[CentOS] DNS is confusing! (I really need some help understanding!)

Kemp, Larry Larry.Kemp at usmetrotel.com
Fri Oct 9 17:35:04 UTC 2009

Like Bowie said, the owner of the IP block (under ARIN or whatever registry you are under) must provide the reverse (IP-Number to host.domain) resolution. I have customers that we issue /30's and larger to that want to run their own mail-server for example. As the ISP under ARIN I simply give them an A or CNAME record so that when they send email, the IP that is visible as their email server is registered in my Reverse DNS as a valid host. ARIN does this to control SPAM, they make the IP block owner responsible. Also not 100% sure exactly what you are trying to do. 

-----Original Message-----
From: centos-bounces at centos.org [mailto:centos-bounces at centos.org] On Behalf Of Bowie Bailey
Sent: Friday, October 09, 2009 12:17 PM
To: centos at centos.org
Subject: Re: [CentOS] DNS is confusing! (I really need some help understanding!)

ML wrote:
> OK, I am confused and DNS is the reason.
> So, Comcast, 13 public IP's bound to my modem.

13 IPs for your modem?  Is this a business account or something?

> Each public IP has a DNS name from comcast (they assign it  
> automatically) like:
> --> 173-13-167-209-sfba.hfc.comcastbusiness.net
> I created a DNS entry at GoDaddy for that is  
> 'inhouse.theindiecompanyllc.com'

You can use Godaddy to set DNS for 'inhouse.theindiecompanyllc.com' to, but to do the reverse, you will need Comcast to delegate
DNS authority to you for the reverse domains.  (For just a few of them,
you would be better off asking Comcast to set the reverse names for you
... don't know if they will do that or not).

> When eth0 is alive, I see that it tells me my name is 173-13-167-209-sfba.hfc.comcastbusiness.net 
>   since I am using a comcast DNS Server.

Doesn't matter what server you use, they should all return this since
Comcast is in control here.

> So if I setup my own internal caching name server for  
> then I have a third name or can I just use the GoDaddy DNS entry I  
> gave it? Do I then set my systems to use my local name server for  
> their information and not Comcast? How does that effect internet  
> browsing where I dont have entries for google.com, etc in my local  
> name server?

I think you are confusing forward and reverse DNS lookups.

A forward DNS lookup is this:

$ host buc.com
buc.com has address

A reverse lookup is this:

$ host localhost domain name pointer menu.buc.com.

The forward lookup is handled by your Godaddy DNS.  This is all that is
needed for letting someone get to your server by name.  The reverse
lookup must be handled by whoever controls the IP addresses.  This is
usually your ISP (Comcast).  Changing the reverse lookup may not be
necessary depending on what you are trying to do.

If you tell us what you are trying to accomplish we may be able to offer
further suggestions.

> Lastly, If I have two NICS (one is and second private  
> internal IP) For my local name server I would have two entries, one  
> for public and one for the private card so I can do something like  
> 'inhouse.local' and get intranet or ssh inhouse.local, etc correct?

I'm not sure what you are trying to say here.  The two NICs should not
matter as long as you have them configured correctly.  The system will
use whichever NIC it requires to get to the IP address given to it by
the DNS (or host file, or whatever).

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