Enable Active Directory authentication over SMB for Linux clients accessing Azure Files

For more information on supported options and considerations, see Overview of Azure Files identity-based authentication options for SMB access.

Azure Files supports identity-based authentication over Server Message Block (SMB) for Linux virtual machines (VMs) using the Kerberos authentication protocol through the following methods:

• On-premises Windows Active Directory Domain Services (AD DS)
• Microsoft Entra Domain Services

In order to use AD DS, you must sync your AD DS to Microsoft Entra ID using Microsoft Entra Connect.

Applies to

File share type	SMB	NFS
Standard file shares (GPv2), LRS/ZRS
Standard file shares (GPv2), GRS/GZRS
Premium file shares (FileStorage), LRS/ZRS

Linux SMB client limitations

You can't use identity-based authentication to mount Azure File shares on Linux clients at boot time using fstab entries because the client can't get the Kerberos ticket early enough to mount at boot time. However, you can use an fstab entry and specify the noauto option. This won't mount the share at boot time, but it will allow a user to conveniently mount the file share after they log in using a simple mount command without all the parameters. You can also use autofs to mount the share upon access.

Prerequisites

Before you enable AD authentication over SMB for Azure file shares, make sure you've completed the following prerequisites.

• A Linux VM running Ubuntu 18.04+, or an equivalent RHEL or SLES VM. If running on Azure, the VM must have at least one network interface on the VNET containing Microsoft Entra Domain Services. If using an on-premises VM, your AD DS must be synced to Microsoft Entra ID.
• Root user or user credentials to a local user account that has full sudo rights (for this guide, localadmin).
• The Linux VM must not have joined any AD domain. If it's already a part of a domain, it must first leave that domain before it can join this domain.
• A Microsoft Entra tenant fully configured, with domain user already set up.

Installing the samba package isn't strictly necessary, but it gives you some useful tools and brings in other packages automatically, such as samba-common and smbclient. Run the following commands to install it. If you're asked for any input values during installation, leave them blank.

sudo apt update -y
sudo apt install samba winbind libpam-winbind libnss-winbind krb5-config krb5-user keyutils cifs-utils

The wbinfo tool is part of the samba suite. It can be useful for authentication and debugging purposes, such as checking if the domain controller is reachable, checking what domain a machine is joined to, and finding information about users.

Make sure that the Linux host keeps the time synchronized with the domain server. Refer to the documentation for your Linux distribution. For some distros, you can do this using systemd-timesyncd. Edit /etc/systemd/timesyncd.conf with your favorite text editor to include the following:

[Time]
NTP=onpremaadint.com
FallbackNTP=ntp.ubuntu.com

Then restart the service:

sudo systemctl restart systemd-timesyncd.service

Enable AD Kerberos authentication

Follow these steps to enable AD Kerberos authentication. This Samba documentation might be helpful as a reference.

Make sure the domain server is reachable and discoverable

1. Make sure that the DNS servers supplied contain the domain server IP addresses.

systemd-resolve --status

Global 
          DNSSEC NTA: 10.in-addr.arpa
                      16.172.in-addr.arpa
                      168.192.in-addr.arpa
                      17.172.in-addr.arpa
                      18.172.in-addr.arpa
                      19.172.in-addr.arpa
                      20.172.in-addr.arpa
                      21.172.in-addr.arpa
                      22.172.in-addr.arpa
                      23.172.in-addr.arpa
                      24.172.in-addr.arpa
                      25.172.in-addr.arpa
                      26.172.in-addr.arpa
                      27.172.in-addr.arpa
                      28.172.in-addr.arpa
                      29.172.in-addr.arpa
                      30.172.in-addr.arpa
                      31.172.in-addr.arpa
                      corp
                      d.f.ip6.arpa
                      home
                      internal
                      intranet
                      lan
                      local
                      private
                      test 

Link 2 (eth0) 
      Current Scopes: DNS 
       LLMNR setting: yes 
MulticastDNS setting: no 
      DNSSEC setting: no 
    DNSSEC supported: no 
         DNS Servers: 10.0.2.5 
                      10.0.2.4 
                      10.0.0.41
          DNS Domain: domain1.contoso.com 

2. If the command worked, skip the following steps and proceed to the next section.

3. If it didn't work, make sure that the domain server IP addresses are pinging.

ping 10.0.2.5

PING 10.0.2.5 (10.0.2.5) 56(84) bytes of data.
64 bytes from 10.0.2.5: icmp_seq=1 ttl=128 time=0.898 ms
64 bytes from 10.0.2.5: icmp_seq=2 ttl=128 time=0.946 ms

^C 

--- 10.0.2.5 ping statistics --- 
2 packets transmitted, 2 received, 0% packet loss, time 1002ms
rtt min/avg/max/mdev = 0.898/0.922/0.946/0.024 ms

4. If the ping doesn't work, go back to prerequisites, and make sure that your VM is on a VNET that has access to the Microsoft Entra tenant.

5. If the IP addresses are pinging but the DNS servers aren't automatically discovered, you can add the DNS servers manually. Edit /etc/netplan/50-cloud-init.yaml with your favorite text editor.

# This file is generated from information provided by the datasource.  Changes
# to it will not persist across an instance reboot.  To disable cloud-init's
# network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        eth0:
            dhcp4: true
            dhcp4-overrides:
                route-metric: 100
            dhcp6: false
            match:
                macaddress: 00:22:48:03:6b:c5
            set-name: eth0
            nameservers:
                addresses: [10.0.2.5, 10.0.2.4]
    version: 2

Then apply the changes:

sudo netplan --debug apply 

6. Winbind assumes that the DHCP server keeps the domain DNS records up-to-date. However, this isn't true for Azure DHCP. In order to set up the client to make DDNS updates, use this guide to create a network script. Here's a sample script that lives at /etc/dhcp/dhclient-exit-hooks.d/ddns-update.

#!/bin/sh 

# only execute on the primary nic
if [ "$interface" != "eth0" ]
then
    return
fi 

# When you have a new IP, perform nsupdate
if [ "$reason" = BOUND ] || [ "$reason" = RENEW ] ||
   [ "$reason" = REBIND ] || [ "$reason" = REBOOT ]
then
   host=`hostname -f`
   nsupdatecmds=/var/tmp/nsupdatecmds
     echo "update delete $host a" > $nsupdatecmds
     echo "update add $host 3600 a $new_ip_address" >> $nsupdatecmds
     echo "send" >> $nsupdatecmds

     nsupdate $nsupdatecmds
fi 

Connect to Microsoft Entra Domain Services and make sure the services are discoverable

1. Make sure that you're able to ping the domain server by the domain name.

ping contosodomain.contoso.com

PING contosodomain.contoso.com (10.0.2.4) 56(84) bytes of data.
64 bytes from pwe-oqarc11l568.internal.cloudapp.net (10.0.2.4): icmp_seq=1 ttl=128 time=1.41 ms
64 bytes from pwe-oqarc11l568.internal.cloudapp.net (10.0.2.4): icmp_seq=2 ttl=128 time=1.02 ms
64 bytes from pwe-oqarc11l568.internal.cloudapp.net (10.0.2.4): icmp_seq=3 ttl=128 time=0.740 ms
64 bytes from pwe-oqarc11l568.internal.cloudapp.net (10.0.2.4): icmp_seq=4 ttl=128 time=0.925 ms 

^C 

--- contosodomain.contoso.com ping statistics ---
4 packets transmitted, 4 received, 0% packet loss, time 3016ms
rtt min/avg/max/mdev = 0.740/1.026/1.419/0.248 ms 

2. Make sure you can discover the Microsoft Entra services on the network.

nslookup
> set type=SRV
> _ldap._tcp.contosodomain.contoso.com.

Server:         127.0.0.53
Address:        127.0.0.53#53

Non-authoritative answer: 

_ldap._tcp.contosodomain.contoso.com service = 0 100 389 pwe-oqarc11l568.contosodomain.contoso.com.
_ldap._tcp.contosodomain.contoso.com service = 0 100 389 hxt4yo--jb9q529.contosodomain.contoso.com. 

Set up hostname and fully qualified domain name (FQDN)

1. Using your text editor, update the /etc/hosts file with the final FQDN (after joining the domain) and the alias for the host. The IP address doesn't matter for now because this line will mainly be used to translate short hostname to FQDN. For more details, see Setting up Samba as a Domain Member.

127.0.0.1       contosovm.contosodomain.contoso.com contosovm
#cmd=sudo vim /etc/hosts   
#then enter this value instead of localhost "ubuntvm.contosodomain.contoso.com UbuntuVM" 

2. Now, your hostname should resolve. You can ignore the IP address it resolves to for now. The short hostname should resolve to the FQDN.

getent hosts contosovm

127.0.0.1       contosovm.contosodomain.contoso.com contosovm

dnsdomainname

contosodomain.contoso.com

hostname -f

contosovm.contosodomain.contoso.com 

Set up krb5.conf

1. Configure /etc/krb5.conf so that the Kerberos key distribution center (KDC) with the domain server can be contacted for authentication. For more information, see MIT Kerberos Documentation. Here's a sample /etc/krb5.conf file.

[libdefaults]
        default_realm = CONTOSODOMAIN.CONTOSO.COM
        dns_lookup_realm = false
        dns_lookup_kdc = true

Set up smb.conf

1. Identify the path to smb.conf.

sudo smbd -b | grep "CONFIGFILE"

   CONFIGFILE: /etc/samba/smb.conf

2. Change the SMB configuration to act as a domain member. For more information, see Setting up samba as a domain member. Here's a sample smb.conf file.

[global]
   workgroup = CONTOSODOMAIN
   security = ADS
   realm = CONTOSODOMAIN.CONTOSO.COM

   winbind refresh tickets = Yes
   vfs objects = acl_xattr
   map acl inherit = Yes
   store dos attributes = Yes

   dedicated keytab file = /etc/krb5.keytab
   kerberos method = secrets and keytab

   winbind use default domain = Yes 

   load printers = No
   printing = bsd
   printcap name = /dev/null
   disable spoolss = Yes

   log file = /var/log/samba/log.%m
   log level = 1

   idmap config * : backend = tdb
   idmap config * : range = 3000-7999

   idmap config CONTOSODOMAIN : backend = rid
   idmap config CONTOSODOMAIN : range = 10000-999999

   template shell = /bin/bash
   template homedir = /home/%U 

3. Force winbind to reload the changed config file.

sudo smbcontrol all reload-config

Join the domain

1. Use the net ads join command to join the host to the Microsoft Entra Domain Services domain. If the command throws an error, see Troubleshooting samba domain members to resolve the issue.

sudo net ads join -U contososmbadmin    # user  - garead

Enter contososmbadmin's password:

Using short domain name -- CONTOSODOMAIN
Joined 'CONTOSOVM' to dns domain 'contosodomain.contoso.com' 

2. Make sure that the DNS record exists for this host on the domain server.

nslookup contosovm.contosodomain.contoso.com 10.0.2.5

Server:         10.0.2.5
Address:        10.0.2.5#53

Name:   contosovm.contosodomain.contoso.com
Address: 10.0.0.8

If users will be actively logging into client machines or VMs and accessing the Azure file shares, you need to set up nsswitch.conf and configure PAM for winbind. If access will be limited to applications represented by a user account or computer account that need Kerberos authentication to access the file share, then you can skip these steps.

Set up nsswitch.conf

1. Now that the host is joined to the domain, you need to put winbind libraries in the places to look for when looking for users and groups. Do this by updating the passwd and group entries in nsswitch.conf. Use your text editor to edit /etc/nsswitch.conf and add the following entries:

passwd:         compat systemd winbind
group:          compat systemd winbind

2. Enable the winbind service to start automatically on reboot.

sudo systemctl enable winbind

Synchronizing state of winbind.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable winbind

3. Then, restart the service.

sudo systemctl restart winbind
sudo systemctl status winbind

winbind.service - Samba Winbind Daemon
   Loaded: loaded (/lib/systemd/system/winbind.service; enabled; vendor preset: enabled)
   Active: active (running) since Fri 2020-04-24 09:34:31 UTC; 10s ago
     Docs: man:winbindd(8)
           man:samba(7)
           man:smb.conf(5)
 Main PID: 27349 (winbindd)
   Status: "winbindd: ready to serve connections..."
    Tasks: 2 (limit: 4915)
   CGroup: /system.slice/winbind.service
           ├─27349 /usr/sbin/winbindd --foreground --no-process-group
           └─27351 /usr/sbin/winbindd --foreground --no-process-group

Apr 24 09:34:31 contosovm systemd[1]: Starting Samba Winbind Daemon...
Apr 24 09:34:31 contosovm winbindd[27349]: [2020/04/24 09:34:31.724211,  0] ../source3/winbindd/winbindd_cache.c:3170(initialize_winbindd_cache)
Apr 24 09:34:31 contosovm winbindd[27349]:   initialize_winbindd_cache: clearing cache and re-creating with version number 2
Apr 24 09:34:31 contosovm winbindd[27349]: [2020/04/24 09:34:31.725486,  0] ../lib/util/become_daemon.c:124(daemon_ready)
Apr 24 09:34:31 contosovm systemd[1]: Started Samba Winbind Daemon.
Apr 24 09:34:31 contosovm winbindd[27349]:   STATUS=daemon 'winbindd' finished starting up and ready to serve connections 

4. Make sure that the domain users and groups are discovered.

getent passwd contososmbadmin

contososmbadmin:*:12604:10513::/home/contososmbadmin:/bin/bash

getent group 'domain users'

domain users:x:10513: 

If the above doesn't work, check if the domain controller is reachable using the wbinfo tool:

wbinfo --ping-dc

Configure PAM for winbind

1. You need to place winbind in the authentication stack so that domain users are authenticated through winbind by configuring PAM (Pluggable Authentication Module) for winbind. The second command ensures that the homedir gets created for a domain user upon first login to this system.

sudo pam-auth-update --enable winbind
sudo pam-auth-update --enable mkhomedir 

2. Ensure that the PAM authentication config has the following arguments in /etc/pam.d/common-auth:

grep pam_winbind.so /etc/pam.d/common-auth

auth    [success=1 default=ignore]      pam_winbind.so krb5_auth krb5_ccache_type=FILE cached_login try_first_pass 

3. You should now be able to log in to this system as the domain user, either through ssh, su, or any other means of authentication.

su - contososmbadmin
Password:

Creating directory '/home/contososmbadmin'.
contososmbadmin@contosovm:~$ pwd
/home/contososmbadmin
contososmbadmin@contosovm:~$ id
uid=12604(contososmbadmin) gid=10513(domain users) groups=10513(domain users),10520(group policy creator owners),10572(denied rodc password replication group),11102(dnsadmins),11104(aad dc administrators),11164(group-readwrite),11165(fileshareallaccess),12604(contososmbadmin) 

Verify configuration

To verify that the client machine is joined to the domain, look up the FQDN of the client on the domain controller and find the DNS entry listed for this particular client. In many cases, <dnsserver> is the same as the domain name that the client is joined to.

nslookup <clientname> <dnsserver>

Next, use the klist command to view the tickets in the Kerberos cache. There should be an entry beginning with krbtgt that looks similar to:

krbtgt/[email protected]

If you didn't configure PAM for winbind, klist might not show the ticket entry. In this case, you can manually authenticate the user to get the tickets:

wbinfo -K contososmbadmin

You can also run the command as a part of a script:

wbinfo -K 'contososmbadmin%SUPERSECRETPASSWORD'

Mount the file share

After you've enabled AD (or Microsoft Entra ID) Kerberos authentication and domain-joined your Linux VM, you can mount the file share.

For detailed mounting instructions, see Mount the Azure file share on-demand with mount.

Use the following additional mount option with all access control models to enable Kerberos security: sec=krb5. Username and password must be omitted when sec=krb5 is in use.

Other mount options

Single-user versus multi-user mount

In a single-user mount use case, the mount point is accessed by a single user of the AD domain and isn't shared with other users of the domain. Each file access happens in the context of the user whose krb5 credentials were used to mount the file share. Any user on the local system who accesses the mount point will impersonate that user.

In a multi-user mount use case, there's still a single mount point, but multiple AD users can access that same mount point. In scenarios where multiple users on the same client will access the same share, and the system is configured for Kerberos and mounted with sec=krb5, consider using the multiuser mount option.

File permissions

File permissions matter, especially if both Linux and Windows clients will access the file share. To convert file permissions to DACLs on files, use a default mount option such as file_mode=<>,dir_mode=<>. File permissions specified as file_mode and dir_mode are only enforced within the client. The server enforces access control based on the file's or directory's security descriptor.

File ownership

File ownership matters, especially if both Linux and Windows clients will access the file share. Choose one of the following mount options to convert file ownership UID/GID to owner/group SID on file DACL:

• Use a default such as uid=<>,gid=<>
• Configure UID/GID mapping via RFC2307 and Active Directory (nss_winbind or nss_sssd)

File attribute cache coherency

Performance is important, even if file attributes aren't always accurate. The default value for actimeo is 1 (second), which means that the file attributes are fetched again from the server if the cached attributes are more than 1 second old. Increasing the value to 60 means that attributes are cached for at least 1 minute. For most use cases, we recommend using a value of 30 for this option (actimeo=30).

For newer kernels, consider setting the actimeo features more granularly. You can use acdirmax for directory entry revalidation caching and acregmax for caching file metadata, for example acdirmax=60,acregmax=5.

Next step

For more information on how to mount an SMB file share on Linux, see:

• Mount SMB Azure file share on Linux