[Mailman-Developers] Author_is_list option in upcoming mailman 2.1.16

[Stephen J. Turnbull]

Mark Sapiro writes:

 > Franck has assured me that this feature can be useful even in the
 > absence of the DNS and MTA changes necessary to DKIM sign outgoing
 > list mail,

That's nonsense.  There's no reason to do this in the absence of
correct DKIM signatures by Mailman or its MTA, and every reason
(starting with RFCs 724, 733, 822, 2822, and 5322) not to do so.  Of
course if the point is to violate the RFCs, then this will still
violate the RFCs without the MTA and DNS changes.  But surely that's
not what Franck means by "useful".

The whole point of this option is to allow lists to do what we've come
to expect them to do (discard or quarantine attachments, add header
and footer text, munge Subject) while still presenting a valid DKIM
signature to the receiver.

 > Also, it seems that an installation would want to validate in some way
 > incoming mail before taking responsibility, even in a minor way, for
 > resending it.

Too late.  The reason we should consider adding this feature at all is
that the big email providers are heading in the direction of imposing
that responsibility whether list owners want it or not.

The only real alternative to DKIM signingby Mailman or its MTA is to
pass the original message through, optionally with additional headers
(eg, RFC 2369), but otherwise verbatim, ensuring valid DKIM existing
signatures won't be corrupted.

There is an alternative to From-munging, though, which is to
encapsulate the whole message (either as message/rfc822 MIME type, or
as a one-message digest).  Then the Mailman host can DKIM sign the
wrapper message without invalidating the signature on the main
message.  In theory this could also be done with a multipart/mixed
(*not* multipart/digest), with a structure like

    multipart/mixed
        text/plain        ; Mailman list header
        message/rfc822
        text/plain        ; Mailman list footer

I have no idea what MUAs would do with that, though. :-(

 > All of this leads me to think that making this available to list
 > owners should be an installation decision rather than being done by
 > default.

If the Mailman host is using DKIM, then list owners will definitely
want the option available.  So site owners should have to make a
conscious decision to shut it off.  On the other hand, since it does
involve a serious systematic RFC violation, I think it would be a good
idea to eliminate the DEFAULT_AUTHOR_IS_LIST option.  Ie, require list
owners to set it explicitly, or site owners to hack code.

See my reply to Franck for more detailed comments on the actual
proposed changes.

Steve