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Troubleshooting a DNS Failure Troubleshooting DNS
page 6-2 OmniSwitch Troubleshooting Guide September 2005
Troubleshooting a DNS Failure
Starting the Troubleshooting Procedure
If you try to use DNS resolution and it does not resolve, or connect from the switch with error such as
“unknown host” take the following steps.
Verify IP connectivity from the switch in question to the DNS server by pinging the server (destination) in
question from the switch (source) by its IP address. If successful, move on to layer 7 DNS or Name resolu-
tion issue. If ping fails, verify IP configuration. If ping is successful, verify that UDP port 53 is not being
filtered.
Layer 7 DNS or Name Resolution Issue
First verify that the switch is configured properly by using the show dns CLI command. Using this
command will show you the current settings and whether it is enabled and properly configured.
-> show dns
Resolver is : enabled
domainName : Alcatel.com
nameServer(s): 10.255.10.254
: 11.255.10.254
: 12.255.10.254
If there is more than one server on the network, make sure that the switch is pointing to the proper DNS
server(s).
If it is configured properly, then verify that you can still ping the server(s) by IP address; if successful ping
by name. If a ping by IP works but name doesn’t, verify the spelling of the name and that the proper
domain has been specified (labdevice.Alcatel.com).
If configuration appears ok to this point you may want to look at the DNS server to verify that the name
you are entering is configured in the server and is active, so that it will know how to respond properly. Can
another device use the DNS server to resolve the name in question? What about resolution of names in
other domains? If the server configuration appears ok, and other devices work with that server, then you
can take a trace (sniff) to see if the request is being sent to the server and what the server is responding
with.
A proper request and response will look similar to the following:
• In the request you should see a DLC header that has the Mac address of the switch as the source and
the MAC address of the DNS server as the destination.
• Next you will see the IP header, which should state that the protocol is UDP (17), the source IP address
should be the switch, and the destination IP address should be the DNS server.
• Next you have a UDP header that should have the DNS destination port of 53 (source port would vary).
The last portion is the DNS header, which should indicate the ID number (the response will have the
same number); it will show you the name the switch is asking to resolve to an IP address.