Refresh Your DNS Cache

Sometimes, when we have a blog published to a Google Custom Domain, which depends very heavily upon DNS configuration for success, we'll make a change to the domain DNS configuration and notice that our readers can see the change, but we can't.
Once again, at 6:23, someone was able to see my blog and leave a comment

Yet, moments after I tried, I was still getting the 404 error. So there seems to be some intermittent activity!


Let's look at the domain DNS configurations. First, current:

mydomain.com. 14400 IN A 216.21.239.197
www.mydomain.com. 14400 IN A 216.21.239.197

Then, new:

mydomain.com. 14400 IN A 64.233.179.121
mydomain.com. 14400 IN A 72.14.207.121
www.mydomain.com. 14400 IN CNAME ghs.google.com.

A TTL of 14400 seconds (4 hours) is a bit extreme. I see mostly 3600 seconds (1 hour), sometimes 900 seconds (15 minutes), in most DNS configurations. If the TTL for your domain is 4 hours, it could be about that long after you make the change, before you start seeing any change. I say "about", because the age of the configuration information, as cached on your computer or server when you make the change, is relevant. If the information is 15 minutes old, then with a TTL of 14400, you'll have to wait another 13500 seconds, or 3 3/4 hours. If the information is 3 hours old, you'll wait merely another hour.

But, you can hasten that, if the cache on your computer is the bottleneck. From a command window, enter
ipconfig /flushdns
If the DNS cache in question is on your computer, it's gone now and the next time that you try for your custom domain, it should come up in its new configuration.

Note 2 Caveats
  • If you have a caching non-authoritative DNS server that your computer retrieves DNS directly from, you'll still have to wait. The "ipconfig" command works only under Microsoft Windows, and only on the computer with the cache. If the cache is upstream from you, you're going to have to remain patient.
  • Whether or not your local DNS cache, and your upstream non-authoritative DNS server gets the updates, you still can't predict the update status of the Google non-authoritative DNS servers.


Knowing your DNS setup is a good idea, when working with custom domains. And allowing for DNS latency at Google is essential.

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