accept-intercept-request works fine when the client browser has been manually configured to use the proxy server. However, when in transparent mode (where traffic from port 80 is NATTED to the proxy port) and the browser is unaware that is being proxyed, authentication requests are not done correctly and the browser displays a "this page is not available" mesg.
Not sure what the fix is for this if there is one.
to reproduce the error on any linux distribution...
1. add accept-interface-requests to the proxy config
2. execute the following iptables command as root
iptables -t nat -N natcensor
iptables -t nat -I prerouting_rule -j natcensor
iptables -t nat -A natcensor -p tcp --dport 80 -j REDIRECT --to-port 3128 (where 3128 is the port the proxy is listening on)
3. confirm that you can browse the internet while confirming that the proxy is being used by checking the logs. The browser should "normal" non-proxy mode.
4. configure an upstream proxy that requires authentication
5. browse to a page that requires the use of the proxy.
at this point you should see the failure.
6. change the configuration of the browser to manually use the proxy.
7. confirm that the authentication request window pops up on the browser when using the upstream proxy.
problem discovered using privoxy 3.0.21 under OpenWRT.