Nmap is the king of port scanners. Billed as a network-exploration and security-auditing tool, it scans entire networks or a single host. Nmap reveals what hosts are available on the network and what services (via open ports) are being offered, detects operating systems and attempts to detect firewalls. The software is very flexible, allowing for all sorts of scan combinations. For example, you could do full TCP connect scans, half connect (TCP SYN only), ACK-only scans and UDP scans. You can also scale back the scan speed from 0.3 seconds between probes to 5 minutes per probe. This lets you avoid overloading a machine, while also sneaking by some IDS systems. Nmap will help you detect unauthorized or unknown services (for example, if someone who shouldn't need to is running IIS) or discover what can get through your firewalls. Nmap got its 15 minutes of fame when it was featured in The Matrix: Reloaded. Suggested by Michael J. DeMaria, Mike Fratto. Platform: Windows, Linux, Mac OS X, others. Price: Freeware, open source (GPL). www.insecure.org/nmap/
Sam Spade
If you have an IP or DNS address and want to find out as much information as possible about it, SamSpade is the first place to go. Pop over to the Web site and see traceroutes, whois, IP whois (find out who owns an IP address), netblock owners and so forth. This is especially useful for tracking down obfuscated URLs, spammers, attackers and trolls. The site is very easy to use and works with almost any browser. Suggested by Ron Anderson, Michael J. DeMaria, Mike Fratto. Platform: Web site, Windows. Price: Freeware. samspade.org
Netcat
Netcat is a versatile utility that reads and writes data over network connections. It's very useful for debugging and incorporating into scripts. You make Netcat "talk" with network servers and clients simply by typing in the protocol commands or redirecting commands from another source. It works similar to telnet, but also can run in server mode, over UDP or TCP, and can transmit binary data. If you've ever encountered a network problem and wondered what was happening with the client or the server, this tool provides an easy way to see what they're saying. Suggested by Mike Lee. Platform: Linux. Price: Freeware, open source (GPL). netcat.sourceforge.net
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