Client Authentication:Centos via sssd/ldap

	Warning:
	This how-to should be validated by Daniel before you use it!!.

	Warning:
	This is based upon limited testing and a small number of users.

Introduction

This how-to shows how to configure a SME-server (>=8b6) and a client Centos >= 5 for a LDAP based SSSD authentication of the client machine on the configured user accounts of the SME.

The main advantage in comparaison to nss_ldap is that the authentication informations stay in the cache and the authentication can therefore furter work, even in offline mode (when the server not available).

Nevertheless, the creation of a local user with the admin rights is recommanded for the emergency case.

These lines are a translation of the method given by Daniel: https://wikit.firewall-services.com/doku.php/tuto/ipasserelle/authentification/centos_sssd_on_sme. Many thanks to him for it.

In this how-to: we assume that:

the host name of the SME is "sme-server" and the domain is "domain.tld".

Configuration of the SME-server

There is quite no necessary configuration of the SME.

• The only thing to do is to create a user (named "auth" in this how-to) via the server-manager and to give him a valid password ("something_very_secret" in the how-to).

It is not required to make "auth" member of any group.

• In addition, it is recommended to install and configure PHPki in order to make the managing of the self-created certificates easier.

Configuration of the client CentOS

Manage the CA of the SME

after having installed phpki, go to https://www.domain.tld/phpki and download on the client machine the certificate of authority (ca.crt).

Place a copy of it or of another CA into /etc/phpki/tls/certs/ and give the 644 permissions:

cp ~/download/ca.crt /etc/phpki/tls/certs/
chmod 644 /etc/phpki/tls/certs/ca.crt

Install the required package

First of all, install the required package:

yum install sssd

Configure SSSD

The configuration is made by the file /ets/sssd/sssd.conf.

• At the beginning of this file, the used domain has to be set. In sssd, a domain can be taken as a source of content. it is possible to set several domains in order of priority.
• And deeper in the file, we will add the configuration of the domain

If the file doesn't exist by default it has to be created and it needs to get the permissions 600 to allow the daemon to start:

cat <<'_EOF' > /etc/sssd/sssd.conf
[sssd]
config_file_version = 2
services = nss, pam
domains = LDAP

[nss]

[pam]

[domain/LDAP]
id_provider = ldap
auth_provider = ldap
ldap_schema = rfc2307
ldap_uri = ldap://sme-server.domain.tld
ldap_default_bind_dn = uid=auth,ou=Users,dc=domain,dc=tld
ldap_default_authtok = something_very_secret
ldap_default_authtok_type = password
ldap_search_base = dc=domain,dc=tld
ldap_user_search_base = ou=Users,dc=domain,dc=tld
ldap_group_search_base = ou=Groups,dc=domain,dc=tld
ldap_user_object_class = inetOrgPerson
ldap_user_gecos = cn
ldap_tls_reqcert = hard
ldap_tls_cacert = /etc/pki/tls/certs/ca.crt
ldap_id_use_start_tls = true
# uncomment below if the SME is a “iPasserelle”
#ldap_user_shell = desktopLoginShell
# comment below if the SME is a “iPasserelle”
override_shell = /bin/bash
cache_credentials = true
enumerate = true
# It is possible to filter the logins via a LDAP-filer
# by commenting the both lines below.
# In this exemple, only the users member of the group netusers
# will be valid on this host.
# posixMemberOF is a parameter only for a iPasserelle
#access_provider = ldap
#ldap_access_filter = (|(posixMemberOf=admins)(uid=backup))
_EOF

chmod 600 /etc/sssd/sssd.conf

	Tip:
	Make sure that the file /etc/pki/tls/certs/ca.crt contains the CA that has sign the certificate of the SME (if PHPki is used, a version > 0,82-13 is required).

Configure nss

To allow nss to use sssd, you should check that sss is set as a source for users and groups in the /etc/nsswitch.conf.

[...]
passwd:     files sss
shadow:     files sss
group:      files sss
[...]
netgroup:   files sss

	Tip:
	This has been done automatically from CentOS 7

Check

getent passwd

should show the ldap-users. If it doen't work, you should start debugging by running sssd in interactiv mode (with sssd -i -d 5 for exemple).

Configure pam

pam must be configured on order to use sssd as a source too:

CentOS 5

rm -f /etc/pam.d/system-auth
cat <<'EOF' > /etc/pam.d/system-auth
#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth        required      pam_env.so
auth        sufficient    pam_unix.so nullok try_first_pass
auth        requisite     pam_succeed_if.so uid >= 500 quiet
auth        sufficient    pam_sss.so use_first_pass
auth        required      pam_deny.so
 
account     required      pam_unix.so
account     sufficient    pam_succeed_if.so uid < 500 quiet
account     [default=bad success=ok user_unknown=ignore] pam_sss.so
account     required      pam_permit.so
 
password    requisite     pam_cracklib.so try_first_pass retry=3
password    sufficient    pam_unix.so md5 shadow nullok try_first_pass use_authtok 
password    sufficient    pam_sss.so use_authtok
password    required      pam_deny.so
 
session     optional      pam_keyinit.so revoke
session     required      pam_limits.so
session     optional      pam_mkhomedir.so skel=/etc/skel umask=0077
session     [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid
session     required      pam_unix.so
session     optional      pam_sss.so
EOF

CentOS 6 / 7

rm -f /etc/pam.d/system-auth
cat <<'EOF' > /etc/pam.d/system-auth
#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth        required      pam_env.so
auth        sufficient    pam_unix.so nullok try_first_pass
auth        requisite     pam_succeed_if.so uid >= 500 quiet
auth        sufficient    pam_sss.so use_first_pass
auth        required      pam_deny.so
 
account     required      pam_unix.so
account     sufficient    pam_succeed_if.so uid < 500 quiet
account     [default=bad success=ok user_unknown=ignore] pam_sss.so
account     required      pam_permit.so
 
password    requisite     pam_cracklib.so try_first_pass retry=3 type=
password    sufficient    pam_unix.so sha512 shadow nullok try_first_pass use_authtok
password    sufficient    pam_sss.so use_authtok
password    required      pam_deny.so
 
session     optional      pam_keyinit.so revoke
session     required      pam_limits.so
session     optional      pam_mkhomedir.so skel=/etc/skel umask=0077
session     [success=1 default=ignore] pam_succeed_if.so service in crond quiet use_uid
session     required      pam_unix.so
session     optional      pam_sss.so
EOF
rm -f /etc/pam.d/password-auth
ln -sf system-auth /etc/pam.d/password-auth

That's all. It has only to be tested now. When it works, the daemon sssd should be enabled at start:

chkconfig sssd on

or

systemctl enable sssd