DNS Resolution

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Here's the scenario:

I have a pretty basic LAN that utilizes a gateway router to get to the  
internet.  Some machines are making use of DHCP to get their ip address  
and DNS server assignments.

A few machines have static ip addresses, are they are sometimes cut off  
from the router by a mechanical switch.  (I did not really like that  
scenario, but it was a necessary compromise.)

Those machines have their DNS server set to the router's ip address.  
This seems to work well enough for some URLs, but others go unresolved.  
  I could manually plug the current DNS server addresses into those  
static machines, but they will eventually get out of date.

Is there some way I can use static ip addresses, but still have those  
machines dynamically query for DNS server addresses?


Re: DNS Resolution

Grinder wrote:
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There's two separate fields here.
But this doesn't do the right thing.  If you ask for DNS info
from my router, it just returns the router address as the DNS
gateway. And then the router forwards the DNS request, to the
primary and secondary info it got from the ISP.


So you can't fix it that way.

I used to type the ISP DNS addresses into
my local network control panel, but that
broke so many times (as you note), it
wasn't funny. I stopped doing that.
My first ISP also had the irritating habit
of rebooting their primary and secondary DNS boxen
*at the same time*. Recovery time was around
fifteen minutes or so.

If I do ipconfig /all, it tells me the DNS server is
my router. And it doesn't tell me about the two DNS
servers actually being used (from the ISP). nslookup
has the same behavior - claims my router resolved the
DNS request.


You could statically assign, as per here.
Leaving a lasting record of resolved IP addresses
to future generations :-( But at least, using
isn't likely to move.


Before using stuff like this though, you want to
thoroughly review the down sides of using public DNS.


Sorry I don't have a good solution. Extracting
the info from the web browser interface on the
router, doesn't sound like a lot of fun, but I suppose
that's a (remote) possibility. Then you'd have to figure
out how to script a way to insert it into the network settings.
And get it to take immediately.


Re: DNS Resolution

On 7/26/2013 5:21 PM, Paul wrote:
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Well, shit, that's sort of what I figured.

My backup plan is to hang the "sometimes disconnected" off of their own  
device with DHCP services, and have that device run into the gateway  
router.  Whenever they connect the networks together, they can bounce  
that device and everyone should get up to date information.

The mystery device will end up being a router, but I'm not sure if DHCP  
will work properly on it if I just use the LAN side of that router.  
Will a router be able to pickup, and pass along, the DNS settings of the  
internet upstream if it's connected through one of it's LAN ports?

Re: DNS Resolution

Grinder wrote:
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As far as I know, DHCP works from branch to branch in your network
tree. So if you tied a router to a router, the second router uses
the gateway on the first router, and the second router tells its
LAN machines, to use its own gateway.

The solution wouldn't be so bad, for the machines
given static addresses for their IP. Who's to know ? :-)
As long as it is working, most people don't care.
What are the odds that or its secondary, stop working ?

If you tie a single computer directly to an ADSL modem
(computer terminates PPPOE protocol), the computer still
uses DHCP, but gets the primary and secondary DNS directly
from the ISP. But that is hardly of more than academic interest,
as it doesn't solve any problems. And if you were to use ICS
(internet connection sharing off a second LAN interface),
I doubt the downstream nodes end up any better off than they
do with a router solution.

Maybe someone else will come up with a clever plan :-)
My LAN skills are limited, to finding that control panel :-)


Re: DNS Resolution

On 7/26/2013 9:29 PM, Paul wrote:
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I'll try it Paul, thanks.

Re: DNS Resolution

Grinder wrote:

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first what is the reason for some of the machines having static ip address?
I would say an access list might solve your problem!

Re: DNS Resolution

On 7/31/2013 4:16 AM, Darklight wrote:
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The static part of the network is behind a mechanical switch that  
disconnects them from the gateway router.   It's not really what I would  
call ideal, but the client want to be able to physically detach that  
part of the network for "security reasons."

Can you elaborate on "access list?"

Re: DNS Resolution

Grinder wrote:

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ok it's been a while since i done ciso networking.
Access list's allow you to control incoming and out going  
traffic. So if you want to limit access to part of the network you
can. I will recommend you read a book called. Accessing the wan by cisco.

There are four books in the cisco series that covers every thing about  

If you get this book go to the index and you will see what you are looking  
for under A.

There is a free app called gns3 which will run under windows or linux
that allows you to practice on a virtual network. All be it with cisco
routers and switches. This app is cpu intensive so be carefull that you  
don't make the network to big.

before you try gns3 go to youtube and look how to set it up.

see example below:


This will allow you to learn or test a network with the setting's you
want to use.

I don't know how much you know but it will be a steep learning curve.

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