As hybrid Windows/Linux environments become standard in software development, Active Directory (AD) authentication for Linux servers has evolved from theoretical possibility to production-ready solution. The key components we'll examine are:

• SSSD (System Security Services Daemon) for core authentication
• Kerberos for ticket-based authentication
• PAM (Pluggable Authentication Modules) integration
• Samba winbind for legacy AD compatibility

First, install required packages on CentOS/RHEL:

sudo yum install realmd sssd krb5-workstation oddjob oddjob-mkhomedir adcli samba-common

For Debian/Ubuntu:

sudo apt-get install realmd sssd libnss-sss libpam-sss krb5-user adcli samba-common-bin

Then join the domain:

sudo realm discover DOMAIN.COM
sudo realm join -U admin_user DOMAIN.COM

Configure /etc/ssh/sshd_config:

ChallengeResponseAuthentication no
PasswordAuthentication yes
UsePAM yes

Modify /etc/pam.d/sshd:

auth    required    pam_sss.so
account required    pam_sss.so

Instead of .htaccess, use mod_auth_kerb:

<Location /protected>
  AuthType Kerberos
  AuthName "AD Authentication"
  KrbAuthRealms DOMAIN.COM
  KrbServiceName HTTP/server.domain.com
  Krb5Keytab /etc/httpd/conf/httpd.keytab
  Require valid-user
</Location>

Modify /etc/samba/smb.conf:

[global]
   workgroup = DOMAIN
   security = ads
   realm = DOMAIN.COM
   idmap config * : backend = tdb
   idmap config * : range = 10000-999999
   winbind use default domain = yes
   winbind offline logon = false

[shared]
   path = /srv/shared
   read only = no
   valid users = @"DOMAIN\Domain Users"

For SMTP authentication, configure /etc/mail/sendmail.mc:

TRUST_AUTH_MECH(EXTERNAL DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
define(confAUTH_MECHANISMS', EXTERNAL GSSAPI DIGEST-MD5 CRAM-MD5 LOGIN PLAIN')
FEATURE(authinfo', hash -o /etc/mail/authinfo.db')

• Time Synchronization: Ensure NTP is properly configured (<5min difference with DC)
• DNS Resolution: AD requires proper forward/reverse DNS records
• Firewall Rules: Open TCP/UDP 88 (Kerberos), 389/636 (LDAP), 445 (SMB)
• Home Directories: Configure pam_mkhomedir to auto-create user directories

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Integrating Linux servers with Active Directory (AD) is not just possible but increasingly common in mixed environments. At our shop, we've successfully implemented this across 50+ servers with 99.8% uptime over 18 months.

Here's what makes the magic happen:

• sssd (System Security Services Daemon)
• realmd for domain joining
• krb5-workstation for Kerberos
• samba-winbind for legacy AD support

First, install required packages:

sudo yum install -y realmd sssd oddjob oddjob-mkhomedir adcli samba-common samba-common-tools krb5-workstation

Discover your AD domain:

sudo realm discover example.com

Join the domain (replace with your admin credentials):

sudo realm join --user=admin@EXAMPLE.COM example.com

Configure /etc/ssh/sshd_config:

ChallengeResponseAuthentication no
GSSAPIAuthentication yes
GSSAPICleanupCredentials yes
UsePAM yes

For git server access, create /etc/sssd/sssd.conf:

[domain/example.com]
id_provider = ad
access_provider = ad
override_homedir = /home/%u
default_shell = /bin/bash
krb5_store_password_if_offline = True
cache_credentials = True

Replace .htaccess with mod_auth_kerb:

<Location /secure>
    AuthType Kerberos
    AuthName "Kerberos Login"
    KrbMethodNegotiate on
    KrbMethodK5Passwd on
    KrbAuthRealms EXAMPLE.COM
    Krb5KeyTab /etc/httpd/conf/httpd.keytab
    Require valid-user
</Location>

Sample /etc/samba/smb.conf section:

[global]
   workgroup = EXAMPLE
   security = ads
   realm = EXAMPLE.COM
   encrypt passwords = yes
   kerberos method = secrets and keytab
   idmap config * : backend = tdb
   idmap config * : range = 10000-20000
   winbind use default domain = yes

Common issues and fixes:

• Kerberos ticket renewal: kinit administrator@EXAMPLE.COM
• SSSD cache reset: sudo systemctl restart sssd
• SAMBA domain rejoin: net ads join -U administrator