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CHapTER 31 Troubleshooting Network Issues
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Most services allow the administrator to specify a port number other than the
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default. If the service does not respond to the default port number, verify that the service has not been configured to use a different port number. You can run Netstat on the server to list listening ports. For more information, see the section titled Netstat earlier in this chapter.
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For a list of common port numbers, see the section titled How to Troubleshoot Network Connectivity Problems later in this chapter .
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Testing Service Connectivity
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After you have identified the port number for the service, you can use PortQry to test connectivity to that service . To test connectivity to a service, open a command prompt and run the following command .
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portqry n destination e portnumber
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For example, to test HTTP connectivity to www.microsoft.com, type the following command at the command line .
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portqry -n www.microsoft.com e 80
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This command produces output similar to the following .
Querying target system called: www.microsoft.com Attempting to resolve name to IP address... Name resolved to 10.209.68.190 TCP port 80 (http service): LISTENING
The destination might be a host name, computer name, or IP address . If the response includes LISTENING, the host responded on the specified port number . If the response includes NOT LISTENING or FILTERED, the service you are testing is not available .
note
Netcat is a great non-Microsoft tool for testing connectivity to specific ports or de-
termining on which ports a computer is listening for connections. Netcat is an open-source tool freely available from http://netcat.sourceforge.net/.
Determining available Remote Management protocols
When troubleshooting a computer remotely, you might need to determine which remote management protocols are available . PortQry can test the default port numbers for common remote management protocols and identify which protocols are available . To determine which management protocols are available on a remote host, open a command prompt and run the following command .
portqry n destination o 32,139,445,3389
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CHapTER 31
This command queries the remote host to determine whether Telnet Server, NetBIOS, Common Internet File System (CIFS), and the Remote Desktop are available . diReCt FRoM tHe SoURCe
Specifying the Source Port
Tim Rains, program Manager
Windows Networking
he portqry sp option allows you to specify which source port you want to use for the connectivity test. Use this parameter to specify the initial source port to
use when you connect to the specified TCp and UDp ports on the destination computer. This functionality is useful to help you test firewall or router rules that filter ports based on their source ports.
The following PortQry output indicates that the remote system will respond to NetBIOS, CIFS, and Remote Desktop requests, but not to Telnet requests .
Querying target system called: 192.168.1.200 Attempting to resolve IP address to a name... IP address resolved to CONTOSO-SERVER TCP port 32 (unknown service): NOT LISTENING TCP port 139 (netbios-ssn service): LISTENING TCP port 445 (microsoft-ds service): LISTENING TCP port 3389 (unknown service): LISTENING
diReCt FRoM tHe SoURCe
Why PortQry Is great
Tim Rains, program Manager
Windows Networking
he real advantage that portQry has over Telnet Client and other such tools is the support for UDp-based services. Telnet Client can help test connectivity only on
TCp ports, but you can use portQry to test UDp ports as well as TCp ports. The UDp ports that portQry can test include Lightweight Directory access protocol (LDap), Remote procedure Calls (RpCs), DNS, NetBIOS Name Service, Simple Network Management protocol (SNMp), Microsoft Internet Security and acceleration (ISa) Server, Microsoft SQL Server 2000 Named Instances, Trivial File Transfer protocol (TFTp), and Layer Two Tunneling protocol (L2Tp).
CHapTER 31 Troubleshooting Network Issues
Route
All IP-based networked devices, including computers, have routing tables . Routing tables describe the local network, remote networks, and gateways that you can use to forward traffic between networks . In networks with a single gateway, the routing table is very simple and indicates that local traffic should be sent directly to the local network, whereas traffic for any network other than the LAN should be sent through the gateway . However, some networks have multiple gateways . For example, you might have two gateways on a LAN: one that leads to the Internet and another that leads to a private network . In that case, the local computer s routing table must describe that specific networks are available through the internal gateway and all other networks are available through the Internet gateway .
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