LDAP authentication
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:
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
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.
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.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, sddm, 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.
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
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.