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I need a Linux command to list all free open ports for use in an application

lsof -i TCP| fgrep LISTEN

Does not seen to be helping as the Ports it lists are not necessarily free for use. How do I list free open ports not in use?

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migrated from stackoverflow.com Jan 8 '13 at 11:29

This question came from our site for professional and enthusiast programmers.

    
// , What if netstat is not available on the host? – Nathan Basanese Dec 14 '15 at 6:35
up vote 129 down vote accepted
netstat -lntu

as replied by @askmish will give you list of services running on your system on tcp and udp ports where

  • -l = only services which are listening on some port
  • -n = show port number, don't try to resolve the service name
  • -t = tcp ports
  • -u = udp ports
  • -p = name of the program

You don't need the 'p' parameter as you're only interested in getting which ports are free and not which program is running on it.

This only shows which ports on your system are used up, though. This doesn't tell you the status of your network e.g. if you're behind NAT and you want some services to be accessible from outside. Or if the firewall is blocking the port for outside visitors. In that case, nmap comes to the rescue. WARNING: Use nmap only on networks which are under your control. Also, there are firewall rules which can block nmap pings, you'll have to fiddle around with options to get correct results.

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This command will list open network ports and the processes that own them:

netstat -lnptu

you can thereafter filter the results to your exact specs.

You could also use nmap for more granular results about ports.

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1  
The -p flag requires root privileges for some processes, so it would be sudo netstat -lnptu – klaus se Oct 30 '14 at 1:17

All opened ports including response traffic:

netstat -tuwanp 2>/dev/null | awk '{print $4}' | sort | uniq -c | wc -l
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2  
A list of just unique port numbers and only IPv4: netstat -tuwanp4 | awk '{print $4}' | grep ':' | cut -d ":" -f 2 | sort | uniq – Aaron C. de Bruyn Oct 9 '15 at 20:13
    
+1 for showing how to filter and extract the numbers from the result. Edited to remove stderr output from netstat (which adds a header to the result in Ubuntu). – datashaman Apr 4 at 6:17
    
Hmm, on second thoughts. -1 for not answering the question. – datashaman Apr 4 at 6:18

The following command will work on any Unix which outputs in the same format as Ubuntu / Debian - where the local address is in the column 4 and the output includes a 2 line header at the top. If either of those numbers is different, tweak the awk command below.

If you want IPv4 only:

netstat -lnt | awk 'NR>2{print $4}' | grep -E '0.0.0.0:' | sed 's/.*://' | sort -n | uniq

If you want IPv6 only:

netstat -lnt | awk 'NR>2{print $4}' | grep -E ':::' | sed 's/.*://' | sort -n | uniq

If you want both together:

netstat -lnt | awk 'NR>2{print $4}' | grep -E '(0.0.0.0:|:::)' | sed 's/.*://' | sort -n | uniq

The command outputs a list of port numbers that are listening on all interfaces. If you want to list all ports that are listening on localhost interface, then use something like this:

netstat -lnt | awk 'NR>2{print $4}' | grep -E '(127.0.0.1:|::1:)' | sed 's/.*://' | sort -n | uniq
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Since net-tools is deprecated, you can use the ss command instead of netstat if netstat is not present on your machine:

ss -lntu

should work similarly to

netstat -lntu

according to the built-in help:

-n, --numeric       don't resolve service names
-l, --listening     display listening sockets
-t, --tcp           display only TCP sockets
-u, --udp           display only UDP sockets
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