Paranoid Penguin - Linux VPNs with OpenVPN

Connect safely to the mother ship with a Linux VPN.
Some Linux VPN Choices

Nowadays, a number of good VPN solutions exist for Linux. Some commercial products, of course, release Linux versions of their proprietary VPN client software (so many more than when I began this column in 2000!).

In the IPsec space, there are Openswan, which spun off of the FreeS/WAN project shortly before the latter ended; Strongswan, another FreeS/WAN spin-off; and NETKEY (descended from BSD's KAME), which is an official part of the Linux 2.6 kernel and is controlled by userspace tools provided by the ipsec-tools package. All of these represent IPsec implementations for the Linux kernel. Because IPsec is an extension of the IPv4 protocol, any IPsec implementation on any operating system must be integrated into its kernel.

vpnc is an open-source Linux client for connecting to Cisco VPN servers (in the form of Cisco routers, Cisco ASA firewalls and so forth). It also works with Juniper/Netscreen VPN servers.

Although I don't recommend either due to PPTP's security design flaws, PPTP-linux and Poptop provide client and server applications, respectively, for Microsoft's PPTP protocol. Think it's just me? Both PPTP-linux's and Poptop's maintainers recommend that you not use PPTP unless you have no choice! (See Resources for links to the PPTP-linux and Poptop home pages.)

And, of course, there's OpenVPN, which provides both client and server support for SSL/TLS-based VPN tunnels, for both site-to-site and remote-access use.

Introduction to OpenVPN

All the non-PPTP Linux VPN tools I just mentioned are secure and stable. I focus on OpenVPN for the rest of this series, however, for two reasons. First, I've never covered OpenVPN here in any depth, but its growing popularity and reputation for security and stability are such that the time is ripe for me to cover it now.

Second, OpenVPN is much simpler than IPsec. IPsec, especially IPsec on Linux in either the client or server context, can be very complicated and confusing. In contrast, OpenVPN is easier to understand, get working and maintain.

Among the reasons OpenVPN is simpler is that it doesn't operate at the kernel level, other than using the kernel's tun and tap devices (which are compiled in the default kernels of most mainstream Linux distributions). OpenVPN itself, whether run as a VPN server or client, is strictly a userspace program.

In fact, OpenVPN is composed of exactly one userspace program, openvpn, that can be used either as a server dæmon for VPN clients to connect to or as a client process that connects to some other OpenVPN server. Like stunnel, another tool that uses SSL/TLS to encapsulate application traffic, the openssl dæmon uses OpenSSL, which nowadays is installed by default on most Linux systems, for its cryptographic functions.

OpenVPN, by the way, is not strictly a Linux tool. Versions also are available for Windows, Solaris, FreeBSD, NetBSD, OpenBSD and Mac OS X.