DNS question

Kevin Darcy kcd at daimlerchrysler.com
Wed Aug 7 20:20:51 UTC 2002


Joe Chancellor wrote:

> I am trying to search a range of IP addresses for webpages.  I got the
> script running correctly, but I think I am not using either the right
> command or possibly do not know the correct command line options.
>
> This works:
>
> C:\>host msn.com
> msn.com has address 207.68.172.246
> C:\>host 207.68.172.246
> 246.172.68.207.in-addr.arpa domain name pointer email.msn.com.
> 246.172.68.207.in-addr.arpa domain name pointer msn.ca.
> 246.172.68.207.in-addr.arpa domain name pointer msn.com.
> 246.172.68.207.in-addr.arpa domain name pointer msn.net.
> C:\>
>
> but why does this example not work:
>
> C:\>host firstname.com
> firstname.com has address 66.60.188.100
> C:\>host 66.60.188.100
> Host 100.188.60.66.in-addr.arpa not found: 3(NXDOMAIN)
> C:\>
>
> I am pretty new to DNS and Linux, so I am a little confused.  Any help is
> appreciated.  Don't let the C prompt fool you it is a Linux box.  Just did
> it to mess with my boss.

What exactly is confusing you? The fact that the name wasn't found, or the
fact that the reported missing name was 100.188.60.66.in-addr.arpa instead of
66.80.188.100?

The name wasn't found because apparently the domain owner didn't bother
creating a reverse (address-to-name) record to match their forward
(name-to-address) record. There is actually no requirement to do this,
although it's considered a Good Practice to do so.

The xx.xx.xx.xx.in-addr.arpa convention is just a way of attaching the
reverse DNS namespace to a "special" branch of the overall DNS namespace. The
reason that the order of octets is reversed is because DNS names are scanned
left-to-right from more specific elements (e.g. hostnames or the ubiquitous
"www") to more general elements (e.g. top-level domains like .com, .net or
..org), whereas the octet representation of addresses goes from more general
elements (e.g. a pre-CIDR "A" class address prefix) to more specific elements
(e.g. the address of a specific node on a specific subnet). Humans seem to
prefer parsing numbers and textual information in opposite directions for
some reason -- consider telephone numbers versus street addresses -- and thus
a reversal is necessary when mapping addresses to these
"special" in-addr.arpa DNS names.


- Kevin




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