This service allows your machine to ignore ICMP echo-request packets (pings) unless the remote client includes a specific keyword (and only that keyword) as the payload, at which point it will send back an ICMP echo-reply containing some custom data.
This was written for one of the challenges in the Pixels Camp quizshow qualifiers, so it may or may not prove useful in other situations.
The ping-responder.py script can be run directly on the machine, or it can be run inside a container. In any case the host must be prevented from sending ICMP echo-reply packets. On Linux this means changing a kernel tunable, like so (in CentOS 7.x):
$ echo net.ipv4.icmp_echo_ignore_all=1 | sudo tee /etc/sysctl.d/z01-disable_echo_reply.conf >/dev/null
$ sudo systemctl restart systemd-sysctl
Now, to run the service inside a container (the recommended way, for a bit of extra safety):
$ sudo docker build -t ping-responder:latest .
$ sudo docker run --read-only -d --restart always --net=host \
--init --name ping-responder ping-responder:latest \
./ping-responder.py -i eth0 -t "text in the request" -s "text in the response"
Replace eth0 by the network interface you want to use on the host and note that --net=host is required otherwise it would only respond to pings sent from within the container network.
On November 1st, 2020, we modified this service for a small homage to Sean Connery — who had just died on October 31st — as the challenge mentioned above involved one of his films. This was deployed into a standalone VM using a Makefile and tested with an Ubuntu VM bootstrapped with Vagrant. You can find both of these things at the root of the repository as a deployment example.