HTTP/2 is the new version of the well known HTTP protocol which has been at the venerable 1.1 since late last century. Version 2 was derived out of Google’s SPDY protocol and it’s a binary protocol over the text based 1.1. It introduces a bunch of improvements including reducing latency, multiplexing, and server push. There’s some useful improvements that will be great for things like apps that use WebSockets. The Apache httpd daemon has included complete support for HTTP/2 since the 2.4.17 release in the form of mod_http2.
First you should configure your site with SSL, I suggest using LetsEncrypt/certbot as documented in this Fedora Magazine article.
Then you need to make sure the module is loaded, at least in Fedora 25 this is enabled in /etc/httpd/conf.modules.d/00-base.conf by default:
LoadModule http2_module modules/mod_http2.so
Then you just need to enable the protocol in either the general configuration or in specific VirtualHost directives for specific sites:
# for a https server Protocols h2 http/1.1 # for a http server Protocols h2c http/1.1
Then it’s just a systemctl restart httpd to make the changes take effect.
To test whether you’re serving over HTTP/2 you can test using this HTTP/2 testing site or with the OpenSSL client (check for “ALPN protocol: h2” in the output) with the following command:
openssl s_client -alpn h2 -connect HOSTNAME:443
Note: HTTP/2 is not currently supported in the httpd shipped in RHEL.