Problem in IO::Socket syntax?

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

I've written a short script that checks to see if a remote server is listening on a specific TCP port. Its in the early stages, so some error checking needs to be built in. For now though, I'm confused by an error that I am getting when I test it using localhost as the guinea pig.

      1 #!/usr/bin/perl -w
      2 
      3 # $Id:$
      4 
      5 # check to see if server is listening on a tcp port
      6 
      7 use strict;
      8 use IO::Socket;
      9 
     10 my @servers = qw(localhost);
     11 
     12 my %ports =     (
     13                         telnet  => "23",
     14                         smtp    => "25",
     15                         pop3    => "110",
     16                         named   => "53",
     17                         http    => "80",
     18                         https   => "443",
     19                 );
     20 
     21 my $proto = "tcp";
     22 my $disco = 0;
     23 
     24 for my $host(@servers) {
     25 
     26   my $ip = gethostbyname($host);
     27   my $ip_addr = inet_ntoa($ip);
     28 
     29   for my $service(keys %ports) {
     30 
     31     my $disco = 0;
     32 
     33     my $checkport = IO::Socket::INET->new(
     34       PeerAddr  => "$ip_addr",
     35       PeerPort  => "$ports{$service}",
     36       Proto     => "$proto",
     37       Timeout   => '0')
     38         or $disco = "1";
     39 
     40     print "$host FAILED to repond on $service $ports{service}\
+n"
     41       if $disco;
     42 
     43     close $checkport;
     44 
     45   }
     46 }
[download]

When I run the script, I get:

Use of uninitialized value in concatenation (.) at ./chkport.pl line 4
+0.
localhost FAILED to repond on telnet 
Can't use an undefined value as a symbol reference at ./chkport.pl lin
+e 43.
[download]

i'm staring at the print statement and the close statement. Both seem to be correct to the best that I can tell. Your input is well appreciated as always.

humbly -c

Comment on Problem in IO::Socket syntax? Select or Download Code

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Re: Problem in IO::Socket syntax? by Ryszard (Priest) on Mar 30, 2002 at 23:03 UTC
`print "$host FAILED to repond on $service $ports{service}\n"` becomes `print "$host FAILED to respond on $service $ports{$service}\n"` Other than that, do you have a firewall on localhost that may prevent connections?	[reply] [d/l] [select]
Re: Re: Problem in IO::Socket syntax? by c (Hermit) on Mar 31, 2002 at 00:13 UTC
Thanks for correcting my poor grammar ;-) To answer your question and the previous, there is no local firewall, and I intentionally included telnet in the list of services because I know that it is not enabled on this host. Really just testing to make certain I get the output I desire when a service is down. I just don't understand the error that was being spit out. I don't see the concantenation that its referring to. -c	[reply]
Re: Re: Re: Problem in IO::Socket syntax? by Ryszard (Priest) on Mar 31, 2002 at 00:22 UTC
If i understand correctly (and I havent run your code) you should be using the variable `$service` in your hash, rather than the hard reference `service`. Your hash `%ports` has no key of `service`.	[reply] [d/l] [select]
Re: Problem in IO::Socket syntax? by lestrrat (Deacon) on Mar 30, 2002 at 22:52 UTC
The only thing I can say is that while there are a few things that I would do differently and that there's a small typo on line 40, your code works for me. Are you absolutely sure that you can access the telnet port on that machine that you're accessing?	[reply]
Re: Re: Problem in IO::Socket syntax? by c (Hermit) on Mar 31, 2002 at 00:16 UTC
telnet is not enabled on the host i am testing on, so the output of telnet failing isnt what throws me. i dont see where the error complaining about the concatenation is coming from however. thanks -c	[reply]
Re: Re: Re: Problem in IO::Socket syntax? by lestrrat (Deacon) on Mar 31, 2002 at 01:22 UTC
Oh. I understand that you were probably frustrated not know where on earth the word "concatnation" would come out of your code, but *you didn't say that that* was your problem*. Wouldn't you agree it's really hard to tell what you were asking? ;) Anyway, I think Ryszard got the answer correctly... basically, embedding variables in string implicitly invokes the concat* op, so your line 40 is equivalent of: `print $host . " FAILEd to repond on " . $service . " " . $ports{ se +rvice } . "\n";` [download] So you see that if `$port{ service }` was undefined, you would get a concatnation warning. If you're interested, check out the output of `perl -MO=Terse yourscript.pl`	[reply] [d/l] [select]
Re: Problem in IO::Socket syntax? by flocto (Pilgrim) on Mar 31, 2002 at 00:54 UTC
Well, AFAIK IO::Socket returns undef upon failure, and will have a hard time closing an undefined variable. In this case you should change your code to: `if ($disco) { print "$host FAILED to respond on $service $ports{$service}\n"; } else { close $checkport; }` [download] I couldn't find IO::Socket's return value when failing in it's documentation, but that it calls connect(), and this returns false when failing. I can't imagine that it (IO::Socket) would return a socket even when failing to open one, so my guess is that this is your mistake. I don't use IO::Socket all that often, so I might be wrong here.. Hope this helps ;) -octo-	[reply] [d/l]
(tye)Re: Problem in IO::Socket syntax? by tye (Sage) on Mar 31, 2002 at 06:59 UTC
And when it fails, please also report why it failed. How to find out why it failed varies by module, but for IO::Socket, I'm pretty sure it is the common $! that you want to report in the error message. - tye (but my friends call me "Tye")	[reply]
Re: Problem in IO::Socket syntax? by DigitalKitty (Parson) on Mar 31, 2002 at 18:13 UTC
Hi. I'm glad the problem was solved but just for reference, if you are using a linux machine ( w/ 2.2 kernel ), you can enable or disable the telnet daemon by editing your inetd.conf file. Just remove the '#' sign that appears in front of the name of the service ( telnet ). Then restart the service and you should be able to telnet to your local machine. If you are using a 2.4 kernel, the file is called xinetd. It's more secure than the 2.2 implementation but serves the same purpose essentially. Thanks, -DigitalKitty	[reply]