Martins Blog

Trying to explain complex things in simple terms

Open VPN for the Road Warrior

Posted by Martin Bach on June 15, 2011

UPDATE

I have updated the post on 05-DEC-2013 to be relevant for openvpn-2.2.2-9.5.1.x86_64 on OpenSuSE 12.3. Instead of Xen I switched to KVM as it was easier to implement. I left the by now outdated versions of the Oracle software as they were, they don’t matter. The Xen “virtual machine” is called domU, KVM calls them VM which sounds more familiar. So simply substitute VM for domU :) Note that I’m NOT using libvirt to manage the networks, otherwise the configuration would be different. The network configuration relies entirely on the bridges provided by Linux and set up in YAST2

INTRODUCTION

As a consultant it is important to have a test lab, something which is your own, where you can play with new versions and concepts to your heart’s delight without disturbing anyone else. Or worse, causing problems for the customer. For this reason I like to have an Internet facing machine which I can connect to from anywhere. In case the corporate network doesn’t let you out, consider getting mobile broadband on a PAYG basis-it works a dream!

I have blogged about my system a number of times, with special emphasis on RHEL 5.x and 6.x. Unlike many other Oracle scientists I do not use Virtual Box or VMWare for virtualisation, but rather Xen. When I started looking at para-virtualisation I looked at Oracle VM but at the time it was lacking features I wanted such as iSCSI provided storage. OpenSuSE is a great distribution which offers a dom0 kernel out of the box, and this is what I went for. My lab can support a four node 11.2.0.2.2 cluster plus two Grid Control domUs, which is more than enough for me to work with. And although it’s busy, it doesn’t make working with the machine impossible.

For a long time I was very happy with my machine, and SSH access was all I needed. But when moving to Vista/Windows 7 a problem became apparent: I could no longer use port-forwarding to access my samba server on my backup domU. Microsoft added some other software to listen on the required port so I started looking at OpenVPN as a solution. This article assumes that you are familiar with OpenVPN-if you are not then you might want to have a look at the documentation. The howto is a a great starting point:

http://openvpn.net/index.php/open-source/documentation/howto.html

Actually, the OpenVPN tutorial is just fantastic, and it is all you need to set your clients and server up. What it failed to explain is how to access your machines behind the gateway. My dom0 has a public IP address to which OpenVPN binds. This public IP is assigned to my first network bridge, br0. I have three more virtual bridges in the system, not related to a physical interface (“empty bridge”). VMWare calls such a network a “host only” network. I am using br1 as the public interface for my domUs, br2 is used as a private interconnect. The final one, br3 is either  the storage network for my iSCSI nodes or-for my 11.2.0.2.2 systems it serves as an alternative interface to the HAIP resource.

So all in all a reasonable setup I think. The trouble is that I didn’t know how to access my database servers in the br1 subnet. This is another one of those tips that look so obvious, but took literally hours to work out. I am not concerned with the private networks br2 and br3, after all those are not meant to be used anyway. The simplified view on my network is shown in this figure:

The workstation is my laptop-obviously.The figure is not 100% correct-the device openVPN connections go to is called tun0, but it’s related to my br0 device.

Prerequisits

The host must have IP forwarding enabled for this to work. Check /proc/sys/net/ipv4/ip_forward – if that returns 0 then forwarding is not enabled.

IPTABLES

First of all, I needed to allow communication from device tun0, to which openVPN listens to, to be forwarded to br1. The following iptables rules do exactly that:

# iptables -I FORWARD -i tun0 -o br1 -j ACCEPT
# iptables -I FORWARD -i br1 -o tun0 -j ACCEPT

Translated into English these 2 lines instruct the firewall to ACCEPT traffic into the FORWARD chain from device tun0 to br1. In other words route the packet to the internal network. The second line allows traffic from the internal network to traverse the tunnel device tun0.

With this in place, you can instruct openVPN to push routes to the client. In my case this is done in the configuration file used to start the openVPN daemon in server mode. My config file, server.conf, contains this line:

push “route 192.168.99.0 255.255.255.0”

When a client connects to the OpenVPN server, this route is automatically added to the local routing table with the correct gateway. Note that on Windows with UAC enabled you have to start OpenVPN as administrator or otherwise it won’t be able to initialise the TAP device properly.

Are we there yet? I’m afraid not.

This is only half the story as I found out the hard way. After what seems like endless troubleshooting routing tables (which were correct all the time!) I found that I had to set a default gateway on the domUs, pointing to the dom0’s address of br1. This is as simple as updating /etc/sysconfig/network-scripts/ifcfg-ethx, and adding this line to it:

GATEWAY=x.x.x.x

Either use your command line skills to add the default route or – like I did – bring the interface down and up again. Again, where gateway is the IP address of the br1 device on the dom0.

Hope this saves you a few hours of troubleshooting.

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