LDAP authentication

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Introduction and Concepts

This is a guide on how to configure an Arch Linux installation to authenticate against an LDAP directory. This LDAP directory can be either local (installed on the same computer) or network (e.g. in a lab environment where central authentication is desired).

The guide is divided into two parts. The first part deals with how to setup an OpenLDAP server that hosts the authentication directory. The second part deals with how to setup the NSS and PAM modules that are required for the authentication scheme to work on the client computers. If you just want to configure Arch to authenticate against an already existing LDAP server, you can skip to the second part.

NSS and PAM

NSS (which stands for Name Service Switch) is a system mechanism to configure different sources for common configuration databases. For example, /etc/passwd is a file type source for the passwd database.

PAM (which stands for Pluggable Authentication Modules) is a mechanism used by Linux (and most *nixes) to extend its authentication schemes based on different plugins.

So to summarize, we need to configure NSS to use the OpenLDAP server as a source for the passwd, shadow and other configuration databases and then configure PAM to use these sources to authenticate its users.

LDAP Server Setup

Installation

Install the OpenLDAP server and configure the server and client. After you have completed that, return here.

Set up access controls

To make sure that no-one can read the (encrypted) passwords from the LDAP server, but still allowing users to edit some of their own select attributes (such as own password and photo), create and import the following LDIF and restart slapd.service afterwards:

Note: Alter the domain components "example" and "org" to your needs
dn: olcDatabase={1}mdb,cn=config
changetype: modify
replace: olcAccess
olcAccess: {0}to attrs=cn,givenName,sn,userPassword,shadowLastChange,mail,loginShell,photo by self write by anonymous auth by dn.base="cn=Manager,dc=example,dc=org" write by * none
olcAccess: {1}to * by self read by dn.base="cn=Manager,dc=example,dc=org" write by * read

Populate LDAP Tree with Base Data

Create a temporary file called base.ldif with the following text.

base.ldif
# example.org
dn: dc=example,dc=org
dc: example
o: Example Organization
objectClass: dcObject
objectClass: organization

# Manager, example.org
dn: cn=Manager,dc=example,dc=org
cn: Manager
description: LDAP administrator
objectClass: organizationalRole
objectClass: top
roleOccupant: dc=example,dc=org

# People, example.org
dn: ou=People,dc=example,dc=org
ou: People
objectClass: top
objectClass: organizationalUnit

# Groups, example.org
dn: ou=Group,dc=example,dc=org
ou: Group
objectClass: top
objectClass: organizationalUnit

Add it to your OpenLDAP tree:

$ ldapadd -D "cn=Manager,dc=example,dc=org" -W -f base.ldif

Test to make sure the data was imported:

$ ldapsearch -x -b 'dc=example,dc=org' '(objectclass=*)'

Adding users

To manually add a user, create an .ldif file like this:

user_joe.ldif
dn: uid=johndoe,ou=People,dc=example,dc=org
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: inetOrgPerson
objectClass: posixAccount
objectClass: shadowAccount
uid: johndoe
cn: John Doe
sn: Doe
givenName: John
title: Guinea Pig
telephoneNumber: +0 000 000 0000
mobile: +0 000 000 0000
postalAddress: AddressLine1$AddressLine2$AddressLine3
userPassword: {CRYPT}xxxxxxxxxx
labeledURI: https://archlinux.org/
loginShell: /bin/bash
uidNumber: 9999
gidNumber: 9999
homeDirectory: /home/johndoe/
description: This is an example user

The xxxxxxxxxx in the userPassword entry should be replaced with the value in /etc/shadow or use the slappasswd command. Now add the user:

$ ldapadd -D "cn=Manager,dc=example,dc=org" -W -f user_joe.ldif

You can add a group similarly with

group_joe.ldif
dn: cn=joe,ou=Group,dc=example,dc=org
objectClass: top
objectClass: posixGroup
cn: joe
gidNumber: 9999
Note: You can automatically migrate all of your local accounts (and groups, etc.) to the LDAP directory using PADL Software's Migration Tools.

Client Setup

Install the OpenLDAP client as described in OpenLDAP. Make sure you can query the server with ldapsearch.

Depending on your target, choose either online-only or online and offline authentication.

Online Authentication

NSS Configuration

NSS is a system facility which manages different sources as configuration databases. For example, /etc/passwd is a file type source for the passwd database, which stores the user accounts.

Install the nss-pam-ldapd package.

Edit /etc/nsswitch.conf which is the central configuration file for NSS. It tells NSS which sources to use for which system databases. We need to add the ldap directive to the passwd, group and shadow databases, so be sure your file looks like this:

passwd: files ldap
group: files ldap
shadow: files ldap

Edit /etc/nslcd.conf and change the base and uri lines to fit your ldap server setup.

Edit the binddn and the bindpw if your LDAP server requires a password. Make sure you change the permission of your /etc/nslcd.conf to 0600 for nslcd to start properly.

Start nslcd.service using systemd.

You now should see your LDAP users when running getent passwd on the client.

PAM Configuration

The basic rule of thumb for PAM configuration is to include pam_ldap.so wherever pam_unix.so is included. Arch moving to pambase has helped decrease the amount of edits required. For more details about configuring pam, the RedHat Documentation is quite good. You might also want the upstream documentation for nss-pam-ldapd.

Tip: If you want to prevent UID clashes with local users on your system, you might want to include minimum_uid=10000 or similar on the end of the pam_ldap.so lines. You will have to make sure the LDAP server returns uidNumber fields that match the restriction.
Note: Each facility (auth, session, password, account) forms a separate chain and the order matters. Sufficient lines will sometimes "short circuit" and skip the rest of the section, so the rule of thumb for auth, password, and account is sufficient lines before required, but after required lines for the session section; optional can almost always go at the end. When adding your pam_ldap.so lines, do not change the relative order of the other lines without good reason! Simply insert LDAP within the chain.

First edit /etc/pam.d/system-auth. This file is included in most of the other files in pam.d, so changes here propagate nicely. Updates to pambase may change this file.

Make pam_ldap.so sufficient at the top of each section, except in the session section, where we make it optional.

/etc/pam.d/system-auth
auth      sufficient pam_ldap.so
auth      required  pam_unix.so     try_first_pass nullok
auth      optional  pam_permit.so
auth      required  pam_env.so

account   sufficient pam_ldap.so
account   required  pam_unix.so
account   optional  pam_permit.so
account   required  pam_time.so

password  sufficient pam_ldap.so
password  required  pam_unix.so     try_first_pass nullok sha512 shadow
password  optional  pam_permit.so

session   required  pam_limits.so
session   required  pam_unix.so
session   optional  pam_ldap.so
session   optional  pam_permit.so

Then edit both /etc/pam.d/su and /etc/pam.d/su-l identically. The su-l file is used when the user runs su --login.

Make pam_ldap.so sufficient at the top of each section but below pam_rootok, and add use_first_pass to pam_unix in the auth section.

/etc/pam.d/su
#%PAM-1.0
auth      sufficient    pam_rootok.so
auth      sufficient    pam_ldap.so
# Uncomment the following line to implicitly trust users in the "wheel" group.
#auth     sufficient    pam_wheel.so trust use_uid
# Uncomment the following line to require a user to be in the "wheel" group.
#auth     required      pam_wheel.so use_uid
auth      required	pam_unix.so use_first_pass
account   sufficient    pam_ldap.so
account   required	pam_unix.so
session   sufficient    pam_ldap.so
session   required	pam_unix.so

To enable users to edit their password, edit /etc/pam.d/passwd:

/etc/pam.d/passwd
#%PAM-1.0
password        sufficient      pam_ldap.so
#password       required        pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
#password       required        pam_unix.so sha512 shadow use_authtok
password        required        pam_unix.so sha512 shadow nullok
Create home folders at login

If you want home folders to be created at login (eg: if you are not using NFS to store home folders), edit /etc/pam.d/system-login and add pam_mkhomedir.so to the session section above any "sufficient" items. This will cause home folder creation when logging in at a tty, from ssh, xdm, kdm, gdm, etc. You might choose to edit additional files in the same way, such as /etc/pam.d/su and /etc/pam.d/su-l to enable it for su and su --login. If you do not want to do this for ssh logins, edit system-local-login instead of system-login, etc.

/etc/pam.d/system-login
...top of file not shown...
session    optional   pam_loginuid.so
session    include    system-auth
session    optional   pam_motd.so          motd=/etc/motd
session    optional   pam_mail.so          dir=/var/spool/mail standard quiet
-session   optional   pam_systemd.so
session    required   pam_env.so
session    required   pam_mkhomedir.so skel=/etc/skel umask=0077
/etc/pam.d/su-l
...top of file not shown...
session         required        pam_mkhomedir.so skel=/etc/skel umask=0077
session         sufficient      pam_ldap.so
session         required        pam_unix.so
Enable sudo

To enable sudo from an LDAP user, edit /etc/pam.d/sudo. You will also need to modify sudoers accordingly.

/etc/pam.d/sudo
#%PAM-1.0
auth      sufficient    pam_ldap.so
auth      required      pam_unix.so  try_first_pass
auth      required      pam_nologin.so

You will also need to add in /etc/openldap/ldap.conf the following:

/etc/openldap/ldap.conf
sudoers_base ou=sudoers,dc=example,dc=org

Online and Offline Authentication with SSSD

SSSD is a system daemon. Its primary function is to provide access to identity and authentication remote resource through a common framework that can provide caching and offline support to the system. It provides PAM and NSS modules, and in the future will D-BUS based interfaces for extended user information. It provides also a better database to store local users as well as extended user data.

Note: You must configure StartTLS for your LDAP server, or SSSD will not work

Install the sssd package.

SSSD Configuration

If it does not exist create /etc/sssd/sssd.conf.

/etc/sssd/sssd.conf
[sssd]
config_file_version = 2
services = nss, pam
domains = LDAP

[domain/LDAP]
cache_credentials = true
enumerate = true

id_provider = ldap
auth_provider = ldap

ldap_uri = ldap://server1.example.org, ldap://server2.example.org
ldap_search_base = dc=example,dc=org
ldap_id_use_start_tls = true
ldap_tls_reqcert = demand
ldap_tls_cacert = /etc/openldap/certs/cacerts.pem
chpass_provider = ldap
ldap_chpass_uri = ldap://server1.example.org
entry_cache_timeout = 600
ldap_network_timeout = 2

# OpenLDAP supports posixGroup, uncomment the following two lines
# to get group membership support (and comment the other conflicting parameters)
#ldap_schema = rfc2307
#ldap_group_member = memberUid

# Other LDAP servers may support this instead
ldap_schema = rfc2307bis
ldap_group_member = uniqueMember

The above is an example only. See sssd.conf(5) for the full details.

Finally set the file permissions chmod 600 /etc/sssd/sssd.conf otherwise sssd will fail to start.

NSCD Configuration

Disable caching for passwd, group and netgroup entries in /etc/nscd.conf as it will interfere with sssd caching.

Keep caching enabled for hosts entries otherwise some services may fail to start.

/etc/nscd.conf
# Begin /etc/nscd.conf
[...]
enable-cache		passwd		no
[...]
enable-cache		group		no
[...]
enable-cache		hosts		yes
[...]
enable-cache		netgroup	no
[...]
# End /etc/nscd.conf

NSS Configuration

Edit /etc/nsswitch.conf as follows:

/etc/nsswitch.conf
# Begin /etc/nsswitch.conf

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

publickey: files

hosts: files dns myhostname
networks: files

protocols: files
services: files
ethers: files
rpc: files

netgroup: files

# End /etc/nsswitch.conf

PAM Configuration

The first step is to edit /etc/pam.d/system-auth as follows:

/etc/pam.d/system-auth
#%PAM-1.0

auth sufficient pam_sss.so forward_pass
auth required pam_unix.so try_first_pass nullok
auth optional pam_permit.so
auth required pam_env.so

account [default=bad success=ok user_unknown=ignore authinfo_unavail=ignore] pam_sss.so
account required pam_unix.so
account optional pam_permit.so
account required pam_time.so

password sufficient pam_sss.so use_authtok
password required pam_unix.so try_first_pass nullok sha512 shadow
password optional pam_permit.so

session     required      pam_mkhomedir.so skel=/etc/skel/ umask=0077
session required pam_limits.so
session required pam_unix.so
session optional pam_sss.so
session optional pam_permit.so
Note: If you happen to use GNOME/Keyring: adding 'sufficient' in the beginning of the PAM stack results in Gnome Keyring not being unlocked. For a solution, look at Advanced PAM Configuration.

These PAM changes will apply to fresh login. To also allow the su command to authenticate through SSSD, edit /etc/pam.d/su:

/etc/pam.d/su
#%PAM-1.0
auth            sufficient        pam_rootok.so

auth sufficient   pam_sss.so      forward_pass
auth            required        pam_unix.so

account [default=bad success=ok user_unknown=ignore authinfo_unavail=ignore] pam_sss.so
account         required        pam_unix.so

session         required        pam_unix.so
session optional pam_sss.so
Enable sudo

Edit /etc/pam.d/sudo as follows:

/etc/pam.d/sudo
#%PAM-1.0
auth           sufficient      pam_sss.so
auth           required        pam_unix.so try_first_pass
auth           required        pam_nologin.so

Also add sudo service to the list of enabled services and the search base in /etc/sssd/sssd.conf:

/etc/sssd/sssd.conf
[sssd]
...
services = nss, pam, sudo
...

[domain/LDAP]
...
ldap_sudo_search_base = ou=sudoers,dc=example,dc=org
...

Alternately, configure sudo to allow the desired LDAP users to use sudo.

Password Management

In order to enable users to change their passwords using passwd edit /etc/pam.d/passwd as follows:

/etc/pam.d/passwd
#%PAM-1.0
password        sufficient      pam_sss.so
#password       required        pam_cracklib.so difok=2 minlen=8 dcredit=2 ocredit=2 retry=3
#password       required        pam_unix.so sha512 shadow use_authtok
password        required        pam_unix.so sha512 shadow nullok

Start/enable sssd.service.

You should now be able to see details of your ldap users with getent passwd username or id username.

Once you have logged in with a user the credentials will be cached and you will be able to login using the cached credentials when the ldap server is offline or unavailable.

See also