Moniker

The Code

So, let's dive right into the code. Lines 3-6 load our modules. Getopt::Std is one module I haven't mentioned yet. It's included with the standard distribution of Perl and handles command line arguments so you don't have to. A nifty little package indeed.

Line 9 sets the number of seconds we'll wait to get a response from the server we're testing. In line 10, the email address that will be set in the From: field of the email is assigned to the $monikeraddr variable. Line 11 sets the $smtpserver variable which holds the hostname of the smtp server we'll send email alerts through.

Lines 13-22 build a multi-dimensional hash that contains the service name, port, the string that's printed once a connection has been established, and the string that we expect from the remote service.

Next, we move into the main body of the code. First we initialize the hash that will hold the value of the command line switches (line 25). Then, we call getopt, which was imported from the Getopt::Std module (line 26).

The next thing we do in lines 27-30 is check the command-line arguments to make sure values were specified for the -h (host) and -s (service). If they weren't, we remind the user of the syntax. We also validate the service they specified to make sure it's listed in our services hash.

In lines 32-45 we set several lexically scoped variables (lines 32-37), create a new instance of the Net::Telnet module (line 36), and create a connection to the remote host (lines 40-45). You'll notice that this last section of code is surrounded by an eval. What eval does in this case is catch an error, if one occurs when $sock->open is called on line 41, and puts the error code or message into the $@ variable. Then, on line 48, we execute the &alert subroutine if $@ contains a value. If an error does not occur, we can assume that connection has been established with the remote service.

On line 51, we print the string that's defined in our services hash (lines 13-22) if it exists. This is necessary for services like www where the server expects some input before it give us some output. In the case of HTTP, we send it the string HEAD / HTTP/1.0\n\n which requests the HEAD information of the root document on the Web server.

Lastly, we wait for the string specified in the services hash that relates to the particular service we're testing (line 54). In the case of the www service, we're looking for the string 200 which the server returns to let us know the request was successful. if all goes well, we print the string Service is operational. on line 56.

Lines 59-82 contain the alert subroutine which sends an email to the address specified by the -e switch on the command-line. This switch is optional, so if it's omitted, the error is simply printed to STDOUT. To send the email, we used the sendmail function which was imported from the Mail::Sendmail module. We simply pass it the email header values and the function does the rest. The nice thing about this module is that it doesn't require sendmail. It's an completely independent implementation of the SMTP protocol, so you can send the email through any SMTP server you like.