Network Security Hacks

Tunnel PPP

Use PPP and SSH to create a secure VPN tunnel.

There are so many options to choose from when creating a VPN or tunneled connection that it’s mind-boggling. You may not be aware that all the software you need to create a VPN is probably already installed on your Unix machines—namely PPP and SSH daemons.

You might have used PPP back in the day to connect to the Internet over a dial-up connection, so you may be wondering how the same PPP can operate over SSH. Well, when you used PPP in conjunction with a modem, it was talking to the modem through what the operating system presented as a TTY interface, which is, in short, a regular terminal device. The PPP daemon on your end would send its output to the TTY, which the operating system would send out the modem and across the telephone network until it reached the remote end, where the same thing would happen in reverse.

The terminals that you use to run shell commands on (e.g., the console, an xterm, etc.) use pseudo-TTY interfaces, which are designed to operate similarly to TTYs. Because of this, PPP daemons can also operate over pseudo-TTYs. So, you can replace the serial TTYs with pseudo-TTYs, but you still need a way to connect the local pseudo-TTY to the remote one. Here’s where SSH comes into the picture.

You can create the actual PPP connection in one quick command. For instance, if you wanted to use the IP 10.1.1.20 for your local end of the connection and 10.1.1.1 on the remote end, you could run a command similar ...