DNS hosts hold the zones for their domains and answer requests with the records from the zones for those domains. When you make changes to your zone, you are making changes to the host. A resolver is a DNS server that will send requests to other DNS servers for the records from their zones to answer the requests that it receives. These sorts of requests are called recursive requests. When you connect to the internet through your Internet Service Provider ISP , your ISP will provide you with two or more resolvers responsible for handling the recursive DNS requests sent by your computer as you use the internet.
Since most DNS records don't change very often, most resolvers are configured to cache or store the results of previous lookups and respond to subsequent requests from the cached results for a period of time until the resolver decides that the cached copy is too old to be trusted.
Propagation is the period it takes for the record cached on all resolvers everywhere to expire. One technique to reduce the time it takes for changes to propagate is to reduce the TTL value in the current zone before making changes; however, the change in the TTL on the record itself will take the length of time specified in the original TTL value to propagate before propagation period is lowered for further changes. Every time you use a domain name on your computer its DNS information will be stored in the cache.
Even if a record is changed, DNS servers will continue working with its formal value from cache until this time has passed. This is the essence of DNS propagation — it is the time required for DNS servers worldwide to update their cached information for a domain name. It is influenced by the TTL of DNS records that might have changed, but there are also other factors that could come into play.
Some internet service providers ignore the TTL of the DNS settings in order to reduce the load on their servers and increase users' surfing speed. Instead, they update the DNS records only once every two to three days. If this is the case, it can take longer for the changes to take effect. Help Center. After the change has been made, we put the TTL back to 4 hours. Most applications see the change right away, but a few users will call in with problems and need to reboot.
The Wikipedia article also has a good discussion on "propagation": "Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. You can see these TTL values for yourself with the nslookup command. I always tell my users it'll be 48 hours for full propagation, to compensate for all the issues mentioned above. Besides the TTL something that you control, see Brian Clapper's excellent advice , and possible longer caching times inside some applications, there is also the synchronization time between the authoritative name servers.
If you're talking Windows and you're talking internal, it depends on the original TTL. When we knew ahead of time that we were going to make a change, we would set the TTL on the A record low Then once the change was made, we increased the TTL back to a more normal amount.
If you're talking about on the Internet, all bets are off. There are some caching domain controllers that we have seen completely ignore TTL, as already mentioned. In those cases we've gone with a general rule of 48 hours. I've seen on average hours for most people. However, i still use 7 days as a rule of thumb for a complete changeover. My experience is that DNS changes can take upwards of eight or more hours, but this is all dependant on how long a client will cache their DNS settings.
Most clients will work with the TTL that you set. I recently changed the IP addresses of our web sites. We had to leave servers up and running on the old IP addresses for weeks in order to respond to requests. It could be greater than the TTL of the replaced record: Many clients ignore the TTL when its too low, or bound it to some other value like an hour.
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