Reader favorites: 10 great free network tools

From sniffing to mapping to monitoring, these utilities perform surprisingly sophisticated tasks

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Wireshark can provide deep inspection of hundreds of protocols, and more are added with each release. It can also import traces from other programs (tcpdump, Cisco IDS, Microsoft Network Monitor and Network General to name a few) so analyzing information from other sources is a breeze. It runs on Windows, Linux, Mac OS and other operating systems.

If you are going to administer a network, big or small, a protocol analyzer is a necessary tool. Wireshark fits the bill.

The Dude

Knowing that services are available on your network is a good thing, but knowing when services go down as soon as (or better yet before) your users and customers do is essential. The Dude is a network management package that excels in so many facets it must be tried to be believed that so much can be offered by a freeware tool.

The Dude easily displays meaningful graphics summarizing service uptime.

The Dude easily displays meaningful graphics summarizing service uptime.

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After installation, like many network management packages, The Dude begins with a network discovery process. You input the IP address range or network to discover plus the type of discovery (such as ping or services). This produces a basic network map from which you may customize types of monitoring. The color of the network device's model changes from green to orange if a service goes down and red if all connectivity is lost.

Monitoring includes simple pings, services based on TCP port number, SNMP probes and the ability to log into machines to acquire more specific data. The Dude comes with a preconfigured services set so as to not overwhelm monitoring, but it's trivial to add user-customized services. While it can do so, The Dude isn't designed for discovering services offered by machines on your network. For that you'll want Nmap, which is discussed later.

Without decent notification attributes though, network management packages lose usefulness. This isn't a problem for The Dude. In addition to the map, you can configure a variety of notification modes, from pop-up windows to e-mail messages. In one test, I manually shut off access to MySQL on my Linux Snort IDS box. The Dude popped up a flag and sent me a customized e-mail within a few seconds. You may wish to tweak probe intervals because a lot of false positives would be a distraction.

The Dude Web interface allows for network management without loading The Dude client via a Secure Sockets Layer connection.

The Dude Web interface allows for network management without loading The Dude client via a Secure Sockets Layer connection.

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The Dude comes as a standard client/server package. You can run the client and server on one computer, or run the server on one computer and connect to it from another machine. It also offers a Web interface (http and/or https) for remote access. Various accounts can be created, from a read-only version for help-desk type operations to full administrative access for network managers.

The Dude has so many features and is so versatile that it easily can fit into just about any network monitoring environment. With the ability to nearly instantaneously inform a network administrator of problems, it can be a very cost-effective support tool that your end users will be glad you implemented.

Nmap/Zenmap

Nmap output, as displayed by Zenmap, provides valuable information concerning services offered by hosts on your network.

Nmap output, as displayed by Zenmap, provides valuable information concerning services offered by hosts on your network.

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Nmap is one of those programs that has been around so long it's virtually considered a staple of a networker's bag of tools. But even though the functionality of Nmap has remained strong, it has grown beyond a Linux-based command-line tool. Today's Nmap provides quick information using a crisp graphical user interface (GUI) called Zenmap.

Nmap's function is simple: discover what ports are open on a target machine or range of target machines. Knowing what ports are open is helpful for many reasons. Not sure how many Web servers are running in your environment? Worried the firewall configuration you pushed out with Group Policy isn't effective? Then run Nmap, concentrating on those ports you assume are blocked by your firewall. Concerned that your users' machines may be running a Trojan known to listen on TCP port 25192? Then perform an Nmap scan (behind firewalls) for that port on your entire address space.

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