( Principal conversion issues

Info Catalog ( Kerberos 4 issues ( Kerberos 4 issues ( Converting a version 4 database
 7.1 Principal conversion issues
 First, Kerberos 4 and Kerberos 5 principals are different. A version 4
 principal consists of a name, an instance, and a realm. A version 5
 principal has one or more components, and a realm (the terms "name" and
 "instance" are still used, for the first and second component,
 respectively).    Also, in some cases the name of a version 4 principal
 differs from the first component of the corresponding version 5
 principal. One notable example is the "host" type principals, where the
 version 4 name is `rcmd' (for "remote command"), and the version 5 name
 is `host'. For the class of principals that has a hostname as instance,
 there is an other major difference, Kerberos 4 uses only the first
 component of the hostname, whereas Kerberos 5 uses the fully qualified
 Because of this it can be hard or impossible to correctly convert a
 version 4 principal to a version 5 principal (1). The biggest problem is
 to know if the conversion resulted in a valid principal. To give an
 example, suppose you want to convert the principal `'.
 The `rcmd' name suggests that the instance is a hostname (even if there
 are exceptions to this rule). To correctly convert the instance `foo'
 to a hostname, you have to know which host it is referring to. You can
 to this by either guessing (from the realm) which domain name to
 append, or you have to have a list of possible hostnames. In the
 simplest cases you can cover most principals with the first rule. If you
 have several domains sharing a single realm this will not usually work.
 If the exceptions are few you can probably come by with a lookup table
 for the exceptions.
 In a complex scenario you will need some kind of host lookup mechanism.
 Using DNS for this is tempting, but DNS is error prone, slow and unsafe
 Fortunately, the KDC has a trump on hand: it can easily tell if a
 principal exists in the database. The KDC will use
 `krb5_425_conv_principal_ext' to convert principals when handling to
 version 4 requests.
 ---------- Footnotes ----------
 (1) the other way is not always trivial either, but usually easier
 (2) at least until secure DNS is commonly available
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