Re: [art] not an erratum in RFC 6376, was Argh!!!! onto https://www.rfc-editor.org/errata.php

["Murray S. Kucherawy" <[e-mail]>]

On Thu, Mar 21, 2024 at 8:30 AM Steffen Nurpmeso <[e-mail]> wrote:

> But .. that is what happens in real life software?  Maybe one
> should even add a paragraph stating that, and mention the Sendmail
> milter protocol, which possibly is the culprit of what happens.
> (Though _why_ that is, since the other possibilities would have
> worked out just equally well?  Ie, literal bytes or simple
> stripping.  I cannot tell, i was not around anything email around
> 2006-2011 when that seems to have been developed.)
>
> I did ask on -dkim@ and implemented as written and said, but DKIM
> verification fails for such multiline headers from milter
> communication if RFC 6376 is followed exactly.  (Thankfully DKIM
> has a certificate test mode built-in, otherwise some reputation
> downgrade (if at all possible in my case) would surely have kicked
> in!  It is a great standard!)
>

milter provides multiline headers separated by LFs, not CRLFs.  I don't
recall why it was done this way, but that's how it's been since the
beginning.  This is in the milter API documentation, which is (or used to
be) packaged in the sendmail source tarball.  You can find it online too;
for example:

https://www.ibm.com/docs/en/aix/7.2?topic=functions-xxfi-header-callback-function

libopendkim is meant to work in any environment, so it makes no assumptions
and expects RFC-compliant input.   Therefore, opendkim (the filter, which
uses the milter API to act on messages in transit) has to convert those LFs
to CRLFs before passing multi-line headers to the library if you want
things to work properly.  If you don't, there's a good chance the header
value won't be canonicalized properly and you're going to have problems.

The library has a flag you can set to convert bare CRs and LFs to CRLFs if
you want it to do that, but that's a convenience feature meant to deal with
broken customers; as John correctly points out, it is undefined what one is
supposed to do when encountering those in the wild.  The risk here is that
if you have it do that conversion, but what goes out over the wire is not
also converted the same way, the signature will never verify.

-MSK