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@c $Id: programming.texi 22071 2007-11-14 20:04:50Z lha $
@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::
* File formats::
@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 it's 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
Heimdal includes a implementation of the Kerberos crypto framework,
all crypto operations.
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 it's 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, File formats, 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.
@node File formats, , API differences to MIT Kerberos, Programming with Kerberos
@section File formats
This section documents the diffrent file formats that are used in
Heimdal and other Kerberos implementations.
@subsection keytab
The keytab binary format is not a standard format. The format has
evolved and may continue to. It is however understood by several
Kerberos implementations including Heimdal, MIT, Sun's Java ktab and
are created by the ktpass.exe utility from Windows. So it has
established itself as the defacto format for storing Kerberos keys.
The following C-like structure definitions illustrate the MIT keytab
file format. All values are in network byte order. All text is ASCII.
@example
keytab @{
uint16_t file_format_version; /* 0x502 */
keytab_entry entries[*];
@};
keytab_entry @{
int32_t size;
uint16_t num_components; /* subtract 1 if version 0x501 */
counted_octet_string realm;
counted_octet_string components[num_components];
uint32_t name_type; /* not present if version 0x501 */
uint32_t timestamp;
uint8_t vno8;
keyblock key;
uint32_t vno; /* only present if >= 4 bytes left in entry */
@};
counted_octet_string @{
uint16_t length;
uint8_t data[length];
@};
keyblock @{
uint16_t type;
counted_octet_string;
@};
@end example
All numbers are stored in network byteorder (big endian) format.
The keytab file format begins with the 16 bit file_format_version which
at the time this document was authored is 0x502. The format of older
keytabs is described at the end of this document.
The file_format_version is immediately followed by an array of
keytab_entry structures which are prefixed with a 32 bit size indicating
the number of bytes that follow in the entry. Note that the size should be
evaluated as signed. This is because a negative value indicates that the
entry is in fact empty (e.g. it has been deleted) and that the negative
value of that negative value (which is of course a positive value) is
the offset to the next keytab_entry. Based on these size values alone
the entire keytab file can be traversed.
The size is followed by a 16 bit num_components field indicating the
number of counted_octet_string components in the components array.
The num_components field is followed by a counted_octet_string
representing the realm of the principal.
A counted_octet_string is simply an array of bytes prefixed with a 16
bit length. For the realm and name components, the counted_octet_string
bytes are ASCII encoded text with no zero terminator.
Following the realm is the components array that represents the name of
the principal. The text of these components may be joined with slashs
to construct the typical SPN representation. For example, the service
principal HTTP/www.foo.net@@FOO.NET would consist of name components
"HTTP" followed by "www.foo.net".
Following the components array is the 32 bit name_type (e.g. 1 is
KRB5_NT_PRINCIPAL, 2 is KRB5_NT_SRV_INST, 5 is KRB5_NT_UID, etc). In
practice the name_type is almost certainly 1 meaning KRB5_NT_PRINCIPAL.
The 32 bit timestamp indicates the time the key was established for that
principal. The value represents the number of seconds since Jan 1, 1970.
The 8 bit vno8 field is the version number of the key. This value is
overridden by the 32 bit vno field if it is present. The vno8 field is
filled with the lower 8 bits of the 32 bit protocol kvno field.
The keyblock structure consists of a 16 bit value indicating the
encryption type and is a counted_octet_string containing the key. The
encryption type is the same as the Kerberos standard (e.g. 3 is
des-cbc-md5, 23 is arcfour-hmac-md5, etc).
The last field of the keytab_entry structure is optional. If the size of
the keytab_entry indicates that there are at least 4 bytes remaining,
a 32 bit value representing the key version number is present. This
value supersedes the 8 bit vno8 value preceeding the keyblock.
Older keytabs with a file_format_version of 0x501 are different in
three ways:
@table @asis
@item All integers are in host byte order [1].
@item The num_components field is 1 too large (i.e. after decoding, decrement by 1).
@item The 32 bit name_type field is not present.
@end table
[1] The file_format_version field should really be treated as two
separate 8 bit quantities representing the major and minor version
number respectively.
@subsection Heimdal database dump file
Format of the Heimdal text dump file as of Heimdal 0.6.3:
Each line in the dump file is one entry in the database.
Each field of a line is separated by one or more spaces, with the
exception of fields consisting of principals containing spaces, where
space can be quoted with \ and \ is quoted by \.
Fields and their types are:
@example
Quoted princial (quote character is \) [string]
Keys [keys]
Created by [event]
Modified by [event optional]
Valid start time [time optional]
Valid end time [time optional]
Password end valid time [time optional]
Max lifetime of ticket [time optional]
Max renew time of ticket [integer optional]
Flags [hdb flags]
Generation number [generation optional]
Extensions [extentions optional]
@end example
Fields following these silently are ignored.
All optional fields will be skipped if they fail to parse (or comprise
the optional field marker of "-", w/o quotes).
Example:
@example
fred@@EXAMPLE.COM 27:1:16:e8b4c8fc7e60b9e641dcf4cff3f08a701d982a2f89ba373733d26ca59ba6c789666f6b8bfcf169412bb1e5dceb9b33cda29f3412:-:1:3:4498a933881178c744f4232172dcd774c64e81fa6d05ecdf643a7e390624a0ebf3c7407a:-:1:2:b01934b13eb795d76f3a80717d469639b4da0cfb644161340ef44fdeb375e54d684dbb85:-:1:1:ea8e16d8078bf60c781da90f508d4deccba70595258b9d31888d33987cd31af0c9cced2e:- 20020415130120:admin@@EXAMPLE.COM 20041221112428:fred@@EXAMPLE.COM - - - 86400 604800 126 20020415130120:793707:28 -
@end example
Encoding of types are as follows:
@table @asis
@item keys
@example
kvno:[masterkvno:keytype:keydata:salt]@{zero or more separated by :@}
@end example
kvno is the key version number.
keydata is hex-encoded
masterkvno is the kvno of the database master key. If this field is
empty, the kadmin load and merge operations will encrypt the key data
with the master key if there is one. Otherwise the key data will be
imported asis.
salt is encoded as "-" (no/default salt) or
@example
salt-type /
salt-type / "string"
salt-type / hex-encoded-data
@end example
keytype is the protocol enctype number; see enum ENCTYPE in
include/krb5_asn1.h for values.
Example:
@example
27:1:16:e8b4c8fc7e60b9e641dcf4cff3f08a701d982a2f89ba373733d26ca59ba6c789666f6b8bfcf169412bb1e5dceb9b33cda29f3412:-:1:3:4498a933881178c744f4232172dcd774c64e81fa6d05ecdf643a7e390624a0ebf3c7407a:-:1:2:b01934b13eb795d76f3a80717d469639b4da0cfb644161340ef44fdeb375e54d684dbb85:-:1:1:ea8e16d8078bf60c781da90f508d4deccba70595258b9d31888d33987cd31af0c9cced2e:-
@end example
@example
kvno=27,@{key: masterkvno=1,keytype=des3-cbc-sha1,keydata=..., default salt@}...
@end example
@item time
Format of the time is: YYYYmmddHHMMSS, corresponding to strftime
format "%Y%m%d%k%M%S".
Time is expressed in UTC.
Time can be optional (using -), when the time 0 is used.
Example:
@example
20041221112428
@end example
@item event
@example
time:principal
@end example
time is as given in format time
principal is a string. Not quoting it may not work in earlier
versions of Heimdal.
Example:
@example
20041221112428:bloggs@@EXAMPLE.COM
@end example
@item hdb flags
Integer encoding of HDB flags, see HDBFlags in lib/hdb/hdb.asn1. Each
bit in the integer is the same as the bit in the specification.
@item generation:
@example
time:usec:gen
@end example
usec is a the microsecond, integer.
gen is generation number, integer.
The generation can be defaulted (using '-') or the empty string
@item extensions:
@example
first-hex-encoded-HDB-Extension[:second-...]
@end example
HDB-extension is encoded the DER encoded HDB-Extension from
lib/hdb/hdb.asn1. Consumers HDB extensions should be aware that
unknown entires needs to be preserved even thought the ASN.1 data
content might be unknown. There is a critical flag in the data to show
to the KDC that the entry MUST be understod if the entry is to be
used.
@end table
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