Posts Tagged ‘encrypted tunnel’

How to make GRE tunnel iptables port redirect on Linux

Saturday, August 20th, 2011

Reading Time: 2minutes
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 -p tcp -m tcp --dport 1723 -j DNAT --to-destination
linux-router:~# /sbin/iptables -A PREROUTING -p gre -j DNAT --to-destination

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

Also the IP address of 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 using the port 1723 and host IP is possible.
Hope this is helpful. Cheers 😉

How to make pptp VPN connection to use IPMI port (IPKVM / Web KVM) on Debian Linux

Wednesday, July 27th, 2011

Reading Time: 3minutes
If you have used KVM, before you certainly have faced the requirement asked by many Dedicated Server Provider, for establishment of a PPTP (mppe / mppoe) or the so called Microsoft VPN tunnel to be able to later access via the tunnel through a Private IP address the web based Java Applet giving control to the Physical screen, monitor and mouse on the server.

This is pretty handy as sometimes the server is not booting and one needs a further direct access to the server physical Monitor.
Establishing the Microsoft VPN connection on Windows is a pretty trivial task and is easily achieved by navigating to:

Properties > Networking (tab) > Select IPv4 > Properties > Advanced > Uncheck "Use default gateway on remote network".

However achiving the same task on Linux seemed to be not such a trivial, task and it seems I cannot find anywhere information or precise procedure how to establish the necessery VPN (ptpt) ms encrypted tunnel.

Thanksfully I was able to find a way to do the same tunnel on my Debian Linux, after a bunch of experimentation with the ppp linux command.

To be able to establish the IPMI VPN tunnel, first I had to install a couple of software packages, e.g.:

root@linux:~# apt-get install ppp pppconfig pppoeconf pptp-linux

Further on it was necessery to load up two kernel modules to enable the pptp mppe support:

root@linux:~# modprobe ppp_mppe
root@linux:~# modprobe ppp-deflate

I’ve also enabled the modules to be loading up during my next Linux boot with /etc/modules to not be bother to load up the same modules after reboot manually:

root@linux:~# echo ppp_mppe >> /etc/modules
root@linux:~# echo ppp-deflate >> /etc/modules

Another thing I had to do is to enable the require-mppe-128 option in /etc/ppp/options.pptp.
Here is how:

root@linux:~# sed -e 's$#require-mppe-128$require-mppe-128$g' /etc/ppp/options.pptp >> /tmp/options.pptp
root@linux:~# mv /tmp/options.pptp /etc/ppp/options.pptp
root@linux:~# echo 'nodefaultroute' >> /etc/ppp/options.pptp

In order to enable debug log for the ppp tunnel I also edited /etc/syslog.conf and included the following configuration inside:

root@linux:~# vim /etc/syslog.conf
news.none;mail.none -/var/log/debug
root@linux:~# killall -HUP rsyslogd

The most important part of course is the command line with ppp command to connect to the remote IP via the VPN tunnel ;), here is how I achieved that:

root@linux:~# pppd debug require-mppe pty "pptp --nolaunchpppd" file /etc/ppp/options.pptp user My_Dedi_Isp_Given_Username password The_Isp_Given_Password

This command, brings up the ppp interface and makes the tunnel between my IP and the remote VPN target host.

Info about the tunnel could be observed with command:

ifconfig -a ppp
ppp0 Link encap:Point-to-Point Protocol
inet addr: P-t-P: Mask:
RX packets:7 errors:0 dropped:0 overruns:0 frame:0
TX packets:12 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:3
RX bytes:70 (70.0 B) TX bytes:672 (672.0 B)

One more thing before I could finally access the IPMI’s web interface via the private IP was to add routing to the private IP address via the tunnel other side IP address:

# P-t-P IP address
ip route add dev ppp0

Now logically one would thing the Web interface to login and use the Java Applet to connect to the server would be accessible but no IT wasn’t !

It took me a while to figure out what is the problem and if not the guys in ##networking helped me I would never really find out why and were inaccessible.

Strangely enough both ports 80 and 443 were opened on and it seems like working, however though I can ping both and there was no possible way to access with TCP traffic.

Routing to the Microsoft Tunnel was fine as I’ve double checked all was fine except whether I tried accessing the IPMI web interface the browser was trying to open the URL and keeps opening like forever.

Thanksfully after a long time of futile try outs, a tip was suggested by a good guy in freenode nick named ne2k

To make the TCP connection in the Microsoft Tunnel work and consequently be able to access the webserver on the remote IPMI host, one needs to change the default MTU set for the ppp0 tunnel interface.
Here is how:

ip link set ppp0 mtu 1438

And tadam! It’s done now IPKVM is accessible via or web interface. Horay ! 🙂