Summary#

Just some notes. I have my days where I do this a lot. But, just in case I don’t and forget down the road. Might as well have this available.

These are generally useful when I’m troubleshooting DHCP issues and IP issues on Windows workstations. I haven’t provided any output. But, if I feel it’s needed. I will soon.

Note: Most commands will need to be executed as Administrator. With the exception to commands that are used to gather information. Depending on policy. Adminstrator privileges may still be needed.

Showing IP Information#

I’ll provide a quick link to how to show your IP interface information on Windows systems soon. But, in brief. If you would like to show your network adapters on Windows devices. The following command can be used.

ipconfig

To see the bigger picture (the full TCP/IP configuration for all adapters). The /all flag can be used to broaden the scope of information provided.

ipconfig /all

Release#

Sends a DHCPRELEASE message to the DHCP server on the network. This will release the current DHCP configuration; discarding the IP configuration for all adapters on the system.

This operation is performed on all adapters that are configured to obtain an IP configuration automatically.

ipconfig /release

If you know what adapter you need to perform these actions on. The following command can be used. Replace "Adapter Name" with the name of your adapter.

ipconfig /release "Adapter Name"

Renew#

Renews the DHCP configuration for all adapters on the system. Will do so for all adapters on the system configured to obtain an IP configuration automagically.

ipconfig /renew

Much like the release flag. You can specify the adapter using the same method. Replace "Adapter Name" with the name of the adapter you’re performing the work on.

ipconfig /renew "Adapter Name"

Show Resolver Cache#

To view the contents of the DNS client resoler cache. The /displaydns flag can be used. This cache contains frequently used DNS queries dynamically. Used for reference before it references the DNS server. This is useful if you need to confirm suspicions of cache poisoning and stale chache entries.

ipconfig /displaydns

FlushDNS#

Flush the DNS resolver cache on the local system. This will discard invalid/broken entries that have been added to the DNS cache dynamically.

This is generally useful when you have a stubborn DNS cache entry or poisoned DNS cache.

One scenario where this can help is if an IPSec tunnel goes down and that tunnel hosts a domain for a sister company that provides services to you. While that sister company also uses their public top-level domain as their internal domain name. When that tunnel goes down. DNS will resolve to the public provider poisoning the cache of your server with a public address that will potentially timeout. So with that being said it’s usually best practice to use a subdomain if you must use your top-level doamin for internal use.

ipconfig /flushdns

Conclusion#

That concludes the notes for today. This is generally a troubleshooting step when you’re troubleshooting DHCP on a Windows network interface. Should be useful in the future.