[Dovecot] Exposing global (default) sieve script through Managesieve

[Matthijs Kooijman]

Hi folks,

I'm setting up a dovecot server with managesieve support. I'd like to
offer spamfiltering through a Sieve script to my users by default,
but still allow them to modify the filtering rules through Managesieve.

I found the sieve_global_path configuration option, which seems perfect
for what I want. I can configure a default script there, which will work
for all users until they set upload their own sieve script using
managesieve.

However, when configured like this, the user experience isn't quite
perfect. When users open the managesieve interface on their client,
there is no trace of the default filters, so users might think the
spamfiltering is done in some other manner. Now, when they create a
filtering rule (e.g., to sort out mail to mailing lists), that rule will
overwrite the default spamfiltering rule causing all the spam to spill
into the user's mailbox. I'm afraid that most users won't realize they
have to manually recreate the spamfiltering rule to fix this. Also, they
might not know how to write the rule, even if they do...

I've considered a few existing ways to fix this:
 - Use sieve_before / sieve_after to make sure that the default script
   is always executed, in addition to any user-supplied scripts. This
   removes the surprise, but removes the option for users to tweak the
   spamfiltering rules.
 - Don't use sieve_global_path, but instead distribute the default
   script to each user's homedir on user creation. This prevents making
   changes to the default script for existing users and in my setup,
   user creation and (mail)homedir creation are nicely separated through
   an LDAP directory, I'd rather not go this route.
 - When using the Roundcube webmail application as the IMAP client, I
   can point Roundcube at the default sieve script. Now, when Roundcube
   sees there are no scripts through ManageSieve, it shows a (fake)
   "default" script with the correct contents. As soon as the user
   changes this script or creates a new script, it is actually uploaded
   to Dovecot, causing the edited script to be used instead of the
   global script.

   This option has the user experience I'm looking for, but having this
   out-of-band connection from Roundcube to the default script
   configured with dovecot is ugly (and tricky, since these run on
   different hosts in my setup). The biggest problem is of course that
   this only works for Roundcube, not for any other IMAP client my users
   might use.

So, I was wondering: Wouldn't it make sense for the managesieve plugin
to do something similar to roundcube: When the user has no sieve script
configured, let it fake a single "default" script, showing the contents
of the global script?

Since the ManageSieve protocol doesn't seem to support any way to flag
this situation, it would be fooling the clients a bit, but I'm not sure
if that's really a problem.

While the user has not script named "default" in his sieve_dir:
 - include a script called "default" in the LISTSCRIPTS output.
 - return the contents of the sieve_global_path in the GETSCRIPT
   "default" command.
 - remove any sieve symlink after a SETACTIVE "default" command (as if
   SETACTIVE "" was given). This causes dovecot to fall back to the
   sieve_global_path script.
 - the DELETESCRIPT "default" command should fail. This might confuse
   clients and users, since it is listed in LISTSCRIPTS but cannot be
   deleted, but I think most users will understand they can't delete the
   default script.
 - RENAMESCRIPT "default" "some_name" should copy the sieve_global_path
   script into the user's sieve_dir. This will effectively copy the
   script instead of renaming it (since it will still be magically
   listed in LISTSCRIPTS), so that might be confusing.

All other commands work just like they do now (in particular, 
PUTSCRIPT "default" uploads a script called "default" into the user's
sieve_dir, preventing all of the above from applying.

As noted above, this change might cause some confusion, but I think that
is manageable. On additional thing is that running SETACTIVE "" will not
completely disable sieve processing (as would be expected), but will
(again) cause the sieve_global_path script to be run. This is already
the case currently, though, and should probably be considered a separate
problem (whose root cause is the lack of a difference between "no script
script configured yet" and "active script disabled", both remove the
sieve symlink). Also, this problem might be a feature in some setups, so
fixing it might not be so easy...


So, any thoughts on this? Any fundamental problems I'm missing? (Not-so)
obvious alternatives?

Gr.

Matthijs
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