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Allow Kerberos Principals in getpwnam() calls

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Problem Statement

When using Kerberos/GSSAPI authentication for a service running on a GNU/Linux host strictly speaking not a POSIX user of the GNU/Linux system is authenticated but a Kerberos principal. I.e. authentication is successful for every Kerberos principal with a valid ticket for the service running on the GNU/Linux host. This is done intentional to keep Kerberos independent of the operating system of the host. But it creates the problem of mapping Kerberos principals to POSIX users.

Basic mappings are integrated in the MIT Kerberos library. By default the realm part of the Kerberos principal is stripped off and what remains is considered as a POSIX user name. The administrator can enhance this by adding some minimal regular-expression operations in /etc/krb5.conf. Additionally the user can create a .k5login file in his home directory and add all Kerberos principals which should be allowed to log in with his identity. All those methods do not scale in environments with multiple realm and cross-realm trust.

To allow more advance mapping schemes a plugin interface was introduced in MIT Kerberos version 1.12.

If the Kerberos principal is available SSSD will store it in its cache in the related user object. The Kerberos principal can be retrieved by looking it up in the central IdM system (LDAP/IPA/AD). If the Kerberos principal is not available but Kerberos authentication is configured SSSD will guess it by adding the configured realm or domain name to the POSIX user name. If authentication is successful with this principal it is stored in the cache as well.

A plugin which looks up the Kerberos principal in SSSD and gets the POSIX user entry back would provide a reliable mapping and scale across multiple realms and trusts because SSSD supports it.

Use case

In Windows environments, the user typically logs in using his UPN. Implementing this feature would reach parity with how Windows users are used to log in.

Implementing the localauth plugin will not only help the feature of looking up UPNs but will make it easier for administrator to configure client machines in a trust scenario as mapping will be done inside sssd instead of the auth_to_local rule in krb5.conf or a .k5login file.

Overview of the Solution

Implementation details

Currently the NSS responder expects that the argument of the getpwnam() call is a user name, either fully qualified or the short version without a domain name. The name is evaluated with the help of regular expression and split into a user and a domain component. By default the ‘@’ character is one of the characters to separate the user and the domain component in a fully qualified user name. This collides with the usage of the ‘@’ character in Kerberos principals, because here it is used to separate the user and the realm component.

One way to solve this is to introduce a special prefix tag, e.g. ‘:princ:’ to indicate that the remainder of the string should be considered as a Kerberos principal and not be split as fully qualified user names. While this would work for the localauth plugin use-case there are other potential use-cases where this would not be possible. E.g. if SSSD should allow AD user to use their UPN (see <http://technet.microsoft.com/en-us/library/cc739093(v=WS.10).aspxfor details). This UPN is build by joining the user logon name and a UPN suffix with an ‘@’ character. I think it cannot be expected from the AD users to add a prefix to this name and pam_sss cannot do it either because it does not know it is a fully qualified name or a UPN.

Especially with the second use-case, we should process the argument of getpwnam() like a fully qualified user name first. If no matching user was found during this pass SSSD can take the original input, check if it contains an ‘@’ character and search the configured backends for a matching Kerberos principal or UPN. It has to be noted that in theory there might be a user with the fully qualified name abc@domain.name and the Kerberos principal def@domain.com and a user with the fully qualified name def@domain.name and the Kerberos principal abc@domain.name. In this case getpwnam(“abc@domain.name”) will always return the entry for the user with the fully qualified name abc@domain.name even if the input was meant as Kerberos principal. This is even possible with Active Directory, i.e. the pre-Windows 2000 name and the user logon name of different users can be the same. I would say that those cases are rare and can be considered as broken configuration.

If the NSS responder decides that the given argument should be considered as a Kerberos principal and was not able to find a matching principal in the cache it can iterate over the configured backends and send a GETACCOUNTINFO request for a user with a new filter type, e.g. BE_FILTER_PRINCIPAL. The LDAP based ID backend which wants to support this new filter type can process it like a any user request but have to use an appropriate search filter.

The localauth plugin will be implemented according to the Kerberos documentation. The plugin can be enabled manually by the admin, but it’s more user-friendly to enable the plugin automatically. To this end, SSSD will gain a new option, tentatively called krb5_localauth_snippet_path. When this option is enabled, a configuration snippet similar to the following is generated into a /var/lib/sss/pubconf/krb5.include.d directory that is already sourced by krb5.conf: :

[plugins]
 localauth = {
  module = sssd:/usr/lib/sssd/modules/sssd_krb5_localauth_plugin.so
  enable_only = sssd
 }

Additional notes

The SSSD cache knows two attributes for principals “userPrincipalName” and “canonicalUserPrincipalName”. The first is used to save the data read from the LDAP server. The second is used if the TGT contains a different principal than the one used to request the TGT, i.e. if the original principal was an alias. While searching principals in the cache both should be respected. Currently “userPrincipalName” is already declared CASE_INSENSITIVE for searched in the cache. “canonicalUserPrincipalName” should be declared the same way to make searches consistent.

Configuration Changes

A new option, tentatively called krb5_localauth_snippet_path will be added to sssd’s Kerberos provider. When this option is set (mostly via SSSD default values, not administrator’s change), then SSSD will generate a file that will be automatically included in krb5.conf and the localauth plugin will be enabled.

How to test

To make sure that a getent passwd user@domain.name search for the Kerberos principal user@domain.name and not for a fully qualified name the domain name in sssd.conf should differ from the realm name in the principal. Probably the easiest configuration is to use the ldap ID provider and make sure the targeted LDAP server has a Kerberos principal attribute set for the users and the ldap_user_principal option points to the corresponding attribute name. :

...
[domain/default]
id_provider = ldap
ldap_user_principal = krbPrincipalName
...

Now the fully qualified names end with ‘@default’ while the Kerberos principal are defined by the LDAP entries. E.g. if there is a user ‘testabc’ with the principal testabc@MY.REALM the command getent passwd testabc@default will return the POSIX user entry searched with the fully qualified name. getent passwd testabc@MY.REALM will return the same entry but now search with the Kerberos principal.

Additionally, logging in as a Windows user using GSSAPI should succeed without requiring password with stock krb5.conf on an IPA client when IPA-AD trust is established, as the following sequence illustrates: :

kinit aduser@AD.DOMAIN.COM
ssh `hostname` -l aduser@AD.DOMAIN.COM

Previously, this required either a .k5login file or an elaborate auth_to_local rule.

Author(s)

Sumit Bose <sbose@redhat.com>