Re: nscd and DNS cache

JD <jd1008@xxxxxxxxx> · Wed, 16 May 2012 13:52:54 -0600

On 05/16/2012 10:39 AM, Ed Greshko wrote:

On 05/17/2012 12:28 AM, Greg Woods wrote:

On Thu, 2012-05-17 at 00:13 +0800, Ed Greshko wrote:

  when you do a look up on www.cnn.com it will return 4 IP
addresses.  Now, since bind would have that in its cache it wouldn't have to send out
a query.  What I don't know is if an application would make a request would the list
be returned in the same order every time to the requesting application?   In other
words, if the TTL is not set low, would that defeat the round robin technique.

Unfortunately, there is no guarantee that a client resolver will
actually use the IP addresses in the order they are presented by the DNS
server. Nothing in the DNS spec requires them to do so.

Yeah....  I know there is no spec...  I'm just expecting the clients to be "dumb" and
take the first one in the stack.  :-) :-)

Interesting things to investigate.....if I really had the time.

My experience says that DNS round robining is actually a poor method of
load balancing. I'm surprised to see a large site like CNN resorting to
this (if that's really what they are doing this for). Perhaps in
combination with a low TTL and a modified DNS server, they can send out
a completely different set of IPs every few minutes, and achieve a sort
of crude load balancing that way, but I think load balancing works
better if you just send out a single IP and use a load balancer that you
can control, such as LVS (Linux Virtual Server) that can farm out
incoming connections to a single virtual address out to multiple real
addresses.

Yes...  I suppose one also has to ask if the load balancing is meant to be server or
network balancing.

Well, after running dnsmasq with the configuration I just emailed,

I see the following behavior of firefox vs. running nslookup on command 

line.

FF, even after resolving google.com only a minute ago, is still spinning 

saying:

lookup up www.google.com
whereas , on the command line, I run nslookup www.google.com
and almost instantly, I get
Server:        127.0.0.1
Address:    127.0.0.1#53

Non-authoritative answer:
Name:    google.com
Address: 74.125.239.1
Name:    google.com
Address: 74.125.239.2
Name:    google.com
Address: 74.125.239.3
Name:    google.com
Address: 74.125.239.4
Name:    google.com
Address: 74.125.239.5
Name:    google.com
Address: 74.125.239.6
Name:    google.com
Address: 74.125.239.7
Name:    google.com
Address: 74.125.239.8
Name:    google.com
Address: 74.125.239.9
Name:    google.com
Address: 74.125.239.14
Name:    google.com
Address: 74.125.239.0

and FF is still spinning waiting for the resolution.

Does anyone see this discrepancy?

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