Using the firewall as a web proxy.

Question

I want to configure my Sophos Firewall so that only Firefox can access the internet from a PC, and all traffic must go through the Sophos explicit web proxy (configured on port 3128). Direct HTTP/HTTPS traffic from the PC should be blocked entirely. 
 When I configure Firefox to use the proxy, web pages are blocked unless direct HTTP/HTTPS traffic (ports 80/443) from the PC to the WAN is allowed. This defeats the purpose of forcing traffic through the proxy. If I block ports 80 and 443, the proxy stops working even though traffic to port 3128 is explicitly allowed. 
 What am i missing? 
 Thanks

Michael Dunn · Accepted Answer

It is actually slightly more complicated. There still needs to be a rule for HTTP/HTTPS for the proxy to talk externally - otherwise you will get the page attempting to load and then timeout with a sophos generated error page.

In effect you have two connections
PC <--> SFOS <--> website

The PC to SFOS is 3128 and needs a firewall rule.
The SFOS to website is 80/443 and needs a firewall rule. Web policy is not applied to traffic generated from the web proxy.

The firewall rules are re-evaluated on the external connection. Otherwise web proxy can be used to bypass some firewall rules (such as GeoIP counry blocks).

Recommendation is

Rule 1 - Service 3128. Web policy what you want.
Rule 2 - Service HTTP/HTTPS. Web policy Block All.

Note: Transparent mode traffic will hit the Block All policy, so they get a block page. However exceptions are still applied, therefore they will still be able to do things like get to microsoft.com (for windows updates). You can watch the Web Filter log for connections that are using that firewall rule and determine if you want to allow/block them via exception.

If I recall there are some other possible setups that block transparent HTTP/HTTPS more completely but are more complex. And that customers found that some updaters and things just do not support direct mode so they found that supporting a little bit of transparent mode was nice to have.

It is suggested that if you are doing this you may want to use WPAD/PAC to deploy the proxy settings. That way any browser configured for proxy "automatically detect settings" picks it up. That will also catch a few non browsers as well.

Michael Dunn · Answer

Because you are doing HTTPS decryption you need to have the sophos CA installed.

Because this changes the way websites are signed (such as foxbusiness), websites that are configured with HSTS will complain. The website told the browser "Dont ever let the certificate change".

Take a look at the FAQ.
Sophos Firewall: HTTPS Decrypt and Scan FAQ

Offhand I do not know how to clear HSTS for the entire browser, I presume sales/support knows. Google results tend to focus on how to clear for single websites.

Using the firewall as a web proxy.

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