There are two methods for authenticating RHEL 6 to Active Directory, Both methods are completely different & should be configured as per the requirement & environment.

NSLCD
nslcd will authenticate against Active Directory using the LDAP/Kerberos method, if you have Unix Service for Windows configured Unix Attributes set for windows user. 

SSSD
SSSD works in way similar to NSLCD, but it tries to cache the user credentials to provide authentication in case authentication server is unavailable. It also requires unix service for windows & unix attributes for users.


Check the below steps In order to configure RHEL-6 with AD using the LDAP / Kerberos method.

NSLCD :-

- Install the RHEL-6 with default packages
- Install the below packages using yum.
    nss-pam-ldapd-0.7.5-3.el6.x86_64
    nss-pam-ldapd-0.7.5-3.el6.i686
    pam_ldap-185-5.el6.x86_64
    pam_ldap-185-5.el6.i686
- Make changes to the below files.
[root@vm150 ~]# cat /etc/nslcd.conf 
uri ldap://10.65.208.43/
base dc=gsslab,dc=pnq,dc=redhat,dc=com
binddn cn=Administrator,cn=Users,dc=gsslab,dc=pnq,dc=redhat,dc=com
bindpw RedHat1!
# Mappings for Active Directory
pagesize 1000
referrals off
filter passwd (&(objectClass=user)(!(objectClass=computer))(uidNumber=*)(unixHomeDirectory=*))
map    passwd uid              sAMAccountName
map    passwd homeDirectory    unixHomeDirectory
map    passwd gecos            displayName
filter shadow (&(objectClass=user)(!(objectClass=computer))(uidNumber=*)(unixHomeDirectory=*))
map    shadow uid              sAMAccountName
map    shadow shadowLastChange pwdLastSet
filter group  (objectClass=group)
map    group  uniqueMember     member
ssl no
# tls_cacertdir /etc/openldap/cacerts

[root@vm150 ~]# cat /etc/sysconfig/authconfig |grep LEGACY
FORCELEGACY=yes

- Run "authconfig-tui" and select "Use LDAP" for "User Information" and "Use Kerberos" for "Authentication

- In LDAP Settings TAB
On the ldap Settings step. Leave the use TLS option unselected put the AD servers fully qualified domain name or Ip Address in and the base DN.

- In Kerberos Settings TAB 
On the kerberos Settings page enter the AD servers Realm, also list the AD servers  fully qualified domain name or Ip Address for the KDC and Admin Server.

This will result out in Starting nslcd daemon.

Verify 

[root@vm150 ~]# id u1
uid=10001(u1) gid=10000(Domain Users) groups=10000(Domain Users),10001(group1),10002(group2),909(group3),10011(group4),10012(group5),10013(group6),10014(group7),10015(group8),10016(group9),10005(group10),10006(group11),10007(group12),10008(group13),10009(group14),10010(group15),10018(group17),1111(Training),10020(TelnetClients)
[root@vm150 ~]# getent passwd u1
u1:*:10001:10000:user1:/home/u1:/bin/sh


SSSD :- 

- Install the RHEL - 6 with default packages.

Make changes to /etc/sssd/sssd.conf

[root@vm172 ~]# cat /etc/sssd/sssd.conf  |grep -v ^# |grep -v ^$
[sssd]
config_file_version = 2
reconnection_retries = 3
sbus_timeout = 30
services = nss, pam
domains = default
[nss]
filter_groups = root
filter_users = root
reconnection_retries = 3
[pam]
reconnection_retries = 3
[domain/default]
ldap_default_authtok_type = password
ldap_id_use_start_tls = False
cache_credentials = True
ldap_group_object_class = group
ldap_search_base = dc=gsslab,dc=pnq,dc=redhat,dc=com
chpass_provider = krb5
ldap_default_authtok = RedHat1!
id_provider = ldap
auth_provider = krb5
ldap_default_bind_dn = cn=Administrator,cn=Users,dc=gsslab,dc=pnq,dc=redhat,dc=com
ldap_user_gecos = displayName
debug_level = 0
ldap_uri = ldap://10.65.208.43/
krb5_realm = GSSLAB.PNQ.REDHAT.COM
krb5_kpasswd = 10.65.208.43
ldap_schema = rfc2307bis
ldap_force_upper_case_realm = True
ldap_user_object_class = person
ldap_tls_cacertdir = /etc/openldap/cacerts
krb5_kdcip = 10.65.208.43

1. Run the authconfig-tui tool. Select ldap under the "User Information" section and Kerberos under the "Authentication" Section.
2. On the ldap Settings step. Leave the use TLS option unselected put the AD servers fully qualified domain name in and the base DN.
3. On the kerberos Settings page enter the AD servers Realm, also list the AD servers  fully qualified domain name for the KDC and Admin Server.

This will result in restarting sssd daemon.

Verify :-
[root@vm172 ~]# id u1
uid=10001(u1) gid=10000(Domain Users) groups=10000(Domain Users),10020(TelnetClients),10021(group20),10022(group19),10023(group18),10018(group17),10010(group15),10009(group14),10008(group13),10007(group12),10006(group11),10005(group10),10016(group9),10015(group8),10014(group7),10013(group6),10012(group5),10011(group4),909(group3),10002(group2),10001(group1)
[root@vm172 ~]# getent passwd u1
u1:*:10001:10000:user1:/:/bin/sh