Converting between version 4 and version 5 principals can at best be described as a mess.
A version 4 principal consists of a name, an instance, and a realm. A version 5 principal consists of one or more components, and a realm. In some cases also the first component/name will differ between version 4 and version 5. Furthermore the second component of a host principal will be the fully qualified domain name of the host in question, while the instance of a version 4 principal will only contain the first part (short hostname). Because of these problems the conversion between principals will have to be site customized.
krb5_425_conv_principal_ext() will try to convert a version 4 principal, given by
name,
instance, and
realm, to a version 5 principal. This can result in several possible principals, and if
func is non-NULL, it will be called for each candidate principal.
func should return true if the principal was “good”. To accomplish this,
krb5_425_conv_principal_ext() will look up the name in
krb5.conf. It first looks in the
v4_name_convert/host subsection, which should contain a list of version 4 names whose instance should be treated as a hostname. This list can be specified for each realm (in the
realms section), or in the
libdefaults section. If the name is found the resulting name of the principal will be the value of this binding. The instance is then first looked up in
v4_instance_convert for the specified realm. If found the resulting value will be used as instance (this can be used for special cases), no further attempts will be made to find a conversion if this fails (with
func). If the
resolve parameter is true, the instance will be looked up with
gethostbyname(). This can be a time consuming, error prone, and unsafe operation. Next a list of hostnames will be created from the instance and the
v4_domains variable, which should contain a list of possible domains for the specific realm.
On the other hand, if the name is not found in a
host section, it is looked up in a
v4_name_convert/plain binding. If found here the name will be converted, but the instance will be untouched.
This list of default host-type conversions is compiled-in:
v4_name_convert = {
host = {
ftp = ftp
hprop = hprop
imap = imap
pop = pop
rcmd = host
smtp = smtp
}
}
It will only be used if there isn't an entry for these names in the config file, so you can override these defaults.
krb5_425_conv_principal() will call
krb5_425_conv_principal_ext() with
NULL as
func, and the value of
v4_instance_resolve (from the
libdefaults section) as
resolve.
krb5_524_conv_principal() basically does the opposite of
krb5_425_conv_principal(), it just doesn't have to look up any names, but will instead truncate instances found to belong to a host principal. The
name,
instance, and
realm should be at least 40 characters long.