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I'm not sure what the right answer is just yet, but I'll offer some
of my thoughts FWIW.
I think the fundamental question is whether the mailing list is the
originator of the messages its members receive or whether the
original author is. This question has come up in other contexts
before, and I don't think it's ever been answered satisfactorily. A
quick search through DKIM archives seems to indicate that this
question has come up there too, and I think answering it will help us
understand what we should be doing here.
On Feb 1, 2007, at 3:00 PM, Mark Sapiro wrote:
> Consider that while Mailman doesn't do all of these things to every
> message, it can do any of the following:
[munge the original message]
From the DKIM FAQ:
- What is the purpose of DKIM?
DKIM lets an organization take responsibility for a message.
The organization taking responsibility is a handler of the
message, either as its originator or as an intermediary. Their
reputation is the basis for evaluating whether to trust the
message for delivery.
I think you can make a legitimate case that Mailman is the originator
of messages its members receive. The message is certainly different
than the one the original poster sent to the list, and Mailman is
clearly an intermediary. Perhaps the message has only been munged in
very trivial ways, but it's also possible to munge it in ways that
could potentially be viewed as spammy. For example, what if a site
decides to put some advertisements in the footer?
If you take this view then it seems reasonable to say that it is the
mailing list's system that "take[s] responsibility for a message."
Sure, the mailing list system could verify the DKIM headers on the
message it receives, but ultimate, it is up to the mailing list
system to decide whether that message (or some derivative of that
message) gets transmitted to its recipients.
Or looked at another way, if I send a message through a mailing list,
I wouldn't want to vouch for whatever comes out the other end because
I don't know what they're going to do to my original content. Maybe
then, it's correct for the DKIM signature on the copied message to be
broken because what recipients got was /not/ the message I sent, and
I don't know how it was munged. But that view implies that I am the
originator of the recipient's message. I am, sort of, but also sort
of not.
I'm not convinced that DKIM is really designed to handle the mailing
list use case. It seems to me that it was designed to handle point-
to-point messages, not messages that flow through an intermediary,
because it's not an enveloping system. Contrast that with S/MIME or
OpenPGP. I can sign the message I send from my mailer and that could
be preserved through the transformations that Mailman performs, with
Mailman wrapping my original in its own signature if it wanted to.
Practically speaking, if we can't come up with a consensus on the
interpretation of which "organization [should] take responsibility
for" the actual message that recipients receive, then what would be
the right thing to do? (Note that this answer is different depending
on whether we're talking about Mailman 2.1 or some future version.)
When this came up before I statement my preference not to make a
"strip DKIM headers" selectable by the list owner. I still prefer
this for Mailman 2.1 because doing so would clearly be a new
feature. Maybe a future version could treat the DKIM header the way
it treats the RFC 2369 headers, with a separate selector for List-
Post. Ideally, we'd have a more general way to decide which headers
get cleansed and which new ones get added. But that's for the future.
One elaboration you /might/ be able to get away with in Mailman 2.1
occurs to me as I look at Mark's list:
> - Add text to the beginning of the message body (msg_header)
> - Add text to the end of the message body (msg_footer)
> - Remove text from the beginning of the message body (Approved: line)
> - Add additional MIME parts to a multipart message (msg_header,
> msg_footer)
> - Convert a single part message to multipart in order to add
> msg_header/msg_footer
> - Remove parts from a multipart message (content filtering)
> - Convert an HTML part to plain text (content filtering)
> - Decode a base64 or quoted-printable encoded part and perhaps
> re-encode it with a different encoding.
> - Change or delete various headers including Subject:, To:, From:
> - Replace some MIME parts with URLs of where they were stored and
> flatten the entire message into a single plain text message
> (scrubber).
> - Probably other things I'm overlooking.
If you could identify the message transformations that break the
signature, then you could remove the signature. If the signature of
the outgoing message were still valid because Mailman didn't touch
any part of the message affecting the signature, then you could keep
it. The implementation of this would be fairly simple; the hard part
is writing the code to verify a DKIM signature and parse the
selectors (IIUC the specification) to figure out which of the above
transformations would break the signature. That might be enough to
not do it in Mailman 2.1.
I'm not sure how much I like that anyway, so comments are definitely
welcome. After mulling over this post for an hour ;) I'm starting to
believe that it's the mailing list system that needs to vouch for the
messages its recipients receive. Of course, it could be Mailman
doing the DKIM signing, or it could be Mailman's outgoing MTA, etc.
But, ISTM Mailman is ultimately deciding what goes into the list
copy, so it is responsible for it.
- -Barry
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