DNS question

**trickster** · 10-31-2002, 03:18 PM

If I add more than one DNS to my network settings, when trying to resolve domain names, will it check the secondary DNS or tertiary if it can't find the host on the Primary DNS, or are the extra DNS' just in case the Primary is down?

I am doing a VPN connection to work. Of course, my ISP's DNS' do not have names of my work's internal hosts. If I set my internet connection with my ISP's DNS and add the internal addresses for my work DNS', after doing a VPN, will that let me resolve my internal hosts?

(I know it is kinda misleading...)

**Compunuts** · 10-31-2002, 04:35 PM

Yes, it will work .....

Secondary DNSs are there for that purpose. So if the host can't be found in Primary DNS, the request is came back negative and your computer will send another request to secondary DNS server.

I used to set my DNS entries from two companies, my current ATTBI connection and my former DSL provider's ( Telocity.com ) DNS so if one is down, I still can resolve hosts. ;D

**trickster** · 10-31-2002, 06:54 PM

Thank you. I will give that a try.

**tolstoy** · 11-01-2002, 12:51 AM

Actually, chances are that your primary DNS will not have the host entries you are looking for, unless they have already been cached by it. In the event that your primary DNS server does not know the answer, your secondary DNS server will not be queried. AFAIK, it will only be queried in the event that the primary nameserver is offline.

DNS works like this:

Say you are looking for the ip address of foo.bar.com. Your computer will query your primary DNS. If your primary DNS does not know where that host is located (in most cases it will not), it will then query a top-level nameserver (a .com nameserver for instance). That nameserver will then know how to locate the bar.com domain, since the .com nameservers are authorative for everything in the .com domain namespace. The .com server itself will not know the address of foo.bar.com, but it will know how to locate bar.com's nameservers, which will most likely hold the appropriate resource records you are looking for. Once the actual resourse record is located, it is then passed back up the tree to your ISPs nameserver. Then it is handed down to the client resolver on your box.

Does this make any sense?

Your primary DNS server will cache this response, but will probably not hold onto it forever. In the event that this name server does not know the answer, your secondary DNS server WILL NOT be queried. If you run tcpdump, you can see this happen on the wire. The primary namserver will respond to your computer, telling it that that host does not exist, and your computer will believe this response.

As far as VPN is concerned, I don't know much about the details of its implementation, but I'm sure the software handles traffic negoiation between its endpoints in its own way.

**trickster** · 11-01-2002, 12:57 AM

That's ok. That is not the case for WINS it seems. I added the domain's WINS and it resolved right up.

**tolstoy** · 11-01-2002, 01:01 AM

[quote author=trickster link=board=4;threadid=5483;start=0#52919 date=1036115857]
That's ok. That is not the case for WINS it seems. I added the domain's WINS and it resolved right up.

[/quote]

WINS will definately do the trick. If you are on an NT4 domain, I don't think DNS will ever even be queried. I'm pretty sure NT defaults to NetBIOS over TCP/IP.