In Apache web server, Modproxy module for  redirecting connections and not a single module its a collection of them. This post describe about mod_proxy and modproxyhttp only.

mod_proxy  : This is a main module implements that managing collections and redirections

modproxyhttp: This mod_proxy_http module implements to proxy features for HTTP and HTTPS protocols.

Install Apache server if is not yet installed in Ubuntu,

# sudo apt-get update
# sudo apt-get install apache2
# sudo service apache2 restart

How to Enable Proxy:

You can enable these modules using a2enmod. For example:
# a2enmod proxy

The above command will create a symlink  /etc/apache2/mods-enabled pointing to  /etc/apache2/mods-available.

If you want to disable a module use a2dismod, This will help you to remove symlink from the mods_enabled directory 
# a2dismod proxy

Then, open a apache2 configure file by any text editor vim or vi,
# vim /etc/apache2/apache2.conf

add the below lines to enable modules and also make sure that the moudles are available in /usr/lib/apache2/modules

LoadModule proxy_module /usr/lib/apache2/modules/
LoadModule proxy_http_module /usr/lib/apache2/modules/

Save that updated file

Restart your apache2 server using below command,
# service apache2 restart

To make ensure that the modules are enabled in Apache,
# apache2ctl -M | grep proxy

 proxy_module (shared)
 proxy_http_module (shared)