Configure SSSD to authenticate with a Windows 2008 Domain Server – Note: WIP

Posted on by phong

https://fedorahosted.org/sssd/wiki/Configuring%20sssd%20to%20authenticate%20with%20a%20Windows%202008%20Domain%20Server

Synopsis

This describes how to configure SSSD to authenticate with a Windows 2008 Domain Server. This guide is a work in progress.

Windows 2008 Server Setup

The domain to be configured is ad.example.com using realm AD.EXAMPLE.COM, the Windows server is server.ad.example.com, and the client host where SSSD is running is client.ad.example.com. Reboot Windows during installation and setup when prompted and complete the needed steps as Administrator.

Operating System Installation

  • Boot from the Windows 2008 Server DVD
  • Install Windows 2008 Server using the hostname server.ad.example.com
  • Make sure server.ad.example.com is in DNS

Domain Configuration

  • In Server Manager add the Active Directory Domain Services role
  • Create a new domain named ad.example.com
  • Required: in Server Manager add the Identity Management for UNIX Role Service for Active Directory Domain Services, use the domain name for the NIS domain name
    • Currently SSSD won’t work with AD in a meaningful fashion without this (this might change if #995 and #996 are solved)

Enabling LDAP Searches

In order to allow SSSD to do LDAP searches for user information in AD SSSD must be configured to bind with SASL/GSSAPI or DN/password. GSSAPI is recommended.

Using SASL/GSSAPI Binds for LDAP Searches

Create the service keytab for the host running SSSD on AD. Either do this with Samba, or using Windows. Samba is recommended.

Creating Service Keytab with Samba

On the Linux client with properly configured /etc/krb5.conf (see below) and suitable /etc/samba/smb.conf:

/etc/samba/smb.conf
[global]
   workgroup = EXAMPLE
   client signing = yes
   client use spnego = yes
   kerberos method = secrets and keytab
   log file = /var/log/samba/%m.log
   password server = AD.EXAMPLE.COM
   realm = EXAMPLE.COM
   security = ads
  • net ads join -U Administrator
    • Or do kinit Administrator first and use -k instead of -U Administrator
  • Additional principals can be created later with net ads keytab add if needed.

You don’t need a Domain Administrator account to do this, you just need an account with sufficient rights to join a machine to the domain. This is a notable advantage of this approach over generating the keytab directly on the AD controller. If you’re using NFS you may want to specify a different createupn argument here. This does not cause any problems for sssd. This would be done using:

net ads join createupn=”nfs/client.ad.example.com@AD.EXAMPLE.COM” -U Administrator

Creating Service Keytab on AD

Do not do this step if you’ve already created a keytab using Samba.

On the Windows server:

  • Open Users & Computers snap-in
  • Create a new Computer object named client (i.e., the name of the host running SSSD)
  • On the command prompt:
    • setspn -A host/client.ad.example.com@AD.EXAMPLE.COM client
    • setspn -L client
    • ktpass /princ host/client.ad.example.com@AD.EXAMPLE.COM /out client-host.keytab /crypto all /ptype KRB5_NT_PRINCIPAL -desonly /mapuser AD\client$ /pass *
      • This sets the machine account password and UPN for the principal
      • If you create additional keytabs for the host add -setpass -setupn for the above command to prevent resetting the machine password (thus changing kvno) and to prevent overwriting the UPN
  • Transfer the keytab created in a secure manner to the client as /etc/krb5.keytab and make sure its permissions are correct:
    • chown root:root /etc/krb5.keytab
    • chmod 0600 /etc/krb5.keytab
    • restorecon /etc/krb5.keytab

See the Linux Client Setup section for verifying the keytab file and the example sssd.conf below for the needed SSSD configuration.

Using DN/Password Binds for LDAP Searches

Not generally recommended but see the example sssd.conf below.

Adding a Group

  • Open Administrative Tools -> Active Directory Users and Computers
  • Browse to ad.example.com, then to Users
  • Right click on Users and Create a New Group named unixusers
  • Double click on the unixusers group then switch to the UNIX Attributes tab
  • Select the NIS Domain created earlier
  • Set the GID as appropriate

Adding a User

  • Open Administrative Tools -> Active Directory Users and Computers
  • Browse to ad.example.com, then to Users
  • Right click on Users and Create a New User named aduser
  • Make sure User must change password at next logon and Account is disabled are unchecked
  • Double click on the aduser group then switch to the UNIX Attributes tab
  • Select the NIS Domain created earlier
  • Set the UID as appropriate
  • Set the Login Shell to /bin/bash
  • Set the Home Directory to /home/aduser
  • Set Primary Group Name/GID to unixusers

Linux Client Setup

  • Install sssd package on the Linux client machine
  • Make configuration changes to the files below
  • Start the sssd service

/etc/krb5.conf

[logging]
 default = FILE:/var/log/krb5libs.log

[libdefaults]
 default_realm = AD.EXAMPLE.COM
 dns_lookup_realm = true
 dns_lookup_kdc = true
 ticket_lifetime = 24h
 renew_lifetime = 7d
 rdns = false
 forwardable = yes

# You may also want either of:
# allow_weak_crypto = true
# default_tkt_enctypes = arcfour-hmac

[realms]
# Define only if DNS lookups are not working
# AD.EXAMPLE.COM = {
#  kdc = server.ad.example.com
#  admin_server = server.ad.example.com
# }

[domain_realm]
# Define only if DNS lookups are not working
# .ad.example.com = AD.EXAMPLE.COM
# ad.example.com = AD.EXAMPLE.COM

Make sure kinit aduser@AD.EXAMPLE.COM works properly. Add the Windows server IP/hostname to /etc/hosts only if needed.

If using SASL/GSSAPI to bind to AD also test that the keytab is working properly:

klist -ke

kinit -k CLIENT$@AD.EXAMPLE.COM

If you generated your keytab with a different createupn argument, it’s possible this won’t work and the following works instead. This is absolutely fine as far as sssd is concerned, and you can instead generate a ticket for the upn you have created:

kinit -k -t /etc/krb5.keytab ‘nfs/client.ad.example.com@AD.EXAMPLE.COM’

Now using this credential you’ve just created try fetching data from the server with ldapsearch (in case of issues make sure /etc/openldap/ldap.conf does not contain any unwanted settings):

/usr/bin/ldapsearch -H ldap://server.ad.example.com/ -Y GSSAPI -N -b “dc=ad,dc=example,dc=com” “(&(objectClass=user)(sAMAccountName=aduser))”

By using the credential from the keytab, you’ve verified that this credential has sufficient rights to retrieve user information.

After both kinit and ldapsearch work properly proceed to actual SSSD configuration.

/etc/sssd/sssd.conf

Example sssd.conf configuration, additional options can be added as needed.

[sssd]
config_file_version = 2
domains = ad.example.com
services = nss, pam
debug_level = 0

[nss]

[pam]

[domain/ad.example.com]
# Unless you know you need referrals, turn them off
ldap_referrals = false
# Uncomment if you need offline logins
# cache_credentials = true
enumerate = false

id_provider = ldap
auth_provider = krb5
chpass_provider = krb5
access_provider = ldap

# Uncomment if service discovery is not working
#ldap_uri = ldap://server.ad.example.com/

# Comment out if not using SASL/GSSAPI to bind
ldap_sasl_mech = GSSAPI
# Uncomment and adjust if the default principal host/fqdn@REALM is not available
#ldap_sasl_authid = nfs/client.ad.example.com@AD.EXAMPLE.COM

# Define these only if anonymous binds are not allowed and no keytab is available
# Enabling use_start_tls is very important, otherwise the bind password is transmitted
# over the network in the clear
#ldap_id_use_start_tls = True
#ldap_default_bind_dn = CN=binduser,OU=user accounts,DC=ad,DC=example,DC=com
#ldap_default_authtok_type = password
#ldap_default_authtok = bindpass

ldap_schema = rfc2307bis

ldap_user_search_base = ou=user accounts,dc=ad,dc=example,dc=com
ldap_user_object_class = user

ldap_user_home_directory = unixHomeDirectory
ldap_user_principal = userPrincipalName

ldap_group_search_base = ou=groups,dc=ad,dc=example,dc=com
ldap_group_object_class = group

ldap_access_order = expire
ldap_account_expire_policy = ad
ldap_force_upper_case_realm = true

# Uncomment if dns discovery of your AD servers isn't working.
#krb5_server = server.ad.example.com
krb5_realm = AD.EXAMPLE.COM

# Probably required with sssd 1.8.x and newer
krb5_canonicalize = false

# Perhaps you need to redirect to certain attributes?
# ldap_user_object_class = user
# ldap_user_name = sAMAccountName
# ldap_user_uid_number = msSFU30UidNumber
# ldap_user_gid_number = msSFU30GidNumber
# ldap_user_gecos = displayName
# ldap_user_home_directory = msSFU30HomeDirectory
# ldap_user_shell = msSFU30LoginShell
# ldap_user_principal = userPrincipalName
# ldap_group_object_class = group
# ldap_group_name = cn
# ldap_group_gid_number = msSFU30GidNumber

NSS/PAM Configuration

Depending on your distribution you have different options how to enable SSSD.

Fedora/RHEL

Use authconfig to enable SSSD, install oddjob-mkhomedir to make sure home directory creation works with SELinux:

authconfig –enablesssd –enablesssdauth –enablemkhomedir –update

Debian/Ubuntu?

Install libnss-sss and libpam-sss to have SSSD added as NSS/PAM provider in /etc/nsswitch.conf and /etc/pam.d/common-* configuration files. Add pam_mkhomedir.so to PAM session configuration manually. Restart SSSD after these changes.

Configure NSS/PAM manually

Manual configuration can be done with the following changes. The PAM example file paths are from Debian/Ubuntu?, in Fedora/RHEL corresponding manual configuration should be done in /etc/pam.d/system-auth and /etc/pam.d/password-auth.

/etc/nsswitch.conf

More maps will be available later (see at least tickets #359, #360, #900, #901, #929).

#
# /etc/nsswitch.conf
#

passwd:         files sss
shadow:         files sss
group:          files sss

hosts:          files dns

bootparams:     files

ethers:         files
netmasks:       files
networks:       files
protocols:      files
rpc:            files
services:       files

netgroup:       files sss

publickey:      files

automount:      files
aliases:        files
/etc/pam.d/common-auth

Right after the pam_unix.so line, add

auth         sufficient    pam_sss.so use_first_pass
/etc/pam.d/common-account

Right after the pam_unix.so line, add

account      [default=bad success=ok user_unknown=ignore] pam_sss.so
/etc/pam.d/common-password

Right after the pam_unix.so line, add

password     sufficient    pam_sss.so use_authtok
/etc/pam.d/common-session

Just before the pam_unix.so line, add

session      optional      pam_mkhomedir.so

Right after the pam_unix.so line, add

session      optional      pam_sss.so

Understanding Kerberos & Active Directory

It is important to understand that (unlike Linux MIT based KDC) Active Directory based KDC divides Kerberos principals into two groups:

  • User Principals – usually equals to the sAMAccountname attribute of the object in AD. In short, User Principal is entitled to obtain TGT (ticket granting ticket). User Principals could be hence used to generate a TGT via kinit -k <principalname>
  • Service Principals – represents which Kerberized service can be used on the computer in question. Service principals CANNOT be used to obtain a TGT -> cannot be used to grant an access to Active Directory controller for example.

Each user object in Active Directory (understand that a computer object in AD is de-facto user object as well) can have:

  • maximum of 2 User Principal Names (UPN). One is pre-defined by its sAMAccountName LDAP attribute (mentioned above, for computer objects it has a form of “<hostname>$”) and second by its UserPrincipalName? string attribute
  • many Service Principal Names (typically one for each Kerberized service we want to enable on the computer) defined by the ServicePrincipalName? (SPN) list attribute. The attributes can be seen/set using the ADSIedit snap-in for example.

Optional Final Test

You may have made iterative changes to your setup while learning about SSSD. To make sure that your setup actually works, and you’re not relying on cached credentials, or cached LDAP information, you may want to clear out the local cache. Obviously this will erase local credentials, and all cached user information, so you should only do this for testing, and while on the network with network access to the AD servers.

service sssd stop; rm -f /var/lib/sss/db/*; service sssd start

If all looks well on your system after this, you know that sssd is able to use the kerberos and ldap services you’ve configured.

Further reading

Please see the following article on Technet site (​http://technet.microsoft.com/en-us/library/cc772815%28WS.10%29.aspx) for more in-depth Kerberos understanding

Written by phong

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