I know that. But I can have some users that uses the transparent proxy and some other users that have configured the proxy. I want to authenticate the latest group while both of them need to pass the squidguard rules.
The problem is that I can't enable in the proxy configuration transparent proxy and user authentication at the same time with default config. I was able to configure that using custom parameters.
The way I did it is setting the proxy as transparent one with default parameters, and adding the auth rules just before the call to squidguard, all of this inside the squid custom parameters.
I also added a forward rule in the firewall, that catches all the traffic from local net (192.168.x.x/xxx) to some other net that is not the local net (! 192.168.x.x/xxx) to port 80, then I forwarded this traffic to localhost port 80, and all worked just like I wanted, so it is possible to do that with some tweaking, but I will be wonderful to have this parameters on the default configuration.
I think that is almost possible also to have authentication with transparent proxy this way: if the browser is authenticated, then it pass OK, if not, then, instead of the "error page" it is shown a "login to proxy page", after that, if logged OK, a rule is configured on the proxy that do a relation between browser IP and username for X minutes.