Information Technology Reference
In-Depth Information
You can test SMTP, IMAP, and POP connectivity using any Telnet application that allows a port number
to be specified. Telnet to the IP address of the destination server using ports 25, 143, and 110
respectively. You should see a hello message, which indicates that you have connectivity to that server.
Telnet
If the Telnet to a particular server fails from one host, try connecting from a router and several other
devices. If when Telnetting to a server you do not receive a login prompt, you will want to check the
following:
•
Are you able to do a reverse DNS lookup on the client's address? Many Telnet servers will not allow
connections from IP addresses that have no DNS entry. This is a common problem for
DHCP-assigned addresses in which the administrator has not added DNS entries for the DHCP
pools.
•
It is possible that your Telnet application cannot negotiate the appropriate options and therefore will
not connect. On a Cisco router, you can view this negotiation process using
debug telnet
.
•
It is possible that Telnet is disabled or has been moved to a port other than 23 on the destination
server.
Troubleshooting Domain Name Server Problems
It is possible for IP connectivity to work but for DNS name resolution to fail. To troubleshoot this
situation, use one of the following methods to determine whether DNS is resolving the name of the
destination:
•
Ping the destination by name, and look for an error message indicating that the name could not be
resolved.
•
If you are working on a UNIX machine, use
nslookup
<fully-qualified domain name>
to perform
a DNS lookup on the destination. If it is successful, the host's address should be displayed:
unix%
nslookup www.somedomain.com
Server: localhost
Address: 127.0.0.1
Non-authoritative answer:
Name: www.somedomain.com
Address: 10.1.1.1
If
nslookup
fails, the following output resembles the following sample output:
unix%
nslookup www.somedomain.com
Server: localhost
Address: 127.0.0.1
*** localhost can't find www.notvalid.com: Non-existent host/domain
If DNS correctly resolves the host's name, go to the section “Narrowing Down the Problem Domain,”
earlier in this chapter, to start troubleshooting again. Otherwise, continue troubleshooting as follows:
1.
Determine which name server you are using; this can be found in different places on each operating
system, so if you are unsure of how to find it, consult the device's manual. For examples:
-
On a Cisco router, type
show run
and look for the
name-server
.
-
On Windows 95 or 98, use
winipcfg.exe
.
-
On Windows 2000 or NT, use
ipconfig.exe
.
