| Admin Tip #142: Manage the DNS resolver cache with IPCONFIG |
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W2K and XP's workstation DNS client resolves host names to support web browsers, mail clients, and other apps where you used a host name rather than IP addresses. The Windows DNS resolver caches the results of DNS queries including queries that fail to resolve). Each DNS record has a time-to-live value that determines when the record should be refreshed with another query. Until the TTL expires, Windows will respond with the cached query rather than perform a new query.
What this means is the when you're troubleshooting connection and DNS problems, it's important to flush the DNS cache and force Windows to generate a new query. ipconfig can be used to view and/or clear the resolver cache, among other things. The following command displays the contents of the resolver cache :
E:\Documents and Settings\Wayne>ipconfig /displaydns
Windows IP Configuration
Record Name . . . . . : udns1.ultradns.net
Record Type . . . . . : 1
Time To Live . . . . : 37577
Data Length . . . . . : 4
Section . . . . . . . : Additional
A (Host) Record . . . : 204.69.234.1
Record Name . . . . . : udns2.ultradns.net
Record Type . . . . . : 1
Time To Live . . . . : 37577
Data Length . . . . . : 4
Section . . . . . . . : Additional
A (Host) Record . . . : 204.74.101.1
dns.sprintip.com
----------------------------------------
Record Name . . . . . : dns.sprintip.com
Record Type . . . . . : 1
Time To Live . . . . : 230
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 208.25.104.7
dnsla.carsdirect.com
----------------------------------------
Record Name . . . . . : dnsla.carsdirect.com
Record Type . . . . . : 1
Time To Live . . . . : 1719
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 12.129.202.53
ns2.cts-bv.com
----------------------------------------
Record Name . . . . . : ns2.cts-bv.com
Record Type . . . . . : 1
Time To Live . . . . : 1194
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . :
1.0.0.127.in-addr.arpa
----------------------------------------
Record Name . . . . . : 1.0.0.127.in-addr.arpa.
Record Type . . . . . : 12
Time To Live . . . . : 83255
Data Length . . . . . : 4
Section . . . . . . . : Answer
PTR Record . . . . . : localhost
www.eastminsterdallas.org
----------------------------------------
Record Name . . . . . : www.eastminsterdallas.org
Record Type . . . . . : 1
Time To Live . . . . : 2083
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 207.70.181.116
stats.cts-bv.nl
----------------------------------------
Record Name . . . . . : stats.cts-bv.nl
Record Type . . . . . : 1
Time To Live . . . . : 1194
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . :
www.lynchwiles.com
----------------------------------------
Record Name . . . . . : www.lynchwiles.com
Record Type . . . . . : 1
Time To Live . . . . : 420
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 203.15.67.163
auth00.ns.uu.net
----------------------------------------
Record Name . . . . . : auth00.ns.uu.net
Record Type . . . . . : 1
Time To Live . . . . : 1961
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 198.6.1.65
ns-3.amazon.com
----------------------------------------
Record Name . . . . . : ns-3.amazon.com
Record Type . . . . . : 1
Time To Live . . . . : 1961
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . :
ns1.wdc.pnap.net
----------------------------------------
Record Name . . . . . : NS1.WDC.PNAP.NET
Record Type . . . . . : 1
Time To Live . . . . : 859
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 216.52.126.1
is-it-true.org
----------------------------------------
Record Name . . . . . : is-it-true.org
Record Type . . . . . : 1
Time To Live . . . . : 27814
Data Length . . . . . : 4
Section . . . . . . . : Answer
A (Host) Record . . . : 66.34.10.74
Section . . . . . . . : Additional
A (Host) Record . . . : 216.221.160.10
Use this command to flush the resolver cache:
ipconfig /flushdns
To check out all the options:
E:\Documents and Settings\Wayne>ipconfig /?
USAGE:
ipconfig [/? | /all | /renew [adapter] | /release [adapter] |
/flushdns | /displaydns | /registerdns |
/showclassid adapter |
/setclassid adapter [classid] ]
where
adapter Connection name
(wildcard characters * and ? allowed, see examples)
Options:
/? Display this help message
/all Display full configuration information.
/release Release the IP address for the specified adapter.
/renew Renew the IP address for the specified adapter.
/flushdns Purges the DNS Resolver cache.
/registerdns Refreshes all DHCP leases and re-registers DNS names
/displaydns Display the contents of the DNS Resolver Cache.
/showclassid Displays all the dhcp class IDs allowed for adapter.
/setclassid Modifies the dhcp class id.
The default is to display only the IP address, subnet mask and
default gateway for each adapter bound to TCP/IP.
For Release and Renew, if no adapter name is specified, then the IP address
leases for all adapters bound to TCP/IP will be released or renewed.
For Setclassid, if no ClassId is specified, then the ClassId is removed.
Examples:
> ipconfig ... Show information.
> ipconfig /all ... Show detailed information
> ipconfig /renew ... renew all adapters
> ipconfig /renew EL* ... renew any connection that has its
name starting with EL
> ipconfig /release *Con* ... release all matching connections,
eg. "Local Area Connection 1" or
"Local Area Connection 2"