DDNS TTL in 4.3

[Doug Barton]

Howdy,

In 4.2 and previous it was true that the TTL sent for DDNS requests was
1/2 the default lease time. In 4.3 it seems that there is no TTL value
sent, so the DDNS records simply inherit the zone default TTL. That's
not necessarily a bad thing, but it's different, and I need to know how
to advise my customers if it turns out they were depending on the old
behavior.

Where can I find documentation on this change? And if it was discussed
previously, my apologies for missing the memo.

Doug

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[Tony Finch]

Doug Barton <[hidden email]> wrote:
>
> In 4.2 and previous it was true that the TTL sent for DDNS requests was 1/2
> the default lease time. In 4.3 it seems that there is no TTL value sent, so
> the DDNS records simply inherit the zone default TTL.

The DNS UPDATE protocol does not allow you to omit the TTL so this
explanation cannot be right! Also the zone default TTL is just an
abbreviation allowed by the master file format, and it doesn't exist
in other parts of the DNS protocol.

I think what you are observing is that isc-dhcp by default caps the TTL to
one hour, which happens to be the same as your default TTL.

There is an item in the release notes:

https://source.isc.org/cgi-bin/gitweb.cgi?p=dhcp.git;a=blob;f=RELNOTES;hb=HEAD#l683

and a helpful comment in the source code:

https://source.isc.org/cgi-bin/gitweb.cgi?p=dhcp.git;a=blob;f=server/ddns.c;hb=HEAD#l345

Tony.
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[Doug Barton]

On 4/22/15 3:17 AM, Tony Finch wrote:

> Doug Barton <[hidden email]> wrote:
>>
>> In 4.2 and previous it was true that the TTL sent for DDNS requests was 1/2
>> the default lease time. In 4.3 it seems that there is no TTL value sent, so
>> the DDNS records simply inherit the zone default TTL.
>
> The DNS UPDATE protocol does not allow you to omit the TTL so this
> explanation cannot be right! Also the zone default TTL is just an
> abbreviation allowed by the master file format, and it doesn't exist
> in other parts of the DNS protocol.
>
> I think what you are observing is that isc-dhcp by default caps the TTL to
> one hour, which happens to be the same as your default TTL.
>
> There is an item in the release notes:
>
> https://source.isc.org/cgi-bin/gitweb.cgi?p=dhcp.git;a=blob;f=RELNOTES;hb=HEAD#l683
>
> and a helpful comment in the source code:
>
> https://source.isc.org/cgi-bin/gitweb.cgi?p=dhcp.git;a=blob;f=server/ddns.c;hb=HEAD#l345

Thanks Tony!


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