MAM silently fails if MUC uses one domain for multiple hosts

[erebion]

Environment

• ejabberd version: 26.04
• Erlang version: Eshell V16.3.1
• OS: Linux (Debian)
• Installed from: official deb

Configuration (only if needed): grep -Ev '^$|^\s*#' ejabberd.yml

  mod_mam:
  ##   ## Mnesia is limited to 2GB, better to use an SQL backend
  ##   ## For small servers SQLite is a good fit and is very easy
  ##   ## to configure. Uncomment this when you have SQL configured:
    db_type: sql
    cache_life_time: 240d
    default: always
    assume_mam_usage: true
  mod_muc:
    host: tardis.erebion.eu 
    access:
      - allow
    access_admin:
      - allow: admin
      - allow: components
    access_create: muc_create
    access_persistent: muc_create
    default_room_options:
      anonymous: true
      mam: true
      public_list: false
    max_users: 1000000

Errors from error.log/crash.log

No errors!

Bug description

Two virtual hosts are configured:

• erebion.eu (main virtual host)
• ejabberd.erebion.eu (mainly used for testing the Matrix GW and other modules before I use them with the main virtual host)

MUC is hard-coded to use tardis.erebion.eu

MAM requests lead to:

<!-- Outgoing Wed Apr 22 00:38:17 2026 (erebion) -->
<iq to="yamusuda@tardis.erebion.eu" type="set" xmlns="jabber:client" id="9e953dfa-2323-49c9-957e-49d18b1b2d58">
  <query xmlns="urn:xmpp:mam:2">
    <x xmlns="jabber:x:data" type="submit">
      <field type="hidden" var="FORM_TYPE">
        <value>urn:xmpp:mam:2</value>
      </field>
    </x>
  </query>
</iq>

<!-- Outgoing Wed Apr 22 00:38:17 2026 (erebion) -->
<r xmlns="urn:xmpp:sm:3" />

<!-- Incoming Wed Apr 22 00:38:17 2026 (erebion) -->
<a h="501" xmlns="urn:xmpp:sm:3" />

<!-- Incoming Wed Apr 22 00:38:17 2026 (erebion) -->
<iq xmlns="jabber:client" xml:lang="en" to="erebion@erebion.eu/gajim.B2V5ANPR" from="yamusuda@tardis.erebion.eu" type="result" id="9e953dfa-2323-49c9-957e-49d18b1b2d58">
  <fin complete="true" xmlns="urn:xmpp:mam:2">
    <set xmlns="http://jabber.org/protocol/rsm">
      <count>0</count>
    </set>
  </fin>
</iq>

Note that there are messages in the database.

Database:

MariaDB [ejabberd]> select server_host from archive where username='yamusuda@tardis.erebion.eu' limit 1;
+---------------------+
| server_host         |
+---------------------+
| ejabberd.erebion.eu |
+---------------------+
1 row in set (6.285 sec)

MariaDB [ejabberd]> 

• This should be set to erebion.eu, but instead is set to ejabberd.erebion.eu, which is incorrect.
• This virtual host was never part of this MUC, it has never had a user joined! It should not show up in connection to this MUC.

Expected behaviour:

• No silent breakage.
• The database should not contain an incorrect value.
• Perhaps there should be an error, if a hard-coded MUC domain cannot be used for multiple virtual hosts

This was found out during debugging with @weiss in the ejabberd MUC. (Thanks a lot!)

Non dev perspective:

• I learned that I cannot use one hard-coded domain for both virtual hosts
• However, MAM should not silently fail by returning a count of "zero messages available"
• It shouldn't save database entries with the wrong host. Perhaps there should be an error explaining that this configuration does not work and just exit.
• I'm not entirely sure how this would be solved best, but currently this is hard to track down for someone who like me is not an ejabberd dev.

[erebion]

@weiss Did I get that summary right?

[weiss]

Yes, you described the cause of the empty MAM response accurately. As I mentioned in the ejabberd channel, your desired setup can be specified by moving the mod_muc configuration into an append_host_config block, giving other local domains access to that MUC service and possibly announcing it via mod_disco.

This is not an ejabberd bug. Your point of reporting it as an issue is that you'd expect ejabberd to log a warning if the same static MUC host is configured for multiple domains, or something?

[erebion]

As I mentioned in the ejabberd channel, your desired setup can be specified by moving the mod_muc configuration into an append_host_config block, giving other local domains access to that MUC service and possibly announcing it via mod_disco.

Sure, but that does not solve the issue that MAM silently fails by returning a count of zero messages, when there are, in fact, messages.

[erebion]

This is not an ejabberd bug. Your point of reporting it as an issue is that you'd expect ejabberd to log a warning if the same static MUC host is configured for multiple domains, or something?

I would expect it to produce an error, as this obviously is not a working setup.

EDIT: Something like "ERROR: Static MUC domain configured for multiple virtual hosts, this does not work."

EDIT 2: I get that this does not work, however, one hould not have to debug this for a long time and then use SQL queries and an ejabberd dev helping out to figure out that this is incorrect and just cannot work.