Top 10 free open source tools for network admins

From troubleshooting DNS queries and misbehaving network apps to keeping your configurations and passwords organized, these tools have you covered

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Dig can ask for a typical name query, replying with an IP address when you give it a domain name. You can also do a reverse lookup: By using the -x switch and giving it an IP address, Dig it will return the corresponding domain name for that IP address. The -t switch lets you specify the type of query you're making, so you can ask for mail server records (MX), name server records (NS), text records (TXT), and more.

If you are sporadically getting incorrect replies to your DNS queries, it's possible that one of your DNS servers has a different set of DNS records than the others. With Dig, you can run the same query against each of your DNS servers to find out which one is providing the erroneous replies. Just give Dig the DNS server's address with the @ symbol in front:

dig @4.2.2.2 www.yourdomain.com

Are you troubleshooting DNS problems with servers that use transaction signatures? Dig lets you specify a TSIG key to use for your queries. Dig also lets you tailor IPv6-only queries to help you troubleshoot IPv6-specific problems.

Dig is a part of the client utilities of the BIND project. It is not generally installed by default, but is readily available on all Unix, Linux, and BSD variants, including Mac OS X. A Windows version is available too.

Top free open source tools for network admins: Nmap

Carrie Moss used it in "The Matrix Reloaded." Crackers, hackers, and network admins alike rely on it, and every networking consultant better have Nmap installed on his or her computer. Nmap is available for nearly every platform imaginable and is amazingly useful as a network and security analysis tool.

Nmap is a lightweight security scanner that's heavy on utility. Nmap can perform tasks as simple as a ping sweep to see which IP addresses are active and responding, as well as carry off complex scripts to scan your systems for known vulnerabilities. Another fun feature of Nmap is the ability to analyze the reply packets it receives from a host to determine which OS the host is running.

Nmap is most commonly used to see which services or ports are open or available on a host. It supports both TCP and UDP scanning. You can give it a single host to scan or a CIDR (Classless Inter-Domain Routing) block or an entire list of hosts and networks from a file. A dizzying range of options allows you to specify which types of packets to send out and to see which hosts are susceptible to various remote attacks. Additionally, Nmap provides several options to bypass firewalls and other network filters that would otherwise block your scans.

Nmap also includes the Nmap Scripting Engine (NSE), which combines custom scripts with existing Nmap functionality to perform more specific discovery and attack analyses than Nmap does by itself. Fyodor and David Fifield gave an excellent talk and demonstration on the NSE at the Black Hat conference in Las Vegas last year. In the demonstration, Fyodor showed the results of Nmap scans against Microsoft company computers that used some of the NSE's MS RPC discovery scripts. The scripts used rpcinfo to gather info such as share names and usernames from the Windows computers. There are 177 NSE scripts available from Nmap.org as of this writing, and because they are user contributed, the list of NSE scripts is expanding at an amazing pace.

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