Checking for Upness (Echo)

Occasionally it's good to know if a server is up and functioning. The echo service is used to make that determination. The code in serverup.pl shows how one checks the upness of two servers.

Caution Windows 95 (and perhaps other operating systems) can't use the SIGALRM interrupt signal. This might cause problems if you use this script on those systems because the program will wait forever when a server does not respond.

serverup.pl operates as follows:

• Turn on the warning compiler option.
• Load the Socket module.
• Turn on the strict pragma.
• Display a message if the miles.cs.cf.ac.uk server is reachable.
• Display a message if the sentinel.cs.cf.ac.uk server is reachable.
• Declare the echo() function.
• Get the host and timeout parameters from the paramter array. If no timeout parameter is specified, 5 seconds wil be used.
• Declare some local variables.
• Get the tcp protocol and echo port numbers.
• Get the server's Internet address.
• If $serverAddr is undefined then the name of the server was probably incorrect and an error message is displayed.
• Check to see if the script is running under Windows 95.
• If not under Windows 95, store the old alarm handler function, set the alarm handler to be an anonymous function that simply ends the script, and set an alarm to go off in $timeout seconds.
• Initialize the status variable to true.
• Create a socket called ECHO.
• Initialize $packedFormat with format specifiers.
• Connect the socket to the remote server.
• Close the socket.
• Check to see if the script is running under Windows 95.
• If not under Windows 95, reset the alarm and restore the old alarm handler function.
• Return the status.

The Perl code for serverup.pl is:

#!/usr/bin/perl -w


use Socket;
use strict;


print "miles.planet.net is up.\n" if echo('miles.planet.net');
print "sentinel.planet.net is up.\n" if echo('sentinel.planet.net');


sub echo {
    my($host)    = shift;
    my($timeout) = shift || 5;


    my($oldAlarmHandler, $status);


    my($proto)      = getprotobyname("tcp")        || 6;
    my($port)       = getservbyname("echo", "tcp") || 7;
    my($serverAddr) = (gethostbyname($host))[4];


    return(print("echo: $host could not be found, sorry.\n"), 0)
        if ! defined($serverAddr);


    if (0 == Win32::IsWin95) {
        $oldAlarmHandler = $SIG{'ALRM'};
        $SIG{'ALRM'} = sub { die(); };
        alarm($timeout);
    }


    $status = 1;    # assume the connection will work.


    socket(ECHO, AF_INET(), SOCK_STREAM(), $proto)
        or die("socket: $!");
    $packFormat = 'S n a4 x8';   # Windows 95, SunOs 4.1+
    #$packFormat = 'S n c4 x8';   # SunOs 5.4+ (Solaris 2)


    connect(ECHO, pack($packFormat, AF_INET(), $port, $serverAddr))
        or $status = 0;


    close(ECHO);


    if (0 == Win32::IsWin95) {
        alarm(0);
        $SIG{'ALRM'} = $oldAlarmHandler;
    }


    return($status);
}

This program will display:

echo: miles.cs.cf.ac.uk could not be found, sorry.
sentinel.cs.cf.ac.uk is up.

When dealing with the echo service, you only need to make the connection in order to determine that the server is up and running. As soon as the connection is made, you can close the socket.

Most of the program should be pretty familiar to you by now. However, you might not immediately realize what return statement in the middle of the echo() function does. The return statement is repeated here:

return(print("echo: $host could not be found, sorry.\n"), 0)
        if ! defined($serverAddr);

The statement uses the comma operator to execute two statements where normally you would see one. The last statement to be evaluated is the value for the series of statements. In this case, a zero value is returned.

The return statement could also be done written like this:

if (! defined($serverAddr) {
    print("echo: $host could not be found, sorry.\n")
    return(0);
}