source: heimdal/trunk/doc/kerberos4.texi@ 4

@c $Id$

@node Kerberos 4 issues, Windows compatibility, Things in search for a better place, Top
@comment  node-name,  next,  previous,  up
@chapter Kerberos 4 issues

Kerberos 4 KDC and KA server have been moved.

For more about AFS, see the section @xref{AFS}.

@menu
* Principal conversion issues::  
* Converting a version 4 database::  
@end menu

@node Principal conversion issues, Converting a version 4 database, Kerberos 4 issues, Kerberos 4 issues
@section 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 @samp{rcmd} (for ``remote command''), and the
version 5 name is @samp{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 hostname.

Because of this it can be hard or impossible to correctly convert a
version 4 principal to a version 5 principal @footnote{the other way is
not always trivial either, but usually easier}. 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 @samp{rcmd.foo}.

The @samp{rcmd} name suggests that the instance is a hostname (even if
there are exceptions to this rule). To correctly convert the instance
@samp{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
@footnote{at least until secure DNS is commonly available}.

Fortunately, the KDC has a trump on hand: it can easily tell if a
principal exists in the database. The KDC will use
@code{krb5_425_conv_principal_ext} to convert principals when handling
to version 4 requests.

@node Converting a version 4 database, , Principal conversion issues, Kerberos 4 issues
@section Converting a version 4 database

If you want to convert an existing version 4 database, the principal
conversion issue arises too.

If you decide to convert your database once and for all, you will only
have to do this conversion once. It is also possible to run a version 5
KDC as a slave to a version 4 KDC. In this case this conversion will
happen every time the database is propagated.  When doing this
conversion, there are a few things to look out for. If you have stale
entries in the database, these entries will not be converted. This might
be because these principals are not used anymore, or it might be just
because the principal couldn't be converted.

You might also see problems with a many-to-one mapping of
principals. For instance, if you are using DNS lookups and you have two
principals @samp{rcmd.foo} and @samp{rcmd.bar}, where `foo' is a CNAME
for `bar', the resulting principals will be the same. Since the
conversion function can't tell which is correct, these conflicts will
have to be resolved manually.

@subsection Conversion example

Given the following set of hosts and services:

@example
foo.se          rcmd
mail.foo.se     rcmd, pop
ftp.bar.se      rcmd, ftp
@end example

you have a database that consists of the following principals:

@samp{rcmd.foo}, @samp{rcmd.mail}, @samp{pop.mail}, @samp{rcmd.ftp}, and
@samp{ftp.ftp}.

lets say you also got these extra principals: @samp{rcmd.gone},
@samp{rcmd.old-mail}, where @samp{gone.foo.se} was a machine that has
now passed away, and @samp{old-mail.foo.se} was an old mail machine that
is now a CNAME for @samp{mail.foo.se}.

When you convert this database you want the following conversions to be
done:
@example
rcmd.foo         host/foo.se
rcmd.mail        host/mail.foo.se
pop.mail         pop/mail.foo.se
rcmd.ftp         host/ftp.bar.se
ftp.ftp          ftp/ftp.bar.se
rcmd.gone        @i{removed}
rcmd.old-mail    @i{removed}
@end example

A @file{krb5.conf} that does this looks like:

@example
[realms]
        FOO.SE = @{
                v4_name_convert = @{
                        host = @{
                                ftp = ftp
                                pop = pop
                                rcmd = host
                        @}
                @}
                v4_instance_convert = @{
                        foo = foo.se
                        ftp = ftp.bar.se
                @}
                default_domain = foo.se
        @}
@end example

The @samp{v4_name_convert} section says which names should be considered
having an instance consisting of a hostname, and it also says how the
names should be converted (for instance @samp{rcmd} should be converted
to @samp{host}). The @samp{v4_instance_convert} section says how a
hostname should be qualified (this is just a hosts-file in
disguise). Host-instances that aren't covered by
@samp{v4_instance_convert} are qualified by appending the contents of
the @samp{default_domain}.

Actually, this example doesn't work. Or rather, it works to well. Since
it has no way of knowing which hostnames are valid and which are not, it
will happily convert @samp{rcmd.gone} to @samp{host/gone.foo.se}. This
isn't a big problem, but if you have run your kerberos realm for a few
years, chances are big that you have quite a few `junk' principals.

If you don't want this you can remove the @samp{default_domain}
statement, but then you will have to add entries for @emph{all} your hosts
in the @samp{v4_instance_convert} section.

Instead of doing this you can use DNS to convert instances. This is not
a solution without problems, but it is probably easier than adding lots
of static host entries. 

To enable DNS lookup you should turn on @samp{v4_instance_resolve} in
the @samp{[libdefaults]} section.

@subsection Converting a database

The database conversion is done with @samp{hprop}. You can run this
command to propagate the database to the machine called
@samp{slave-server} (which should be running a @samp{hpropd}).

@example
hprop --source=krb4-db --master-key=/.m slave-server
@end example

This command can also be to use for converting the v4 database on the
server:

@example
hprop -n --source=krb4-db -d /var/kerberos/principal --master-key=/.m | hpropd -n
@end example