Re: DNS and subdomains

[Harry Hoffman via NANOG <nanog () nanog org>]

Thanks everyone for the wonderful answers! The thoughts in my head were not
aligned with reality ;-)

I had a thought similar to what Bill describes, that everywhere there's a
dot (.) there's a subdomain. I also had the thought/assumption that the
data to the left of the leftmost dot (.) was essentially the hostname. It's
been a while since I've regularly been into DNS but I thought I remembered
it that way. But then seeing some of the OSINT data, that seemed to throw
that idea out the window as they appear to use the publicsuffix.org site to
divine suffix and domain and then consider everything to the left of that
as a subdomain. I've now updated my understanding.

Cheers,
Harry

On Mon, Feb 24, 2025 at 9:47 PM William Herrin <bill () herrin us> wrote:

> On Mon, Feb 24, 2025 at 5:58 PM Harry Hoffman via NANOG <nanog () nanog org>
> wrote:
> > In working with several OSINT sources for domain processing it seems
> like the way domains and subdomains are processed essentially equates
> subdomains with FQDNs.
>
> Hi Harry,
>
> I don't understand what you mean. Do you mean how do local resolvers
> expand local names (e.g. "server") in FQDNs
> ("server.examplecompany.com")?
>
>
> > Has this become common practice? Is there a definitive way to determine
> subdomains? I seem to recall that "older" dns server software wouldn't
> allow this but it could be that my memory is faulty.
>
> Subdomain is kinda a fuzzy question. Everywhere there's a dot there's
> a "subdomain," but the only technical meanings those dots have is
> that:
>
> 1. There *might* be a DNS delegation there.
> 2. The resolver's primitive internal compression algorithm can break
> the name in parts there when composing the query or response packet.
>
> There's another similar question you might be trying to ask: how do
> you determine whether a DNS name has been delegated from one zone file
> at one server to another zone file at another server? That has a more
> precise answer with technical meaning: If a NS and SOA records exist
> at the "dot" boundary then there's a delegation. If not, then there
> isn't.
>
> For example, I have a host named "cat.p.dirtside.com." "dirtside.com"
> is a delegation of "com" because there's an NS record for
> "dirtside.com." However, "p.dirtside.com" is NOT a delegation of
> "dirtside.com" because there's no NS record. "cat.p" is simply a name
> within the "dirtside.com" zone file.
>
> Regards,
> Bill Herrin
>
>
> --
> William Herrin
> bill () herrin us
> https://bill.herrin.us/