Posts Tagged ‘iptables firewall’

How to make NAT enable hosts in a local network to access the internet, create port forwarding to local IPs behind the router using iptables

Tuesday, August 23rd, 2011

I’m bulding new iptables firewall on one Linux server. The Debian GNU/Linux is required to act as firewall do Network Adress Translation for a small network of office PCs as well as forward some of the inbound ports to hosts from the local network located behind the router.

The local network besides the router had an IP addressing in the class C network e.g. (192.168.1.1-255)

First I procceded and enabled the Network Address Translation via the Linux kernel variable:

linux:~# sysctl -w net.ipv4.ip_forward=1
linux:~# echo 'net.ipv4.ip_forward=1' >> /etc/sysctl.conf

Initially I even forgot to switch on the net.ipv4.ip_forward to 1 (by default this value is set to 0) – GNU/Linux’s default network behaviour is not predetermined to act as network router.
However, since I haven’t configured Network Address Translation for quite some time it completely slipped my mind!

Anyways next the actual iptables rule which makes NAT possible I used is:

linux:~# /sbin/iptables -t nat -A POSTROUTING -s 192.168.1.0/24 ! -d 192.168.1.0/24 -j SNAT --to-source xxx.xxx.xxx.xxx

Whether xxx.xxx.xxx.xxx is the External IP address assigned to the router on eth0

With this very simple rules now Network the local network is capable of accessing the Internet withotu problem.

It’s a good time to say that still many system administrators, still erroneously use MASQUERADE rules instead of SNAT .
IP MASQUERADING is an ancestry from ipchains and these days should be completely abandonded, especially where no often change of primary IP address to access the internet is made.
For dial-ups or other kind of networking, where the IP addresses are often changed still IP MASQUERADING might be a good idea though.

My next goal was to make the Linux router to do port forwarding of Traffic which arrives on port 80 to a IIS server assigned with a local IP address of 192.168.1.5
I did the webserver (port 80), port forwarding from IP xxx.xxx.xxx.xxx to 192.168.1.5 with the iptables rule:

linux:~# /sbin/iptables -t nat -A PREROUTING -d xxx.xxx.xxx.xxx/32 -p tcp -m tcp --dport 80 -j DNAT --to-destination 192.168.1.5:80

There was a requirement to do port forwarding for a Windows remote Desktop running on standard port 3389 from the router to the internal Windows IP address running the IIS webserver, however the company required me to only allow access to the rdesktop 3389 port to certain real IP addresses.
Initially I thought about using the above PREROUTING rule which makes the port redirection to the IIS server and only change port 80 to port 3389 , and then use filter table INPUT chain rules like:

/sbin/iptables -A INPUT -s xx1.xx2.xx3.xx4,1xx,2xx,3xx,4xx,xxx.xxx.xxx.xxx -p tcp -m tcp --dport 3389 -j ACCEPT/sbin/iptables -A INPUT -p tcp -m tcp --dport 3389 -j REJECT --reject-with icmp-port-unreachable
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However this did not work out, so I decided to give a try to do the same within the filter table using the FORWARD chain, like so:

FORWARD/sbin/iptables -A FORWARD -p tcp -m tcp -s xx1.xx2.xx3.xx4,1xx,2xx,3xx,4xx,xxx.xxx.xxx.xxx -p tcp -m tcp --dport 3389 -j ACCEPT
/sbin/iptables -A FORWARD -p tcp -m tcp --dport 3389 -j REJECT --reject-with icmp-port-unreachable

Adding this rules did not added any filtering to the forwarded remote desktop port. I suspected that somehow probably my above PREROUTING nat rules are read before any other rules and therefore automatically allows any IP address to port fortward traffic.
I’ve checked the iptables documentation and it seems my guess was partially right.

When some kind of network traffic enters the iptables firewall it first goes through the PREROUTING channel and then the traffic flows in a certain order.
iptables packet flow diagram

The iptables network packets flow is clearly seen in above’s diagram a thorough looks gives a very good idea on how packet is being processed by iptables

Finally as I couldn’t think about a good solution on how to only filter the port redirected traffic, which always firstly entered in the POSTROUTING chain, I’ve consulted with the guys in irc.freenode.net in #Netfilter.

I’m quite thanksful as a guy nicknamed Olipro has given me a pretty good picture on the port forwarding POSTROUTING problem and has provided me with a very logical easy and great fix.
He suggested that I only do port forwarding for certain IP addresses instead of allowing all IP addresses and then lookup for a way to allow only some of them and filter the rest.

The iptables rule to restrict the incoming traffic to the remote desktop forwarded port 3389 to few only allowed IP addresses looks like so:

linux:~# /sbin/iptables -t nat -A PREROUTING -d xxx.xxx.xxx.xxx/32 -s xx1.xx2.xx3.xx4,1xx,2xx,3xx,4xx,xxx.xxx.xxx.xxx -p tcp -m tcp –dport 3389 -j DNAT –to-destination 192.168.1.5:3389

Now the three sample IPs passed xx1.xx2.xx3.xx4,1xx,2xx,3xx,4xx,xxx.xxx.xxx.xxx has added to port forward traffic on 3389 to 192.168.1.5

By the way I did not know that newer versions of iptables support passing by multiple IP addresses to the –source or –destination IP. This is really great feature I’ve learned from the good guys from #Netfilter. However one should be careful when using the multiple IPs with -s or -d, it’s really important that the passed consequent IPs has no space between the , delimiter.

Now that’s all my task is completed. All computerse inside the Network 192.168.1.1-255 on the Linux router freely can access the Internet, all IPs are also capable to access the IIS server located behind the NAT as well as only certain IPs are capable of accessing to the IIS remote desktop.
Hope the article helps somebody 😉

How to make GRE tunnel iptables port redirect on Linux

Saturday, August 20th, 2011

I’ve recently had to build a Linux server with some other servers behind the router with NAT.
One of the hosts behind the Linux router was running a Window GRE encrypted tunnel service. Which had to be accessed with the Internet ip address of the server.
In order < б>to make the GRE tunnel accessible, a bit more than just adding a normal POSTROUTING DNAT rule and iptables FORWARD is necessery.

As far as I’ve read online, there is quite of a confusion on the topic of how to properly configure the GRE tunnel accessibility on Linux , thus in this very quick tiny tutorial I’ll explain how I did it.

1. Load the ip_nat_pptp and ip_conntrack_pptp kernel module

linux-router:~# modprobe ip_nat_pptp
linux-router:~# modprobe ip_conntrack_pptp

These two modules are an absolutely necessery to be loaded before the remote GRE tunnel is able to be properly accessed, I’ve seen many people complaining online that they can’t make the GRE tunnel to work and I suppose in many of the cases the reason not to be succeed is omitting to load this two kernel modules.

2. Make the ip_nat_pptp and ip_nat_pptp modules to load on system boot time

linux-router:~# echo 'ip_nat_pptp' >> /etc/modules
linux-router:~# echo 'ip_conntrack_pptp' >> /etc/modules

3. Insert necessery iptables PREROUTING rules to make the GRE tunnel traffic flow

linux-router:~# /sbin/iptables -A PREROUTING -d 111.222.223.224/32 -p tcp -m tcp --dport 1723 -j DNAT --to-destination 192.168.1.3:1723
linux-router:~# /sbin/iptables -A PREROUTING -p gre -j DNAT --to-destination 192.168.1.3

In the above example rules its necessery to substitute the 111.222.223.224 ip address withe the external internet (real IP) address of the router.

Also the IP address of 192.168.1.3 is the internal IP address of the host where the GRE host tunnel is located.

Next it’s necessery to;

4. Add iptables rule to forward tcp/ip traffic to the GRE tunnel

linux-router:~# /sbin/iptables -A FORWARD -p gre -j ACCEPT

Finally it’s necessery to make the above iptable rules to be permanent by saving the current firewall with iptables-save or add them inside the script which loads the iptables firewall host rules.
Another possible way is to add them from /etc/rc.local , though this kind of way is not recommended as rules would add only after succesful bootup after all the rest of init scripts and stuff in /etc/rc.local is loaded without errors.

Afterwards access to the GRE tunnel to the local IP 192.168.1.3 using the port 1723 and host IP 111.222.223.224 is possible.
Hope this is helpful. Cheers 😉

Enabling Active FTP connections on CentOS 5.5

Tuesday, January 4th, 2011

If you experience problems with making your CentoOS 5.5 work with active ftp connections , e.g. every connection you make to the FTP needs to be in a passive mode or the file transfer or FTP directory listing doesn’t initialize at all, here is how you can solve it:

Edit the file /etc/sysconfig/iptables-config and change their the line:

IPTABLES_MODULES="ip_conntrack_netbios_ns"

to look like:

IPTABLES_MODULES=”ip_conntrack_netbios_ns ip_nat_ftp ip_conntrack_ftp”

Adding the two modules ip_nat_ftp and ip_conntrack_ftp will instruct the CentOS’s /etc/init.d/iptables firewall rules to initialize the kernel modules ip_nat_ftp and ip_conntrack_ftp

This modules solves problems with Active FTP not working caused by a host running behind a firewall router or behind a NAT.

This will hopefully resolve your issues with Active FTP not working on CentOS.

If loading this two kernel modules doesn’t solve the issues and you’re running vsftpd FTP server, then it’s likely that the Active FTP non-working problems are caused by your VSFTPD configuration.

If that’s the case something that might help is setting in /etc/vsftpd/vsftpd.conf the following variables:

pasv_enable=NO
pasv_promiscuous=YES

Of course as a final step you will need to restart the iptables firewall:

[root@centos: ~]# /etc/init.d/iptables restart
Flushing firewall rules: [ OK ]
Setting chains to policy ACCEPT: filter [ OK ]
Unloading iptables modules: [ OK ]
Applying iptables firewall rules: [ OK ]
Loading additional iptables modules: ip_conntrack_netbios_ns
ip_nat_ftp ip_conntrack_ftp [ OK ]

As you can see the two modules ip_nat_ftp and ip_conntrack_ftp are now loaded as additional modules.
Moreover if you have also modified your vsftpd.conf you need to restart the vsftpd via the init script:

[root@centos: ~]# /etc/init.d/vsftpd restart
Shutting down vsftpd: [ OK ]
Starting vsftpd for vsftpd: [ OK ]

If adding this two modules and adding this two extra variables in vsftpd configuration doesn’t help with making your FTP server to work in Active FTP mode , it’s very likely that the whole troubles comes from the firewall configuration, so an edit of /etc/sysconfig/iptables would be necessary;

To find out if the firewall is the source of the FTP not able to enter active mode, stop your firewall for a while by issuing the cmd:

[root@centos:~]# /etc/init.d/iptables stop

If iptables is the source of thepassive ftp troubles, an iptables rules similar to this should make your firewall allow active ftp connections;

*filter :INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT
[0:0] -A INPUT -i lo -j ACCEPT -A INPUT -d 127.0.0.0/255.0.0.0 -i ! lo -j REJECT –reject-with icmp-port-unreachable
-A INPUT -m state –state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p tcp -m tcp –dport 80 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 44444 -j ACCEPT
-A INPUT -p tcp -m state –state NEW -m tcp –dport 21 -j ACCEPT -A INPUT -p icmp -m icmp –icmp-type 8 -j ACCEPT
-A INPUT -j REJECT –reject-with icmp-port-unreachable -A FORWARD -j REJECT –reject-with icmp-port-unreachable
-A OUTPUT -j ACCEPT -A OUTPUT -p tcp -m tcp –dport 21 -m state –state ESTABLISHED,RELATED -j ACCEPT