Multiple $TTL values

Bob Harold rharolde at umich.edu
Thu Sep 22 19:56:23 UTC 2016


On Thu, Sep 22, 2016 at 3:36 PM, Woodworth, John R <
John.Woodworth at centurylink.com> wrote:

> Hello,
>
>
>
> We’ve recently noticed multiple $TTL values in transferred zonefiles which
> do not exist in the original zonefiles.  They appear to be aggregates of
> TTLs set for individual records and I am definitely a fan of the organized
> look and feel.
>
>
>
> However, I am curious about how they should be interpreted where $ORIGIN
> is concerned.  I just re-read rfc2308 and it states quite simply:
>
> “   All resource records appearing after the directive, and which do not
>
>     explicitly include a TTL value, have their TTL set to the TTL given
>
>     in the $TTL directive. “
>
>
>
> My confusion is $ORIGIN basically defines the default origin while reading
> in the file and creates a mini-universe for interpreting records until
> redefined.  Would a $TTL after a $ORIGIN be encapsulated by and restricted
> to records within that $ORIGIN block?
>
>
>
> My gut tells me no, and to follow the RFC literally (or loosely stated
> “from this point down”) but looking at the file it seems as if the $TTL is
> intended to be for the records within the $ORIGIN only (i.e. it is not
> reset to global value at the end).
>
>
>
> I need to investigate this more on my own but I thought it might prove
> useful to ask the group as part of my research.
>
>
>
>
>
> Thanks in advance,
>
> John
>
>
>
This is a common point of confusion.  DNS does not transfer zoneFILES.
Zone files are read and converted into the in-memory tree structure.  Zones
are sent in wire format from the in-memory tree.  The receiving end
populates its in-memory tree.  It can then convert the information to zone
file format, and write it out, but do not expect it to look anything like
the original zone file. It has no idea what the original file looked like,
or what order the records were in.

$ORIGIN and $TTL only apply to the zone they are in, so no need to reset
them at the end of the file since they cease to exist at that point.  They
apply "from this line down until changed" and are merely a convenience to
shorten the size of the file.

-- 
Bob Harold
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