Paranoid Penguin - Linux VPNs with OpenVPN, Part IV

Use dangerous local-area networks without fear with OpenVPN.
Other Client Settings

Nearly all of the rest of the directives in Listing 2 are ones I already covered in Parts II and III of this series when dissecting Listing 1, the server configuration file. The client-side settings for those directives are even the same as specified on the server.

In the case of user nobody and group nogroup, which tell the openvpn command to run with unprivileged user and group identities after initialization, make sure the user account nobody and the group nogroup exist on your client system. Because both typically exist on most Linux systems, they probably do already. If not, you either can create them or change the directives to point to some other, existing dæmon account or group name. In no event should you change either to any user or group name used for actual human-use login accounts!

The only other directive worth mentioning here is mute-replay-warnings, which I didn't include in the server.ovpn file. Declaring this directive (without any argument) tells OpenVPN not to log events associated with its anti-packet-replay mechanism, which tends to trigger false alarms if you connect to a wireless network. It won't turn off the actual anti-replay protection; it just suppresses associated local log entries.

Initiating a Tunnel

You've created a client configuration file and put it into your client system's /etc/openvpn directory. You've copied over your CA certificate file, client certificate and key files, and your TA key file. It's time to connect to the server.

Assuming your client system is running Linux (specifically Ubuntu), and assuming that, as in Listing 2, your client configuration file is called client.ovpn, follow these steps the first time you connect to your server:

1) Change your working directory to /etc/openvpn:

bash-$ cd /etc/openvpn

2) Run OpenVPN like this:

bash-$ sudo openvpn --config ./client.ovpn

3) When prompted, enter your client key's passphrase.

Enter Private Key Password: Your passphrase here

Note that in step 2 you started OpenVPN without the --daemon directive, leaving it running in the foreground. If everything works without a hitch, the next time you start OpenVPN, you can use sudo openvpn --daemon --config ./client.ovpn, in which case OpenVPN will run silently after asking you for your client key passphrase. On my Ubuntu client system, OpenVPN logs to the file /var/log/syslog when running in dæmon mode.

Troubleshooting

Hopefully, at this point you've got a working VPN tunnel back to your server. If you don't though, two different mistakes have caused the majority of my own problems using OpenVPN.

First, your tunnel will fail if you fail to copy all necessary files into /etc/openvpn: client configuration file, CA certificate, client certificate, client key and TLS Authentication key. These files also must have appropriate permissions set, and you must run the openvpn command with the appropriate level of privilege (that is, root privilege).

Second, firewall rules on both server and clients must be either disabled (left open) or reconfigured to allow OpenVPN traffic to pass. Telling you how to write such rules easily could occupy half or more of an entire article, especially if I were to cover the art of forcing certain types of traffic to use the tunnel. Suffice it to say, for now, check your iptables rules before you even bother running OpenVPN in server or client mode.

Cross-Platform Notes

In writing this article, I tested both an Ubuntu client and a Windows XP client. Getting the Windows XP client to connect properly was no more difficult than on Ubuntu. It was a simple matter of placing the correct files in the correct directory and tweaking my Windows firewall configuration a little.

On Windows clients, the user and group directives have no effect, as OpenVPN's self-demotion feature is not supported in Windows. Other than that, however, I found the Windows version of OpenVPN to work in a very similar manner as the Linux version.

Conclusion

And that, dear readers, is how you configure OpenVPN to allow yourself to connect back to your home network from an untrusted remote site. This process is, in practice, much easier than my taking four months to describe it implies. Hopefully, my line-by-line dissections of the two configuration files have given you a strong enough understanding of how OpenVPN works for you to explore other usage scenarios.

I may devote one more column to this topic, because Virtual Private Networks are such a powerful tool, and these four installments have covered only a small subset of its potential. Regardless, I hope you've found this series useful and that you have success in your own Virtual Private Linux endeavors. Until next time, be safe!

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