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

[Rick Moen]

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.