But when you see this error you know that your Nagios installation isn’t able to use any external commands. Fear not, this error is simple to over come, and in this article I am going to walk you through the fix.

Assumptions

Figure 1

This how to will assume that you have a working Nagios installation as well as root or sudo access to your server. This article also assumes you have tried to use features that require external commands. If not, let’s take a look at how.

If you point your browser to your Nagios server, click on the Service Detail link and then click on one of the services (such as HTTP or SSH). On this page you will see the Service Commands box. If you click on one of those links and you get the error – you need the fix!

The fix

Saved by zero right? Sort of. The first thing to check is in the /etc/nagios3/nagios.cfg file. Look for a line like:

check_external_commands=1

If that line is either commented out or set to “0″, change it to look like the entry above. Once you have done that, save and close the file. Now restart Nagios with the command sudo /etc/init.d/nagios3 restart. You’re not done yet.

The next step is to open up the /etc/nagios3/cgi.cfg file. You will most likely find these two lines:

authorized_for_all_services=nagiosadmin

authorized_for_all_hosts=nagiosadmin

After these two lines, you need to add the following:

authorized_for_all_host_commands=nagiosadmin

authorized_for_all_service_commands=nagiosadmin

Save and close this file. Now restart Apache with the command sudo /etc/init.d/apache2 restart.

Still not done. Now you have to make a slight change to /etc/groups. Open up this file, with your favorite text editor, and look for a line that looks like:

nagios:x:127:

Change this line to look like:

nagios:x:127:www-data

Now save and close this file. Almost done. But the next step is a bit more challenging the previous.

In the directory /var/lib/nagios3/rw is a file nagios.cmd. This file most likely doesn’t have the correct permissions set. By default only the user nagios has access to these files. Since we can’t change this to the www-data user (it would break other features of Nagios), we need to just add the executable group bit to the file like so:

sudo chmod g+x /var/lib/nagios/rw

That’s it. Now go back to that Service Commands section and click on one of the links. You should no longer get that error.

Final thoughts

You may have thought your Nagios installation was pretty useful. Now that you can execute external commands, you will find your Nagios install to be far more useful.

Now, the only issue with iptraf is that it is old. It hasn’t been updated since 2005 (as far as I can see). But being old doesn’t mean it isn’t useful. The iptraf tool still installs, still runs, and is just as useful as it was when it was still in active development.  And because iptraf is still a useful tool, I thought I would show how it is installed and used on a modern Linux distribution.

Installation

Installation is simple. The iptraf tool should be found in your distribution’s repositories. So open up Synaptic (or gnome-packagekit, or whatever tool you prefer to install with) and follow these steps:

1. Search for “iptraf” (no quotes).
2. Mark iptraf for installation.
3. Click Apply to install.

That’s it. Now you are ready to run the tool.

Usage

Figure 1

Using iptraf requires root (or sudo) privileges. It also requires a terminal window. So open up your favorite terminal window and issue the command sudo iptraf. When you run iptraf you will see information about the license and copyright. All you need to do is press a key to get beyond this screen.

The next screen you will see is the options window. From here you can just start up a traffic capture or you can set some options and/or filters you can gather general or detailed interface statistics, or you can gather statics on a LAN station.

Let’s take a look at creating a filter for monitoring. Let’s say you want to watch web traffic on a web server. To do this scroll down to the Filters entry and hit Enter. In the next screen you can choose from IP, ARP, RARP, and Non-IP. Select IP and then, from the resulting screen, select Define new filter.

Figure 2

The first step to defining the new filter is to give the filter a description. Enter something like “Web Traffic” (no quotes) and hit Enter.  It will now seem like all you have done is create a filter with no rules (just a name). Hit the i key to Insert rules into your new filter. This new screen (see Figure 2) allows you to enter rules for both source and destination. You have to tab around to get to the fields you want. For the IP address(es) just enter the address, for the ports enter the start and ending numbers, and for the protocols you tab to the protocol you want to include and then hit the “y” key (no quotes).

Once you have saved the filter you will find yourself in the Filter list. Follow these steps to start monitoring with your new filter:

1. Hit <Ctrl>x to get out of this screen.
2. Scroll up to Apply filter.
3. Select the new filter and hit Enter.
4. Exit all of the menus until you are back at the main window.
5. Select IP traffic monitor and hit Enter.
6. Select the interface you want to monitor and hit Enter.

Figure 3

Watch the action (see Figure 3) occur in real time. The nice thing about iptraf is that its small size allows for perfect real-time, constant monitoring on a network from a single machine.

Final thoughts

I realize that iptraf is WAY out of date. But the fact that the tool still works as well as it did when it was in active development say a lot about how well this tool was made. And although many might balk at using a tool that is no longer being worked on, in the case of iptraf it’s not so much an issue. It still works and it still works well.

Networx offers several network and bandwidth related tools in its interface. It will place an icon in the Windows system tray after download that will open a bandwidth monitor on left-click and the context menu with all the available modules on a right-click.

The bandwidth monitor will display useful information about the bandwidth consumption of the computer system including daily, weekly and monthly reports, an overview of today’s bandwidth consumption, custom bandwidth reports, dial up sessions and hourly rates. Data can be exported and backed up among other things. This module gives the system administrator a quick overview of the bandwidth consumption of the computer system.

Networx offers additional modules that aid in the network monitoring process. This includes a speedmeter which can measure the network’s (or network devices) throughput. The tool is basically a speed test that will monitor the current network traffic and display the current, average and maximum transfer rates.

It is furthermore possible to setup a bandwidth quota and trigger alerts if 80% of that quota has been used. It is also possible to configure the quota to only cover upload or download bandwidth and set it to be a daily, weekly or monthly quota.

Ping, traceroute and netstat commands are available from the context menu of the network monitoring software as well. The options are quite extensive and allow the user to select an individual networking adapter to be monitored, configure notifications or define on and off peak hours.

Networx might not be the most sophisticated network monitoring software but it proves to be a nice solution for single computer systems.

Three Network Monitor Experts have been released so far, the last less than two weeks ago. Available are the Simple Search, Top Users and TCP Analyzer Experts. Here is a short summary of what these Experts do:

• Simple Search Lets you search frames and comments for raw ASCII text case-insensitively or with regular expressions.
• Top Users Lets you determine the heaviest users of network bandwidth.
• TCP Analyzer This tool analyzes network traces of Transmission Control Protocol (TCP) connections.

The Experts can be downloaded separately and need to be installed before they can be used. It is recommended to close the Network Monitor before installing the add-ons. The Experts become then available in the Experts menu on top of the program.

Microsoft Network Monitor is offered as a free download for 32-bit and 64-bit editions of the Windows operating system.

The system administrator starts by adding new servers that should be monitored to the application. Required information are a unique name, the server address and the monitoring type. Available monitoring types are TCP / IP, Ping or Services. Additional information might be required depending on the type selected. A TCP / IP monitoring for example requires a port that can be added in the same interface.

The events and notifications that can be configured are quite extensive. Events can be configured for positive and negative results. Positive results – meaning the remote server is up and running – are limited to playing a sound or running a command while negative results (server down) include the additional options to reboot the computer or start a service.

Three notification types are available. The administrator can be informed by email, message box or net message.

Specific parameters for every server can be configured in the Configuration View tab. It allows the user to change the monitoring interval, configure the connection timeout, set the mail server and enable the logging of events.

DreamSys Server Monitor can be used as a Internet or network monitoring software. It lacks some of the features of advanced monitoring applications that offer website or database monitoring but could be enough for administrators who do not need that functionality.

The main interface of the monitoring software is divided into a quick server check option at the bottom which can be used to check if services such as HTTP, FTP or POP are running on a IP or domain. The networking monitoring software will automatically display the results in the same interface. Probably more interesting than the quick check function is the option to add servers permanently to the monitoring tool.

The user needs to enter a server name which is used for identification and a server address which is usually an IP address. A list of services are preselected and will be checked unless unchecked by the user. Alerts can be enabled or disabled in the same interface as well. These do have to be configured separately and can inform the user by email, screen message, executing a program or playing a sound.

There is also the possibility to add http url checks next to the default services that can be monitored by Open Monitor. This allows a user to specify an url, port, alerts and words that the page should include or exclude. This can be used for example to verify that a page is up by checking for a word on that page that is always appearing. Can also be used to run scripts that output a simple message in the end, e.g. a database check that is displaying ok if the database is up and running.

Open Monitor is a straightforward, easy to use, network monitoring software that can monitor popular services. Alternative programs are the server monitoring software IT Service View, Newt, the website monitoring software SiteMonitor or the Microsoft Network Monitor.

The information that are displayed after the network scan are – among others – computer names, IP addresses, operating systems, uptime, MAC address, domain name and user name. The collected data can be sorted in numerous ways. The free version of NEWT has a few limitations like only being able to scan and monitor 25 computers, no SNMP device detection or multithreaded IP Range discovery.

Next to the computer analysis it is possible to send a few commands to remote computers in the network. It is possible to send messages, ping computers or to turn them on or off remotely.

The network monitoring software is compatible with most Microsoft operating systems including Windows XP, Windows Vista and even the soon to be released Windows 7.

Update: The network monitoring software Newt has been discontinued by its developers. The project page simply states that NEWT Freeware is no longer available, and that its developers are dedicating all resources to the professional version of the applications.

If you are looking for a network monitoring software you may want to check out the following alternative which we have reviewed in the past:

York – A network monitor for the Windows operating system that automatically filters traffic into groups such as passwords, files or pictures. The program supports logging into files and saving of the raw network traffic into a file for further analysis.

The program is compatible with all 32-bit and 64-bit editions of the Microsoft Windows operating system. Just follow the link above to find out more about it.

The network monitoring software can be used to monitor incoming and outgoing traffic of selected network adapters and connections. Traffic is shown in graphs and logged in files for further processing. Reports are extensive and provide access to daily, weekly, monthly and custom reports. Even better, reports can be exported which can be useful for book keeping among other things.

A realtime network traffic graph visualizes the incoming and outgoing traffic. Notifications can be used to perform actions under certain conditions. They can be configured to run so called actions when network traffic drops or rises above a certain level. Possible actions include running a software, displaying visual and sound alerts or to drop the connection.

Networx includes network information and testing tools like netstat that display all applications currently using the network connection. The application uses roughly 10 Megabytes of computer memory in minimized state.

Update: The program sits quietly in the system tray of the operating system for most of the time it is running. A right-click brings up the program menu where you can display or hide the traffic graph on the desktop and access other tools and settings.

The application is offered as a setup and portable version, ideal for users who carry an USB or troubleshooting DVD with them around when they to tech support. The latest program version is compatible with all recent versions of the Microsoft Windows operating system, including 32-bit and 64-bit editions of Windows Vista and Windows 7, as well as Windows server variants.

NetWorx 5 has been released in 2009, check out our review of the new version here.

One of the main features of the network monitoring software is the ability to automatically scan a network based on a network adapter, device or selected IP range. The software will scan the selected range and display all connected computers and devices after the fast scan.

A click on a network device opens the Proof Scan window that provides access to information such as Ping, Traceroute, Netbios, running network services and SNMP System plus additional links to advanced traceroutes and a graphical ping view.

Other features of the network monitoring software include a hierarchical view of the network, a search, graphs and network trapping. Trapping will alert the user of network status changes, for example when a network device goes online of offline.

One network management feature is the generation of network status reports which are saved in html format. They display a detailed view of IP addresses including graphs.

Update: The Look@Lan project has been discontinued by its developers. The program is no longer updated and likely not compatible anymore with more recent versions of the Windows operating system.

The developers have released a new program called Fing, which in its current development stage, is a command line utility for network and service discovery. Unlike Look@Lan which has only been available for windows, Fing is also available for other platforms including various Linux distributions, Mac OS X and the Android operating system.

Fing’s developers have promised that they will release a version with a graphical user interface in the future. The latest version of Fing can be downloaded from the project homepage. I suggest you check out the documentation posted there to get started.