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Need script to monitor port for active, then send command

I need a script that would continuously monitor one port and when it is active (not listening or waiting) execute a series of commands to standard output.

I am familiar with netstat but am not good with scripting so I could use some help.

Thanks.

My very old script on perl

Code:

#!/usr/bin/perl -w

use Socket;



#net - netstat

#tcp - socket

#udp - deprecated, don't work

my %serv=(8031=>'net', 8041=>'net',8051=>'net',8000=>'net');



sub restart {

    my ($port)=@_;

    if($port eq 8031) {system("/home/kernel/run_vlc_yamal start");}

    if($port eq 8041) {system("/home/kernel/run_vlc_eam22 start");}

    if($port eq 8051) {system("/home/kernel/run_vlc_yamal_c start");}

    if($port eq 8000) {system("rc-config restart icecast");}

    if($port eq 9001) {system("/home/kernel/run_flv2");}

}



sub check {

    my ($port,$prot,$du)=@_;

    if($prot ne 'net')

    {

        if($prot eq 'tcp') {socket(SOCK, PF_INET, SOCK_STREAM, getprotobyname('tcp'));}

        else {socket(SOCK, PF_INET, SOCK_DGRAM, getprotobyname('udp'));}

        my $paddr = sockaddr_in($port, inet_aton("127.0.0.1"));



        $con=0;

        if($prot eq 'udp')

        {

            send(SOCK, 0, 0, $paddr);

            eval {

                local $SIG{ALRM} = sub { die "alarm time out" };

                alarm 5;

                recv(SOCK, $con, 4, 0);

                alarm 0;

                1;

            } or $con=0;

        }

        else

        {

            $con=connect(SOCK, $paddr);

        }

        close SOCK;

    }

    else

    {

        $con=0;

        open(GREP,"-|") || exec "netstat -ln|grep .".$port;

        while (<GREP>){

            if($_=~/.$port /){$con++;}

        }

        close GREP;

    }

    if($port eq 67 && !$con && !$du){

        sleep(10);

        $con=check($port,$prot,$du+1);

    }

    if(!$con)

    {

        restart($port);

    }

    return $con;

}



my %work;

open (F1,"</home/kernel/sock.log");

while(<F1>)

{

    chomp;

    my @F=split / /;

    $work[$F[0]]=$F[1];

}

close(F1);

open (F1,">/home/kernel/sock.log");

while(($port,$prot) = each(%serv))

{

    $con=check($port,$prot,0);

    if($con){

        print F1 "$port 1\n";

        if($work[$port] eq "0"){

            system("/usr/bin/snmptrap -v 1 -c public $host .1.1 `hostname` 51 0 '' .1 s '$port up'");

        }

    }

    else {

        print F1 "$port 0\n";

        if($work[$port] eq "1"){

            system("/usr/bin/snmptrap -v 1 -c public $host .1.1 `hostname` 51 0 '' .1 s '$port down'");

        }

    }

    

}  

close(F1);

Thanks Troop. Do you think you could provide me with the littlest of notations, like what each section does so I don't blow out my server? It looks like this is checking to see if a service is up and if not, restart it, but I cannot be sure. I only need to check one port number and see if it's active, if so, execute a script.

You should change

Code:

my %serv=(8031=>'net', 8041=>'net',8051=>'net',8000=>'net');



sub restart {

    my ($port)=@_;

    if($port eq 8031) {system("/home/kernel/run_vlc_yamal start");}

    if($port eq 8041) {system("/home/kernel/run_vlc_eam22 start");}

    if($port eq 8051) {system("/home/kernel/run_vlc_yamal_c start");}

    if($port eq 8000) {system("rc-config restart icecast");}

    if($port eq 9001) {system("/home/kernel/run_flv2");}

}

...

    if(!$con)

    {

        restart($port);

    }

Code:

my %serv=(your_port=>'net');



sub restart {

    my ($port)=@_;

    if($port eq your_port) {system("your_command");}

}

...

    if($con) { restart($port); }

if a service on port your_port is active then execute your_command.

or you can just simplify to

Code:

#!/usr/bin/perl -w



my $port=your_port;

$con=0;

open(GREP,"-|") || exec "netstat -ln|grep .".$port;

while (<GREP>){

  if($_=~/.$port /){$con++;}

}

close GREP;

if($con) { execute your command } #if port is up

thank you Troop.

When I run it as:

#!/usr/bin/perl -w

my $port=21;
$con=0;
open(GREP,"-|") || exec "netstat -ln|grep .".$port;
while (<GREP>){
if($_=~/.$port /){$con++;}
}
close GREP;
if($con) { print "hello world 21 is in use!" } #if port is up

I receive the error:
Use of uninitialized value $_ in pattern match (m//) at ./check.pl line 7.

Quote:

Use of uninitialized value $_ in pattern match (m//) at ./check.pl line 7.

it's just a warning so code is running

Code:

# cat test.pl 

#!/usr/bin/perl -w



my $port=21;

my $con=0;

open(GREP,"-|") || exec "netstat -ln|grep .".$port;

while (<GREP>){

if(defined $_ && $_=~/.$port /){$con++;}

}

close GREP;

if($con) { print "hello world 21 is in use!"; }



# ./test.pl

hello world 21 is in use!

# killall vsftpd

# ./test.pl

#

Thank you so much for your help, Troop. I think that'll do it!

I may have spoken too soon. I startup my tftpd and see that port 69 is listening. I run this script:

#!/usr/bin/perl -w

my $port=69;
my $con=0;
open(GREP,"-|") || exec "netstat -ln|grep .".$port;
while (<GREP>){
if(defined $_ && $_=~/.$port /){$con++;}
}
close GREP;
if($con) { print "hello world 69 is in use!"; }

I then tftp into the server and GET a file.

The script is still just sitting there, nothing happens

I think I may have spoken too soon.

I fire up the tftpd daemon on port 69, then run the script indicating my $port=69;

Then I tftp and GET a file, and the script just sits there... nothing happens.

Thanks.

post output

Code:

netstat -ln|grep :69

netstat -ln|grep :69

udp 0 0 0.0.0.0:69 0.0.0.0:*

Thanks for your help, Troop. I really appreciate it!

strange

Code:

# cat test.pl

#!/usr/bin/perl -w

my $port=69;

my $con=0;

#open(GREP,"-|") || exec "netstat -ln|grep :".$port;

open(GREP,"-|") || exec "echo 'udp 0 0 0.0.0.0:69 0.0.0.0:*'";

while (<GREP>){

if($_=~/:$port /){$con++;}

}

close GREP;

if($con) { print "hello world 69 is in use!"; }

# ./1.pl

hello world 69 is in use!

Your perl works differently?
try

Code:

# cat test.sh 

#!/bin/sh

PORT=69;

CON=`netstat -ln|grep -c :$PORT`

if [ $CON -gt 0 ]; then

    echo "$PORT is in use";

fi

Thank you! the following does work:

#!/usr/bin/perl -w
my $port=69;
my $con=0;
open(GREP,"-|") || exec "netstat -ln|grep :".$port;
while (<GREP>){
if($_=~/:$port /){$con++;}
}
close GREP;
if($con) { print "hello world 69 is in use!"; }

For my own education can you tell me what $con does, is about? I'm trying to learn. I've got the rest of it.

$con counts cxns. Set to zero initially, (= false in an if() ). If $con gets incremented, a cxn was found, therefore its 'true' in the final if() .

Now using a variation of what works I tried, using ngrep, but it doesn't work.:

#!/usr/bin/perl -w
my $port=69;
my $con=0;
open(NGREP,"-|") || exec "ngrep|grep :".$port;
while (<NGREP>){
if($_=~/:$port /){$con++;}
}
close NGREP;
if($con) { print "Alert! Transfer on port 69!"; }

if I:
ngrep | grep 69

and then GET a file with tftp

I see output so I know ngrep with grep is working....