01-13-2005, 07:07 AM
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Confirmed User
Join Date: Nov 2004
Posts: 541
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Quote:
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Originally Posted by darksoul
altho it might sound weird thats sligthly  |
Is it?
I thought this was about the time it takes for DNS changes to propogate to all the DNS servers.
If it is, sorry 
Quote:
Caching and Time To Live
Because of the huge volume of requests generated by a system like the DNS, the designers wished to provide a mechanism to reduce the load on individual DNS servers. The mechanism devised provided that when a DNS resolver (i.e. client) received a DNS response, it would cache that response for a given period of time. A value (set by the administrator of the DNS server handing our the response) called the time to live, or TTL defines that period of time. Once a response goes into cache, the resolver will consult its cached (stored) answer; only when the TTL expires (or until an administrator manually flushes the response from the resolver's memory) will the resolver contact the DNS server for the same information.
Propagation time
An important consequence of this distributed and caching architecture is that changes to the DNS are not necessarily immediately effective globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.wikipedia.org, and then changes the IP address to which www.wikipedia.org resolves at 12:01pm, the administrator must consider that a person who cached a response with the old value at 12:00pm will not consult the DNS server again until 6:00pm. The period between 12:01pm and 6:00pm in this example is called propagation time, which is best defined as a period of time that begins between whenever you make a change to a DNS record, and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to the DNS: not everyone is necessarily seeing the same thing you're seeing. RFC1537 (http://www.ietf.org/rfc/rfc1537.txt) helps setting it.
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