Code Review: Checking open ports.

Chady has asked for the wisdom of the Perl Monks concerning the following question:

Dear monks,

I'm writing a small utility to monitor certain services on a dozen servers or so. I shouldn't have access on these servers, so I decided for the POP, SMTP and FTP monitoring to just check if their ports are open, since I wouldn't have access to check a logon.

I wrote this snippet:

sub checkports {
    
    my $sock;
    my ($pop, $smtp, $ftp) = @_;
    DEBUG && print "Checking ports : ";
    my %ports = (
            110 => $pop,
            25 => $smtp,
            21 => $ftp
        );
    
    for my $port (keys %ports) {
        DEBUG && print "[$port] ";
        
        $sock = new IO::Socket::INET(PeerAddr => $ports{$port}, PeerPo
+rt => $port, Proto => 'tcp', Timeout=>10);
        if ($sock) {
            $client->{"PORT$port"} = 'U';
            close $sock; 
        } else {
            $client->{"PORT$port"} = 'D';
        }
    }

    DEBUG && print " done.\n";
}
[download]

Since I have a fairly small knowledge in networking, I was wondering if this approach is reasonable; Is there another way to check if the services are up without having to get a username/password and try a real login?

He who asks will be a fool for five minutes, but he who doesn't ask will remain a fool for life.

Chady | http://chady.net/

Comment on Code Review: Checking open ports. Download Code

Replies are listed 'Best First'.
Re: Code Review: Checking open ports. by ozone (Friar) on Jun 09, 2003 at 08:58 UTC
The way this works, you're not really testing that the services actually are functioning. You're just connecting to the port, which is done at the OS level. So your service could've hung, but as far as you're concerned, it's still alive. Also, most services have a no-op type function that returns some sort of standard string you can test for. With SMTP, you could 'say hello' by sending 'HELO bob', and expect to get a response. I can't remember the other ones.	[reply]
Re: Re: Code Review: Checking open ports. by waswas-fng (Curate) on Jun 09, 2003 at 11:30 UTC
Ozone is right, connecting to the port only verifies there is a program that is listening and bound to the port and acepting incoming connections. If your envornment normally sees crashes of those services where they server fine unless they are not running at all this may be "enough" for you. else more active tests are needed and you either need to understand the protocols that you are testing or use off the shelf modules to test functionality such as lwp for HTTP, net::ftp for FTP etc. note that this type of testing usually requires actuall access to the service (user for ftp, access to a web page to download). You need to decide what level of testing makes you comfortable. -Waswas	[reply]
Re: Re: Code Review: Checking open ports. by naChoZ (Curate) on Jun 09, 2003 at 12:15 UTC
With smtp, `helo` should elicit a `250` response code, with pop, `user postmaster` (or other valid login name) should result in a `+OK` response, and for ftp, a `noop` should return a `200` response code. ~~ naChoZ	[reply]
Re: Re: Code Review: Checking open ports. by fsn (Friar) on Jun 09, 2003 at 11:41 UTC
You should be aware that at least sendmail servers logs connection that doesn't do anything useful, ie. only a connect and disconnect, or even a connect, HELO, NOOP and a disconnect. So you better get permission from the server administrator, or he might come back and ask you what you are doing and tell you to stop. It happend to me...	[reply]
Re: Code Review: Checking open ports. by hardburn (Abbot) on Jun 09, 2003 at 13:59 UTC
Instead of making a new program, why not just use nmap (external link)? ---- I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident. -- Schemer Note: All code is untested, unless otherwise stated	[reply]