https://realhanbit.co.kr/books/141
http://www.haproxy.org/download/1.8/doc/proxy-protocol.txt
https://www.haproxy.com/blog/haproxy/proxy-protocol/
https://www.haproxy.com/blog/preserve-source-ip-address-despite-reverse-proxies/
https://roadrunner2.github.io/mod-proxy-protocol/mod_proxy_protocol.html
https://mariadb.com/kb/en/library/proxy-protocol-support/
https://www.slideshare.net/xenonix/haproxy-tcp-source-ip
