[wplug] OT - request for comment on an idea

[Chris Romano]

On 6/3/05, Chester R. Hosey <Chester.Hosey at gianteagle.com> wrote:
> On Fri, 2005-06-03 at 13:57 -0400, Chris Romano wrote:
> 
> > Now from my understanding if a DNS server can contact the primary DNS
> > of the site/name that it is looking for, it will try the secondary
> > DNS
> > for that site/name.  Is this right?  If so, this means that there
> > shouldn't be to much downtime if this where to happen.  I problem is
> > if the IP is cached on the requesting DNS server.  I am not too
> > familiar with DNS yet, so in this case will the site be down until
> > the
> > requesting DNS server refreshes it's cache?
> >
> > So is there a better way of doing this or am I at least somewhat on
> > the right track?  I hope that I explained that well enough.
> >
> > Thanks,
> > Chris
> 
> Actually there are two separate parts that you need to worry about:
> 
> 1) Availability of a DNS server which will translate a domain name to
> one or more IP addresses.
> 
> 2) Ensuring that the services offered by hosts at those addresses fail
> over.
> 
> Regarding #1, yes, a secondary DNS server should provide this service
> should the first one fail. Generally you'd set up the secondary to pull
> information from the primary. Since clients can pull from either server
> and are likely to cache replies, you cannot expect quick DNS changes to
> take effect.
> 
> Furthermore, since clients may cache, you cannot have DNS and HTTP on
> both servers, simply have DNS for each server reply with its own IP for
> the DNS lookup, and expect that DNS failing will cause all clients to
> query the other DNS server, get the active server's IP during name
> resolution, and connect to the proper HTTP server.
> 

Wait ... I know that I might run into problems with caching, that I
have to figure out.  But lets go with the following: (IPs are for ill.
I know they can't really work).

localserver 1 - primary DNS: 192.168.0.1
localserver 2 - webserver: 192.168.0.2
localserver 3 - email: 192.168.0.3

remoteserver 1 - secondary DNS: 10.10.10.1

Under normal situations both localserver 1 and remoteserver 1 point to
192.168.0.2 for www.example.com and 192.168.0.3 is mx for example.com

Say our link goes down. Remote server will start Apache and qmail,
load zones files that say www.example.com -> 10.10.10.1 and mx
example.com -> 10.10.10.1.

Someone requesting www.example.com should get 10.10.10.1 if the DNS is
not cached, and 192.168.0.2 if it is cached, right?  So I could have
Apache and qmail on the machine but dns caching defeats the plan.

Chris