How to secure your own VPS with a ‘secret door knock’

We will be using an innovative technique known as the ‘secret door knock’.

The technique works by running a program on the server that listens for a sequence of attempts to access the back-end of the server. These attempts are made via ports: logical paths for data to pass in and out of a server that can also be understood by other servers.

Each port number defines that way that two servers ‘talk’ to each other. Web traffic is port 80 and email server traffic is port 25, for instance. But there are thousands of ports available.

How does this protect my server?

Under the secret door knock technique, a pre-agreed pattern of attempted entrances is created. The source server knocks on a series of the host server’s ports in the agreed order. The program we mentioned above is listening to this pattern to see if it matches what has been agreed.

If the source server uses the right pattern of knocks - that is, attempts entry via all the right port numbers in the right sequence - then the host server will grant access. If not, the source server’s overtures are spurned.

Why is this a good idea?

The secret door knock might sound like something you would see in an episode of Dad’s Army, and the comparisons with the Home Guard don’t stop there. It is not the most sophisticated line of defence for your server, but it is a very cheap and effective form of protection.

Much like 1970s sitcoms, the secret door knock is seen as a bit uncool and is strangely out of vogue at the moment.

But given the current level of cyber threats to businesses, it is a really useful tool for system admins to consider. It is an extra layer for hackers or cybercriminals to get through.

Would you like to set up a secret door knock for your server? Get in touch if you need our help.