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Apple Pro Training Series: OS X Support Essentials 10.10: Supporting and Troubleshooting OS X Yosemite: General Network Troubleshooting

From the book

Reference 22.1 General Network Troubleshooting

The most important thing to remember about troubleshooting network issues is that it is often not the computer’s fault. There are many other points of failure to consider when dealing with LAN and Internet connection issues. So the second most important thing to remember about troubleshooting network issues is that you need to isolate the cause of the problem before attempting generic resolutions.

To help isolate network issues, you can categorize them into three general areas:

  • Local issues—These are usually related to either improperly configured network settings or disconnected network connections.
  • Network issues—These are by far the hardest to pinpoint, as there could be literally hundreds of points of failure involved. It always helps to be familiar with the physical topology of your network. Start by checking the devices that provide network access closest to your Mac. Something as simple as a bad Ethernet port on a network switch can cause problems. As you move on to investigating devices further away from your Mac, you will find that it’s often easiest to start your investigation using the network diagnostic utilities included with OS X.
  • Service issues —These issues are related to the actual network device or service you are trying to access. For example, the devices providing DHCP or DNS services could be temporarily down or improperly configured. It’s often easy to determine if the problem is with the service alone by testing other network services. If the other network services work, you’re probably not dealing with network or local issues. Again, OS X provides some useful diagnostic tools for testing service availability. Troubleshooting network services is also covered in Lesson 23 “Network Services”.

You will be using three main tools for diagnosing network issues in OS X: Network preferences, Network Diagnostics, and Network Utility.

Network Preference Status

One of the diagnostic tools you should always check first is Network preferences. Network preferences features a dynamically updating list that shows you the current status of any network interface. If a network connection is not working, you will find out about it here first.

Network status indicators are:

  • Green—The connection is active and configured with TCP/IP settings. This, however, does not guarantee that the service is using the proper TCP/IP settings.
  • Yellow—The connection is active but the TCP/IP settings are not properly configured. If you are still experiencing problems with this service, double-check the network settings. If the settings appear sound, move on to the other diagnostic utilities.
  • Red—This status usually indicates either improperly configured network settings or disconnected network interfaces. If this is an always-on interface, check for proper physical connectivity. If this is a virtual or Point-to-Point Protocol connection, double-check the settings and attempt to reconnect.

Common Network Issues

A good starting point for resolving network issues is to quickly verify some of the most common causes. You can think of this list as items you should check every time you’re having an issue. To put it another way, verify common issues before hunting down exotic ones. This includes verifying Ethernet connectivity, Wi-Fi connectivity, DHCP services, and DNS services.

Ethernet Connectivity Issues

For well over a century, those supporting any electronic device have heeded these words: “Check the cable first!” If you’re using an Ethernet connection, always verify the physical connection to the system, and if possible verify the entire Ethernet run back to the switch. If that’s not possible, try swapping your local Ethernet cable or use a different Ethernet port.

You should also verify the Ethernet status from Network preferences, as detailed in the next section. Also, keep an eye out for substandard Ethernet cabling or problematic switching hardware. A symptom of these issues would be a large amount of packet errors, which you can verify with Network Utility, as covered later in “Reference 22.2 Network Utility”.

You may also find that while the Ethernet switch registers a link, Network preferences still shows the link as down. This issue may be resolved by manually setting a slower speed in the advanced hardware settings of Network preferences, as covered in Lesson 21 “Advanced Network Configuration”.

Wi-Fi Connectivity Issues

A modern version of “Check the cable first!” would certainly be “Check the Wi-Fi first!” After all, when using Wi-Fi networking, the wireless signal represents the “physical” network connection. Start by verifying you are connected to the correct SSID from the Wi-Fi status menu or Network preferences. Note that the Wi-Fi status menu in the following screenshot sports an exclamation point (!), indicating there is a problem with the wireless network.

The Wi-Fi status menu can also serve as a diagnostic tool if you hold down the Option key when choosing this menu item. This view shows connection statistics for the currently selected Wi-Fi network. Of particular note is the Transmit Rate, which shows (in megabits per second) the current data rate for the selected Wi-Fi network. The Wi-Fi status menu is capable of other diagnostic tricks, including helping you quickly identify network issues and opening the Wireless Diagnostics application.

The Wireless Diagnostics application requires administrative authentication, but it can collect a huge amount of information about the Mac system’s wireless and network configuration. Opening the Wireless Diagnostics application reveals an assistant interface. The primary goal of this assistant is to create a compressed archive of all the relevant files that would help experienced technical support staff diagnose a tricky connection issue. Of course, you can certainly expand the archive generated by the Wireless Diagnostics application and explore the contents on your own. However, the details of the collected items are beyond the scope of this guide.

You’ll also find a variety of advanced wireless network utilities in the Wireless Diagnostics application. Anytime you have the application open, you can reveal these additional utilities from the Window menu.

Again, use of these advanced wireless tools is beyond the scope of this guide. However, when working with wireless vendors or support specialists in trying to resolve tricky wireless issues, these wireless utilities are extremely valuable. For example, the Performance window provides a real-time view of the radio signal quality. With the wireless performance utility open, you can physically move a Mac portable device around an area to identify wireless “dead zones.”

DHCP Service Issues

Most client network connections are configured automatically via DHCP. If the DHCP server has run out of available network addresses, or there is no DHCP service available, as is the case with small ad hoc networks, the client automatically generates a self-assigned address. Sometimes this automatic assignment of addressing is referred to as “link-local addressing,” but Network preferences shows it as Self-Assigned.

Self-assigned addresses are always in the IP address range of with a subnet mask of The network client automatically generates a random self-assigned address and then checks the local network to make sure no other network device is using that address.

Once a unique self-assigned address is established, the network client can establish connections only with other network devices on the local network. Consequently, a client configured with a self-assigned address may be able to communicate with other devices on the LAN, but it doesn’t have access to WAN or Internet resources.

DNS Service Issues

Aside from TCP/IP settings, DNS is a requirement for most network services. As always, you should start by verifying the DNS server configuration in Network preferences. Remember, in most cases the topmost network service interface is the primary one, and as such is used for all DNS resolution. The exception is if the primary network service is lacking a router configuration, in which case DNS resolution falls to the next fully configured network service interface.

Though it’s rare, the OS X DNS resolution services can sometimes cache out-of-date DNS information and return inaccurate results. If you suspect your DNS issues are due to old information, you can either restart the system or flush the DNS service caches. You can find out more about this process from Apple Support article HT202516, “OS X: How to reset the DNS cache.”

Network Diagnostics

OS X includes Network Diagnostics Assistant to help you troubleshoot common network issues. Some networking applications automatically open this assistant when they encounter a network issue. You can also open it manually by clicking the Assist Me button at the bottom of Network preferences, and then clicking the Diagnostics button.

Network Diagnostics Assistant asks you a few simple questions about your network setup, and then, based on your answers, it runs a battery of tests to determine where the problem might be occurring. Test results are displayed using colored indicators on the left side of the window. If there are problems, the assistant makes suggestions for resolution.

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