Paranoid Penguin - Linux VPNs with OpenVPN, Part III
Now that I've covered a sample server configuration file in depth, let's fire up our OpenVPN dæmon in server mode! This, as you'll see, is the easy part.
OpenVPN uses a single command, openvpn, for everything. Precisely what any given OpenVPN instance does depends on how you start it. As you've already seen, some startup parameters, like --show-ciphers, cause the openvpn command to give certain information and then exit. Other parameters tell it to remain active, listening for incoming client connections (--mode server) or attempting to establish and maintain a tunnel to some server, as a client (--mode client).
If you execute openvpn with the --config parameter followed by the name of a configuration file, OpenVPN will start itself configured with all parameters in that file. For example, you could create a configuration file containing just the parameter show-ciphers (parameters must start with a -- if specified in a command line, but the -- is omitted for all parameters within configuration files).
More commonly, as with Listing 1, we use configuration files for server-mode and client-mode startup. I mentioned that the server helper directive expands into a list of other parameters; the first of these is mode server.
Thus, to start OpenVPN as a persistent server dæmon running the configuration file /etc/openvpn/server.ovpn, shown in Listing 1, use this command:
sudo openvpn --config ./server.ovpn
Note the relative path for the file server.ovpn. If that file resides in /etc/openvpn, you'd need to run the above command from within that directory. Note also the use of sudo. On non-Ubuntu systems, you might instead su to root before running this command. Regardless, OpenVPN must be run as root in order to read its server key file, to open the tun device and so forth, even though as configured in Listing 1 it subsequently will demote itself to user nobody and group ID nogroup.
Did you notice I omitted the --daemon flag on that command line? Again, you can use that flag to tell OpenVPN to run in the background (like a quiet, well-behaved dæmon) and log its messages to /var/log/daemon.log, but you first may want to make sure everything's working properly.
At this point, I had hoped I'd be able to give you a detailed walk-through of client configuration, but I'm out of space for now, so that will need to wait until next time. But, I won't leave you completely hanging. Listing 2 shows a sample client configuration file, client.ovpn, that corresponds to Listing 1's server.ovpn file.
Listing 2. Client's iwazaru.ovpn File
client dev tun proto udp remote 220.127.116.11 1194 resolv-retry infinite nobind user nobody group nogroup persist-key persist-tun mute-replay-warnings ca ca.crt cert minion.crt key minion.key ns-cert-type server tls-auth ta.key 1 cipher BF-CBC comp-lzo verb 3 mute 20
Much of this should be familiar. Other parts you can figure out via the openvpn(8) man page. In the meantime, feel free to experiment. To run OpenVPN in client mode on a client computer, use this command:
sudo openvpn --config ./iwazaru.ovpn --daemon openvpn-client
One parting tip for you experimenters: you'll need to disable or reconfigure any local iptables (firewall) rules you've got running on either your server or client systems. I'll discuss iptables considerations in the next column in this series, and I'll continue where we left off this time. Until then, be safe!
Mick Bauer (email@example.com) is Network Security Architect for one of the US's largest banks. He is the author of the O'Reilly book Linux Server Security, 2nd edition (formerly called Building Secure Servers With Linux), an occasional presenter at information security conferences and composer of the “Network Engineering Polka”.
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