Network Troubleshooting
Updated September 12, 2002
Created June 1, 2001

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Notes for Linux:

• If you have pings that don't work right, try temporarily moving your /etc/resolv.conf out of the way.



• Check your hostname


You should be able to execute the following command:

hostname

If none of the following commands work then you need to check your hostname:

hostname -a
hostname -s
hostname -d
hostname -f

To fix your hostname problem choose one of the following methods based on how your IP address is configured:


• Static IP Address


If you have a static IP address, then you should have lines in /etc/hosts that reads similar to the following:

127.0.0.1    localhost.localdomain  localhost
192.168.0.1  server1.mydomain.org   server1

• Dynamic IP Address


If you have a dynamic IP address, then you should have a line in /etc/hosts that reads similar to the following:

127.0.0.1    server1.mydomain.org   server1 localhost

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Windows

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Linux

/sbin/ifconfig
ifconfig
ifconfig -a
netstat -rn
netstat -r
netstat
route
ping -c 2 -b 192.138.0.255 | sort; arp -a
ping
arp
traceroute
telnet / ftp / rsh / ssh / http

• ping disappears until about 30 sequences
- If a ping doesn't show any progress until about 30 sequences, then starts showing about 1 per second, then check /etc/resolv.conf. Most likely the dns server is inaccessible - even if you are only pinging an IP and not a name. Also you may notice before you get to the 30 sequences you will have 50%, 75%, and probably 90% failure rates if you stop early and examine the results. Rename /etc/resolv.conf and see if your pings get better.
• ping shows high failure rate
- see the above entry. Check /etc/resolv.conf, you are probably not able to get to the nameserver listed in /etc/resolv.conf. Rename /etc/resolv.conf and see if your pings get better.

General Notes on Network Troubleshooting:

1. Do you have an IP address?



• (Linux/Unix): ifconfig
• (Unix): ifconfig -a
• (NT): ipconfig
• (Win 9x): winipcfg


2. Can you ping others in your local subnet?



• ping 192.168.0.1

Note: If the above command doesn't work in Linux, then rename or move /etc/resolv.conf out of the way temporarily.


3. Can you ping your local broadcast?



• (Linux): ping -b 192.168.0.255
• (Win/NT): ping 192.168.0.255


4. Do you have one and only one default gw listed in your route table



• (Linux/Unix): netstat -rn
• (NT / Win9x): route print


5. Can you ping the IP of the default gateway?



• ping 192.168.0.1


6. Can you numerically ping something on the other side of the default gateway?



• ping 192.168.20.1


7. Is your system correctly pointing to a nameserver?



• (Linux/Unix): cat /etc/resolv.conf
• (Win9x): winipcfg
• (NT): ipconfig /all


8. Can you ping the IP of the nameserver?



• ping 192.168.0.100


9. Can you ping other systems by name?



• ping www.geocities.com

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Linux General Notes:

date

date newdate

hwclock --utc --systohc