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

Hello fellow monks,

I need to solve a little, almost unnoticeable, but annoying problem. To test if a socket on a machine is open and there's something listening behind, i just try to connect with 

        [ ...snippet... ]
        my $port = shift;
        my $proto = getprotobyname('tcp');
        my $iaddr = inet_aton( '172.16.1.5' );
        my $paddr = sockaddr_in($port, $iaddr);

        socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "Cannot pr
+epare socket: $!";

        eval {

                connect(SOCKET, $paddr) || die "Can't connect to $port
+";

        };

        close SOCKET || warn "Cannot close socket: $!";
        [ ...snippet... ]

[download]

All goes well, but the fact that when I close the socket, client side, on the server side it bothers to stay on TIME_WAIT. I'd guessed that closing the socket, the stream itself had been over and the server socket closed, but so it doesn't seem. Am I missing something? Is there any explicit way to send the ack to the server so that it will close the socket opened for my testing connection?

Thanks in advance for any kind answer.

Replies are listed 'Best First'.
Re: TCP Socket and TIME_WAIT
by Khen1950fx (Canon) on Aug 28, 2012 at 00:07 UTC
    Your use of the bareword SOCKET should be my $socket. Don't forget to declare your subs. 
    #!/usr/bin/perl

BEGIN {
    $|  = 1;
    $^W = 1;
}

use strict;
use warnings;
use Socket qw/PF_INET SOCK_STREAM inet_aton sockaddr_in/;

my $port  = shift;
my $proto = getprotobyname('tcp');
my $iaddr = inet_aton('172.16.1.5');
my $paddr = sockaddr_in( $port, $iaddr );

socket( my $socket, PF_INET, SOCK_STREAM, $proto )
  || die "Cannot prepare socket: $!";

eval { 
    die "Can't connect to $port" 
      unless connect( my $socket, $paddr ); 
};

shutdown($socket, 2);
sleep 1;
exit 0;

    [download]
[reply]
[d/l]
[select]

      Wonderful! That's what I was looking for!

      Many thanks!

[reply]
Re: TCP Socket and TIME_WAIT
by dave_the_m (Monsignor) on Aug 28, 2012 at 02:04 UTC
    TIME_WAIT is an integral part of the TCP protocol state, and the socket will always enter TIME_WAIT for a period in the the host which first closed the socket.

    See the TCP state diagram

    Dave.

[reply]

      Sure, I know, but it wasn't clear to me how to achieve an active close, instead a passive one for which, on that system, the socket would last on TIME_WAIT for some time.

      Thanks though!

[reply]

Back to Seekers of Perl Wisdom