanks eoke.
I made the one change needed and ran the script and it works great with no issues.

 

by: RiggedPosted on 2009-09-02 at 00:21:38ID: 25238614

Sweet, the people are happy with the results - now to do some security testing. WOOHOO - I thought this will take allot longer to get working - EOKE your the man, and I actually understand what you did in the script.

 

by: eokePosted on 2009-09-02 at 02:11:58ID: 25239130

Hi Rigged,

Thank you for your kind words.

Just a word of warning your iptables configuration will not currently survive a reboot.  One way to get around this is to use iptables-save to save your currently running configuration and then call iptables-restore during boot up.
 

20120131-EE-VQP-002

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