Azure AD Domain Services for Linux authentication

NOTE: I do not mean for this to be an exhaustive guide on how you should implement security on your *nix infrastructure, so make sure you’ve thought this out. There are also some larger Azure AD design considerations you should research more before proceeding as well.

A customer with on-premises Active Directory infrastructure syncing to Azure Active Directory (AAD from here on), for Office 365 primarily, wanted to enable AD authentication on their *nix systems in Azure. The customer was adamant they did not:

• want to deploy/pay for any Windows systems if it could be avoided
• want the *nix infra reaching back over the VPN to on-prem
• want to deploy any 3rd party software (ie: Centrify, or something like that)
• want to deploy a shadow OpenLDAP+Kerberos system

To fulfill these requirements, enter stage right: Azure AD Domain Services.

This would be a new adventure for me, since most people have/are willing to deploy windows in Azure. This tool essentially provides, what amounts to, read-only Domain Controller services without the overhead of a Windows operating system. It then synchronizes (ONE-WAY!!!) from AAD, which is being fed from the on-premises AD infra. You should read the docs more, but while its a one-way push, you can do some things like create OU’s and other fun stuff… but thats way out of scope from the spirit of this post.

1.) The first step down this road is to deploy Azure AD Domain Services (lets shorten this to from here on to ADDS). There are some things to consider here, and as stated, I am not going to be exhaustive. At the time of this writing, it would appear you can only have one ADDS per tenant, so think long and hard about where you decide to site this.

Some things I had to think about and decisions I made when I spun ADDS up (just as a small example):

• Where is my customer primarily hosting their VM’s? (US Central)
• Do I want to put it inside its own Virtual Network? (YES)
• Review firewall configuration required.
• Create peering from *nix machine Virtual Networks to the ADDS Virtual Network.
• Change subnets the *nix machines run in to use ADDS for DNS.
• Do I have any strange/internal DNS considerations to worry about? (Thankfully, NO)
• Will the customer want to administer ADDS beyond the Azure console in the cloud? (Thankfully, NO. Otherwise they would HAVE to deploy a Windows machine in Azure, see: https://aka.ms/aadds-admindomain)

After thinking about those, and many other things, ADDS spun up without drama in about 30 minutes. I then did some of my prerequisite DNS changes in various subnets.

2.) The next obvious step is, configure the *nix machines to use this new ADDS functionality you deployed. Here is a very rough guide on how I did this. Again, there is so much nuance to this… you really need to plan this out and lab it up like any SSO project. This is bare bones as it gets here.

2a.) Reconfigure the machine to use the new ADDS DNS ip’s. In my case, the machines were all using DHCP so nothing to do except bounce the lease. If you are managing DNS outside of DHCP, you will have o “do the thing” with nmcli, resolv.conf… whatever applies to your environment.

2b.) Install the necessary packages. In my demo, I’m on CentOS 7.x:

# yum install -y realmd oddjob oddjob-mkhomedir sssd samba-common-tools adcli krb5-workstation

2c.) Configure the machine to use ADDS:

We have a couple different paths we can take here: realm or adcli. In my case, realm as A-OK for my use case. XXXXX == being ADDS name of course.

[root@XXXXXX-www01 ~]# realm join XXXXXX.onmicrosoft.com -U adminuser@XXXXXX.ONMICROSOFT.COM -v
* Resolving: _ldap._tcp.XXXXXX.onmicrosoft.com
* Performing LDAP DSE lookup on: 10.0.3.5
* Successfully discovered: XXXXXX.onmicrosoft.com
Password for adminuser@XXXXXX.ONMICROSOFT.COM:
* Required files: /usr/sbin/oddjobd, /usr/libexec/oddjob/mkhomedir, /usr/sbin/sssd, /usr/bin/net
* LANG=C LOGNAME=root /usr/bin/net -s /var/cache/realmd/realmd-smb-conf.RQ0NB0 -U adminuser@XXXXXX.ONMICROSOFT.COM ads join XXXXXX.onmicrosoft.com
Using short domain name — XXXXXX
Joined ‘XXXXXX-WWW01’ to dns domain ‘XXXXXX.onmicrosoft.com’
* LANG=C LOGNAME=root /usr/bin/net -s /var/cache/realmd/realmd-smb-conf.RQ0NB0 -U adminuser@XXXXXX.ONMICROSOFT.COM ads keytab create
* /usr/bin/systemctl enable sssd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/sssd.service to /usr/lib/systemd/system/sssd.service.
* /usr/bin/systemctl restart sssd.service
* /usr/bin/sh -c /usr/sbin/authconfig –update –enablesssd –enablesssdauth –enablemkhomedir –nostart && /usr/bin/systemctl enable oddjobd.service && /usr/bin/systemctl start oddjobd.service
* Successfully enrolled machine in realm

At this point, unless you want to fully qualify your users, you might want to edit your /etc/sssd/sssd.conf and change use_fully_qualified_names to: False and then restart the sssd daemon.

3.) At this point, if all has gone well, you should be able to see the user objects & groups which have flowed from on-prem to ADDS:

[root@XXXXXX-www01 ~]# id testuser
uid=691801111(testuser) gid=691800513(domain users) groups=691800513(domain users),691801104(aad dc administrators),691801113(linux-sudo_all),691800520(group policy creator owners),691801102(dnsadmins),691800572(denied rodc password replication group)

[root@XXXXXX-www01 ~]# groups testuser
testuser : domain users aad dc administrators linux-sudo_all group policy creator owners dnsadmins denied rodc password replication group

At this point, … time to start configuring things like your allowed ssh groups, sudoers … all that fun stuff. Extra points if you manage your SSH keys in a secure and automated fashion as well for another layer of security.

Some final thoughts:

If you’ve used Azure AD Connect, you know there can be lag issues for the impatient. ADDS adds yet another layer into the mix. On top of everything else, you’re now waiting for AAD to synchronize into ADDS too. So between on-prem -> AAD -> ADDS … it can be a while for changes/new objects to become a reality. Keep this in mind, and if this is a non-starter for you… look for another solution.

This customer pretty much doesn’t even allow interactive access to their production *nix infra (a good thing!), so this was really just needed for the basics if the … you know what … hit the fan.

My customer was a small shop, so the pricing worked out to be about (at the time of this post) around $110/mo for ADDS. They considered this a huge win over a Windows VM + the overhead of managing and securing it.

Til next time…