Mailman must not strip DKIM-Signature headers

[James Ralston]

I reviewed the discussion that led to the decision to have Mailman strip DomainKey-Signature and DKIM-Signature headers:

http://mail.python.org/pipermail/mailman-developers/2006-January/018537.html

This may have been the correct action to take in 2006. But since then, the DKIM standard has been finalized (RFC4871), and it contains:

3.5. The DKIM-Signature Header Field

   The DKIM-Signature header field SHOULD be treated as though it were a
   trace header field as defined in Section 3.6 of [RFC2822], and hence
   SHOULD NOT be reordered and SHOULD be prepended to the message.

RFC2822 defers to RFC2821, which has been replaced by RFC5321, which states:

4.4. Trace Information

   An Internet mail program MUST NOT change or delete a Received: line
   that was previously added to the message header section. SMTP
   servers MUST prepend Received lines to messages; they MUST NOT change
   the order of existing lines or insert Received lines in any other
   location.

Ergo, Mailman MUST NOT strip DKIM-Signature headers. Even if Mailman knows that actions it will take with the message will invalidate one or more DKIM-Signature headers, those (now-invalid) signatures MUST be left intact. DKIM-Signature headers must be treated exactly like trace (Received) headers.

This isn't simply a case of doing the right thing just for standards compliance: although DKIM states that MTAs can't treat the lack of a DKIM signature any differently than an invalid DKIM signature, the presence of a DKIM-Signature header (even invalid) is a consumable item for MUAs; e.g., a token for Bayesian anti-spam systems. (For example, we currently receive *zero* spam with forged DKIM-Signature headers for our own domain. Zero. Thus, even an invalid DKIM-Signature header is incredibly useful to have.)

The fix for this is simple; just remove this line from Mailman/Handlers/CleanseDKIM.py:

    del msg['dkim-signature']

As for DomainKeys, although the DomainKeys protocol is now historical, RFC4870 states:

3.5.2. Determining Whether an Email Should Be Signed

   A signer MUST NOT sign an email that already contains a "DomainKey-
   Signature:" header unless a "Sender:" header has been added that was
   not included in the original signature. The most obvious case where
   this occurs is with mailing lists.

   A signer SHOULD NOT remove an existing "DomainKey-Signature:" header.

The difficulty here is that the signer (downstream from Mailman) cannot guarantee that a Sender header was added that was not included in the original signature, but it is forbidden from adding another DomainKey-Signature header if that wasn't the case. I suspect this is why many (most?) DomainKeys signers simply refrain from signing if a DomainKey-Signature header is already present, which is the behavior that Joe Peterson observed, and led to the decision to unconditionally strip all DomainKey-Signature and DKIM-Signature headers.

Thus, I think Mailman stripping DomainKey-Signature headers is probably still the best choice; leaving them will all but guarantee that any downstream DomainKey-signer will refrain from generating a DomainKey-Signature. This isn't optimal, of course, but implementation ambiguities like this are why DomainKeys is now historical.

But Mailman absolutely needs to cease stripping DKIM-Signature headers. (And really, CleanseDKIM.py should be renamed to CleanseDomainKeys.py.)

[Mark Sapiro (msapiro)]

This was fixed in Mailman 2.1.10 by adding the following to Defaults.py along with the code to implement it.

# Some list posts and mail to the -owner address may contain DomainKey or
# DomainKeys Identified Mail (DKIM) signature headers <http://www.dkim.org/>.
# Various list transformations to the message such as adding a list header or
# footer or scrubbing attachments or even reply-to munging can break these
# signatures. It is generally felt that these signatures have value, even if
# broken and even if the outgoing message is resigned. However, some sites
# may wish to remove these headers by setting this to Yes.
REMOVE_DKIM_HEADERS = No