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VPN with SSH

As of version 4.3, OpenSSH can use the tun/tap device to encrypt a tunnel. This is very similar to other TLS based VPN solutions like OpenVPN. One advantage with SSH is that there is no need to install and configure additional software. Additionally the tunnel uses the SSH authentication like pre shared keys. The drawback is that the encapsulation is done over TCP which might result in poor performance on a slow link. Also the tunnel is relying on a single (fragile) TCP connection. This technique is very useful for a quick IP based VPN setup. There is no limitation as with the single TCP port forward, all layer 3/4 protocols like ICMP, TCP/UDP, etc. are forwarded over the VPN. In any case, the following options are needed in the sshd_conf file:

PermitRootLogin yesPermitTunnel yes

Single P2P connection

Here we are connecting two hosts, hclient and hserver with a peer to peer tunnel. The connection is started from hclient to hserver and is done as root. The tunnel end points are 10.0.1.1 (server) and 10.0.1.2 (client) and we create a device tun5 (this could also be an other number). The procedure is very simple:

• Connect with SSH using the tunnel option -w
• Configure the IP addresses of the tunnel. Once on the server and once on the client.

Connect to the server

Connection started on the client and commands are executed on the server.

Server is on Linux

cli># ssh -w5:5 root@hserversrv># ifconfig tun5 10.0.1.1 netmask 255.255.255.252   # Executed on the server shell

Server is on FreeBSD

cli># ssh -w5:5 root@hserversrv># ifconfig tun5 10.0.1.1 10.0.1.2                  # Executed on the server shell

Configure the client

Commands executed on the client:

cli># ifconfig tun5 10.0.1.2 netmask 255.255.255.252   # Client is on Linuxcli># ifconfig tun5 10.0.1.2 10.0.1.1                  # Client is on FreeBSD

The two hosts are now connected and can transparently communicate with any layer 3/4 protocol using the tunnel IP addresses