RE: [Nolug] ISP failover w/DNS

Hi Chris.

I've seen a few low-end units which don't, but most do.

Oh, and as you surmised, the local machines should use the router for their
DNS lookups. Actually, this is true in general, even on a LAN with only one
feed. You don't want to hard code the vendor's DNS resolvers into all your
machines. Before someone says it. The extra time this adds is negligible.

Interleaving would help, if you have to, yes. But first find out if either
of the vendors allows external access (use by anyone, not just those in
their own address space). Most ISP's don't, but some do.

Also, you could consider using some of the "public" DNS resolvers which are
open to all, like the ones which Google offers. I have found this to be a
bad idea, in general, since the extra latency to reach them surpasses any
benefit.

Finally, you could set up your own DNS resolver inside the LAN.

Enjoy,

John

    John Souvestre - New Orleans LA - (504) 454-0899

From: owner-nolug@stoney.kellynet.org
[mailto:owner-nolug@stoney.kellynet.org] On Behalf Of Chris Jones
Sent: Saturday, May 14, 2011 1:43 pm
To: nolug@nolug.org
Subject: Re: [Nolug] ISP failover w/DNS

I kind of figured that was the case, as I've never seen a failover router
that didn't have a built in forwarder. But in the case where you didn't
have any other option, I was thinking the first way to list the DNS servers
would probably give you the least amount of delay at least. Like I said,
purely hypothetical, though. You're probably right though, I bet every
router that does failover does this already.

On Sat, May 14, 2011 at 1:38 PM, John Souvestre <johns@sstar.com> wrote:

Hi Chris.

Any time you reference a DNS resolver that you can't reach you will
experience a delay before trying the next DNS resolver in the list. You
want to avoid this situation!

Any router you use should for a load balancing or failover function should
have at least a DNS forwarder built in. Furthermore, when the external DNS
resolvers are defined in it, they should be per input feed. Thus the router
knows which external DNS resolvers to use based on which line(s) is up.

Regards,

John

    John Souvestre - New Orleans LA - (504) 454-0899
<tel:%28504%29%20454-0899>

From: owner-nolug@stoney.kellynet.org
[mailto:owner-nolug@stoney.kellynet.org] On Behalf Of Chris Jones
Sent: Saturday, May 14, 2011 1:28 pm
To: nolug@nolug.org
Subject: [Nolug] ISP failover w/DNS

I was thinking about a project I'm working on for a client of mine. They're
going to have 2 ISP's piped into their network, and use a router that does
failover. I probably won't even need to do this in my case, but I was just
thinking if setting up dual internet w/ failover becomes more common, this
is a situation that could easily be encountered. Most of my clients that
would ever want internet redundancy probably have their own DNS server
anyways. :) I know some of you guys work for ISP's, and there are even a
couple of CCNP's on here I believe. I consider myself very knowledgeable
about TCP/IP, but probably nothing like what you have to know to get a CCNP
or CCIE. (actually, CCNA/CCNP is probably what I should work on next)

The thing I was thinking about, is if you have 2 ISP's, each one has their
own DNS servers. Let's hypothetically say the ISP's are Cox and AT&T (they
aren't actually, for geographical reasons), and each ISP has 2 DNS servers.
Set Cox as the primary ISP, and if that goes down, it fails over to the
slower AT&T line. If you set the computers behind the firewall to Cox's DNS
servers, when it fails over to AT&T, it will probably lose access to Cox's
DNS servers. So, what would be the "best practices" way to combine the two
sets of DNS servers on a network? Obviously most operating systems will let
you put even more than 2 DNS servers, so you can just list them all, which
is probably fine to do if the router doesn't have its own built in DNS
forwarder. (so you could set the PC's to just use 192.168.1.1 as their DNS,
and the router will route the DNS requests on its own)

But, what order should you do it for best performance and reliability? I'm
thinking one of two orders:

Order #1:
Cox DNS 1
AT&T DNS 1
Cox DNS 2
AT&T DNS 2

Order #2:
Cox DNS 1
Cox DNS 2
AT&T DNS 1
AT&T DNS 2

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Received on 05/14/11