Diagnosing and resolving name resolution issues


When you can reach a destination using an IP address but not reach a host using a host name, you might have a problem with host name resolution. Typically, name resolution issues have to do with improper configuration of the DNS client or problems with DNS registration. You can use the following tasks to troubleshoot problems with DNS name resolution:

  • Verify DNS configuration
  • Test DNS name resolution with the ping tool
  • Use the nslookup tool to view DNS server responses
  • Display and flush the DNS client resolver cache

On the computer having DNS name resolution problems, verify the following information:

  • Host name
  • The primary DNS suffix
  • DNS suffix search list
  • Connection-specific DNS suffixes
  • DNS servers

You can obtain this information by entering ipconfig /all at a command prompt. To obtain information about which DNS names should be registered in DNS, enter netsh interface IP show DNS.

Computers running windows vista and Windows Server 2008 support DNS traffic over IPv6. By default, IPv6 configures the well-known site-local addresses of DNS servers at FEC0:0:0:FFFF::1, FEC0:0:0:FFFF::2, and FEC0:0:0:FFFF::3. To add the IPv6 addresses of your DNS servers, use the properties of the Internet Protocol Version 6 (TCP/IPv6) component in Network Connections or the netsh interface IPv6 add DNS command. To register the appropriate DNS names as IP address resource records with DNS dynamic update, use the ipconfig /registerDNS command. Computers running windows xp or Windows Server 2003 do not support DNS traffic over IPv6.

TCP/IP checks the DNS client resolver cache before sending DNS name queries. The DNS resolver cache maintains a history of DNS lookups that have been performed when a user accesses network resources using TCP/IP. This cache contains forward lookups, which provide host name to IP address resolution, and reverse lookups, which provide IP address to host name resolution. After a DNS entry is stored in the resolver cache for a particular DNS host, the local computer no longer has to query external servers for DNS information on that host. This enables the computer to resolve DNS requests locally, providing a quicker response.

How long entries are stored in the resolver cache depends on the time to live (TTL) value assigned to the record by the originating server. To view current records and see the remaining TTL value for each record, type ipconfig /displayDNS in an elevated command prompt. These values are given as the number of seconds that a particular record can remain in the cache before it expires. These values are continually being counted down by the local computer. When the TTL value reaches zero, the record expires and is removed from the resolver cache.

Occasionally, you'll find that you need to clear out the resolver cache to remove old entries and enable computers to check for updated DNS entries before the normal expiration and purging process takes place. Typically, this happens because server IP addresses have changed and the current entries in the resolver cache point to the old addresses rather than the new ones. Sometimes the resolver cache itself can get out of sync, particularly when DHCP has been misconfigured.

Note Skilled administrators know that several weeks in advance of the actual change, they should start to decrease the TTL values for DNS records that are going to be changed. Typically, this means reducing the TTL from a number of days (or weeks) to a number of hours, which allows for quicker propagation of the changes to computers that have cached the related DNS records. After the change is completed, Administrators should restore the original TTL value to reduce renewal requests.

In most cases, you can resolve problems with the DNS resolver cache by either flushing the cache or reregistering DNS. When you flush the resolver cache, all DNS entries are cleared out of the cache and new entries are not created until the next time the computer performs a DNS lookup on a particular host or IP address. When you reregister DNS, Windows Server 2008 attempts to refresh all current DHCP leases and then performs a lookup on each DNS entry in the resolver cache. By looking up each host or IP address again, the entries are renewed and reregistered in the resolver cache. You'll generally want to flush the cache completely and allow the computer to perform lookups as needed. Reregister DNS only when you suspect problems with DHCP and the DNS resolver cache.

You can test DNS name resolution by pinging a destination using its host name or fully qualified domain name (FQDN). If an incorrect IP address is shown, you can flush the DNS resolver cache and use the nslookup tool to determine the set of addresses returned in the DNS Name Query Response message.

You can use the IPCONFIG command to flush and reregister entries in the DNS resolver cache by following these steps:

  1. Start an elevated command prompt.
  2. To clear out the resolver cache, type ipconfig /flushDNS at the command line.
  3. To renew DHCP leases and reregister DNS entries, type ipconfig /registerDNS at the command line.
  4. When the tasks are complete, you can check your work by typing ipconfig /displayDNS at the command line.

To start nslookup, enter nslookup at a command prompt. At the nslookup > prompt, use the set d2 command to get detail information about DNS response messages. Then, use nslookup to look up the desired FQDN. Look for A and AAAA records in the detailed display of the DNS response messages.

With IPv6, the DNS client maintains a neighbor's cache of recently resolved link-layer addresses as well as a standard resolver cache. To display the current contents of the neighbor cache, enter netsh interface IPv6 show neighbors. To flush the neighbor's cache, enter netsh interface IPv6 delete neighbors.

For IPv6, the DNS client also maintains a destination cache. The destination cache stores next-hop IPv6 addresses for destinations. To display the current contents of the destination cache, enter netsh interface IPv6 show destinationcache command. To flush the destination cache, enter netsh interface IPv6 delete destinationcache.

Please Donate

RSS Feeds


Copyright © 2023 ashkerala.com. All Rights Reserved.
Google+ Click to listen highlighted text! Powered By GSpeech