http://go7cnxivpns4x2trczyaabxwnrz7bwlxjiverkc32o4mjd5npjukpqid.onion/blog/2021-09-30-hosting-one-domain-on-multiple-servers.html
They contain short scripts, which are executed on request.
One of provided functions is ifportup , which takes a port number and a list of addresses, and returns a random one that is responding. Example Lua record: 300 IN LUA A "ifportup(80, {'x.x.x.x', 'y.y.y.y'})" 3.