[getdns-api] Adding an extension to highlight common DNS errors

[Phil Pennock]

On 2013-01-27 at 18:09 -0800, Paul Hoffman wrote:
> On Jan 27, 2013, at 5:10 PM, Phil Pennock <getdns-api-phil at spodhuis.org> wrote:
> > IP address as hostname field in MX record.
> > 
> > IP address as NS hostname field.  As AFSDB hostname field.
> 
> I feel like I need to take a shower. But now that you say it, I can
> believe that they probably exist in non-trivial sample sizes. Will
> add.

By default, Exim doesn't honour IP addresses in MX records; it crops up
occasionally that someone comes asking why things aren't working and we
point them to our "allow_mx_to_ip" option.

> > There's a theme there: expected value type mismatch, IP incorrectly
> > given, but folks often expect it to work anyway.  Which ties into the
> > CNAME too.
> 
> Can you be more specific here?

Various RR types expect a hostname in some field.  In some cases, they
get IP addresses, in some cases they get CNAMEs instead.  In all the
highlighted examples which have so far been accepted as in-scope and
issues, this is the common theme.  DNS as a protocol makes it possible
to screw up here and for some apps to be lax, but in just about all
other cases it's either a deeper semantic issue or something that might
cause a zonefile to fail to be loaded.

It's this one use-case where the syntax permits "sequence of labels
which are supposed to be one thing" and people can encode, eg, an IP
address (even though it will go as many bytes ASCII-encoded numbers
instead of as a raw 4/16-byte field).

Thus: common theme.

-Phil