When integrating Linux clients with a Windows Server 2008 R2 domain, the kinit command often becomes the first stumbling block. The error message "Realm not local to KDC while getting initial credentials" typically indicates a mismatch between the configured realm and what the KDC expects.

The /etc/krb5.conf file is crucial for proper Kerberos authentication. Let's examine a working configuration:

[libdefaults]
    default_realm = DS.DOMAIN.COM
    dns_lookup_realm = false
    dns_lookup_kdc = true
    ticket_lifetime = 24h
    renew_lifetime = 7d
    forwardable = true

[realms]
    DS.DOMAIN.COM = {
        kdc = dc1.ds.domain.com:88
        kdc = dc2.ds.domain.com:88
        admin_server = dc1.ds.domain.com
        default_domain = ds.domain.com
    }

[domain_realm]
    .ds.domain.com = DS.DOMAIN.COM
    ds.domain.com = DS.DOMAIN.COM

Case Sensitivity Matters: Kerberos realms are typically uppercase (DS.DOMAIN.COM) while DNS names are lowercase (ds.domain.com). Ensure your configuration reflects this.

DNS Configuration Check: Verify your DNS settings with:

nslookup ds.domain.com
dig -t SRV _kerberos._tcp.ds.domain.com
dig -t SRV _ldap._tcp.ds.domain.com

If basic configuration doesn't resolve the issue, try these diagnostic commands:

# Check Kerberos ticket granting
kvno Administrator@DS.DOMAIN.COM

# Verbose kinit attempt
KRB5_TRACE=/dev/stdout kinit Administrator@DS.DOMAIN.COM

# Verify time synchronization (critical for Kerberos)
ntpdate -u dc1.ds.domain.com

On your Windows Server 2008 R2 domain controller:

1. Ensure the SPN (Service Principal Name) is properly registered
2. Verify cross-realm trust settings if applicable
3. Check event logs for Kerberos-related errors

Before attempting kinit again, validate your configuration with:

klist
kdestroy -A
kinit -V Administrator@DS.DOMAIN.COM

Remember that changes to krb5.conf don't require a service restart, but DNS caching might require flushing.

When integrating Ubuntu clients with a Windows Server 2008 R2 domain controller, the Kerberos authentication process must be properly configured. The specific error Realm not local to KDC while getting initial credentials typically indicates a mismatch between the configured realms and the actual KDC (Key Distribution Center) setup.

Your current configuration shows several potential issues that need addressing:

[libdefaults]
default_realm = DS.DOMAIN.COM  # Fixed parameter name
dns_lookup_realm = true
dns_lookup_kdc = true  # Added missing equals sign

[realms]
    DS.DOMAIN.COM = {
        kdc = ds.domain.com:88
        admin_server = ds.domain.com
        default_domain = domain.com
    }

[domain_realm]
    .domain.com = DS.DOMAIN.COM
    domain.com = DS.DOMAIN.COM

Here are the critical corrections needed:

• The parameter should be default_realm not just default
• Missing equals sign in dns_lookup_kdc = true
• Ensure the realm name matches exactly (case-sensitive) between configuration and Active Directory

After making these changes, perform the following checks:

# Verify DNS resolution
host ds.domain.com

# Check Kerberos configuration
klist -e

# Test authentication (interactive mode)
kinit Administrator@DS.DOMAIN.COM

If the issue persists:

1. Verify time synchronization between client and domain controller
2. Check firewall settings for UDP port 88 (Kerberos)
3. Examine the Windows event logs on the domain controller
4. Test with FQDN instead of short hostname

Here's a verified configuration that works with Windows Server 2008 R2:

[libdefaults]
    default_realm = DS.DOMAIN.COM
    dns_lookup_realm = false
    dns_lookup_kdc = true
    ticket_lifetime = 24h
    renew_lifetime = 7d
    forwardable = true

[realms]
    DS.DOMAIN.COM = {
        kdc = dc1.ds.domain.com:88
        kdc = dc2.ds.domain.com:88
        admin_server = dc1.ds.domain.com:749
        default_domain = domain.com
    }

[domain_realm]
    .domain.com = DS.DOMAIN.COM
    domain.com = DS.DOMAIN.COM
    .ds.domain.com = DS.DOMAIN.COM
    ds.domain.com = DS.DOMAIN.COM