Scanning for Random DNS Servers

Posted 1-28-2011 by Matthew Downey
Nmap is such a useful tool. One neat feature is the ability to scan random IP addresses. Someone recently asked me if there was a way to discover random DNS servers. 
sudo nmap -Pn -sS -p 53 -iR 1000 --open | grep -B 4 "53/tcp open"
That will invoke nmap to scan 1000 random IP addresses for an open TCP port 53. It pipes through grep to filter out the garbage (the -B option grabs 4 lines of output before the line that was grepped for).
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