What is CONNECT?

The CONNECT method is used with HTTP to establish a tunnel to a server identified by a given resource. It is commonly used for SSL/TLS encryption.

Principle of operation of CONNECT:

1. Client sends a request: The client (e.g., a browser) sends a POST request to the proxy server with the method CONNECT and the destination address (hostname:port).

   CONNECT example.com:443 HTTP/1.1
   Host: example.com:443
   

2. Proxy establishes a connection: The proxy server receives the request and establishes a TCP connection to the specified destination server (example.com:443).
3. Proxy responds to the client: If the connection is successfully established, the proxy sends a response with the status code 200 OK. This indicates that the tunnel is ready.

   HTTP/1.1 200 Connection Established
   

4. Establishing the tunnel: After receiving the 200 OK response, the client and destination server can freely exchange data through the tunnel established by the proxy. In this case, the proxy acts as a relay, simply forwarding data between the client and the server without analyzing its content (unless it is a specialized HTTPS proxy with a Man-in-the-Middle function).

Main applications:

• HTTPS connections through a proxy: The most common scenario. CONNECT allows the client to establish an encrypted connection with an HTTPS server through a proxy that cannot (or should not) decrypt the traffic.
• Other protocols through a proxy: Can be used for tunneling other protocols that operate over TCP, although this is less typical.

Advantages:

• Security: Allows end-to-end encryption in HTTPS, even when using a proxy.
• Simplicity for the proxy: The proxy does not need to understand the specifics of the protocols tunneled through CONNECT.

Disadvantages:

• Increased delays: The additional step of establishing a connection with the proxy may add slight delays.
• Proxy blocking: Some proxy servers may restrict or block the use of the CONNECT method for certain ports or addresses.