Apache Proxy: No protocol handler was valid

This can happen if you don't have mod_proxy_http enabled

sudo a2enmod proxy_http

For me to get my https based load balancer working, i had to enable the following:

sudo a2enmod ssl
sudo a2enmod proxy
sudo a2enmod proxy_balancer
sudo a2enmod proxy_http

For my Apache2.4 + php5-fpm installation to start working, I needed to activate the following Apache modules:

sudo a2enmod proxy
sudo a2enmod proxy_fcgi

No need for proxy_http, and this is what sends all .php files straight to php5-fpm:

<FilesMatch \.php$>
    SetHandler "proxy:unix:/var/run/php5-fpm.sock|fcgi://localhost"
</FilesMatch>

In my case, I needed proxy_ajp module.

a2enmod proxy proxy_http proxy_ajp 

This was happening for me in my Apache/2.4.18 (Ubuntu) setup. In my case, the error I was seeing was:

... AH01144: No protocol handler was valid for the URL /~socket.io/. If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.

The configuration related to this was:

  ProxyPass /~socket.io/ ws://127.0.0.1:8090/~socket.io/
  ProxyPassReverse /~socket.io/ ws://127.0.0.1:8090/~socket.io/

"No protocol handler was valid for the URL /~socket.io/" meant that Apache could not handle the request being sent to "ws://127.0.0.1:8090/~socket.io/"

I had proxy_http loaded, but also needed proxy_wstunnel. Once that was enabled all was good.