This page describes how to configure Linux systems to connect to an Active Directory domain as the database for users and groups.

sssd or winbind - an introduction

TL;DR: sssd is great and winbind generally sucks, I recommend configuring sssd manually because realmd is a bit unreliable.

AD wasn't designed for Linux, and since most of our machines run Linux we have made things difficult for ourselves. There are two main options for joining Linux machines to AD domains, which are sssd and winbind. Here we will use sssd because it is much cleaner, far more configurable, much better documented so much better in almost every way compared to winbind. realmd is a utility which automatically configures sssd or winbind but it doesn't work especially well in the environment at UCC.

Both of these are services which run in the background and provide a source of user IDs, group IDs, user info, manage authentication and provide group membership info - all of which can be done on Linux systems using the files /etc/passwd (user ID and information listing), /etc/shadow (secure user password storage) and /etc/group (group IDs and membership listing). Active Directory is based on the Windows security model, which is somewhat more complex than the POSIX standard implemented in Linux, so it is the task of software like sssd and winbind to provide a way to map the Windows/AD model onto the Linux/POSIX equivalents.

Notes:

• winbind <= 4.6.0 does not support certain important configuration options. If you need to use winbind, follow the steps here to get a newer version from the package repositories.

• sssd <= 1.15.0 has bugs causing certificate verification to fail regardless of the TLS config, try to get a newer version or fix your AD domain certificate issues.

Manual configuration of sssd

There is a very informative Red Hat article about configuring sssd manually. The following instructions are a functional adaptation. For a more detailed understanding of what these config files and options mean, please start by reading the manpages for sssd, sssd-ad, sssd-krb5, sssd-ldap, sssd.conf, krb5.conf and smb.conf and the above article.

These instructions are confirmed working on Linux Mint 19 (Tara), using sssd version 1.16.1 and samba-common-bin package version 4.7.6.

1. Install the necessary packages and uninstall the bad ones:

   apt-get install samba-common-bin samba-dsdb-modules sssd sssd-ad sssd-krb5 sssd-ldap sssd-tools krb5-user krb5-doc libpam-sss libnss-sss
   apt-get remove winbind realmd libpam-krb5 libpam-ldap libpam-winbind libnss-ldap libnss-winbind

2. Remove any configuration files if they exist and stop services:

   mv /etc/samba/smb.conf /etc/samba/smb.conf.old
   mv /etc/nsswitch.conf /etc/nsswitch.conf.old
   rm /etc/krb5.conf /etc/krb5.keytab /etc/sssd/sssd.conf
   systemctl stop sssd winbind samba-ad-dc smbd nmbd

3. Put the following into various config files:

   • vim /etc/sssd/sssd.conf

     [sssd]
     config_file_version = 2
     domains = ad.v.ucc.asn.au
     services = nss, pam, pac
     
     # domain configuration: see manpages sssd.conf, sssd-ldap, sssd-krb5 and sssd-ad
     # see https://access.redhat.com/articles/3023951
     # needs correct configution for: /etc/nsswitch.conf /etc/samba/smb.conf /etc/resolv.conf `pam-auth-update`
     [domain/AD.V.UCC.ASN.AU]
     enumerate = true
     id_provider = ad
     auth_provider = ad
     chpass_provider = ad
     access_provider = ad
     ldap_id_mapping = false
     
     cache_credentials = true
     # if you want to use a custom CA certificate for AD
     #ldap_tls_cacert = /etc/sssd/ad.v.ucc.asn.au_ca.cert
     # or just allow invalid (self-signed) certificates
     ldap_tls_reqcert = allow
     
     # allow local users to be included in AD groups
     ldap_rfc2307_fallback_to_local_users = true
     
     # Kerberos konfiguration - see also /etc/krb5.conf
     #krb5_use_kdcinfo = true  # let sssd tell krb5 what to do
     krb5_realm = AD.V.UCC.ASN.AU
     krb5_keytab = /etc/krb5.keytab
     
     # which attributes to use for AD object to POSIX mappings
     #ldap_user_name = uid
     #ldap_user_uid_number = uidNumber
     #ldap_user_gid_number = gidNumber
     #ldap_user_object_class = user
     #ldap_schema = ad

   • vim /etc/nsswitch.conf

     # /etc/nsswitch.conf
     
     passwd:         compat systemd sss
     group:          compat systemd sss
     shadow:         compat sss
     gshadow:        files
     
     hosts:          files mdns4_minimal [NOTFOUND=return] dns myhostname
     networks:       files
     
     protocols:      db files
     services:       db files sss
     ethers:         db files
     rpc:            db files
     
     netgroup:       nis sss

   • vim /etc/krb5.conf

     [libdefaults]
         default_realm = AD.V.UCC.ASN.AU
         dns_lookup_realm = true
         dns_lookup_kdc = true
         rdns = false
         ticket_lifetime = 24h
         renew_lifetime = 7d
         forwardable = yes

   • vim /etc/samba/smb.conf

     [global]
             realm = AD.V.UCC.ASN.AU
             workgroup = VUCC
             security = ads
             client signing = mandatory
             client use spnego = yes
             tls enabled = yes
             kerberos method = secrets and keytab
             dedicated keytab file = /etc/krb5.keytab
             log file = /var/log/samba/%m.log

4. Run pam-auth-update and check that SSS authentication is enabled and that winbind authentication (if present) is disabled.

5. Create the machine account in AD and produce a keytab for sssd containing the machine account credentials. If using a user other than Administrator, ensure they have sufficient privileges to join a machine to the domain.

   rm /etc/krb5.keytab
   kinit Administrator
   net ads join --no-dns-updates -k
   # verify the machine account credentials are in the keytab:
   klist -k

6. Start the necessary services:

   systemctl start sssd

7. Verify that you can see the correct user and group info using getent passwd and getent group respectively. The output format is equivalent to the /etc/passwd and /etc/group files.

Automatic configuration of sssd using realmd

Note: realmd is a misnomer - it is not a daemon but simply a utility which automatically generates configuration files for sssd or winbind.

See the official realmd website for more information.

Realmd is supposedly able to generate valid configuration files and automatically detect the necessary configuration settings, and this might work when joining a Linux machine to a normal, functional and correctly configured AD domain using Windows domain controllers, but in the case of the VUCC domain it simply fails to work.

Installation instructions:

1. Install packages: apt install realmd krb5-user sssd-ad sssd-krb5 sssd-tools sssd-ldap samba-common-bin adcli libnss-sss libpam-sss

2. Test to make sure you can connect to the domain: realm discover ad.v.ucc.asn.au

3. Join to the domain using realm join -v -U <user> ad.ucc.gu.uwa.edu.au

   • realmd defaults to using sssd, which is fine
   • It installs any necessary packages.

   • It generates the configuration files /etc/krb5.conf, /etc/samba/smb.conf, /etc/sssd/sssd.conf, modifies /etc/nsswitch.conf, creates a machine account on the domain and populates /etc/krb5.keytab with machine account credentials, and configures PAM to use SSS authentication.

     • The automatically generated configurations are not very clean, contain redundant information and are broken.

     • Comment the line use_fully_qualified_names = True in /etc/sssd/sssd.conf

4. It might just work - if not then verify the config files are correct according to the manual instructions, or just give up and do it manually.

Manual configuration of winbind

This may work, it has not however been updated for VUCC, so proceed with caution.

Based on the instructions from https://wiki.samba.org/index.php/Setting_up_Samba_as_a_Domain_Member and https://wiki.samba.org/index.php/Authenticating_Domain_Users_Using_PAM Before configuring the domain ensure the following:

• Install the required packages: apt install samba winbind krb5-user libpam-krb5 libpam-winbind libnss-winbind

• Ensure the system is configured according to the SOE.

• edit /etc/krb5.conf to point to the new domain:

•    [libdefaults]
          default_realm = ad.ucc.gu.uwa.edu.au
          dns_lookup_realm = false
          dns_lookup_kdc = true

• Make the following /etc/samba/smb.conf:

• [global]
  # Configure the domain infomation
          security = ads
          realm = ad.ucc.gu.uwa.edu.au
          workgroup = UCCDOMAYNE
  
  # use winbind to map users and groups
          winbind enum users = yes
          winbind enum groups = yes
          winbind use default domain = yes
          kerberos method = secrets and keytab
  
  #Config gid/sid mapping based on AD attributes
          winbind nss info = rfc2307
  
          idmap config * : backend = tdb
          idmap config * : range = 13000-17999
          
          #idmap config for UCCDOMAYNE
          idmap config UCCDOMAYNE:backend = ad
          idmap config UCCDOMAYNE:schema_mode = rfc2307
          idmap config UCCDOMAYNE:range = 1-999999
          idmap config UCCDOMAYNE:unix_nss_info = yes
          idmap config UCCDOMAYNE:unix_primary_group = yes

• Join the machine to the domain with: net ads join -U <username>.

• configure pam using pam-auth-update and enable Winbind NT/AD authentication.

• configure nsswitch.conf

• # /etc/nsswitch.conf
  #
  # Example configuration of GNU Name Service Switch functionality.
  # If you have the `glibc-doc-reference' and `info' packages installed, try:
  # `info libc "Name Service Switch"' for information about this file.
  
  passwd:         compat winbind
  group:          compat winbind
  shadow:         files
  gshadow:        files
  
  hosts:          files mdns4_minimal [NOTFOUND=return] dns
  networks:       files
  
  protocols:      db files
  services:       db files 
  ethers:         db files
  rpc:            db files
  
  netgroup:       nis 

• start the services:

• winbindd
  nmbd
  smbd

• Make sure the computer can fetch the domain users and groups with:

• wbinfo -g` and `wbinfo -u`