Before there was native support for Kerberos v5 derived AFS tokens, the krb524 service was used to convert a Kerberos v5 service ticket into a Kerberos v4 service ticket that could in turn be used to construct an AFS authentication token. As of OpenAFS 1.2.8 [14 December 2002], support was added to allow the immediate use of Kerberos v5 tickets as AFS (2b) tokens. This is the first building block necessary to break away from the limitations of Kerberos v4 with AFS. By using Kerberos v5 directly the security holes inherent in Kerberos v4 cross-realm are avoided. Use of cryptographically stronger algorithms for authentication and encryption become a possibility.
Another reason for using Kerberos v5 directly is because the krb524 service runs on port (4444/udp), which has increasingly been blocked by Internet service providers. The port was used to spread a worm which attacked Microsoft Windows in the Summer of 2003. When the port is blocked users find that they are unable to authenticate.
Replacing the Kerberos v4 ticket with a Kerberos v5 ticket is a win in all situations except when the cell name does not match the realm name and the principal names placed into the ACL's are not the principal names from the Kerberos v5 ticket. Unfortunately, some organizations have AFS cell names and Kerberos realm names which differ by more then just typographic case and rely the krb524d service to map the Kerberos v5 client principal name from realm FOO to a Kerberos v4 principal in realm BAR. This allows user@FOO to appear to be user@bar for the purposes of accessing the AFS cell.
To support this mode of operation OpenAFS for Windows versions 1.4.x through 1.6.x supported a registry value, Use524, that forced the use of krb524d within the AFS Authentication Tool and during integrated logon. Previous revisions of this documentation advised that this option should only be used by individuals until such time as their organizations transitioned away from the krb524 service.
Over the last few years all Kerberos distributions have removed support for Kerberos v4. As a result, OpenAFS can no longer support the krb524d service and the functionality has been removed from the source tree for the 1.7.x release.
As an alternative, sites should be aware that the OpenAFS 1.4.x servers permit the use of a secondary realm name that can be treated as equivalent to the cell name for authentication. This functionality can be used to avoid the need for the krb524 service if and only if both realms are managed by the same administrative entity.