An Ajax-Enhanced Web-Based Ethernet Analyzer
It's time to give dns-watcher.rb a spin:
sudo ruby dns-watcher.rb
The output from one such invocation is shown in Figure 1. Note that there are not 50 lines of output, as might be expected. Remember, the program's if statement checks to see whether the captured DNS message is a query going to the server and processes the message only if it is. All other DNS messages are ignored by the program, even though they still contribute to the overall count of DNS packets processed.
To run the analyzer for a longer amount of time, change the NUMPACKETS constant to some value greater than 50. As shown in Figure 1, it took the analyzer just more than 40 seconds to process 50 DNS messages (on my PC, on my network segment—your mileage will vary). It is not unreasonable to assume that changing the constant value to something like 250 could result in several minutes of processing. Obviously, piping the output to a disk file or to less allows you to review any results at your leisure.
With my little analyzer up and running, I started thinking it would be cool if I could provide a Web-based interface to it. As every Web developer knows, long-running, server-bound processes and the Web tend not to go together, as there's nothing worse than waiting at a browser for long periods of time while such a process executes. During the years, a number of solutions to this problem have been proposed, which involve techniques that employ redirection, cookies, sessions and the like. Although such techniques work, I've always thought they were rather clunky, and I've been on the lookout for something more elegant. Having just completed Reuven M. Lerner's excellent series of LJ articles on Ajax programming [see the October, November and December 2006 issues of LJ], I wondered if I could combine my analyzer with an Ajax-enabled Web page, updating a part of the Web page with the output from the analyzer as and when it was generated.
Listing 2. A Simple HTML Web Page That Starts the Analyzer
<html> <head> <title>Start a new DNS Analysis</title> </head> <body> Click to <a href="/cgi-bin/startwatch.cgi">start</a>. </body> </html>
My strategy is simple enough. I provide a starter Web page that starts the network analysis on the Web server as a backgrounded CGI process, and then redirects to another Web page that displays the results in an HTML text-area widget, updating the text area with the results from the network analysis. The little HTML Web page in Listing 2 gets things moving. All this Web page really does is provide a link that, when clicked, calls the startwatch.cgi script. The latter is itself straightforward CGI, written as a bash script. Here's the entire script:
#! /bin/sh echo "Content-type: text/html" echo "" sudo /usr/bin/ruby /var/www/watcher/dns-watcher.rb \ > /var/www/watcher/dns-watcher.log & echo '<html><head>' echo '<title>Fetching results ... </title>' echo '<meta http-equiv="Refresh" content="1;' echo 'URL=/watcher.html">' echo '</head><body>Fetching results ... </body>' echo '<html>'
The key line of script is the one that invokes Ruby and feeds the interpreter the dns-watcher.rb program, redirecting the latter's standard output to a file called dns-watcher.log. Note the trailing ampersand at the end of this command, which runs the analyzer as a background process. The script continues by sending a sort HTML Web page to the browser that redirects to the analysis results page, called watcher.html, which is shown in Listing 3.
Listing 3. The Network Analysis Results Web Page