When DNS changes are made they occur instantly on our nameservers. However, it may be some time before your computer (or your customer’s computers) see the changes. This is because every DNS record has something called a TTL (Time TLive) value. The TTL tells other DNS servers how long to cache a DNS record for before refreshing their local data – it’s important because it controls the amount of DNS traffic on the internet.

Normally DNS records are set with a TTL value of 14400 (it’s in seconds – so this is 4hrs), but some DNS records have a 24Hr TTL value (86400) so it can take up to a day to see the changes take effect.

The TTL delay is compounded by the fact that your operating system (Windows and Macs) both cache DNS data to reduce the number of requests they make to your ISP. On top of that, web browsers also create a local cache.

The knock on effect is that new DNS data can be delayed even longer.

Clearing Windows’ DNS cache

  • Windows 7 – open a command prompt (clicking the Start button, entering cmd and pressing <Return>)
  • Windows 8 – by right clicking the Start screen, and clicking All Apps. Look for the Windows System section, and click on Command Prompt.

Enter the following command (you must be logged in as an administrator) and press the <return> key.

 

ipconfig /flushdns

 

To make sure you are seeing the correct IP addresses for your website, you can see the contents of your local cache with:

 

ipconfig /displaydns

 

Clearing Mac OS X DNS cache

Open the Terminal application from the Applications/Utilities folder or search for it using Spotlight. Depending on which version of Mac OSX you are running, you should type in the following commands, and press the <return> key. You may be asked to enter an administrative username and password – do so, as this is an administrator level task.

For OSX prior to 10.5.1 (Leopard)

sudo lookupd -flushcache

 

Mac OSX 10.5 and 10.6 (Leopard and Snow Leopard)

 

sudo dscacheutil -flushcache

 

You can obtain a full list of all entries in the cache using this command.

 

sudo dscacheutil -cachedump -entries

 

Mac OSX 10.7 and 10.8 (Lion and Mountain Lion)

 

sudo killall -HUP mDNSResponder

 

Clearing Ubuntu/Linux DNS cache

Open the terminal application and enter this command, and press the <return> key.

 

sudo /etc/init.d/nscd restart

 

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