Although it's rare, sometimes Samba crashes or fails to start. When
this happens, restarting it is almost always enough to get it working
again. Since all sorts of authentication problems can occur if one of
the domain controllers is down, it's probably best to just have systemd
automatically restart _samba.service_ if it ever stops for any reason.
The `net cache flush` command does not seem to always work to clear the
identity mapping cache used by winbind. Explicitly moving the file
does, though.
On a new DC, the `idmap.ldb` file does not yet exist the first time
`sysvolsync` runs. This causes a syntax error in the condition that
checks the modification timestamp of the file.
Forcing the PDC lookup to use localhost as the DNS server does not work
when first adding a new domain controller, as the `sysvolsync` script
runs before Samba starts. There isn't much advantage to using the local
DNS server over the system-defined server anyway.
The Samba KDC log file seems to grow rather quickly sometimes, outpacing
the monthly rotation policy. Let's rotate it weekly and keep 4
historical versions.
The `tls cafile` setting in `smb.conf` is not necessary. It is used for
verifying peer certificates for mutual TLS authentication, not to
specify the intermediate certificate authority chain like I thought.
The setting cannot simply be left out, though. If it is not specified,
Samba will attempt to load a file from a built-in default path, which
will fail, causing the server to crash. This is avoided by setting the
value to the empty string.
Samba AD DC does not implement [DFS-R for replication of the SYSVOL][0]
contents. This does not make much of a difference to me, since
the SYSVOL is really only used for Group Policy. Windows machines may
log an error if they cannot access the (basically empty) GPO files, but
that's pretty much the only effect if the SYSVOL is in sync between
domain controllers.
Unfortunately, there is one side-effect of the missing DFS-R
functionality that does matter. On domain controllers, all user,
computer, and group accounts need to have Unix UID/GID numbers mapped.
This is different than regular member machines, which only need UID/GID
numbers for users that will/are allowed to log into them. LDAP entries
only have ID numbers mapped for the latter class of users, which does
not include machine accounts. As a result, Samba falls back to
generating local ID numbers for the rest of the accounts. Those ID
numbers are stored in a local database file,
`/var/lib/samba/private/idmap.ldb`. It would seem that it wouldn't
actually matter if accounts have different ID numbers on different
domain controllers, but there are evidently [situations][1] where DCs
refuse to allocate ID numbers at all, which can cause authentication to
fail. As such, the `idmap.ldb` file needs to be kept in sync.
If we're going to go through the effort of synchronizing `idmap.ldb`, we
might as well keep the SYSVOL in sync as well. To that end, I've
written a script to synchronize both the SYSVOL contents and the
`idmap.ldb` file. It performs a simple one-way synchronization using
`rsync` from the DC with the PDC emulator role, as discovered using DNS
SRV records. To ensure the `idmap.ldb` file is in a consistent state,
it only copies the most recent backup file. If the copied file differs
from the local one, the script stops Samba and restores the local
database from the backup. It then flushes Samba's caches and restarts
the service. Finally, it fixes the NT ACLs on the contents of the
SYSVOL.
Since the contents of the SYSVOL are owned by root, naturally the
synchronization process has to run as root as well. To attempt to limit
the scope of control this would give the process, we use as much of the
systemd sandbox capabilities as possible. Further, the SSH key pairs
the DCs use to authenticate to one another are restricted to only
running rsync. As such, the `sysvolsync` script itself cannot run
`tdbbackup` to back up `idmap.ldb`. To handle that, I've created a
systemd service and corresponding timer unit to run `tdbbackup`
periodically.
I considered for a long time how to best implement this process, and
although I chose this naïve implementation, I am not exactly happy with
it. Since I do not fully understand *why* keeping
the `idmap.ldb` file in sync is necessary, there are undoubtedly cases
where blindly copying it from the PDC emulator is not correct. There
are definitely cases where the contents of the SYSVOL can be updated on
a DC besides the PDC emulator, but again, we should not run into them
because we don't really use the SYSVOL at all. In the end, I think this
solution is good enough for our needs, without being so complicated
[0]: https://wiki.samba.org/index.php?title=SysVol_replication_(DFS-R)&oldid=18120
[1]: https://lists.samba.org/archive/samba/2021-November/238370.html
This work-around is no longer necessary as the default Fedora policy now
covers the Samba DC daemon. It never really worked correctly, anyway,
because Samba doesn't start `winbindd` fast enough for the
`/run/samba/winbindd` directory to be created before systemd spawns the
`restorecon` process, so it would usually fail to start the service the
first time after a reboot.
The KDC service, as managed by Samba, continuously logs to two files
that need to be rotated. The upstream configuration for logrotate only
manages one of these files, and does not correctly signal the service
after rotating, as it expects the service to be managed by systemd
instead of Samba. As such, we need to adjust the configuration to
handle both files and send SIGHUP directly to the process.
Send logs to the systemd journal for easier viewing and disable logging
to a file. Also, the `samba_dc_log_level` variable can control the log
level (0-10, 0 being off, 10 being insane debugging).
I honestly don't remember why the `use rfc2307` setting was only enabled
on the first DC. All DCs seem to need this setting in order to use the
UID/GID numbers from the directory, instead of using auto-generated
numbers.
In order to enable LDAPS/STARTTLS support in Samba, the `tls enabled`
option must be set to `yes` and the `tls keyfile` and `tls certfile`
options must be set to the path of the private key and certificate
files, respectively, that Samba will use. The `samba_tls_enabled`,
`samba_tls_keyfile`, and `samb_tls_certfile` Ansible variables can be
used to control these values.
The service principal name added to `/etc/krb5.keytab` had a trailing
`}` character because of a typo in the Ansible task. This resulted in
GSSAPI authentication failing because server processes could not find
the host key in the key table.
The *samba-dc* role now configures `winbindd` on domain controllers to
support identity mapping on the local machine. This will allow domain
users to log into the domain controller itself, e.g. via SSH.
The Fedora packaging of *samba4* still has some warts. Specifically, it
does not have a proper SELinux policy, so some work-arounds need to be
put into place in order for confined processes to communicate with
winbind.
This commit adjusts the tasks in the *samba-dc* role to use a
conditional include to restrict tasks relating to the BIND9_DLZ plugin
only to hosts that are configured to use it.
The *bind-utils* package contains `dig` and `nsupdate`, which are used
to query and manage DNS records.
The *cyrus-sasl-gssapi* package contains the GSSAPI plugin for
SASL-aware applications, including `ldapsearch`.
The *ldb-tools* package contains `ldbsearch` and other tools for
directly using Samba database files.
The `/var/lib/samba/bind-dns` directory contains files that are
hard-linked to files in the `/var/lib/samba/private` directory. All
paths for a file must have the same context, or `restorecon` will
effectively "toggle" the labels each time it is run.
Evidently, some files in `/var/lib/samba` match multiple file context
rules. Thus, when running `restorecon` against the entire
`/var/lib/samba` directory, files in the `bind-dns` subdirectory may end
up with the wrong label. To work around this issue, `restorecon` is now
run only on that subdirectory to ensure the correct labels are applied.
This is likely to cause problems when a full filesystem relabel is
scheduled.
The *samba-dc* role now supports joining an existing Active Directory
domain as an additional domain controller. The `samba_is_first_dc` variable
controls whether the machine will be provisioned with a new domain (when
true) or added to an existing domain (when false).
Joining an existing domain naturally requires credentials of a user with
permission to add a new DC, the `samba_dc_join_username` and
`samba_dc_join_password` variables can be used to specify them.
Alternatively, if these variables are not defined, then the process will
attempt to use Kerberos credentials. This would require playbooks to
make a ticket-granting-ticket available somehow, such as by executing
`kinit` prior to applying the *samba-dc* role.
The *samba-dc* role installs Samba on the managed node and configures it
as an Active Directory Domain controller. A custom module,
`samba_domain` handles the provisioning using `samba-tool domain
provision` in an idempotent way.
Dustin