If you have installed endian firewall, you can force end users to using proxy services,  end users no need to setup proxy’s setting in their browser client.  It is named Transparent Proxy.  It is one of best feature that owned by squid proxy.

This is step by step to activate tranparent proxy at endian firewall :

1. Login as admin to endian firewall web admin.

2. Click Proxy–> HTTP

3. Tick Tranparent on Green at Endian 2.1.2 or older version.

Endian Firewall 2.1.2 or older

4.  If you use Endian Firewall 2.2, choose Tranparent on Green interface at  Endian 2.2.

Endian Firewall 2.2

5. Don’t forget to restart HTTP Proxy.

endian firewall 2.1.2 or older

endian firewall 2.2

6. Finish.