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Old 11-05-2012, 06:45 PM   #1
Registered: Sep 2012
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Find open ports

Found a script to search for port status open/in-use.
But am unsure of what is happening with the execute command.
exec 3>/dev/tcp/${MACHINE}/${PORT}
if [ $? -eq 0 ]
    echo "Port accepting connections"
    echo " connections not possible"
Does the use of the number 3 have a special meaning?
exec executes files so what is it doing with the 3?
How is that related to /dev/tcp ? Does /dev/tcp require the machine and port ??
Is the dotted quad the local or remote machine address?
Old 11-05-2012, 07:55 PM   #2
Registered: Nov 2003
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If you want to check for open ports, I recommend using 'nmap' which is included in Slackware.

If you want to know how the script works, it probably doesn't on Linux (/dev/tcp doesn't exist?). The exec 3>... tells the shell to open a new stream on file descriptor 3 to the named file or device, and continue through the script with the stream open. This use of 'exec' does not execute anything.
Old 11-06-2012, 09:30 AM   #3
Registered: Jul 2006
Location: latvia
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yes, use nmap
simple usage
nmap localhost
nmap -p0-1000 x.x.x.x

Last edited by zux; 11-06-2012 at 09:30 AM. Reason: edit
Old 11-06-2012, 09:57 AM   #4
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nmap -sV x.x.x.x
where x.x.x.x is your ip adress or hostname.

Old 11-06-2012, 12:29 PM   #5
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The example given by the OP uses simple bash commands to check a certain port. It can also be extended to check all possible ports.


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