VPN over SSH

SSH is an awesome protocol and it can do a lot of things. I found out that OpenSSH has native TUN device support. Unfortunately I was not able to set this up properly. On my Linux boxes this failed with interesting errors. I ended up using strace to find out that there is some permission idiosyncracy.

As I love using pipes in my terminal, I had an idea. What about using socat and piping IP traffic through the SSH tunnel? tldr; it works.

$ sudo socat TUN:,up EXEC:'ssh -l root HOST "socat TUN:,up"'

Let's break this monster down.

socat allocates two entities and connects them via a bidirectional channel (e.g. two pipes for stdin and stdout respectively). Let's have a look at the “ADDRESS TYPES” section in the manpage here: socat(1). Any of these address types can be used. So let's create a TCP listener and print everything to the terminal.

$ socat TCP-LISTEN:1234 -

This command creates a TCP listener at port 1234 and connects it to stdin/stdout of the socat process. Eventually, this is a stupid replacement of nc -l -p 1234.

One step further, socat enables creating arbitrary processes and piping data into them. With the TCP listener example we can pipe data from the network in a subprocess like grep[^1].

$ socat -u TCP-LISTEN:1234 EXEC:'grep foo' &
$ echo "foobar" | nc 1234

The VPN stuff works like the very same. So, let's start slowly[^2]:

$ sudo socat TUN:,up - | hexdump

This command allocates a TUN device, assings the ip address, sets the device into the up state (= enables it), and relays everything to the terminal into hexdump. When you ping the subnet via ping then the IP packets show up. So now, let's send this data over an SSH tunnel.

ssh is awesome as it provides a bidirectional connection (via two pipes) to a remote process. Everything which is written to ssh's stdin is written to the remote process' stdin tunneled via the SSH connection! Well, let's spawn a socat an the remote side as well:

$ sudo socat TUN:,up EXEC:'ssh -l root HOST "socat TUN:,up -"'

The right argument (EXEC:"…") spaws ssh logs into HOST and starts a socat which is connected to a tunnel device. The remote socat needs to write everything to - (stdio) because - is connected to the SSH tunnel. It works! The only problem left is that root is needed on both sides. But this can be solved somehow…

[^1]: Using unidirectional mode with -u to avoid blocking.

[^2]: sudo is needed to allocate a TUN device. There are tricks to do this as a usual user, but I am too lazy to find out…