Registering and Flushing DNS

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. Once 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, which provides 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, in an elevated command prompt, type ipconfig/displaydns. 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 the resolver cache needs to be cleared out 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.

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. Once 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 Vista 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 that there are problems with DHCP and the DNS resolver cache.

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.