From: e8mhpsznamq001@sneakemail.com
To: list@maradns.org
Subject: Common misconceptions about DNS
There is a common misconception about DNS floating around
that it is up to the DNS servers serving a given domain,
such as example.com, to tell all the other DNS servers
in the world when a change happenes to one of the
host names in the domain.
The idea is that such a change is somehow propagated through
the internet is very common. It is also incorrect.
DNS was designed in the bad old days of the internet. The
days when the backone of the internet was a 56k link. The
days when 5 minute timeouts for telnets were needed; a
timeout any shorter would prematurely disconnt someone.
As a result, it uses caching to store data. However, an
ISP's DNS servers do not cache the information about,
say, www.maradns.org, until someone asks the ISP's DNS
server about maradns.org. At which point, the ISP server
will find the DNS server for maradns.org, find out
the IP for maradns.org, and also be told how long
to cache that data.
Clear as mud?
Let me word that differently.
Some enlightened person on the internet wants to find
out more about MaraDNS. So, they whip out Mozilla and
decide to go to maradns.org to find out more about this
wonderful (newly nearly bug-free [TM]) piece of software.
Now, before they can go to maradns.org, they need to find
out where maradns.org is on the net. Which requires a
rather ugly process called DNS. A process so ugly that
most people's home computers don't even bother with
this process. Instead, they ask their friendly local
ISP's DNS servers where they could find maradns.org on
this here internet thingy.
So, the home computer asks the ISP's DNS server "Hey,
where the heck is MaraDNS org".
At this point, the first thing the ISP's DNS server
does is look in their cache. "Hmmm, did someone
recently ask about maradns.org?" asks the ISP's
DNS server. Well, how about that, no one recently
asked about MaraDNS. "Oh dear; I have to find where
it is on the internet now. Oh, God, please not let
this be a host name with out-of-bailiwick name
servers."
So, to make a long sorid story short, eventually the
ISP's DNS server hooks up with the authoritative DNS
servers for maradns.org. You know, getting the
answer straight form the horses mouth and all that.
So, the ISP's DNS server saunters down to the maradns.org
name server and asks where it could find maradns.org.
The process goes like this:
ISP DNS server: Hey, where can I find maradns.org?
maradns.org DNS server: It's at 66.33.48.187; remember
that for one day.
This means now that, for the next 24 hours the ISP's
DNS server will reemember that maradns.org can be
found at 66.33.48.187. This way, the ISP's DNS
server doesn't need to bug the maradns.org DNS
server about where maradns.org can be found.
So, what happens if maradns.org has a sudden change of
address. Well, while maradns.org's DNS server will
give everyone the correct new address, there will be
a lot of DNS servers out there which will remember
the old address for maradns.org for 24 hours.
Because of this, people will be knocking on the wrong
door until the ISP's DNS server realizes its postit
note about where maradns.org can be found (so that
Mr. ISP DNS server doesn't have to go through the
sordid process of bugging Ms. maradns.org DNS server
again) is out of date; forcing Mr. ISP DNS server to
get his lazy butt up out of his desk, find
Ms. MaraDNS, and ask her about the whereabouts
of maradns.org again.
Now, notice something about this entire process.
Ms. MaraDNS did not have to do anything but answer
Mr. ISP DNS server. Ms. MaraDNS did not have
to play the gossip game, where she had to tell
all her friends where maradns.org can be found,
so that those friends can tell their friends, and
so on. Why waste her time being a gossip when all
she has to do is sit around and answer questions
that various ISP's and other recursive DNS servers
ask her?
There. Now hopefully I have purged the idea about
DNS records somehow propagated from everyone's
head. That just ain't how it works.
- Sam
