Unix for Advanced Users - Networking - Dang, the network has gone south!

13. Networking

13.5 Dang, the network has gone south!

Here, we provide a brief checklist of things to check when a machine does not seem to be on the network when it should be.

13.5.1 Check the plug!

Though it seems obvious, the first thing to do is to make sure that the machine is physically connected to the network. Check that the cable is firmly seated at the wall and at your computer.

13.5.2 Check for network activity.

Check to see whether the network interface on your machine sees network traffic. Interface cards usually have lights on them, and one of the should flash as the card detects network traffic. Some setups make use of an adapter (sometimes called a MAU) between the computer and the network cable. The MAU should also have a light that will flash at the presence of network activity.

Lack of network activity indicates that the problem is physical and outside of your computer. The problem could be a bad cable. Try replacing your cable with one that is known to work. If you still see no activity then the problem is either with your ethernet card or the socket into which you are plugging the cable. A known good cable and card will show a bad socket and likewise a known good socket and cable will show a card.

13.5.3 Is the interface configured?

If the network card sees activity, check to see whether the interface has been configured. That is, does the operating system see the card and traffic on it. Run ifconfig with no options. It will list the interfaces that are up and operating. For example, here is sample output from ifconfig from a Sun Sparc station..

lo0: flags=849 mtu 8232
        inet 127.0.0.1 netmask ff000000 
le0: flags=863 mtu 1500
        inet 999.99.99.99 netmask ffffff00 broadcast 999.99.99.255

It shows that two devices are up: lo0 the loopback device and le0 the ethernet device. (Device names vary among Unices.) If the ethernet device is not up it may be for several reasons.

13.5.3.1 Does the kernel recognize the device?

This section does not apply if your networking has been working on this machine in the past (and you haven't changed anything).

The ethernet device cannot be configured if the kernel does not see it properly. You can check to see whether the kernel has detected the device by using dmesg. Look for the appropriate device name in dmesg output. If it is missing, the kernel did not detect the device, and the problem may be that support for the device is not in the kernel or that appropriate kernel modules have not been loaded.

Note that on intel boxes, the kernel may recognize cards even if the irq and io addresses conflict with other devices, but the card will not work.

13.5.3.2 Has an attempt been made to configure the device?

The device may not be up because someone brought it down or because it was not brought up during boot up. If the device definitely ought to be up, you might check the system log for errors. If not, there is no harm in trying to bring it up (see ifconfig and route). If the device comes up, the problem may be solved. If not, be alert for errors at the command prompt that you used to bring it up and the system log.

13.5.4 Where can you ping?

If the device is configured, you should have access to the net, and it is time to test where you can go.

13.5.4.1 Can you ping your neighbor?

Try to ping a host on the same subnet using its numeric IP address. If this fails, your network device may be incorrectly configured. Be sure that your network mask is correct.

A good neighbor to check is your router. It is on your subnet, and the next step will be to try to ping somewhere off of the subnet by going through the router. It will be good to know that the router is up.

13.5.4.3 Can you ping beyond your router?

Try to ping a host outside of your subnet by using its IP address. If this fails, check your routing table to make sure that the default route for packets is to the correct router. Use netstat -r to display routing tables, and if the appropriate route is missing, use route command to establish a route.

If the router is up, and your default gateway is correctly set in the routing table, then there may be a problem with the router itself. Contact the networking authorities to find out if the router is experiencing technical difficulties.

13.5.4.4 Can you ping beyond your router by host name?

If you can ping a host that is beyond the router by IP number but not by hostname, the problem lies in the realm of domain name services.

First, check to see whether the file /etc/resolv.conf contains the IP number of the correct name server or name servers. If so, try pinging the domain name server using its IP number. If you cannot ping the domain name server, contact the networking authorities to be sure that you are using the correct address. If your address is correct, the domain name server may be down.

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