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I've been told to assume it takes as long as 48 hours for a DNS change to propagate throughout the entire Internet, because some DNS servers cache their entries for longer than my TTL.

However, for years and across ISPs and domains, every time I've made a DNS change I see the effects within a couple of hours.

Is it still true that I need to assume a full two days for everyone to see my changes?

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In particular, web crawlers tend to cache DNS for a very long time. I've seen the Baidu spider still hitting the old address three weeks after the TTL expired. I've never seen the Googlebot take more than a couple of days but that's on TTLs of 15 minutes. –  Ladadadada Apr 11 '12 at 21:57
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DNS doesn't propagate. There are issues relating to caching and TTL but that's not propagation. –  John Gardeniers Apr 11 '12 at 23:02
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They DO. There are issues related to caching and the common word for it is propagation. ;) –  Sandman4 Apr 12 '12 at 14:58

3 Answers 3

up vote 7 down vote accepted

It takes until the last cache holding the old data expires and that server fetches the new data.

You have limited control over that with the TTL value on the records, but there are ISPs who disregard cache times, cache everything for the SOA expire or refresh time, assign their own arbitrary value (AOL used to be famous for caching everything for 1 day regardless of any other directives), and probably a few broken implementations that always re-query the data.

Bottom line: It takes as long as it takes.
48 hours is a good rule of thumb that has served the internet well.

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AOL is no longer a major ISP. Is disregarding TTL in widespread practice by current network operators? –  spiffytech Apr 13 '12 at 19:54
    
You'd be surprised how many people in the world still use dial-up ISPs. In any case, blatantly violating the DNS RFCs by ignoring TTLs is still common enough that I take it into account when making DNS changes and decommissioning old services. I always suggest to others that they do the same, in the spirit of "Don't Break The Internet (even if the people it would break for are idiots)". –  voretaq7 Apr 13 '12 at 19:59

Yes, and in some cases it can be much longer.

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I think it's a 80/20 problem. Don't spend 80% of your energy trying to figure out how 20% of Internet users will get to your site. Stick to RFCs and you've done your job. The rest is beyond your control anyway.

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