Web Client Programming Using Perl
HTTPPing.pl sends a HEAD request to the web server to be monitored. The server responds by sending only the HTTP headers. If you are testing only if the server is up, this is all you need to know. It is a good idea to understand the basic HTTP methods, GET, POST and HEAD, when beginning web-client programming. To see how the HEAD command works, we can emulate a browser using telnet. I have a web server running on the machine that I use for development. If you do not have a server running on your local machine, replace localhost with the name of a production web server.
$ telnet www.cpan.org 80Trying 127.0.0.1... Connected to localhost. Escape character is '^]'.
Type the following, then press “enter” twice:
HEAD / HTTP/1.0This is the output returned by my server.
HTTP/1.1 200 OK Date: Fri, 21 May 1999 03:18:26 GMT Server: Apache/1.3.6 (Unix) (Red Hat/Linux) Last Modified: Wed, 0 1999 21:17:54 GMT ETag: "41803-799-370bcb82" Accept-Ranges: bytes Content-Length: 1945 Connection: close Content-Type: text/htmlThis block of text contains the HTTP headers for the requested file. It can be automated using Perl and LWP as shown in Listing 1.
The first line of the script determines which interpreter to use to run the script. If Perl is in a different location on your system, you will need to change your script accordingly. HTTPPing.pl accepts two arguments: the URL to HTTP ping and a debug flag. These arguments are placed into variables. LWP::UserAgent and an HTTP::Request objects are created. The Request is passed to the UserAgent, and a response is returned. The response is examined to see if the server is up. If the site is up, the script exits with a status of 1; if not, it exits with a status of 0. Optionally, determined by the debug parameter, the status of the site is verbosely stated as “up” or “down”.
To ping a site, enter the command:
$ perl HTTPPing.pl http://localhost/ 1 http://localhost is up.
If you have a site that uses SSL and a user name and password, you could use:
$ perl HTTPPing.pl https://username:password@localhost/ 1 https://username:password@localhost/ is up.
Now that you can HTTP ping your site, let's see how to send the alert page. Once that is accomplished, you can tie HTTPPing.pl and the SendPage subroutine together by calling them both from a Perl script running as a service.
Several options are available for sending a page from a Perl script: the Simple Network Paging Protocol (SNPP), TAP (telocator alphanumeric protocol) and proprietary paging interfaces. Many pager services allow SNPP access. Also, a Perl module, Net::SNPP, can make sending a page very simple.
My paging service provides a web-based paging interface. It is an HTML form which collects the recipient's pager PIN and the message, and puts (POST) that information in a CGI script at my paging service provider. Since this is a web-based interface, LWP is a natural for the job. The form requires three values to be posted: the command type, the PIN and the message. The command type is constant for sending pages to my type of pager; the other two values are passed in as arguments.
Note that the request object allows you to create a POST request as well as the already familiar HEAD request. This subroutine is the one I use in my Monitor.pl script. Most paging service providers have web-based paging services (see Resources).
Alternately, if your paging service provider gives you access to an SNPP server, consider using that protocol. Part of the libnet bundle is the Net::SNPP module. It is a great tool for sending pages using the SNPP (RFC 1861). It also has the appeal of being open-standards-based. However, some SNPP servers are so slow that many times, the above method will be superior. Using the Net::SNPP module to send a page is shown in Listing 3.
Integrate HTTPPing.pl and whichever SendPage routine you chose. Monitor.pl repeatedly calls the HTTPPing.pl script at regular intervals. If HTTPPing.pl detects that the site is down, it retries the site to verify. If the site is verified as down, a page is sent. After sending the page, Monitor.pl continues to send HTTP pings to the site. When the site comes back up, another page is sent.
Below is an example of starting Monitor.pl, giving the URL to ping, the pager pin and the delays as arguments. The last argument is an optional debug flag that will show you what is going on inside the script.
perl Monitor.pl http://localhost 1234567 120 120 1
Once Monitor.pl is started, it will patiently test your site at an interval you choose for as long as you like. It is a good idea to start such a script during system boot.
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