On 9/14/2020 1:19 PM, Matej Tyc wrote:
Hello,

I am relatively new to the world of MTAs and MDAs, and I try to set up shared mailboxes.

So far I have somehow succeeded - I have defined a shared namespace and I have managed to create per-mailbox ACL files thanks to the doveadm command.

However, I have been following these resources and there were bits that have puzzled me:

When learning about how ACL work in e.g. https://doc.dovecot.org/settings/plugin/acl/ - when one wishes to use the Global ACL file, how does one link it to a particular user's mailboxes? Examples that are listed in the documentation are far too generic. For example does "* user=foo lrw" imply that all mailboxes of all accounts are shared to the user foo? The doveadm command works only if dovecot is set up with per-mailbox ACL files, so I can't use it to reverse-engineer the correct syntax.


The global ACLs are...global. They apply to all matching mailboxes system-wide. So to answer your question, yes "* user=foo lrw" means all mailboxes of all accounts are shared to the user foo. But...

An interesting aspect to ACLs are dictionaries. I understood it as some kind of cache - if there is no dictionary or it is empty, then shared mailboxes don't work. Conversely, dictionary itself is not enough, one needs actual ACLs set up correctly. Is this a correct understanding?

The ACLs grant/deny access to a specific mailbox - when that mailbox is known to the client. But ACLs are never scanned or iterated over to generate a list of available mailboxes - that's where the dictionary comes in. The dictionary is a list of shared mailboxes - but that's all it is. So when a client queries the server for a list of available mailboxes the dictionary is consulted. The ACLs are then applied for each transaction whenever a client tries to read/write/access/whatever a specific mailbox. So theoretically, if you can manually specify the shared mailbox correctly, no dictionary is required for access.


Next what https://wiki.dovecot.org/SharedMailboxes/Shared and https://wiki.dovecot.org/Dictionary describe is a possibility to reference LDAP data to define an ACL dictionary. Do I understand it correctly that if a LDAP database is the single source of truth, then I don't have to worry about updating dictionaries as long as LDAP itself is up-to-date, but I have to keep ACLs and LDAP in sync manually (or using an application)?

Again, a dictionary is a list of shared mailboxes - not ACL's. You can use any dictionary source Dovecot can read from - but if the dictionary also supports writing then any manipulation of ACLs will automatically update the dictionary.

What the above implies, and I will now state explicitly, is that while global ACLs provide *access* they do not *publish* that access. A dictionary must be manually updated to list those mailboxes.

--
Daniel

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