Some time ago I had the problem of dynamically start virtual machines, when an incoming connection was received in a port. The exact problem was to have a VM that was powered off, and start it whenever an incoming ssh connection was received, and then forward the network traffic to that VM to serve the ssh request. In this way, I could have a server in a cloud provider (e.g. Amazon), and not to spend money if I was not using it.
This problem has been named “the sleeping beauty”, because of the tale. It was like having a sleeping virtual infrastructure (i.e. the sleeping beauty), that will be awaken as an incoming connection (i.e. the kiss) was received from the user (i.e. the prince).
Now I have figured out how to solve that problem, and that is why this time I learned
How to dynamically create on-demand services to respond to incoming TCP connections
The way to solve it is very straightforward, as it is fully based in the socat application.
socat is “a relay for bidirectional data transfer between two independent data channels”. And it can be used to forward the traffic received in a port, to other pair IP:PORT.
A simple example is:
$ socat tcp-listen:10000 tcp:localhost:22 &
And now we can SSH to localhost in the following way:
$ ssh localhost -p 10000
The interesting thing is that socat is able to exec one command upon receiving a connection (using the destination of the relay the address type EXEC or SYSTEM). But the most important thing is that socat will stablish a communication using stdin and stdout.
So it is possible to make this funny thing:
$ socat tcp-listen:10000 SYSTEM:'echo "hello world"' &  11136 $ wget -q -O- http://localhost:10000 hello world $ + Done socat tcp-listen:10000 SYSTEM:'echo "hello world"'
Now that we know that the communication is stablished using stdin and stdout, we can somehow abuse of socat and try this even funnier thing:
$ socat tcp-listen:10000 SYSTEM:'echo "$(date)" >> /tmp/sshtrack.log; socat - "TCP:localhost:22"' &  27421 $ ssh localhost -p 10000 cat /tmp/sshtrack.log mié feb 27 14:36:45 CET 2019 $ + Done socat tcp-listen:10000 SYSTEM:'echo "$(date)" >> /tmp/sshtrack.log; socat - " TCP:localhost:22"'
The effect is that we can execute commands and redirect the connection to an arbitrary IP:PORT.
Now, it is easy to figure how to dinamically spawn servers to serve the incoming TCP resquests. An example to spawn a one-shot web server in port 8080 to serve requests in port 10000 is the next:
$ socat tcp-listen:10000 SYSTEM:'(echo "hello world" | nc -l -p 8080 -q 1 > /dev/null &) ; socat - "TCP:localhost:8080"' &  31586 $ wget -q -O- http://localhost:10000 hello world $ + Done socat tcp-listen:10000 SYSTEM:'(echo "hello world" | nc -l -p 8080 -q 1 > /dev/null &) ; socat - "TCP:localhost:8080"'
And now you can customize your scripts to create the effective servers on demand.
The sleeping beauty application
The sleeping beauty is a system that helps to implement serverless infrastructures: you have the servers aslept (or not even created), and they are awaken (or created) as they are needed. Later, they go back to sleep (or they are disposed).
In the sleeping-beauty, you can configure services that listen to a port, and the commands that socat should use to start, check the status or stop the effective services. Moreover it implements an idle-detection mechanism that is able to check whether the effective service is idle, and if it has been idle for a period of time, stop it to save resources.
Example: In the description of the use case, the command to be used to start the service, will contact Amazon AWS and will start a VM. The command to stop the service will contact Amazon AWS to stop the VM. And the command to check whether the service is idle or not will ssh the VM and execute the command ‘who’.