A records, PTR records, and TTL setting

Wed Jan 10 17:25:12 UTC 2024

Ugh!  It just dawned on me where I saw it.

It was from running the `unbound-control list_local_data` command, and I got a response like this:

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

And so I copied it!

Jon

> On Jan 10, 2024, at 11:18 AM, Jon Murphy <jcmurphy26 at gmail.com> wrote:
> 
> Interesting - I will give it a try.  I don’t know why I included the `IN`.  I think I found an example and copied it!
> 
> I found I could remove the TTL and that led to my question concerning the default TTL or 3600.
> 
> Thank you for the response!
> 
> Jon
> 
> 
>> On Jan 10, 2024, at 7:37 AM, RayG <rgsub1 at btinternet.com> wrote:
>> 
>> 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> 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
>>> 
> 