The features of proxy (what can and what cannot proxy server)

Proxy server (shortly - proxy) is a computer program that is activated on connection to a certain port of a computer (they also say it "bind" to certain port). As any other computer program it has many settings which allow it to disable access to certain places, cut off adverts etc.

What proxy can

• disable access to certain sites, like www.porno.com or www.icq.com;
• disable access to sites, containing banned keywords, like "porno" or even "proxy";
• cut off certain parts on pages (banners);
• disable receiving of files with predefined extensions (*.mp3, *.zip, *.exe, *.rar etc.) and/or predefined sizes (both in Kbs and/or in pixels, for example - 468x60);
• log web surfing activity and send report to system administrator with all web pages visited;
• disable use of any protocols (for example, disable access to "https://..." and/or "ftp://..." sites);
• disable access from any computers to this proxy. In other words it is quite possible that of two near by computers one may work with proxy and another may not;

While using CONNECT method (visiting "https://..." sites, building proxy chains, port mapping through proxy etc.) or using SOCKS proxy:

• completely disable CONNECT method (or turn off SOCKS proxy). Then ICQ will not work and there will be no access to "https://..." sites;
• disable connection through proxy to certain servers, for example, login.icq.com;
• disable connection through proxy to certain ports, for example, port 25 (SMTP), port 6667 (IRC), port 5190 (ICQ);
• disable connection through proxy to all ports except specified, for example, port 443. In this case proxy names "HTTPS proxy";

What proxy cannot

• use heuristic analyzer, i.e. make semantic analysis of page contents and disable all sites with certain materials, despite the keywords;
• filter information, for example, block some pages using keywords while using CONNECT method (or in SOCKS proxy);
• let pass back connections from internet (SOCKS 5 can do it but this is also non-trivial task);

Resume: proxy server is a computer program, so as any program it has its own merits and demerits. If administrator tuned a proxy and forgot about it then proxy server always can be bypassed. Otherwise, your attempts to bypass proxy server are always a battle with sys admin and all your solutions are temporary - until administrator find and close just another "hole".