How does a proxy(like squid) knows if the connection is using http?

Question

I'm thinking of make my own protocol tunneling with http. So, in some cases we have user and password for proxies like squid.

1-How does it know is really http going on?? does it checks the contents of the TCP connection on port 80?? 2-If so then I really need to use http. If dont I can use any TCP connection on port 80?

Answer 1

It checks the content decoding and interpreting the HTTP requests and responses, otherwise the proxy wouldn't know where to forward the request.

HTTPS is a different thing (proxy can't read encrypted ssl connections) and just interprets a single CONNECT line sent before the HTTPS traffic.

More Information: http://en.wikipedia.org/wiki/HTTP_tunnel