Tuesday, 4 March 2014


I recently had a need to check out the communication between a client and a server, as well as act upon the contents. Now, I could have used a Network Sniffer to find that out, but it's hard to have a hook with a Network Sniffer and a Java program. Besides, the level of detail is a bit too high. I was only interested in what data is actually going over the line.

With this in mind, I set about to write my "socket proxy server".

The main reason for this "socket proxy server" is because I both do not control the client and the server, yet I wish to "be involved". One saving grace is that I can change the server IP address on the client.

I tested my socket proxy server by starting it as a proxy to a simple website.

java -cp socketproxy.jar com.tools.socketproxy.Main 8080 www.karchan.org 80

So, now I could connect to http://localhost:8080 and I would see the website provided.

Here is the communication as the socket proxy server saw it. First the messages as sent by the client.
received from client:{GET / HTTP/1.1
Host: localhost:8080
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:27.0) Gecko/20100101 Firefox/27.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Cookie: has_js=1
Connection: keep-alive
If-Modified-Since: Wed, 26 Feb 2014 16:26:34 +0000
If-None-Match: "1393431994"

Consequently the reply from the server.
received from server:{HTTP/1.1 200 OK
Date: Wed, 26 Feb 2014 16:29:04 GMT
Server: Apache/2.2.15 (CentOS)
X-Powered-By: PHP/5.3.3
Expires: Sun, 19 Nov 1978 05:00:00 GMT
Last-Modified: Wed, 26 Feb 2014 16:29:04 +0000
Cache-Control: no-cache, must-revalidate, post-check=0, pre-check=0
ETag: "1393432144"
Link: </node/1>; rel="canonical",</node/1>; rel="shortlink"
X-Generator: Drupal 7 (http://drupal.org)
Connection: close
Transfer-Encoding: chunked
Content-Type: text/html; charset=utf-8

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" version="XHTML+RDFa 1.0" dir="ltr"

The UML diagram above, shows the main Thread classes and how they relate to each other.

Dealing with multi-threaded programming is always hard and dangerous. In our case, the threads are provided by a fixed thread pool in the concurrency package of Java. I also had a List of Messages, so the list was created using Collections.synchronizedList(List list). What helps is that two connections cannot interfere with each other, making threading easier.

You can find the source code (Java 7) and javadoc in the references below.

Currently the functionality is limited, however it should be easy to extend.

For example, the current listener will only be called after the conversation has finished. This makes the current implementation unsuitable for directly interfering in the communication.

I required this socket proxy server for one of my private hobby projects. This will be the subject of one of my next blog posts. See [1].


Github - socketproxy repository
Github - socketproxy javadoc
[1] Google App Engine - First Try

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