Accessing the data center from the cloud with OpenVPN

This post was inspired by a recent exercise I went through at the prompting of my colleague Dan Mesh. The goal was to have Amazon EC2 instances connect securely to servers at a data center using OpenVPN.

In this scenario, we have a server within the data center running OpenVPN in server mode. The server has a publicly accessible IP (via a firewall NAT) with port 1194 exposed via UDP. Cloud instances which run OpenVPN in client mode are connecting to the server, get a route pushed to them to an internal network within the data center, and are then able to access servers on that internal network over a VPN tunnel.

Here are some concrete details about the network topology that I'm going to discuss.

Server A at the data center has an internal IP address of 10.10.10.10 and is part of the internal network 10.10.10.0/24. There is a NAT on the firewall mapping external IP X.Y.Z.W to the internal IP of server A. There is also a rule that allows UDP traffic on port 1194 to X.Y.Z.W.

I have an EC2 instance from which I want to reach server B on the internal data center network, with IP 10.10.10.20.

Install and configure OpenVPN on server A

Since server A is running Ubuntu (10.04 to be exact), I used this very good guide, with an important exception: I didn't want to configure the server in bridging mode, I preferred the simpler tunneling mode. In bridging mode, the internal network which server A is part of (10.10.10.0/24 in my case) is directly exposed to OpenVPN clients. In tunneling mode, there is a tunnel created between clients and server A on a separated dedicated network. I preferred the tunneling option because it doesn't require any modifications to the network setup of server A (no bridging interface required), and because it provides better security for my requirements (I can target individual servers on the internal network and configure them to be accessed via VPN). YMMV of course.

For the initial installation and key creation for OpenVPN, I followed the guide. When it came to configuring the OpenVPN server, I created these entries in /etc/openvpn/server.conf:

server 172.16.0.0 255.255.255.0
push "route 10.10.10.0 255.255.255.0"
tls-auth ta.key 0 

The first directive specifies that the OpenVPN tunnel will be established on a new 172.16.0.0/24 network. The server will get the IP 172.16.0.1, while OpenVPN clients that connect to the server will get 172.16.0.6 etc.

The second directive pushes a static route to the internal data center network 10.10.10.0/24 to all connected OpenVPN clients. This way each client will know how to get to machines on that internal network, without the need to create static routes manually on the client.

The tls_auth entry provides extra security to help prevent DoS attacks and UDP port flooding.

Note that I didn't have to include any bridging-related scripts or other information in server.conf.

At this point, if you start the OpenVPN service on server A via 'service openvpn start', you should see an extra tun0 network interface when you run ifconfig. Something like this:


tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet addr:172.16.0.1  P-t-P:172.16.0.2  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:2 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:168 (168.0 B)  TX bytes:168 (168.0 B)

Also, the routing information will now include the 172.16.0.0 network:

# netstat -rn

Kernel IP routing table

Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface

172.16.0.2      0.0.0.0         255.255.255.255 UH        0 0          0 tun0

172.16.0.0      172.16.0.2      255.255.255.0   UG        0 0          0 tun0

...etc

Install and configure OpenVPN on clients

Here again I followed the Ubuntu OpenVPN guide. The steps are very simple:

1) apt-get install openvpn

2) scp the following files (which were created on the server during the OpenVPN server install process above) from server A to the client, into the /etc/openvpn directory: 

ca.crt

ta.key

client_hostname.crt 

client_hostname.key

3) Customize client.conf:

# cp /usr/share/doc/openvpn/examples/sample-config-files/client.conf /etc/openvpn

Edit client.conf and specify:

remote X.Y.Z.W 1194     (where X.Y.Z.W is the external IP of server A)

cert client_hostname.crt
key client_hostname.key
tls-auth ta.key 1

Now if you start the OpenVPN service on the client via 'service openvpn start', you should see a tun0 interface when you run ifconfig:


tun0      Link encap:UNSPEC  HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet addr:172.16.0.6  P-t-P:172.16.0.5  Mask:255.255.255.255
          UP POINTOPOINT RUNNING NOARP MULTICAST  MTU:1500  Metric:1
          RX packets:2 errors:0 dropped:0 overruns:0 frame:0
          TX packets:2 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:100 
          RX bytes:168 (168.0 B)  TX bytes:168 (168.0 B)

You should also see routing information related to both the tunneling network 172.16.0.0/24 and to the internal data center network 10.10.10.0/0 (which was pushed from the server):

# netstat -rn

Kernel IP routing table

Destination     Gateway         Genmask         Flags   MSS Window  irtt Iface

172.16.0.5      0.0.0.0         255.255.255.255 UH        0 0          0 tun0

172.16.0.1      172.16.0.5      255.255.255.255 UGH       0 0          0 tun0

10.0.10.0       172.16.0.5      255.255.255.0   UG        0 0          0 tun0

....etc

At this point, the client and server A should be able to ping each other on their 172.16 IP addresses. From the client you should be able to ping server A's IP 172.16.0.1, and from server A you should be able to ping the client's IP 172.16.0.6.

Create static route to tunneling network on server B and enable IP forwarding on server A

Remember that the goal was for the client to access server B on the internal data center network, with IP address 10.10.10.20. For this to happen, I needed to add a static route on server B to the tunneling network 172.16.0.0/24, with server A's IP 10.10.10.10 as the gateway:

# route add -net 172.16.0.0/24 gw 10.10.10.10

The final piece of the puzzle is to allow server A to act as a router at this point, by enabling IP forwarding (which is disabled by default). So on server A I did:

# sysctl -w net.ipv4.ip_forward=1

# echo "net.ipv4.ip_forward=1" >> /etc/sysctl.conf

At this point, I was able to access server B from the client by using server B's 10.10.10.20 IP address.

We've just started to experiment with this setup, so I'm not yet sure if it's production ready. I wanted to jot down these things though because they weren't necessarily obvious, despite some decent blog posts and OpenVPN documentation. Hopefully they'll help somebody else out there too.