Question

I am trying to write a script that connects to a list of IPs on port 53 – and I want the result to return only open ports. Here is the script I am running below – I have tried grepping and cutting the output but im not sure I am doing this correctly – I cant seem to pipe the script results to a text file either.

#!/bin/bash

for ip in $(seq 200 254); do
    nc -v 192.168.11.$ip 53 &
done

I apologise for its simplicity I am new – and if the solution is elsewhere

Answer 1

Check for the command’s exit code, it should be zero for a successful connection. Also use the -z option to drop the connection once it has been established.

#!/bin/bash

for ip in $(seq 200 254); do
    nc -z 192.168.11.$ip 53
    if [ $? -eq 0 ]; then
        echo "Hit: 192.168.11.$ip"
    fi
done

If you were using nmap instead of netcat, you could have used this:

nmap 192.168.11.200-254 -p 53

PS. If you’re trying to determine which hosts run DNS server, you should scan for open 53/udp, not 53/tcp (option -u in netcat)