On Thu, Nov 17, 2011 at 8:18 PM, John Gwinner <[email protected]> wrote:
> On our SMTP hosting provider, they have ‘redundant’ DNS entries for the same
> FQDN:
You can certainly have multiple A records associated with a single
domain name. This is often called "round robin DNS", as most
full-service DNS resolvers will rotate the answer record set with each
query.
> My question to the group – can this general idea be used to
> create ‘backup’ web servers?
Sort of.
> I don’t think a client stack will know to query DNS ‘again’
> if the first IP address was down.
Exactly.
There are multiple pieces involved. There's the full-service DNS
resolver you happen to be using -- typically your ISP's nameservers,
or maybe your friendly-neighborhood Active Directory Domain
Controllers. Then there's the client DNS resolver on the client
computer, which may be dumb ("stub resolver"), or caching (as with
Windows). Then there's the actual network application software, i.e.,
the web browser, and whatever libraries it uses.
When multiple A records exist, it's implementation-dependent as to
what happens on the client. Some programs just call gethostbyname()
(or the equivalent) on the name they've been given, use whatever
address comes first, and abort if that fails. Some will try each
address in turn until a connect() succeeds. Some do other things.
Things are further complicated by the fact that most web browsers
are complicated beasts, and do various caching internally. So they
may get an address and stick with it until you quit the browser
program entirely.
So if a web server goes down, it's anyone's guess what clients will
end up doing.
Round robin DNS is more useful for load-balancing, as your SMTP
provider says. Even then, it's not intelligent load-balancing. You
can get unlucky and have a bunch of people using one server, just
because things worked out that way. There's no way to influence the
ordering of the records returned.
> Is this kosher?
It's completely legal in terms of the standards. Whether it's a
good idea or not depends on what you're trying to do. See above. :)
> I couldn’t see a way on the big registrars to have multiple “A” records
> anyway, so it may not be possible, but I thought I’d check.
Most of the web UIs I've seen for managing DNS present a rather
simplified view of things, which usually doesn't reflect how DNS
actually works.
That said, I use DomainMonger.com as my cheap-but-decent
registrar/DNS host, and their web UI allows one to enter multiple A
records per name.
Also, it's worth noting that a registrar is not the same thing as a
DNS hosting provider, although these days many people use the same
company for both. All a registrar does is take your domain name and
some nameservers, and publish that information in the domain registry.
It's those registered nameservers that determine the answers people
get. The registered nameservers can be operated by the registrar, but
they don't have to be. You can run your own, or have another party do
it. Maybe you register with Net Sol, but have your DNS hosting done
by UltraDNS, for example.
-- Ben
~ Finally, powerful endpoint security that ISN'T a resource hog! ~
~ <http://www.sunbeltsoftware.com/Business/VIPRE-Enterprise/> ~
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