Member server in an Active Directory domain

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A Samba server needs to join the Active Directory (AD) domain before it can serve files and printers to Active Directory users. This is different from Network User Authentication with SSSD, where we integrate the AD users and groups into the local Ubuntu system as if they were local.

For Samba to authenticate these users via Server Message Block (SMB) authentication protocols, we need both for the remote users to be “seen”, and for Samba itself to be aware of the domain. In this scenario, Samba is called a Member Server or Domain Member.

See also:
Samba itself has the necessary tooling to join an Active Directory domain. It requires a sequence of manual steps and configuration file editing, which is thoroughly documented on the Samba wiki. It’s useful to read that documentation to get an idea of the steps necessary, and the decisions you will need to make.

Use `realmd` to join the Active Directory domain

For this guide, though, we are going to use the realmd package and instruct it to use the Samba tooling for joining the AD domain. This package will make certain decisions for us which will work for most cases, but more complex setups involving multiple or very large domains might require additional tweaking.

Install `realmd`

First, let’s install the necessary packages:

sudo apt install realmd samba

In order to have the joined machine registered in the AD DNS, it needs to have an FQDN set. You might have that already, if running the hostname -f command returns a full hostname with domain. If it doesn’t, then set the hostname as follows:

sudo hostnamectl hostname <yourfqdn>

For this guide, we will be using j1.internal.example.fake, and the AD domain will be internal.example.fake.

Verify the AD server

Next, we need to verify that the AD server is both reachable and known by running the following command:

sudo realm discover internal.example.fake

This should provide an output like this, given our setup:

internal.example.fake
  type: kerberos
  realm-name: INTERNAL.EXAMPLE.FAKE
  domain-name: internal.example.fake
  configured: no
  server-software: active-directory
  client-software: sssd
  required-package: sssd-tools
  required-package: sssd
  required-package: libnss-sss
  required-package: libpam-sss
  required-package: adcli
  required-package: samba-common-bin

realm is suggesting a set of packages for the discovered domain, but we will override that and select the Samba tooling for this join, because we want Samba to become a Member Server.

Join the AD domain

Let’s join the domain in verbose mode so we can see all the steps:

sudo realm join -v --membership-software=samba --client-software=winbind  internal.example.fake

This should produce the following output for us:

 * Resolving: _ldap._tcp.internal.example.fake
 * Performing LDAP DSE lookup on: 10.0.16.5
 * Successfully discovered: internal.example.fake
Password for Administrator:
 * Unconditionally checking packages
 * Resolving required packages
 * Installing necessary packages: libnss-winbind samba-common-bin libpam-winbind winbind
 * LANG=C LOGNAME=root /usr/bin/net --configfile /var/cache/realmd/realmd-smb-conf.A53NO1 -U Administrator --use-kerberos=required ads join internal.example.fake
Password for [INTEXAMPLE\Administrator]:
Using short domain name -- INTEXAMPLE
Joined 'J1' to dns domain 'internal.example.fake'
 * LANG=C LOGNAME=root /usr/bin/net --configfile /var/cache/realmd/realmd-smb-conf.A53NO1 -U Administrator ads keytab create
Password for [INTEXAMPLE\Administrator]:
 * /usr/sbin/update-rc.d winbind enable
 * /usr/sbin/service winbind restart
 * Successfully enrolled machine in realm

Note:
This command also installed the libpam-winbind package, which allows AD users to authenticate to other services on this system via PAM, like SSH or console logins. For example, if your SSH server allows password authentication (PasswordAuthentication yes in /etc/ssh/sshd_config), then the domain users will be allowed to login remotely on this system via SSH.
If you don’t expect or need AD users to log into this system (unless it’s via Samba or Windows), then it’s safe and probably best to remove the libpam-winbind package.

Until bug #1980246 is fixed, one extra step is needed:

Configure /etc/nsswitch.conf by adding the word winbind to the passwd and group lines as shown below:
```
passwd:         files systemd winbind
group:          files systemd winbind
```
Now you will be able to query users from the AD domain. Winbind adds the short domain name as a prefix to domain users and groups:
```
$ getent passwd INTEXAMPLE\\Administrator
INTEXAMPLE\administrator:*:2000500:2000513::/home/administrator@INTEXAMPLE:/bin/bash
```
You can find out the short domain name in the realm output shown earlier, or inspect the workgroup parameter of /etc/samba/smb.conf.

Common installation options

When domain users and groups are brought to the Linux world, a bit of translation needs to happen, and sometimes new values need to be created. For example, there is no concept of a “login shell” for AD users, but it exists in Linux.

The following are some common /etc/samba/smb.conf options you are likely to want to tweak in your installation. The smb.conf(5) man page explains the % variable substitutions and other details:

home directory
template homedir = /home/%U@%D
(Another popular choice is /home/%D/%U)
login shell
template shell = /bin/bash
winbind separator = \
This is the \ character between the short domain name and the user or group name that we saw in the getent passwd output above.
winbind use default domain
If this is set to yes, then the domain name will not be part of the users and groups. Setting this to yes makes the system more friendly towards Linux users, as they won’t have to remember to include the domain name every time a user or group is referenced. However, if multiple domains are involved, such as in an AD forest or other form of domain trust relationship, then leave this setting at no (default).

To have the home directory created automatically the first time a user logs in to the system, and if you haven’t removed libpam-winbind, then enable the pam_mkhomedir module via this command:

sudo pam-auth-update --enable mkhomedir

Note that this won’t apply to logins via Samba: this only creates the home directory for system logins like those via ssh or the console.

Export shares

Shares can be exported as usual. Since this is now a Member Server, there is no need to deal with user and group management. All of this is integrated with the Active Directory server we joined.

For example, let’s create a simple [storage] share. Add this to the /etc/samba/smb.conf file:

[storage]
    path = /storage
    comment = Storage share
    writable = yes
    guest ok = no

Then create the /storage directory. Let’s also make it 1777 so all users can use it, and then ask samba to reload its configuration:

sudo mkdir -m 1777 /storage
sudo smbcontrol smbd reload-config

With this, users from the AD domain will be able to access this share. For example, if there is a user ubuntu the following command would access the share from another system, using the domain credentials:

$ smbclient //j1.internal.example.fake/storage -U INTEXAMPLE\\ubuntu
Enter INTEXAMPLE\ubuntu's password:
Try "help" to get a list of possible commands.
smb: \>

And smbstatus on the member server will show the connected user:

$ sudo smbstatus

Samba version 4.15.5-Ubuntu
PID     Username     Group        Machine                                   Protocol Version  Encryption           Signing
----------------------------------------------------------------------------------------------------------------------------------------
3631    INTEXAMPLE\ubuntu INTEXAMPLE\domain users 10.0.16.1 (ipv4:10.0.16.1:39534)          SMB3_11           -                    partial(AES-128-CMAC)

Service      pid     Machine       Connected at                     Encryption   Signing
---------------------------------------------------------------------------------------------
storage      3631    10.0.16.1     Wed Jun 29 17:42:54 2022 UTC     -            -

No locked files

You can also restrict access to the share as usual. Just keep in mind the syntax for the domain users. For example, to restrict access to the [storage] share we just created to only members of the LTS Releases domain group, add the valid users parameter like below:

[storage]
    path = /storage
    comment = Storage share
    writable = yes
    guest ok = no
    valid users = "@INTEXAMPLE\ LTS Releases"

Choose an `idmap` backend

realm made some choices for us when we joined the domain. A very important one is the idmap backend, and it might need changing for more complex setups.

User and group identifiers on the AD side are not directly usable as identifiers on the Linux site. A mapping needs to be performed.

Winbind supports several idmap backends, and each one has its own man page. The three main ones are:

Choosing the correct backend for each deployment type needs careful planing. Upstream has some guidelines at Choosing an idmap backend, and each man page has more details and recommendations.

The realm tool selects (by default) the rid backend. This backend uses an algorithm to calculate the Unix user and group IDs from the respective RID value on the AD side. You might need to review the idmap config settings in /etc/samba/smb.conf and make sure they can accommodate the number of users and groups that exist in the domain, and that the range does not overlap with users from other sources.

For example, these settings:

idmap config * : range = 10000-999999
idmap config intexample : backend = rid
idmap config intexample : range = 2000000-2999999
idmap config * : backend = tdb

Will reserve the 2,000,000 through 2,999,999 range for user and group ID allocations on the Linux side for the intexample domain. The default backend (*, which acts as a “globbing” catch-all rule) is used for the BUILTIN user and groups, and other domains (if they exist). It’s important that these ranges do not overlap.

The Administrator user we inspected before with getent passwd can give us a glimpse of how these ranges are used (output format changed for clarity):

$ id INTEXAMPLE\\Administrator
uid=2000500(INTEXAMPLE\administrator)
gid=2000513(INTEXAMPLE\domain users)
groups=2000513(INTEXAMPLE\domain users),
       2000500(INTEXAMPLE\administrator),
       2000572(INTEXAMPLE\denied rodc password replication group),
       2000519(INTEXAMPLE\enterprise admins),
       2000518(INTEXAMPLE\schema admins),
       2000520(INTEXAMPLE\group policy creator owners),
       2000512(INTEXAMPLE\domain admins),
       10001(BUILTIN\users),
       10000(BUILTIN\administrators)