[sf-lug] www.sf-lug.org & sf-lug.org - not looking so good on The Internet

[jim]

Truly clarifying! Thanks. Worth reading for anyone 
learning about DNS. 



On Tue, 2013-02-05 at 20:26 -0800, Rick Moen wrote:
> Quoting Jim Stockford (jim at systemateka.com):
> 
> > http://www.sf-lug.org 
> > better? 
> 
> Jim, as I was trying to say upthread:  _Yes._  Except for people whose
> local nameservers still are supplying the old DNS returned value as
> cached data because it's already there and within the published Time To
> Live TTL) of 7200 seconds = 2 hours.
> 
> Herewith, one of my periodic efforts to reduce Linux users' confusion
> about DNS matters.  (It's not really mysterious.)
> 
> You said:  'I think I fixed it at about 7 PM PST Tuesday 20130205.'
> And so you did.  Specifically, the two authoritative nameservers for the
> sf-lug-org domain, ns41.worldnic.com and ns42.worldnic.com, had until
> then been publishing this line for 'www.sf-lug.org':
> 
> www.sf-lug.org.         7200    IN      A       208.69.40.247
> 
> You changed that to this, switching to the same target IP address used
> for sf-lug.com:
> 
> www.sf-lug.org.         7200    IN      A       208.96.15.252
> 
> 
> The '7200' you see there is the TTL, the Time To Live value associated
> with (and sent out with) this DNS record.  TTL means 'Please consider
> this data stale and presumptively no good if it's older than this number
> of seconds.'
> 
> You updated the published value at, let's say for the sake of
> illustration, exactly 7:00 PM local time.  However, a bunch of SF-LUG
> people such as you, Machiel Paoli, me, and various others had been
> test-loading the Web page, thereby causing our local DNS nameservers to
> look up the DNS value during the 6 PM hour.  So, those local nameservers
> of ours have lodged within their caches what the nameservers believe to
> be still-valid answers to the question 'What does the A record for
> www.sf-lug.org point to?', those being still valid because they were
> fetched less than 7200 seconds ago from the authoritative nameservers.
> 
> The worldwide DNS system works largely because of extremely pervasive
> local caching, which is both a blessing and a curse.  It's a blessing in
> that it's the only thing preventing worldwide DNS from burning down from
> excess traffic.  It's a curse in that most DNS answers people use are
> cached values that are in no way guaranteed to necessarily match the
> current values offered by the domains' authoritative DNS servers.
> 
> Where DNS is concerned, the 'dig' command is your kung-fu.  You can
> answer just about any question with it.  Please see my earlier post for
> some examples.
> 
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