README Reference https://github.com/stefano-lupo/Java-Proxy-Server

Improvements from the Reference

• Support any HTTP protocol including Post, Put etc.
• Support any format of data by streaming in byte format.
• Better multi-thread control using thread pool
• Cleaner codes (still have a room to improve)

The Proxy Server

A proxy server is a server that sits between the client and the remote server in which the client wishes to retrieve files from.All traffic that originates from the client, is sent to the proxy server and the proxy server makes requests to the remote server on the client’s behalf. Once the proxy server receives the required files, it then forwards them on to the client. This can be beneficial as it allows the proxy server administrator some control over what the machines on its network can do. For example, certain websites may be blocked by the proxy server, meaning clients will not be able to access them. It is also beneficial as frequently visited pages can be cached by the proxy server. This means that when the client (or other clients) make subsequent requests for any files that have been cached, the proxy can issue them the files straight away, without having to request them from the remote server which can be much quicker if both the proxy and the clients are on the same network. Although these files are known to be contained in the proxy’s cache, it is worth noting that the clients have no knowledge of this and may be maintaining their own local caches. The benefit of the proxy cache is when multiple clients are using the proxy and thus pages cached due to one client can be accessed by another client.

The Implementation

1. Main: To run main which arguments can be passed for websites to block. It also supports cache control and server shutdown.
2. ProxyServer: It is a singleton class that runs only one Server Socket to receive any request from the client.
3. RequestHandler: It implements Runnable interface which runs in a unique thread when a request is received.
4. Helper: helper methods used by request handler.

The RequestHandler Class

The RequestHandler class is responsible for servicing the requests that come through to the proxy. The RequestHandler examines the request received and services the request appropriately. The requests can be subdivided into three main categories - HTTP GET requests, HTTP GET requests for file contained in the cache and HTTPS CONNECT requests.

HTTP GET

These are the standard requests made when a client attempts to load a webpage. Servicing these requests is a simple task:

• Parse out the URL associated with the request.
• Create a HTTP connection to this URL.
• Echo the client’s GET request to the remote servr.
• Echo the server’s response back to the cliet.
• Save a local copy of the file into the proxy’s cache.

HTTP GET for File in Cache

As before, these are the typical requests made by clients, only in this case, the file is contained in the proxy’s cache.

• Parse out the URL associated with the request
• Hash the URL and use this as the key for the HashMap data structure.
• Open the resulting file for reading.
• Echo the contents of the file back to the client.
• Close the file.

HTTPS Requests

HTTPS connections make use of secure sockets (SSL). Data transferred between the client and the server is encrypted. This is widely used in the financial sector in order to ensure secure transactions, but is becoming increasingly more widespread on the internet. However at first glance it poses a problem for proxy servers: How is the proxy to know what to do with this encrypted data coming from the client? In order to overcome this problem, initially, another type of HTTP request is made by the client, a CONNECT request. This request is standard HTTP and thus is unencrypted and contains the address of who the client wants to create a HTTPS connection with and this can be extracted by the proxy. This is a process known as HTTP Connect Tunneling and works as follows:

• Client issues a CONNECT Request
• Proxy extracts the destination URL.
• Proxy creates a standard socket connection to the remote server specified by the URL.
• If successful, the proxy sends a ‘200 Connection Established ‘ response to the client, indicating that the client can now begin to transmit the encrypted data to the proxy.
• The proxy then simultaneously forwards any data sent to it from the client to the remote server, and any data received from the remote server back to the client. All of this data will be encrypted and thus the proxy cannot cache or even interpret the data.