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48.6.9. Setting Up Cross Realm Authentication

Cross-realm authentication is the term which is used to describe situations in which clients (typically users) of one realm use Kerberos to authenticate to services (typically server processes running on a particular server system) which belong to a realm other than their own.
For the simplest case, in order for a client of a realm named A.EXAMPLE.COM to access a service in the B.EXAMPLE.COM realm, both realms must share a key for a principal named krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM, and both keys must have the same key version number associated with them.
To accomplish this, select a very strong password or passphrase, and create an entry for the principal in both realms using kadmin. 
~]# kadmin -r A.EXAMPLE.COM
kadmin: add_principal krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM
Enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Re-enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" created.
kadmin:	quit
~]# kadmin -r B.EXAMPLE.COM
kadmin: add_principal krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM
Enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Re-enter password for principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM":
Principal "krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM" created.
kadmin: quit
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Use the get_principal command to verify that both entries have matching key version numbers (kvno values) and encryption types.

Warning

Security-conscious administrators may attempt to use the add_principal command's -randkey option to assign a random key instead of a password, dump the new entry from the database of the first realm, and import it into the second. This will not work unless the master keys for the realm databases are identical, as the keys contained in a database dump are themselves encrypted using the master key.
Clients in the A.EXAMPLE.COM realm are now able to authenticate to services in the B.EXAMPLE.COM realm. Put another way, the B.EXAMPLE.COM realm now trusts the A.EXAMPLE.COM realm, or phrased even more simply, B.EXAMPLE.COM now trusts A.EXAMPLE.COM.
This brings us to an important point: cross-realm trust is unidirectional by default. The KDC for the B.EXAMPLE.COM realm may trust clients from the A.EXAMPLE.COM to authenticate to services in the B.EXAMPLE.COM realm, but the fact that it does has no effect on whether or not clients in the B.EXAMPLE.COM realm are trusted to authenticate to services in the A.EXAMPLE.COM realm. To establish trust in the other direction, both realms would need to share keys for the krbtgt/A.EXAMPLE.COM@B.EXAMPLE.COM service (take note of the reversed in order of the two realms compared to the example above).
If direct trust relationships were the only method for providing trust between realms, networks which contain multiple realms would be very difficult to set up. Luckily, cross-realm trust is transitive. If clients from A.EXAMPLE.COM can authenticate to services in B.EXAMPLE.COM, and clients from B.EXAMPLE.COM can authenticate to services in C.EXAMPLE.COM, then clients in A.EXAMPLE.COM can also authenticate to services in C.EXAMPLE.COM, even if C.EXAMPLE.COM doesn't directly trust A.EXAMPLE.COM. This means that, on a network with multiple realms which all need to trust each other, making good choices about which trust relationships to set up can greatly reduce the amount of effort required.
Now you face the more conventional problems: the client's system must be configured so that it can properly deduce the realm to which a particular service belongs, and it must be able to determine how to obtain credentials for services in that realm.
First things first: the principal name for a service provided from a specific server system in a given realm typically looks like this: 
service/server.example.com@EXAMPLE.COM
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In this example, service is typically either the name of the protocol in use (other common values include ldap, imap, cvs, and HTTP) or host, server.example.com is the fully-qualified domain name of the system which runs the service, and EXAMPLE.COM is the name of the realm.
To deduce the realm to which the service belongs, clients will most often consult DNS or the domain_realm section of /etc/krb5.conf to map either a hostname (server.example.com) or a DNS domain name (.example.com) to the name of a realm (EXAMPLE.COM).
Having determined which to which realm a service belongs, a client then has to determine the set of realms which it needs to contact, and in which order it must contact them, to obtain credentials for use in authenticating to the service.
This can be done in one of two ways.
The default method, which requires no explicit configuration, is to give the realms names within a shared hierarchy. For an example, assume realms named A.EXAMPLE.COM, B.EXAMPLE.COM, and EXAMPLE.COM. When a client in the A.EXAMPLE.COM realm attempts to authenticate to a service in B.EXAMPLE.COM, it will, by default, first attempt to get credentials for the EXAMPLE.COM realm, and then to use those credentials to obtain credentials for use in the B.EXAMPLE.COM realm.
The client in this scenario treats the realm name as one might treat a DNS name. It repeatedly strips off the components of its own realm's name to generate the names of realms which are "above" it in the hierarchy until it reaches a point which is also "above" the service's realm. At that point it begins prepending components of the service's realm name until it reaches the service's realm. Each realm which is involved in the process is another "hop".
For example, using credentials in A.EXAMPLE.COM, authenticating to a service in B.EXAMPLE.COM:


A.EXAMPLE.COM → EXAMPLE.COM → B.EXAMPLE.COM

  • A.EXAMPLE.COM and EXAMPLE.COM share a key for krbtgt/EXAMPLE.COM@A.EXAMPLE.COM
  • EXAMPLE.COM and B.EXAMPLE.COM share a key for krbtgt/B.EXAMPLE.COM@EXAMPLE.COM
Another example, using credentials in SITE1.SALES.EXAMPLE.COM, authenticating to a service in EVERYWHERE.EXAMPLE.COM:


SITE1.SALES.EXAMPLE.COM → SALES.EXAMPLE.COM → EXAMPLE.COM → EVERYWHERE.EXAMPLE.COM

  • SITE1.SALES.EXAMPLE.COM and SALES.EXAMPLE.COM share a key for krbtgt/SALES.EXAMPLE.COM@SITE1.SALES.EXAMPLE.COM
  • SALES.EXAMPLE.COM and EXAMPLE.COM share a key for krbtgt/EXAMPLE.COM@SALES.EXAMPLE.COM
  • EXAMPLE.COM and EVERYWHERE.EXAMPLE.COM share a key for krbtgt/EVERYWHERE.EXAMPLE.COM@EXAMPLE.COM
Another example, this time using realm names whose names share no common suffix (DEVEL.EXAMPLE.COM and PROD.EXAMPLE.ORG):


DEVEL.EXAMPLE.COM → EXAMPLE.COM → COM → ORG → EXAMPLE.ORG → PROD.EXAMPLE.ORG

  • DEVEL.EXAMPLE.COM and EXAMPLE.COM share a key for krbtgt/EXAMPLE.COM@DEVEL.EXAMPLE.COM
  • EXAMPLE.COM and COM share a key for krbtgt/COM@EXAMPLE.COM
  • COM and ORG share a key for krbtgt/ORG@COM
  • ORG and EXAMPLE.ORG share a key for krbtgt/EXAMPLE.ORG@ORG
  • EXAMPLE.ORG and PROD.EXAMPLE.ORG share a key for krbtgt/PROD.EXAMPLE.ORG@EXAMPLE.ORG
The more complicated, but also more flexible, method involves configuring the capaths section of /etc/krb5.conf, so that clients which have credentials for one realm will be able to look up which realm is next in the chain which will eventually lead to the being able to authenticate to servers.
The format of the capaths section is relatively straightforward: each entry in the section is named after a realm in which a client might exist. Inside of that subsection, the set of intermediate realms from which the client must obtain credentials is listed as values of the key which corresponds to the realm in which a service might reside. If there are no intermediate realms, the value "." is used.
Here's an example: 
[capaths]
A.EXAMPLE.COM = {
	B.EXAMPLE.COM = .
	C.EXAMPLE.COM = B.EXAMPLE.COM
	D.EXAMPLE.COM = B.EXAMPLE.COM
	D.EXAMPLE.COM = C.EXAMPLE.COM
}
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In this example, clients in the A.EXAMPLE.COM realm can obtain cross-realm credentials for B.EXAMPLE.COM directly from the A.EXAMPLE.COM KDC.
If those clients wish to contact a service in theC.EXAMPLE.COM realm, they will first need to obtain necessary credentials from the B.EXAMPLE.COM realm (this requires that krbtgt/B.EXAMPLE.COM@A.EXAMPLE.COM exist), and then use those credentials to obtain credentials for use in the C.EXAMPLE.COM realm (using krbtgt/C.EXAMPLE.COM@B.EXAMPLE.COM).
If those clients wish to contact a service in the D.EXAMPLE.COM realm, they will first need to obtain necessary credentials from the B.EXAMPLE.COM realm, and then credentials from the C.EXAMPLE.COM realm, before finally obtaining credentials for use with the D.EXAMPLE.COM realm.

Note

Without a capath entry indicating otherwise, Kerberos assumes that cross-realm trust relationships form a hierarchy.
Clients in the A.EXAMPLE.COM realm can obtain cross-realm credentials from B.EXAMPLE.COM realm directly. Without the "." indicating this, the client would instead attempt to use a hierarchical path, in this case:


A.EXAMPLE.COM → EXAMPLE.COM → B.EXAMPLE.COM

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