Windows Server 2008 R2 : Troubleshoot IPV6 (part 2) - Check the Routing Table for IPV6, Flush the DNS Cache

3. Check the Routing Table for IPV6

If you find yourself unable to connect to remote resources using IPV6, one of the things you will want to check is the routing table. Specifically, you will be looking for routes that have been incorrectly identified or erroneously entered into the routing table. Use ROUTE PRINT, as shown in Figure 4.

NETSTAT -R
NETSH INTERFACE IPV6 SHOW ROUTE

Each of these commands will show you the IPV6 entries on the routing table. To correct or enter a missing route, you will need to use the following:

NETSH INTERFACE IPV6 SET ROUTE, ROUTE ADD, or ROUTE CHANGE.

It is also possible to remove erroneous or incorrect routes using this:

NETSH INTERFACE IPV6 DELETE ROUTE command or ROUTE DELETE.

In each of these cases, it is important that you have a clear understanding of what the correct routing table entries should look like and that you are able to recognize entries that are not correct or are simply not there. As we discussed earlier, you really need a good knowledge of the way things are supposed to work in your network infrastructure in order to troubleshoot them effectively.

Figure 4. Routing table displayed using ROUTE PRINT

4. Validate DNS Name Resolution for IPV6 Addresses

If the IPV6 addressing configuration and response checks out, you will want to move up and check on the resolution of host names to TCP/IP addresses, which means DNS. DNS resolves host names to IP addresses for both IPV4 and IPV6. You can perform some simple tasks to ensure that IPV6 host name to IP address resolution is occurring properly.

First verify that your DNS server has been configured to resolve host names to IPV6 addresses and that it is acting upon name resolution requests that it receives. To begin, use the HOSTNAME utility to check the host name of the server and to check the DNS suffix.

Next, open the DNS Manager tool, and verify that all your configured DNS servers appear on the DNS Manager's list of authoritative servers. You can also use the DNS Manager to check the process of forwarding in the event that a host name cannot be resolved to an IP address on the local DNS server. If you need to make changes to the DNS suffix or to connection-specific DNS suffix information, you can do it using DNS Manager.

5. Flush the DNS Cache

Each IPV6 client maintains a list of recently resolved DNS to IPV6 addresses. This list is called the DNS resolver cache. If for some reason a record in the cache had an incorrect address for a given host name, it would limit connectivity. In cases like this, you can flush the contents of the DNS resolver cache using IPCONFIG /FLUSHDNS. See Figure 5.

Figure 5. Results of IPCONFIG /FLUSHDNS

This command will remove all entries from the cache and force the machine to resolve the address with recursive queries sent to the local DNS server hierarchy and get the correct host name to IP address information.

You can quickly check for the function of DNS resolution using the PING tool. PING can be used in conjunction with IP addresses, host names, or FQDNs. For example:

PING Computer 1

or

PING www.microsoft.com

6. Test IPV6 TCP Connections

So, what if everything works from an IP perspective but you still cannot get a TCP connection to occur between systems? In the majority of cases, this is a problem with packet filtering. You will need to check the same locations for TCP filtering. Since you will be checking your filters when you are validating IP connectivity, it makes sense to check for TCP filters at the same time. If you didn't check for TCP filtering earlier, it is time to do it now.

One of the easiest ways to check TCP connections is with the TELNET tool. TELNET is a command-line tool that establishes TCP connections between systems. TELNET uses a syntax similar to the PING command; simply use the TELNET command followed by the IPV6 address.