User:Tbw/Active Directory Integration With SSSD
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.
- SSSD: SSSD provides a set of daemons to manage access to remote directories and authentication mechanisms.
Configuration
Active Directory Configuration
This section works with the default configuration of Windows Server 2012 R2.
Updating the GPO
[To be removed if not confirmed]
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:
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.
Configuring NTP
Read Time synchronization to configure an NTP service.
On the NTP servers configuration, use the IP addresses for the AD servers, as they typically run NTP as a service. Alternatively, you can use other known NTP servers provided the Active directory servers sync to the same stratum.
Ensure that the service is configured to sync the time automatically very early on startup.
Kerberos
Let us assume that your AD is named example.com. Let us 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
allow_weak_crypto = trueto 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.
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] workgroup = EXAMPLE password server = pdc.example.com realm = EXAMPLE.COM security = ads server string = %h ArchLinux Host kerberos method = secrets and keytab
Join the Domain
You need an AD Administrator account to do this. Let us 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'
Configuring, starting and testing services
Configuring and starting SSSD
Configure SSSD to connect to the Active Directory domain:
/etc/sssd/sssd.conf
[sssd] domains = example.com config_file_version = 2 services = nss, pam default_domain_suffix = example.com [domain/example.com] ad_domain = example.com krb5_realm = EXAMPLE.COM cache_credentials = True id_provider = ad krb5_store_password_if_offline = True default_shell = /bin/bash ldap_id_mapping = True use_fully_qualified_names = True fallback_homedir = /home/%u access_provider = ad full_name_format = %1$s
SSSD requires permission 600 on sssd.conf to start
chmod 600 /etc/sssd/sssd.conf
Enable and start the SSSD deamon sssd.service
.
Configuring NSSwitch
Next we will need to modify the NSSwitch configuration, which 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 (sss
) as additional sources for Users and Groups.
/etc/nsswitch.conf
passwd: compat mymachines systemd sss group: compat mymachines systemd sss shadow: compat sss
Testing SSSD
To ensure that our host is able to query the domain for users and groups, we test nsswitch settings by issuing the 'id' command. The following output shows that user information was retried from Active Directory:
# id test.user@ad.example.com
uid=1897201121(test.user@ad.example.com) gid=1897200513(test.user@ad.example.com) groups=1897200513(domain users@ad.example.com),1897201115(test.group1@ad.example.com),1897201125(test.group2@ad.example.com)
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_winbind.so
into the authentication process.
The Arch Linux PAM configuration keeps the central auth process in /etc/pam.d/system-auth
. Starting with the stock configuration from pambase
, change it like this:
system-auth
"auth" section
Find the line:
auth required pam_unix.so ...
Delete it, and replace with:
auth [success=1 default=ignore] pam_localuser.so auth [success=2 default=die] pam_sss.so auth [success=1 default=die] pam_unix.so nullok auth requisite pam_deny.so
"account" section
Find the line:
account required pam_unix.so
Keep it, and add this below:
account [success=1 default=ignore] pam_localuser.so account required pam_sss.so
"password" section
Find the line:
password required pam_unix.so ...
Delete it, and replace with:
password [success=1 default=ignore] pam_localuser.so password [success=2 default=die] pam_sss.so password [success=1 default=die] pam_unix.so sha512 shadow password requisite pam_deny.so
"session" section
Find the line:
session required pam_unix.so
Keep it, and add this line immediately above it:
session required pam_mkhomedir.so skel=/etc/skel/ umask=0022
Below the pam_unix line, add these:
session [success=1 default=ignore] pam_localuser.so session required pam_sss.so
passwd
"password" section
In order for logged-in Active Directory users to be able to change their passwords with the 'passwd' command, the file /etc/pam.d/passwd
must include the information from system-auth.
Find the line:
password required pam_unix.so sha512 shadow nullok
Delete it, and replace with:
password include system-auth
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.
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!
Earlier we skipped configuration of the shares. Now that things are working, go back to /etc/samba/smb.conf
, and add the exports for the host that you want available on the windows network.
/etc/samba/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 keyword 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