How to check for open port in Linux on multiple servers together ?

Suppose you have a list of servers, and you want to check for a particular port on which server its open.

So if you telnet each server on that port manually so that will take a lot of time.

In this post, we will look how to check for open ports in Linux through a script to get this job done quickly.

 

I. Check on one serve for port open, say port number 53 for DNS.

### telnet server-name port-number
# telnet ngelinux5 53

[root@ngelinux001 ~]# nc -zv dns1-serv 53
Ncat: Version 7.50 ( https://nmap.org/ncat )

Ncat: Connection timed out.

 

II. Create a file with list of servers.

[root@ngelinux001 ~]# cat /tmp/tempfile2
dns2-serv
f5dns1
dns3
dns1-serv
{-output-truncated-}

 

III. Now we check if port 53 is open.
If open, we redirect server-name to a new file tempfile3 so in last we have a list of servers.

[root@ngelinux001 ~]# for i in `cat /tmp/tempfile2`; do nc -zv $i; if [ $? -eq 0 ]; then echo $i >> /tmp/tempfile3; fi; done
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connection timed out.
Ncat: Version 7.50 ( https://nmap.org/ncat )

 

IV. To get server list where the port is found opened.

# cat /tmp/tempfile3
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