the page https://www.elastic.co/guide/en/logstash/current/ls-to-ls.html explains, step by step, how to setup your infrastructure to use Lumberjack for LogStash to LogStash communication.
However, a key point is missing. The certificates generated with the openssl command
openssl req -x509 -batch -nodes -newkey rsa:2048 -keyout lumberjack.key -out lumberjack.cert -subj /CN=localhost
expire after one month.
I think it is worth mentioning.