Automate Multi‑Server Port Scanning with Shell & Netcat

Why automate port checks?

When managing a few servers you can manually test a port with the nc command, but in a cluster this becomes time‑consuming and error‑prone. A shell script combined with nc can probe any number of hosts and ports automatically.

About netcat (nc)

nc

(netcat) is a versatile networking utility that can read from and write to TCP or UDP connections. It supports three main modes: connect, listen, and relay, and its basic syntax is nc [options] host|IP port.

Scanning a single port on multiple servers

1. Create a file server-list.txt with one IP address per line.

# cat server-list.txt
192.168.1.2
192.168.1.3
192.168.1.4
192.168.1.5
192.168.1.6
192.168.1.7

2. Write a script port_scan.sh that loops over the list and runs nc -zvw3 $server 22 to test SSH (port 22).

#!/bin/sh
for server in `more server-list.txt`
do
    nc -zvw3 $server 22
done

3. Make the script executable and run it.

$ chmod +x port_scan.sh
$ sh port_scan.sh
Connection to 192.168.1.2 22 port [tcp/ssh] succeeded!
Connection to 192.168.1.3 22 port [tcp/ssh] succeeded!
... (output for each host)

Scanning multiple ports on multiple servers

1. Re‑use server-list.txt and create port-list.txt containing the ports to test, one per line (e.g., 22 and 80).

# cat port-list.txt
22
80

2. Write a script multiple_port_scan.sh with nested loops: the outer loop iterates over servers, the inner loop over ports.

#!/bin/sh
for server in `more server-list.txt`
do
    for port in `more port-list.txt`
    do
        nc -zvw3 $server $port
        echo ""
    done
done

3. Make it executable and run it.

$ chmod +x multiple_port_scan.sh
$ sh multiple_port_scan.sh
Connection to 192.168.1.2 22 port [tcp/ssh] succeeded!
Connection to 192.168.1.2 80 port [tcp/http] succeeded!
... (output for each host/port pair)

These scripts provide a quick, repeatable way to verify service availability across large server farms without manual intervention.