heimdal/doc/programming.texi

@c $Id$

@node Programming with Kerberos, Migration, Windows 2000 compatability, Top
@chapter Programming with Kerberos

First you need to know how the Kerberos model works, go read the
introduction text (@pxref{What is Kerberos?}).

@menu
* Kerberos 5 API Overview::     
* Walkthrough of a sample Kerberos 5 client::  
* Validating a password in a server application::  
* API differences to MIT Kerberos::  
@end menu

@node Kerberos 5 API Overview, Walkthrough of a sample Kerberos 5 client, Programming with Kerberos, Programming with Kerberos
@section Kerberos 5 API Overview

All functions are documented in manual pages.  This section tries to
give an overview of the major components used in Kerberos library, and
point to where to look for a specific function.

@subsection Kerberos context

A kerberos context (@code{krb5_context}) holds all per thread state. All global variables that
are context specific are stored in this structure, including default
encryption types, credential cache (for example, a ticket file), and default realms.

See the manual pages for @manpage{krb5_context,3} and
@manpage{krb5_init_context,3}.

@subsection Kerberos authentication context

Kerberos authentication context (@code{krb5_auth_context}) holds all
context related to an authenticated connection, in a similar way to the
kerberos context that holds the context for the thread or process.

The @code{krb5_auth_context} is used by various functions that are
directly related to authentication between the server/client. Example of
data that this structure contains are various flags, addresses of client
and server, port numbers, keyblocks (and subkeys), sequence numbers,
replay cache, and checksum types.

See the manual page for @manpage{krb5_auth_context,3}.

@subsection Kerberos principal

The Kerberos principal is the structure that identifies a user or
service in Kerberos. The structure that holds the principal is the
@code{krb5_principal}. There are function to extract the realm and
elements of the principal, but most applications have no reason to
inspect the content of the structure.

The are several ways to create a principal (with different degree of
portability), and one way to free it.

See manual page for @manpage{krb5_principal,3} for more information
about the functions.

@subsection Credential cache

A credential cache holds the tickets for a user. A given user can have
several credential caches, one for each realm where the user have the
initial tickets (the first krbtgt).

The credential cache data can be stored internally in different way, each of them for
different proposes.  File credential (FILE) caches and processes based
(KCM) caches are for permanent storage. While memory caches (MEMORY)
are local caches to the local process.

Caches are opened with @manpage{krb5_cc_resolve,3} or created with
@manpage{krb5_cc_gen_unique,3}.

If the cache needs to be opened again (using
@manpage{krb5_cc_resolve,3}) @manpage{krb5_cc_close,3} will close the
handle, but not the remove the cache. @manpage{krb5_cc_destroy,3} will
zero out the cache, remove the cache so it can no longer be
referenced.

See also manual page for @manpage{krb5_ccache,3}

@subsection Kerberos errors

Kerberos errors are based on the com_err library.  All error codes are
32-bit signed numbers, the first 24 bits define what subsystem the
error originates from, and last 8 bits are 255 error codes within the
library.  Each error code have fixed string associated with it.  For
example, the error-code -1765328383 have the symbolic name
KRB5KDC_ERR_NAME_EXP, and associated error string ``Client's entry in
database has expired''.

This is a great improvement compared to just getting one of the unix
error-codes back.  However, Heimdal have an extention to pass back
customised errors messages.  Instead of getting ``Key table entry not
found'', the user might back ``failed to find
host/host.example.com@@EXAMLE.COM(kvno 3) in keytab /etc/krb5.keytab
(des-cbc-crc)''.  This improves the chance that the user find the
cause of the error so you should use the customised error message
whenever its available.

See also manual page for @manpage{krb5_get_error_string,3} and
@manpage{krb5_get_err_text,3}.

@subsection Keytab management

A keytab is a storage for locally stored keys. Heimdal includes keytab
support for Kerberos 5 keytabs, Kerberos 4 srvtab, AFS-KeyFile's,
and for storing keys in memory.

Keytabs are used for servers and long-running services.

See also manual page for @manpage{krb5_keytab,3}

@subsection Kerberos crypto

See also manual page for @manpage{krb5_crypto_init,3},
@manpage{krb5_keyblock,3}, @manpage{krb5_create_checksum,3}, 
and @manpage{krb5_encrypt,3}.

@node Walkthrough of a sample Kerberos 5 client, Validating a password in a server application, Kerberos 5 API Overview, Programming with Kerberos
@section Walkthrough of a sample Kerberos 5 client

This example contains parts of a sample TCP Kerberos 5 clients, if you
want a real working client, please look in @file{appl/test} directory in
the Heimdal distribution.

All Kerberos error-codes that are returned from kerberos functions in
this program are passed to @code{krb5_err}, that will print a
descriptive text of the error code and exit. Graphical programs can
convert error-code to a human readable error-string with the
@manpage{krb5_get_err_text,3} function.

Note that you should not use any Kerberos function before
@code{krb5_init_context()} have completed successfully. That is the
reason @code{err()} is used when @code{krb5_init_context()} fails.

First the client needs to call @code{krb5_init_context} to initialise
the Kerberos 5 library. This is only needed once per thread
in the program. If the function returns a non-zero value it indicates
that either the Kerberos implementation is failing or its disabled on
this host.

@example
#include <krb5.h>

int
main(int argc, char **argv)
@{
        krb5_context context;

        if (krb5_context(&context))
                errx (1, "krb5_context");
@end example

Now the client wants to connect to the host at the other end. The
preferred way of doing this is using @manpage{getaddrinfo,3} (for
operating system that have this function implemented), since getaddrinfo
is neutral to the address type and can use any protocol that is available.

@example
        struct addrinfo *ai, *a;
        struct addrinfo hints;
        int error;

        memset (&hints, 0, sizeof(hints));
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_protocol = IPPROTO_TCP;

        error = getaddrinfo (hostname, "pop3", &hints, &ai);
        if (error)
                errx (1, "%s: %s", hostname, gai_strerror(error));

        for (a = ai; a != NULL; a = a->ai_next) @{
                int s;

                s = socket (a->ai_family, a->ai_socktype, a->ai_protocol);
                if (s < 0)
                        continue;
                if (connect (s, a->ai_addr, a->ai_addrlen) < 0) @{
                        warn ("connect(%s)", hostname);
                            close (s);
                            continue;
                @}
                freeaddrinfo (ai);
                ai = NULL;
        @}
        if (ai) @{
                    freeaddrinfo (ai);
                    errx ("failed to contact %s", hostname);
        @}
@end example

Before authenticating, an authentication context needs to be
created. This context keeps all information for one (to be) authenticated
connection (see @manpage{krb5_auth_context,3}).

@example
        status = krb5_auth_con_init (context, &auth_context);
        if (status)
                krb5_err (context, 1, status, "krb5_auth_con_init");
@end example

For setting the address in the authentication there is a help function
@code{krb5_auth_con_setaddrs_from_fd} that does everything that is needed
when given a connected file descriptor to the socket.

@example
        status = krb5_auth_con_setaddrs_from_fd (context,
                                                 auth_context,
                                                 &sock);
        if (status)
                krb5_err (context, 1, status, 
                          "krb5_auth_con_setaddrs_from_fd");
@end example

The next step is to build a server principal for the service we want
to connect to. (See also @manpage{krb5_sname_to_principal,3}.)

@example
        status = krb5_sname_to_principal (context,
                                          hostname,
                                          service,
                                          KRB5_NT_SRV_HST,
                                          &server);
        if (status)
                krb5_err (context, 1, status, "krb5_sname_to_principal");
@end example

The client principal is not passed to @manpage{krb5_sendauth,3}
function, this causes the @code{krb5_sendauth} function to try to figure it
out itself.

The server program is using the function @manpage{krb5_recvauth,3} to
receive the Kerberos 5 authenticator.

In this case, mutual authentication will be tried. That means that the server
will authenticate to the client. Using mutual authentication
is good since it enables the user to verify that they are talking to the
right server (a server that knows the key).

If you are using a non-blocking socket you will need to do all work of
@code{krb5_sendauth} yourself. Basically you need to send over the
authenticator from @manpage{krb5_mk_req,3} and, in case of mutual
authentication, verifying the result from the server with
@manpage{krb5_rd_rep,3}.

@example
        status = krb5_sendauth (context,
                                &auth_context,
                                &sock,
                                VERSION,
                                NULL,
                                server,
                                AP_OPTS_MUTUAL_REQUIRED,
                                NULL,
                                NULL,
                                NULL,
                                NULL,
                                NULL,
                                NULL);
        if (status)
                krb5_err (context, 1, status, "krb5_sendauth");
@end example

Once authentication has been performed, it is time to send some
data. First we create a krb5_data structure, then we sign it with
@manpage{krb5_mk_safe,3} using the @code{auth_context} that contains the
session-key that was exchanged in the
@manpage{krb5_sendauth,3}/@manpage{krb5_recvauth,3} authentication
sequence.

@example
        data.data   = "hej";
        data.length = 3;

        krb5_data_zero (&packet);

        status = krb5_mk_safe (context,
                               auth_context,
                               &data,
                               &packet,
                               NULL);
        if (status)
                krb5_err (context, 1, status, "krb5_mk_safe");
@end example

And send it over the network.

@example
        len = packet.length;
        net_len = htonl(len);

        if (krb5_net_write (context, &sock, &net_len, 4) != 4)
                err (1, "krb5_net_write");
        if (krb5_net_write (context, &sock, packet.data, len) != len)
                err (1, "krb5_net_write");
@end example

To send encrypted (and signed) data @manpage{krb5_mk_priv,3} should be
used instead. @manpage{krb5_mk_priv,3} works the same way as
@manpage{krb5_mk_safe,3}, with the exception that it encrypts the data
in addition to signing it.

@example
        data.data   = "hemligt";
        data.length = 7;

        krb5_data_free (&packet);

        status = krb5_mk_priv (context,
                               auth_context,
                               &data,
                               &packet,
                               NULL);
        if (status)
                krb5_err (context, 1, status, "krb5_mk_priv");
@end example

And send it over the network.

@example
        len = packet.length;
        net_len = htonl(len);

        if (krb5_net_write (context, &sock, &net_len, 4) != 4)
                err (1, "krb5_net_write");
        if (krb5_net_write (context, &sock, packet.data, len) != len)
                err (1, "krb5_net_write");

@end example

The server is using @manpage{krb5_rd_safe,3} and
@manpage{krb5_rd_priv,3} to verify the signature and decrypt the packet.

@node Validating a password in a server application, API differences to MIT Kerberos, Walkthrough of a sample Kerberos 5 client, Programming with Kerberos
@section Validating a password in an application

See the manual page for @manpage{krb5_verify_user,3}.

@node API differences to MIT Kerberos,  , Validating a password in a server application, Programming with Kerberos
@section API differences to MIT Kerberos

This section is somewhat disorganised, but so far there is no overall
structure to the differences, though some of the have their root in
that Heimdal uses an ASN.1 compiler and MIT doesn't.

@subsection Principal and realms

Heimdal stores the realm as a @code{krb5_realm}, that is a @code{char *}.
MIT Kerberos uses a @code{krb5_data} to store a realm.

In Heimdal @code{krb5_principal} doesn't contain the component
@code{name_type}; it's instead stored in component
@code{name.name_type}. To get and set the nametype in Heimdal, use
@manpage{krb5_principal_get_type,3} and
@manpage{krb5_principal_set_type,3}.

For more information about principal and realms, see
@manpage{krb5_principal,3}.

@subsection Error messages

To get the error string, Heimdal uses
@manpage{krb5_get_error_string,3} or, if @code{NULL} is returned,
@manpage{krb5_get_err_text,3}. This is to return custom error messages
(like ``Can't find host/datan.example.com@@EXAMPLE.COM in
/etc/krb5.conf.'' instead of a ``Key table entry not found'' that
@manpage{error_message,3} returns.

Heimdal uses a threadsafe(r) version of the com_err interface; the
global @code{com_err} table isn't initialised.  Then
@manpage{error_message,3} returns quite a boring error string (just
the error code itself).


@c @node Why you should use GSS-API for new applications, Walkthrough of a sample GSS-API client, Validating a password in a server application, Programming with Kerberos
@c @section Why you should use GSS-API for new applications
@c 
@c SSPI, bah, bah, microsoft, bah, bah, almost GSS-API.
@c 
@c It would also be possible for other mechanisms then Kerberos, but that
@c doesn't exist any other GSS-API implementations today.
@c 
@c @node Walkthrough of a sample GSS-API client, , Why you should use GSS-API for new applications, Programming with Kerberos
@c @section Walkthrough of a sample GSS-API client
@c 
@c Write about how gssapi_clent.c works.