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Setting Up Kerberos

  3 minute read.

Last Modified 2022-08-15 14:06 EDT

Kerberos is a web authentication protocol that uses strong cryptography. It proves the identity of both client and server over an insecure network connection.

Kerberos uses realms and keytabs to authenticate clients and servers. A Kerberos realm is an authorized domain that a Kerberos server can use to authenticate a client. Kerberos keytabs allow systems and clients to join an Active Directory or LDAP. Keytabs make it possible to join without entering a password.

TrueNAS allows configuring both Kerberos realms and keytabs.

Kerberos Realms

Your network must contain a Key Distribution Center (KDC) to add a realm. Users can configure Kerberos realms. Go to Directory Services > Kerberos Realms** and click **ADD**. By default, TrueNAS creates a Kerberos realm for the local system.

DirectoryServicesKerberosRealmsAdd

Enter the Realm name and click SUBMIT.

See Kerberos Screens for more information on Kerberos screens and settings.

Kerberos Keytabs

Kerberos keytabs allow systems and clients to join an Active Directory or LDAP. Keytabs make it possible to join without entering a password. A keytab (key table) is a file that stores encryption keys for various authentication scenarios. With keytabs, the TrueNAS system database benefits from this security feature. It does not store the Active Directory or LDAP administrator account password. This could be a security risk in some environments.

When using a keytab, create and use a less privileged account to perform any required queries. The TrueNAS system database stores the password for that account.

Create Keytab on Windows Server for Active Directory

To create the keytab on a Windows Server system, open a command prompt and use the ktpass command:

ktpass -princ USERNAME@REALM.COM -pass PASSWORD -crypto ENCRYPTION TYPE -ptype KRB5_NT_PRINCIPAL -kvno 0 -out c:\PATH\KEYTABNAME.KEYTAB where USERNAME@REALM.COM is the Windows Server user and principal name written in the format username@KERBEROS.REALM. The Kerberos realm is typically in all caps, but the Kerberos realm case should match the realm name. Refer to this note about using /princ for more details.

PASSWORD is the Windows Server user password.

ENCRYPTION TYPE is the cryptographic type you want to use. Setting ENCRYPTION TYPE to ALL allows using all supported cryptographic types. Users can specify each key instead of ALL:

  • DES-CBC-CRC is used for compatibility.
  • DES-CBC-MD5 is used for compatibility and adheres more closely to the MIT implementation.
  • RC4-HMAC-NT uses 128-bit encryption.
  • AES256-SHA1 uses AES256-CTS-HMAC-SHA1-96 encryption.
  • AES128-SHA1 uses AES128-CTS-HMAC-SHA1-96 encryption.

Specifying cryptographic types creates a keytab with enough privileges to grant tickets.

PATH\KEYTABNAME.KEYTAB is the path where you want to save the keytab and the name you want it to have.

ktpass -princ admin@WINDOWSSERVER.NET -pass Abcd1234! -crypto ALL -ptype KRB5_NT_PRINCIPAL -kvno 0 -out c:\kerberos\freenas.keytab

Add Windows Keytab to TrueNAS

After generating the keytab, add it to the TrueNAS system in Directory Services > Kerberos Keytabs > Add Kerberos Keytab.

To instruct the Active Directory service to use the keytab, go to Directory Services > Active Directory and click Advanced Options. Select the installed keytab using the Kerberos Principal dropdown list.

When using a keytab with Active Directory, username and userpass in the keytab should match the Domain Account Name and Domain Account Password fields in Directory Services > Active Directory.

To instruct LDAP to use a principal from the keytab, go to Directory Services > Active Directory. Click Advanced Options, then select the installed keytab using the Kerberos Principal dropdown list.

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