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.
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.
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.
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.. 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.
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.
If the device is configured, you should have access to the net, and it is
time to test where you can go.
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.
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.
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.
Check the plug!
Check for network activity.
Is the interface configured?
lo0: flags=849
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.
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).
Has an attempt been made to configure the device?
Where can you ping?
Can you ping your neighbor?
Can you ping beyond your router?
Can you ping beyond your router by host name?