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Chapter 9. Using canonicalized DNS host names in IdM

Identity Management (IdM) disables DNS canonicalization by default to prevent attackers from redirecting short-name queries to compromised hosts. By enabling this feature, clients can resolve short host names to their canonical forms during Kerberos authentication.

For example, if an attacker controls the DNS server and a host in the domain, the attacker can cause the short host name, such as demo, to resolve to a compromised host, such as malicious.example.com. In this case, the user connects to a different server than expected.

This procedure describes how to use canonicalized host names on IdM clients.

9.1. Adding an alias to a host principal
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Manually add service principal aliases to allow users to access hosts using short names. This method provides a secure alternative to global canonicalization by explicitly linking a short name, like demo, to the full host principal in the Kerberos database.

By default, Identity Management (IdM) clients enrolled by using the ipa-client-install command do not allow to use short host names in service principals. For example, users can use only host/demo.example.com@EXAMPLE.COM instead of host/demo@EXAMPLE.COM when accessing a service.

Learn how to add an alias to a Kerberos principal. Note that you can alternatively enable canonicalization of host names in the /etc/krb5.conf file. For details, see Enabling canonicalization of host names in service principals on clients.

Prerequisites

  • The IdM client is installed.
  • The host name is unique in the network.

Procedure

  1. Authenticate to IdM as the admin user: 

    $ kinit admin
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  2. Add the alias to the host principal. For example, to add the demo alias to the demo.examle.com host principal: 

    $ ipa host-add-principal demo.example.com --principal=demo
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Modify the /etc/krb5.conf file to permit the client to resolve host names via DNS before requesting service tickets. Setting the dns_canonicalize_hostname parameter to true automates name resolution but requires a trusted DNS environment.

NOTE

If you use host principal aliases, as described in Adding an alias to a host principal, you do not need to enable canonicalization.

Prerequisites

  • The Identity Management (IdM) client is installed.
  • You are logged in to the IdM client as the root user.
  • The host name is unique in the network.

Procedure

  • Set the dns_canonicalize_hostname parameter in the [libdefaults] section in the /etc/krb5.conf file to false: 

    [libdefaults]
...
dns_canonicalize_hostname = true
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Choose between using fully qualified domain names (FQDN) or relying on Active Directory NetBIOS compatibility when canonicalization is active. These options determine how the client requests service principals across different IdM and AD environments.

If you set dns_canonicalize_hostname = true in the /etc/krb5.conf file as explained in Enabling canonicalization of host names in service principals on clients, you have the following options when you use a host name in a service principal:

  • In Identity Management (IdM) environments, you can use the full host name in a service principal, such as host/demo.example.com@EXAMPLE.COM.
  • In environments without IdM, but if the RHEL host as a member of an Active Directory (AD) domain, no further considerations are required, because AD domain controllers (DC) automatically create service principals for NetBIOS names of the machines enrolled into AD.
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