This weekend at the SouthEast LinuxFest, I had a talk about how you can authenticate to Fedora using Active Directory credentials that lack Unix attributes. Since newer deployments of the most recent versions of Active Directory no longer give you the ability by default to configure Unix attributes, it is important to know that this is not a show stopper.
In my talk, I showed how SSSD uses ID Mapping by converting an objectSID value from a user object from binary to a human-readable number and then runs that number through an algorithm to generate a UID. It will do the same thing for group objects so that you also have GIDs. Besides the UID and GID, SSSD has the ability to use a ‘fallback’ mode for home directory and shell locations. This way, you can “fill in the blanks” of missing information.
Here is an example user object we used in the demonstration to show this:
ldapsearch -LLL -h coldharbour.win.terranforge.com -D Administrator@WIN.TERRANFORGE.COM -W -b dc=win,dc=terranforge,dc=com samaccountname=youknownothing
dn: CN=Jon Snow,CN=Users,DC=win,DC=terranforge,DC=com
cn: Jon Snow
distinguishedName: CN=Jon Snow,CN=Users,DC=win,DC=terranforge,DC=com
displayName: Jon Snow
name: Jon Snow
As you can see, Jon Snow (youknownothing) lacks four of the things that POSIX compliant systems require a user to have: UID, GID, Home Directory and Shell. However, on a Fedora 23 system that has been joined to the same AD domain, we can successfully see that the user DOES have a UID, GID, Home Directory and Shell:
[root@garden ~]# cat /etc/fedora-release
Fedora release 23 (Twenty Three)
[root@garden ~]# id youknownothing
uid=436801105(youknownothing) gid=436800513(domain users) groups=436800513(domain users)
[root@garden ~]# getent passwd youknownothing
And, we can authenticate as that user to the Fedora system:
[root@garden ~]# ssh youknownothing@localhost
[youknownothing@garden ~]$ id
uid=436801105(youknownothing) gid=436800513(domain users) groups=436800513(domain users) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
This happens successfully because SSSD is converting the binary SID value to a number, turning that number into a UID based off of an algorithm and then filling in whatever attributes are necessary for the POSIX-compliant system to accept the user as valid. The only thing SSSD requires from AD to make this happen is an ‘id’, such as a username and the SID attribute. In sssd.conf, we specify the Shell and Home Directory attributes:
id_provider = ad
ad_server = coldharbour.win.terranforge.com
services = nss, pam
config_file_version = 2
domains = win.terranforge.com
Using the ‘default_shell’ and ‘fallback_homedir’ options means that if SSSD does not find these attributes within AD, it will substitute what you give it. In this case, /bin/bash and /home/%u. This allows you to specify the unixHomeDir and unixShell attributes in AD for a user if you still desire to do so, and SSSD will use those.
To generate an UID and GID based off of the object’s SID value, SSSD’s ID Mapping algorithm is very similar to how Winbind’s autorid backend works. This makes it trivial to move from older Winbind configurations to SSSD and continue to retain original UID and GID values. Using SSSD in this fashion will make the UIDs and GIDs across all systems joined to AD consistent for each user and group, making things like file-sharing hassle-free.