Getting user agent and delivering separate executable based on OS

Let's say I want to send someone a link, and depending on their operating system, they will be prompted to download a different executable (one for macOS, one for Linux, one for Windows). Any ideas for how to get the user agent and then choose which file to deliver based on that info?

xxd -rp | nc -l 80
And wait until a poor fool tries to connect - once this happens you will see their user againt and, then you will be able to manually type the executable that you want to serve in hexadecimal.