How I installed OpenVPN on both my laptop and my server
Ok, to be fair, I followed most parts described here. Some parts may be similar, or even copy/paste-like, but I also tried to explain each point where I spend time to understand, to make it clearer.
Of course, one can recompile from sources, but I just run
Here is how to check immediately if the connection is correctly set:
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Just make sure $Client_IP and $Server_IP are IP addresses on the Internet and Internal ones are private IP addresses not used in any of both networks (10.9.8.1/10.9.8.2 may be a good choice).
OpenVPN will complain on both sides that the connection is not encrypted, and all traffic is transmetted clear-text, this is just to check if there is any firewall around blocking our tunnel.
If you can ping from each part your partener:
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then everything goes just fine. You may then go forward. Otherwise, I would suggest having a look at your firewall rules, either locally (on your box, or your client) or between (enterprise firewall for instance; we will come to this point later).
OpenVPN with static keys
OpenVPN provides a command to generate static keys; and they are directly created with appropriate rights. I usually prefer creating a folder containing them. The key needs to be copied on the same folder on the other side (pre-shared keys).
Here is the configuration files:
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Restart OpenVPN on both side and check again if from each side you can ping the other.
OpenVPN with certificates
Ok, pre-shared keys are nice, but it usually goes better with a PKI and a Certificate Authority (CA) giving guarantee on the identity of both the client and the server.
About best practices, it is better to allocate a third computer, without any network connexion if possible, to play the role of CA. It is always better to create certificates on this computer, even if most of tutorials and documentations make the certificates creation on OpenVPN server itself. Just make sure openvpn package is installed on this computer and create the different certificates:
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Now, create a configuration file for your server:
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$Internal_Subnets is the network you have chosen to bring up the OpenVPN tunnel (here, we choose 10.9.8.0/24). Create log directory and openvpn.log file.
On client side:
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Don’t forget to rename your tun0.conf file to something like tun0.conf.DISABLED, as every *.conf files will be taken into account by the OpenVPN daemon. Restart both client and server, first the server, and then the client, and check the connection is possible (PING both side, as usual).
can’t access LAN behind OpenVPN
At this point, you can ping the next hop, but not any further. Lots of people got stuck here. No lan access, Can’t ping/access servers on local network, LAN behind router not accessible with openvpn, and so on, and so on.
Well, first of all, Server needs to route packets when received.
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Then, client must setup its default gateway to the $Server_Internal_IP. But not for joining the $IP_Server itself! The OpenVPN connects through the Internet, and you need to access your OpenVPN server with the route you usually use to go to the Internet!
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Now, I appear on the VPN server as 10.9.8.2. As a classic internal host with its private IP address. Hence, the gateway (here, the OpenVPN server itself, but not necessary) needs to NAT or Route my IP address. If my OpenVPN server is at the same time my new gateway:
Otherwise, the trafic will go through the OpenVPN server’s gateway (and may need to be NATed elsewhere).
Just a few last things
I hope this cover the most. Here is the script I used with certificate to check out the server configuration with certificates:
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And on client side:
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About enterprise firewall troubles
If neither your own local firewall, nor your server firewall is blocking the trafic, it may be because of your enterprise firewall. Then, we just need to find a weakness to the usual firewall strategy to still go through. The idea here is similar to those developped when dealing with SSH-Tunneling. Keep in mind this lower the global security of your enterprise, and as a sysadmin, I will enjoy fucking you up if I find it through.
The idea is simply to use a port none can’t allow blocking. Such as 80/TCP HTTP (but this one gets easely proxyfied) or 443/TCP HTTPS or 53/UDP DNS. The last two will perfectly work and most of time bypass enterprise firewalls.
- 443/HTTPS force you to use a TCP connection. This is not too bad (usually, pro-UDP says that TCP in TCP would result in performance issue, but this is not so right, YMMV).
- Both need OpenVPN server listens on ports other services usually listen at: If you have a HTTP[S] server, well, you may need to shut it off.
To avoid this, a solution is to fix the OpenVPN source port and create on server side a firewall rule to redirect packets with this chosen port to the OpenVPN service, letting every other connection reaching the target server. This is clearly not optimal, as unlucky people trying to reach you site would end up with your OpenVPN server, without knowing what to do with this, (or glade to discover this unsuspected service). But anyway, these two iptables rules will do the trick:
- I have thought, this would be possible to use the UDP:53 port as well; thinking of many wifi which redirect you to an authentication webpage. Well, if the DNS port is not firewalled (try contacting directly your DNS server with a command such as dig pelicanux.net @18.104.22.168), then you can also use OpenVPN to get Internet for free. In this case, you don’t go through the wifi DNS, and this is possible only if every DNS ports are open to the Internet. But most of time, the DNS port is redirected to this DNS resolver, which will need a DNS query it can understand (respecting the standards, especially RFC1035) in order to process it. (see this superuser post) or similar.