A records, PTR records, and TTL setting

Wed Jan 10 13:37:05 UTC 2024

I think you have the format wrong:

unbound-control.exe" local_data deb12dell.localdomain 60 A 192.168.60.175

unbound-control.exe" local_data 192.168.60.175.in-addr.arpa. 60 PTR deb12dell.localdomain

Works for me.

From: Jon Murphy <jcmurphy26 at gmail.com> 
Sent: Wednesday, January 10, 2024 4:07 AM
To: Fred Morris <m3047-unbound-b3u at m3047.net>
Cc: unbound-users at lists.nlnetlabs.nl
Subject: Re: A records, PTR records, and TTL setting

Fred,

I am sorry for the late response.  Otto informed me to look for your post.  Your post made it to my email but then it just sat.  Ugh!

Thank you for your response!

I added details I did not include in my initial post: 

https://lists.nlnetlabs.nl/pipermail/unbound-users/2024-January/008207.html

And this January 1 update may help.

My comments below,

Jon

On Dec 25, 2023, at 2:45 PM, Fred Morris via Unbound-users <unbound-users at lists.nlnetlabs.nl <mailto:unbound-users at lists.nlnetlabs.nl> > wrote:

Happy holidays...

On Sun, 24 Dec 2023, Jon Murphy via Unbound-users wrote:

[...]
With a simple network (well defined, eh?) I am guessing it does not matter.

It doesn't matter has long as you have one address per name. X-D

I can have one A and one PTR record per network interface.

You forget about the name in that algebra.

So for my "deb12dell.localdomain" device, it is OK to have "two" or each, like this:

deb12dell.localdomain. 60 IN A 192.168.60.175
175.60.168.192.in-addr.arpa. 60 IN PTR deb12dell.localdomain.
deb12dell.localdomain. 60 IN A 192.168.65.180
180.65.168.192.in-addr.arpa. 60 IN PTR deb12dell.localdomain.

The name in that construction doesn't distinguish between interfaces.

The info is coming from the ISC-DHCP system I get a hostname and an IP address.  I can modify the hostname and make the second item something like `deb12dell-1` and then pass that change to unbound.

But this does not feel right…

Right now there are two interfaces. One ethernet and one wireless.  In the future there could be more.

Are both interfaces accessible, everywhere? If not, then half of the attempts to reach deb12dell (using the A records) will initially fail. What happens after that depends on the software. There is no way to query for just one of the A records (unless you set up views or split horizon DNS), the software attempting to connect will have to decide which address to try, and whether or not to try them both.

You've confounded the PTR record case (by using an intentionally unresolvable-on-the-internet name) making it more difficult to discuss, although I'm less concerned about it. Going back to the case of a mail server (a reminder, this isn't in the mail protocol, it's something people do as a practical test for legitimacy), as long as there's only one A record for a name it doesn't matter how many PTR records resolve to it because they'll never be enumerated by any normal DNS query:

example.com. MX 10 server.example.com.
server.example.com. A 10.0.1.111
111.1.0.10.in-addr.arpa. PTR server.example.com.
112.1.0.10.in-addr.arpa. PTR server.example.com.

For me, there is only a LAN or local network.  And only A records and PTR records.

In case you wanted to encode some reliable identity information in the name (there be dragons! a file share would be a bad application of this, at least without some other authentication / access controls) let's say a file share that is only accessible to the "downtown" office you could have a whole bunch of PTR records resolving to downtown.office.localdomain, the A records don't have to agree with the PTR records for that purpose:

joe.downtown.example.com. A 10.0.0.22
jean.downtown.example.com. A 10.0.0.28
22.0.0.10.in-addr.arpa. PTR downtown.example.com.
28.0.0.10.in-addr.arpa. PTR downtown.example.com.

There are two different ways some sort of access control can work in the above scenario, and it's up to the server architect how it works:

1) You list joe.downtown.example.com and jean.downtown.example.com as
  permitted. When the service starts it performs A record lookups and
  stores the addresses 10.0.0.22 and 10.0.0.28 internally. When a
  connection attempt is made, it compares the client's address with
  10.0.0.22 amd 10.0.0.28 to see if it's one of those.

2) You list downtown.example.com as permitted. This FQDN is read, but no
  lookups are performed at service startup. When a connection attempt is
  made, a PTR lookup is performed for the address and it compares the
  FQDN with what it stored at startup.

Wow - this is over my head!  

(I see you picked up on the need for the trailing dot, good for you!)

Thank you!  Based on what I read it seemed like the right thing to make it an absolute path.

Kind of depends on your use case, or application; it's usually not configurable. There are three questions to ask:

1) Is the A record compared to the PTR record for identity?

Yes.

2) Is the A record query made at startup, and the address recorded and
  used going forward?

It may or may not be at start-up since my info comes from ISC-DHCP.  It could happen, or even change, as devices come and go.

3) Is the PTR record query made at access time?

I am not understanding the question.  I now there are reverse lookups.

Thank you!

Jon

--

Fred Morris

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