Routing issues, slow network applications, DNS resolution problems -- a network administrator has to deal with a host of network nuisances on a daily basis. How do you survive when you're constantly under the gun to fix the problems? Like any other professional, you need a solid set of tools.
Not surprisingly, plenty of options exist in the open source camp. Excellent open source software tools are available to help you keep a close watch over your network, as well as meet many other needs of the busy network manager. From monitoring, troubleshooting, and security analysis tools to utilities for keeping track of IP allocations, passwords, and router configurations, here are my top 10 picks of the most essential open source tools for our network admin toolbox -- all free for the downloading.
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This is by no means an exhaustive list of open source networking utilities available, and I've merely touched on their capabilities. Are there other free open source tools that you use regularly but we didn't list here? Leave a comment and let us know!
Top free open source tools for network admins: DigDNS problems plague us all, and they're easily overlooked when troubleshooting, so you need a reliable tool that provides detailed information about how users' DNS queries are being resolved. Why not use the tool made by the Internet Systems Consortium, the same group that produces the BIND DNS server software running the majority of DNS servers worldwide? That tool is Dig.
At the heart of it, Dig is a command-line utility that performs DNS queries. That alone is helpful, but Dig can also tell you most everything about the queries and replies -- you'll sometimes need that extra information to determine why you're getting a strange reply from a DNS server. The default output of Dig provides you with all the data you'll require for troubleshooting: reply/error codes from the server, flags used in the query, a reiteration of your query, the answer to your query, how long the query took, which server it received the reply from, and how much data it received in the reply. Dig can be quite useful when you're trying to diagnose slow network applications, by determining how long it takes a computer to get DNS resolution for the application server's domain name.
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 @188.8.131.52 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: NmapCarrie 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.
If you're a longtime user of Nmap but haven't kept up with Nmap news and releases, you'll want to check out the Zenmap GUI's new network topology feature, which lets you create an interactive network map based on information gathered by Nmap. The map begins with localhost at the center and displays all discovered hosts in concentric rings around it, the rings indicating the number of hops away the hosts are. From there you can shift the focus to another host or get more info by clicking a host's icon in the map. The shape of the icon refers to the type of device, and the size indicates the number of open ports.
All this makes Nmap perfect for checking on IP address usage, scanning for security vulnerabilities, and ensuring your firewalls and routers are operating properly.
Top free open source tools for network admins: KeePassOh the passwords! How many passwords do we have for all the various servers, switches, routers, and other network gear we have to manage? And when we have to change a password, we must be sure to notify all of the other people who have access to that equipment. A good password management system can save valuable time and spare you a lot of hassle.
Enter KeePass, an encrypted database program to store all of your usernames, passwords, access URLs, and more. You can restrict access to the KeePass database with a password, a key file, or both. The password database is encrypted with either AES or the Twofish encryption algorithm, and not as one contiguous file but in 256-bit chunks -- decrypting a single piece of data nets a cracker little or no useful data. Plus KeePass encrypts all the data in its database, not just the passwords, so your usernames, URLs, and other notes are safe as well.
You can create groups for password records to help organize the info if you have a lot of passwords to track. Groups can have subgroups, subgroups can have subgroups, and so on. A search function helps you quickly find the password record you need.
How do you share the KeePass database with coworkers who are running Mac OS X or some other version of Unix? No problem. KeePass is ported to Windows, Mac OS X, various Linuxes, and popular mobile phone platforms, including iPhone, Android, and BlackBerry. Because the KeePass database is stored in a single file, it's easy to distribute among your NOC team. Did I mention that KeePass is portable, needing no installation on Windows or Mac OS X? You can carry it with you on a USB stick or download it to a computer without leaving any unwanted registry entries or library files.
Already using another password manager? KeePass can import your existing password database in formats from a variety of programs such as Password Keeper, CodeWalletPro, and Password Agent. Other file formats are supported through KeePass plug-ins.
Top free open source tools for network admins: IPplanWe network admins must keep track of which IP addresses we have, which are in use, and which are available to be allocated to the systems administrators who always want yet another IP address for their servers. Are you tracking your IP addresses in an Excel spreadsheet? Well, stop! Get the benefits of using a real database with IPplan.
IPplan is not a generic spreadsheet or database application. It is tailored to tracking IP addresses, so it understands and enforces CIDR blocks. Each address record has support for contact info, hardware, DNS name, location, description, MAC address, NAT address tracking, and a generic "additional information" field. You can also upload a file to attach to the IP address record.
IP address records are organized into subnets (CIDR blocks), which are assigned to customers or autonomous systems. Subnets are further organized into network areas or ranges (supernets) within the customer hierarchy. Because IPplan is designed for use by ISPs, it works well for organizing enterprise addresses for companies with multiple locations and complex networks that use multiple AS (Autonomous System) numbers. If you decide to rework the subnets on your network, no problem. IPplan handles changes easily via its split and merge subnet functions, allowing you to split and merge subnets without losing any data.
You can import your existing IP allocation data via a CSV file upload. Alternatively, you can use an XML file created by Nmap to import addresses, or you can define the subnets yourself and have IPplan automatically fill in the DNS names for you. IPplan can import the DNS info via a zone transfer from your DNS servers.
You can set up multiple logins, so the entire network admin team has access to the IPplan database. You could even give the system administrators access to IPplan and require them to request IP addresses from you via the Web GUI.
IPplan is more than just an IP address tracking database. As you can see, you can also use IPplan to manage your DNS records, and IPplan implements "triggers" that allow you to run custom scripts based on actions taken in the Web interface.
Top free open source tools for network admins: Tcpdump and WiresharkWhen things get really hairy and you can't figure out what's going wrong on your network, it's time to pull out Tcpdump. This utility lets you capture the network traffic on a network card and view the packets and frames in real time.
If you're wondering why a browser can't find the Web server, you can fire up Tcpdump and see what's happening. Is the computer sending out DNS queries? Is it receiving a valid reply from the correct DNS server? By viewing the query and reply packets with Tcpdump, you can determine if the DNS server is replying with NXDomain for what should be a perfectly valid domain name or if the user changed the DNS server settings because he thinks that Google's DNS servers "must be faster" than your company's own servers. Or maybe the DNS queries and replies are fine, but the remote Web server is not responding. Then you would see the HTTP request packet leave the computer, but no replies from the Web server.
Tcpdump is a great tool by itself, but pair it with Wireshark, and you have an unbeatable system for troubleshooting network application issues. You can save your Tcpdump packet captures to files and open them in Wireshark for easier analysis. Wireshark gives you a GUI to examine Tcpdump captures and sort the data for more thorough analysis. You can compare time stamps on individual packets to see how long it's taking for a reply to be returned after a request has been made. And if you've synced the system clocks on client and server computers, you can see how long it takes for packets to travel between the two.
If you have a slow internal Web application, you can use Tcpdump and Wireshark to locate the bottleneck. If you see a long delay in the DNS lookup requests and replies but the actual HTTP requests and replies are fast, then you know the trouble lies with the DNS system or the network links to the DNS servers. If the DNS process is working normally, then you'll want to examine how long it takes for client requests to reach the server and how long it takes for the server to reply back. Wherever your network problem lies, Tcpdump and Wireshark can help you put your finger on it.
Books have been written about Tcpdump and Wireshark. Read them and learn all about these two utilities. You'll certainly improve your network troubleshooting game.