Can DNS propagation last less than 24 hours?

Question

Can DNS propagation last less than 24 hours? Most websites say it lasts 24-48 hours, does that mean that it CAN'T last less than 24 hours?

Answer 1

"DNS propagation" means that most ("practically all") nameservers that people on the internet might be using will return the new/changed/current information. This is made up of two times:

1. The time from making the change on the relevant administrative interface until the change is actually made on the authoritative nameserver.

   This can be instantaneous: For example, when I want to change a record for my private domain, the appropriate interface actually is the nameserver config with its eight or so entries.

   Or it can be very slow, because it's a multi-stage process: To change nameserver addresses, for example, you may be updating a registrar's database (let's say joker.com or gandi.net); from the registrar, changes are generally only periodically (every hour, or every eight hours, or whatever) sent to the registry's main database (to Verisign to update .com); from their database, they need to feed the information to the actual nameservers that deal with the zone, which again happens periodically, anywhere between hourly and daily.

2. The time for the change to be known to a particular user's/ISP's nameserver.

   In the best case, this is immediate after step 1. If the ISP has never seen a name before, the standard recursive algorithm will be used to look it up, and the authoritative server will return the freshly updated or added value.

   In the worst case, the ISP's server has previously looked up that name, and will cache the result. Then you get to wait until that cache entry expires; how long that takes depends on the entry's "time to live" (the TTL is a property of the record; setting it is completely up to whoever maintains the entry in the authoritative server) or, if the name did not exist, the "negative cache ttl" set for its containing zone (maintaining that is the responsibility and sole decision of whoever runs that zone).

   An even worse case is when the TTL gets incorrectly handled (too long and tedious for now, but it might take twice as long to expire as it's set - theoretically even longer, but that gets quite rare quite quickly).

   A yet worse case is when of the multiple authoritative servers only one gets updated, and the one that ends up getting asked has problems updating itself - theory aside, all reasonably-large service providers will monitor that and fix or shut off malfunctioning servers, so this may add a few hours for this to happen plus a full TTL if the request got the old, incorrect reply just before that maintenance/fix happened.

If you assume reasonable-sounding values for step 1 (up to 6 hours for registrar-to-registry, daily reload by registry) and common values for TTLs (at least one minute, technically, but commonly between 5 minutes for hosts like www.google.com, who use different addresses for load balancing and traffic management, and anything between 1 and 24 hours for fully-hosted websites), you end up with "between 24 and 48 hours, for most people on the internet".

You may get lucky and experience just a few minutes between making your change and everybody in the world seeing it, or you may get particularly unlucky and have your change ignored by a completely unrelated malfunctioning ISP somewhere, who's users won't see your change until next week.