Active Directory Integration

Warning: Because Arch Linux is a rolling release distribution, it is possible that some of the information in this article could be outdated due to package or configuration changes made by the maintainers. Never blindly follow these or any other instructions. When the instructions say to edit or change a file, consider making a backup copy. Check the date of the last revision of this article.

A key challenge for system administrators of any datacenter is trying to coexisting in Heterogeneous environments. By this we mean the mixing of different server operating system technologies (typically Microsoft Windows & Unix/Linux). User management and authentication is by far the most difficult of these to solve. The most common way of solving this problem is to use a Directory Server. There are a number of open-source and commercial solutions for the various flavors of *NIX; however, few solve the problem of interoperating with Windows. Active Directory (AD) is a directory service created by Microsoft for Windows domain networks. It is included in most Windows Server operating systems. Server computers on which Active Directory is running are called domain controllers.

Active Directory serves as a central location for network administration and security. It is responsible for authenticating and authorizing all users and computers within a network of Windows domain type, assigning and enforcing security policies for all computers in a network and installing or updating software on network computers. For example, when a user logs into a computer that is part of a Windows domain, it is Active Directory that verifies his or her password and specifies whether he or she is a system administrator or normal user.

Active Directory uses Lightweight Directory Access Protocol (LDAP) versions 2 and 3, Kerberos and DNS. These same standards are available as linux, but piecing them together is not an easy task. Following these steps will help you configure an ArchLinux host to authenticate against an AD domain.

This guide explains how to integrate an Arch Linux host with an existing Windows Active Directory domain. Before continuing, you must have an existing Active Directory domain, and have a user with the appropriate rights within the domain to: query users and add computer accounts (Domain Join).

This document is not an intended as a complete guide to Active Directory nor Samba. Refer to the resources section for additional information.

Terminology

If you are not familiar with Active Directory, there are a few keywords that are helpful to know.

• Domain : The name used to group computers and accounts.
• SID : Each computer that joins the domain as a member must have a unique SID or System Identifier.
• SMB : Server Message Block.
• NETBIOS: Network naming protocol used as an alternative to DNS. Mostly legacy, but still used in Windows Networking.
• WINS: Windows Information Naming Service. Used for resolving Netbios names to windows hosts.
• Winbind: Protocol for windows authentication.

Configuration

Active Directory Configuration

Warning: This section has not been validated. Proceed with caution

Updating the GPO

It may be necessary to disable Digital Sign Communication (Always) in the AD group policies. Dive into:

Local policies -> Security policies -> Microsoft Network Server -> Digital sign communication (Always) -> activate define this policy and use the disable radio button.

If you use Windows Server 2008 R2, you need to modify that in GPO for Default Domain Controller Policy -> Computer Setting -> Policies -> Windows Setting -> Security Setting -> Local Policies -> Security Option -> Microsoft network client: Digitally sign communications (always)

Linux Host Configuration

The next few steps will begin the process of configuring the Host. You will need root or sudo access to complete these steps.

Installation

Install the following packages:

• samba
• pam-krb5 ---- error: does not exist in latest version or arch linux!
• pam_pwcheck
• openntpd (or) ntp

Updating DNS

Active Directory is heavily dependent upon DNS. You will need to update /etc/resolv.conf to use one or more of the Active Directory domain controllers:

/etc/resolv.conf

nameserver <IP1>
nameserver <IP2>

Replacing <IP1> and <IP2> with valid IP addresses for the AD servers. If your AD domains do not permit DNS forwarding or recursion, you may need to add additional resolvers.

Note: If your machine dual boots Windows and Linux, you should use a different DNS hostname and netbios name for the linux configuration if both operating systems will be members of the same domain.

Configuring NTP

Read NTPd or OpenNTPD to configure a NTP service. Note that OpenNTPD is no longer maintained.

On the configuration, use the IP addresses for the AD servers. Alternatively, you can use other known NTP servers provided the Active directory servers sync to the same stratum. However, AD servers typically run NTP as a service.

Ensure the daemon is configured to sync automatically on startup.

Kerberos

Let's assume that your AD is named example.com. Let's further assume your AD is ruled by two domain controllers, the primary and secondary one, which are named PDC and BDC, pdc.example.com and bdc.example.com respectively. Their IP adresses will be 192.168.1.2 and 192.168.1.3 in this example. Take care to watch your syntax; upper-case is very important here.

/etc/krb5.conf

[libdefaults]
        default_realm 	= 	EXAMPLE.COM
	clockskew 	= 	300
	ticket_lifetime	=	1d
        forwardable     =       true
        proxiable       =       true
        dns_lookup_realm =      true
        dns_lookup_kdc  =       true
	
[realms]
	EXAMPLE.COM = {
		kdc 	= 	PDC.EXAMPLE.COM
                admin_server = PDC.EXAMPLE.COM
		default_domain = EXAMPLE.COM
	}
	
[domain_realm]
        .kerberos.server = EXAMPLE.COM
	.example.com = EXAMPLE.COM
	example.com = EXAMPLE.COM
	example	= EXAMPLE.COM

[appdefaults]
	pam = {
	ticket_lifetime 	= 1d
	renew_lifetime 		= 1d
	forwardable 		= true
	proxiable 		= false
	retain_after_close 	= false
	minimum_uid 		= 0
	debug 			= false
	}

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

Note: Heimdal 1.3.1 deprecated DES encryption which is required for AD authentication before Windows Server 2008. You'll probably have to add

allow_weak_crypto = true

to the [libdefaults] section.

Creating a Kerberos Ticket

Now you can query the AD domain controllers and request a kerberos ticket (uppercase is necessary):

kinit administrator@EXAMPLE.COM

You can use any username that has rights as a Domain Administrator.

Validating the Ticket

Run klist to verify you did receive the token. You should see something similar to:

# klist

 Ticket cache: FILE:/tmp/krb5cc_0
 Default principal: administrator@EXAMPLE.COM
 
 Valid starting    Expires           Service principal 
 02/04/12 21:27:47 02/05/12 07:27:42 krbtgt/EXAMPLE.COM@EXAMPLE.COM
         renew until 02/05/12 21:27:47

Samba

Samba is a free software re-implementation of the SMB/CIFS networking protocol. It also includes tools for Linux machines to act as Windows networking servers and clients.

Note: The configuration can vary greatly depending on how the Windows environment is deployed. Be prepared to troubleshoot and research

In this section, we will focus on getting Authentication to work first by editing the 'Global' section first. Later, we will go back and add shares.

/etc/samba/smb.conf

[Global]
  netbios name = MYARCHLINUX
  workgroup = EXAMPLE
  realm = EXAMPLE.COM
  server string = %h ArchLinux Host
  security = ads
  encrypt passwords = yes
  password server = pdc.example.com
  idmap uid = 10000-20000
  idmap gid = 10000-20000
 
  #idmap backend = rid

  winbind use default domain = Yes
  winbind enum users = Yes
  winbind enum groups = Yes
  winbind nested groups = Yes
  winbind separator = +
  winbind refresh tickets = yes
  winbind gid = 10000-20000

  template shell = /bin/bash
  template homedir = /home/%D/%U
   
  preferred master = no
  dns proxy = no
  wins server = pdc.example.com
  wins proxy = no

  inherit acls = Yes
  map acl inherit = Yes
  acl group control = yes

  load printers = no
  debug level = 3
  use sendfile = no

We shall now explain to Samba that it shall use the PDC´s database for authentication queries. Again, we use winbindd which is a part of the samba package. Winbind maps the UID and GID of the AD to our Linux-machine. Winbind uses a Unix-implementation of RPC-calls, Pluggable Authentication Modules (aka PAM) and Name Service Switch (NSS) to allow Windows AD and users accessing and to grant permissions on the Linux-machine. The best part of winbindd is, that you don´t have to define the mapping yourself, but only define a range of UID and GID. That´s what we defined in smb.conf.

Update the samba configuration file to enable the winbind daemon

/etc/conf.d/samba

 ##### /etc/conf.d/samba #####
 #SAMBA_DAEMONS=(smbd nmbd)
 SAMBA_DAEMONS=(smbd nmbd winbindd)

Next, configure samba to startup at boot. Read Daemons for more details.

Starting and testing services

Starting Samba

Hopefully, you have not rebooted yet! Fine. If you are in an X-session, quit it, so you can test login into another console, while you are still logged in.

Start Samba (including smbd, nmbd and winbindd):

If you check the processes, you'll see that winbind did not actually start. A quick review of the logs shows that the SID for this host could be obtained from the domain:

# tail /var/log/samba/log.winbindd

[2012/02/05 21:51:30.085574,  0] winbindd/winbindd_cache.c:3147(initialize_winbindd_cache)
  initialize_winbindd_cache: clearing cache and re-creating with version number 2
[2012/02/05 21:51:30.086137,  2] winbindd/winbindd_util.c:233(add_trusted_domain)
  Added domain BUILTIN  S-1-5-32
[2012/02/05 21:51:30.086223,  2] winbindd/winbindd_util.c:233(add_trusted_domain)
  Added domain MYARCHLINUX  S-1-5-21-3777857242-3272519233-2385508432
[2012/02/05 21:51:30.086254,  0] winbindd/winbindd_util.c:635(init_domain_list)
  Could not fetch our SID - did we join?
[2012/02/05 21:51:30.086408,  0] winbindd/winbindd.c:1105(winbindd_register_handlers)
  unable to initialize domain list

Join the Domain

You need an AD Administrator account to do this. Let's assume this is named Administrator. The command is 'net ads join'

# net ads join -U Administrator

Administrator's password: xxx
Using short domain name -- EXAMPLE
Joined 'MYARCHLINUX' to realm 'EXAMPLE.COM'

See screenshot of Active Directory Users and Computers [[1]]

Restart Samba

winbindd failed to start on the first try because we were not yet a domain.

Restart the samba service and winbind should fire up as well.

NSSwitch tells the Linux host how to retrieve information from various sources and in which order to do so. In this case, we are appending Active Directory as additional sources for Users, Groups, and Hosts.

/etc/nsswitch.conf

 passwd:            files winbind
 shadow:            files winbind
 group:             files winbind 
 
 hosts:             files dns wins

Testing Winbind

Let's check if winbind is able to query the AD. The following command should return a list of AD users:

# wbinfo -u

administrator
guest
krbtgt
test.user

• Note we created an Active Directory user called 'test.user' on the domain controller

We can do the same for AD groups:

# wbinfo -g

domain computers
domain controllers
schema admins
enterprise admins
cert publishers
domain admins
domain users
domain guests
group policy creator owners
ras and ias servers
allowed rodc password replication group
denied rodc password replication group
read-only domain controllers
enterprise read-only domain controllers
dnsadmins
dnsupdateproxy

Testing nsswitch

To ensure that our host is able to query the domain for users and groups, we test nsswitch settings by issuing the 'getent' command. The following output shows what a stock ArchLinux install looks like:

# getent passwd

root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/bin/false
daemon:x:2:2:daemon:/sbin:/bin/false
mail:x:8:12:mail:/var/spool/mail:/bin/false
ftp:x:14:11:ftp:/srv/ftp:/bin/false
http:x:33:33:http:/srv/http:/bin/false
nobody:x:99:99:nobody:/:/bin/false
dbus:x:81:81:System message bus:/:/bin/false
ntp:x:87:87:Network Time Protocol:/var/empty:/bin/false
avahi:x:84:84:avahi:/:/bin/false
administrator:*:10001:10006:Administrator:/home/EXAMPLE/administrator:/bin/bash
guest:*:10002:10007:Guest:/home/EXAMPLE/guest:/bin/bash
krbtgt:*:10003:10006:krbtgt:/home/EXAMPLE/krbtgt:/bin/bash
test.user:*:10000:10006:Test User:/home/EXAMPLE/test.user:/bin/bash

And for groups:

# getent group

root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin
adm:x:4:root,daemon
tty:x:5:
disk:x:6:root
lp:x:7:daemon
mem:x:8:
kmem:x:9:
wheel:x:10:root
ftp:x:11:
mail:x:12:
uucp:x:14:
log:x:19:root
utmp:x:20:
locate:x:21:
rfkill:x:24:
smmsp:x:25:
http:x:33:
games:x:50:
network:x:90:
video:x:91:
audio:x:92:
optical:x:93:
floppy:x:94:
storage:x:95:
scanner:x:96:
power:x:98:
nobody:x:99:
users:x:100:
dbus:x:81:
ntp:x:87:
avahi:x:84:
domain computers:x:10008:
domain controllers:x:10009:
schema admins:x:10010:administrator
enterprise admins:x:10011:administrator
cert publishers:x:10012:
domain admins:x:10013:test.user,administrator
domain users:x:10006:
domain guests:x:10007:
group policy creator owners:x:10014:administrator
ras and ias servers:x:10015:
allowed rodc password replication group:x:10016:
denied rodc password replication group:x:10017:krbtgt
read-only domain controllers:x:10018:
enterprise read-only domain controllers:x:10019:
dnsadmins:x:10020:
dnsupdateproxy:x:10021:

Testing Samba commands

Try out some net commands to see if samba can communicate with AD:

# net ads info

[2012/02/05 20:21:36.473559,  0] param/loadparm.c:7599(lp_do_parameter)
  Ignoring unknown parameter "idmapd backend"
LDAP server: 192.168.1.2
LDAP server name: PDC.example.com
Realm: EXAMPLE.COM
Bind Path: dc=EXAMPLE,dc=COM
LDAP port: 389
Server time: Sun, 05 Feb 2012 20:21:33 CST
KDC server: 192.168.1.2
Server time offset: -3

# net ads lookup

[2012/02/05 20:22:39.298823,  0] param/loadparm.c:7599(lp_do_parameter)
  Ignoring unknown parameter "idmapd backend"
Information for Domain Controller: 192.168.1.2

Response Type: LOGON_SAM_LOGON_RESPONSE_EX
GUID: 2a098512-4c9f-4fe4-ac22-8f9231fabbad
Flags:
        Is a PDC:                                   yes
        Is a GC of the forest:                      yes
        Is an LDAP server:                          yes
        Supports DS:                                yes
        Is running a KDC:                           yes
        Is running time services:                   yes
        Is the closest DC:                          yes
        Is writable:                                yes
        Has a hardware clock:                       yes
        Is a non-domain NC serviced by LDAP server: no
        Is NT6 DC that has some secrets:            no
        Is NT6 DC that has all secrets:             yes
Forest:                 example.com
Domain:                 example.com
Domain Controller:      PDC.example.com
Pre-Win2k Domain:       EXAMPLE
Pre-Win2k Hostname:     PDC
Server Site Name :              Office
Client Site Name :              Office
NT Version: 5
LMNT Token: ffff
LM20 Token: ffff

# net ads status -U administrator | less

objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user
objectClass: computer
cn: myarchlinux
distinguishedName: CN=myarchlinux,CN=Computers,DC=leafscale,DC=inc
instanceType: 4
whenCreated: 20120206043413.0Z
whenChanged: 20120206043414.0Z
uSNCreated: 16556
uSNChanged: 16563
name: myarchlinux
objectGUID: 2c24029c-8422-42b2-83b3-a255b9cb41b3
userAccountControl: 69632
badPwdCount: 0
codePage: 0
countryCode: 0
badPasswordTime: 0
lastLogoff: 0
lastLogon: 129729780312632000
localPolicyFlags: 0
pwdLastSet: 129729764538848000
primaryGroupID: 515
objectSid: S-1-5-21-719106045-3766251393-3909931865-1105
...<snip>...

Configuring PAM

Now we will change various rules in PAM to allow Active Directory users to use the system for things like login and sudo access. When changing the rules, note the order of these items and whether they are marked as required or sufficient is critical to things working as expected. You should not deviate from these rules unless you know how to write PAM rules.

In case of logins, PAM should first ask for AD accounts, and for local accounts if no matching AD account was found. Therefore, we add entries to include pam_winbindd.so into the authentication process. Furthermore, we include pam_mkhomedir.so. If an AD user logs in, /home/example/user will be created automatically.

/etc/pam.d/login

#%PAM-1.0
auth            required        pam_securetty.so
auth            requisite       pam_nologin.so
auth            sufficient      pam_unix.so nullok
auth            required        pam_winbind.so use_first_pass use_authtok
auth            required        pam_tally.so onerr=succeed file=/var/log/faillog
# use this to lockout accounts for 10 minutes after 3 failed attempts
#auth           required        pam_tally.so deny=2 unlock_time=600 onerr=succeed file=/var/log/faillog
account         required        pam_access.so
account         required        pam_time.so
account         sufficient      pam_unix.so
account         sufficient      pam_winbind.so use_first_pass use_authtok
password        required        pam_pwcheck.so
password        sufficient      pam_unix.so
password        sufficient      pam_winbind.so use_first_pass use_authtok
#password       required        pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
#password       required        pam_unix.so sha512 shadow use_authtok
session         required        pam_mkhomedir.so skel=/etc/skel/ umask=0022
session         sufficient      pam_unix.so
session         sufficient      pam_winbind.so use_first_pass use_authtok
session         required        pam_env.so
session         required        pam_motd.so
session         required        pam_limits.so
session         optional        pam_mail.so dir=/var/spool/mail standard
session         optional        pam_lastlog.so
session         optional        pam_loginuid.so
-session        optional        pam_ck_connector.so nox11
-session        optional        pam_systemd.so

Testing login

Now, start a new console session (or ssh) and try to login using the AD credentials. The domain name is optional, as this was set in the Winbind configuration as 'default realm'. Please note that in the case of ssh, you will need to modify the /etc/ssh/sshd_config file to allow kerberos authentication (KerberosAuthentication yes).

test.user
EXAMPLE+test.user

Both should work. You should notice that /home/example/test.user will be automatically created. Again, if you are using ssh, you need to add the pam_mkhomedir.so line mentioned above to the /etc/pam.d/sshd file. Log into another session using an linux account. Check that you still be able to log in as root - but keep in mind to be logged in as root in at least one session!

/etc/pam.d/gdm

TODO

Sudo

Another thing to get working is 'sudo'. First add the 'test.user' to /etc/sudoers. You can tweak this later, for now lets test things are working:

/etc/sudoers

If you were to attempt a sudo now, it would fail.

Adjust the sudo file to mark pam_unix as sufficient and add the line for winbind as shown:

/etc/pam.d/sudo

#%PAM-1.0
auth            sufficient      pam_unix.so
auth            required        pam_winbind.so use_first_pass use_authtok
auth            required        pam_nologin.so

Configuring Shares

Earlier we skipped configuration of the shares. Now that things are working, go back to /etc/smb.conf, and add the exports for the host that you want available on the windows network.

/etc/smb.conf

[MyShare]
  comment = Example Share
  path = /srv/exports/myshare
  read only = no
  browseable = yes
  valid users = @NETWORK+"Domain Admins" NETWORK+test.user

In the above example, the keywork NETWORK is to be used. Do not mistakenly substitute this with your domain name. For adding groups, prepend the '@' symbol to the group. Note that Domain Admins is encapsulated in quotes so Samba correctly parses it when reading the configuration file.

See also

• Wikipedia: Active Directory
• Wikipedia: Samba
• Wikipedia: Kerberos
• Samba: Documentation
• Samba Wiki: Samba & Active Directory
• Samba Man Page: smb.conf

Commercial Solutions

• Centrify
• Likewise