samba-mirror/source4/auth/kerberos/kerberos-notes.txt

Copyright Andrew Bartlett <abartlet@samba.org> 2005-2009
Copyright Donald T. Davis <don@mit.edu>  

Released under the GPLv3

Important context for porting to MIT
------------------------------------

This document should be read in conjunction with the Samba4 source code.  
DAL and KDC requirements are expressed (as an implementation against Heimdal's 
HDB abstraction layer) in Samba4's source4/kdc/hdb-samba4.c in particular.
hbd-samba4.c is the biggest piece of samba-to-krb glue layer, so the main
part of the port to MIT is to replace hdb-samba4 with a similar glue layer
that's designed for MIT's code.

PAC requirements are implemeneted in source4/kdc/pac-glue.c

The plugins (both of the above are Heimdal plugins) for the above are loaded 
in source4/kdc/kdc.c

For GSSAPI requirements, see auth/gensec/gensec_gssapi.c (the consumer of 
GSSAPI in Samba4)

For Kerberos requirements, see auth/kerberos/krb5_init_context.c .

Samba has its own credentials system, wrapping GSS creds, just as GSS 
creds wrap around krb5 creds.  For the interaction between Samba4 credentials
system and GSSAPI and Kerberos see auth/credentials/credentials_krb5.c .

AllowedWorkstationNames and Krb5
--------------------------------

Microsoft uses the clientAddresses *multiple value* field in the krb5
protocol (particularly the AS_REQ) to communicate the client's netbios 
name (legacy undotted name, <14 chars)

This is (my guess) to support the userWorkstations field (in user's AD record).
The idea is to support client-address restrictions, as was standard in NT:
The AD authentication server I imagine checks the netbios address against 
this userWorkstations value (BTW, the NetLogon server does this, too).

The checking of this field implies a little of the next question:

Is a DAL the layer we need?
---------------------------

Looking at what we need to pass around, I don't think
the DAL is even the right layer; what we really want 
is to create an account-authorization abstraction layer 
(e.g., is this account permitted to login to this computer,
at this time?).  
Here is how we ended up doing this in Heimdal:
  * We created a separate plugin, with this API:
      typedef struct hdb_entry_ex {
         void *ctx;
         hdb_entry entry;
         void (*free_entry)(krb5_context, struct hdb_entry_ex *);
      } hdb_entry_ex;

  * The void *ctx is a "private pointer," provided by the 'get' method's
    hdb_entry_ex retval.  The APIs below use the void *ctx so as to find 
    additional information about the user, not contained in the hdb_entry 
    structure.  Both the provider and the APIs below understand how to cast 
    the private void *ctx pointer. 

       typedef krb5_error_code
            (*krb5plugin_windc_pac_generate)(void *, krb5_context,
				             struct hdb_entry_ex *, krb5_pac*);
       typedef krb5_error_code
            (*krb5plugin_windc_pac_verify)(void *, krb5_context,
			                   const krb5_principal,
			                   struct hdb_entry_ex *,
			                   struct hdb_entry_ex *,
			                   krb5_pac *);
       typedef krb5_error_code
            (*krb5plugin_windc_client_access)(void *, 
                                              krb5_context, 
                                              struct hdb_entry_ex *, 
                                              KDC_REQ *, krb5_data *);
                                              
  * (The krb5_data* here is critical, so that samba's KDC can return 
    the right NTSTATUS code in the 'error string' returned to the client.
    Otherwise, the windows client won't get the right error message to 
    the user (such as 'password expired' etc).  The pure Kerberos error 
    is not enough)

       typedef struct krb5plugin_windc_ftable {
            int			minor_version;
            krb5_error_code	(*init)(krb5_context, void **);
            void		(*fini)(void *);
            rb5plugin_windc_pac_generate	pac_generate;
            krb5plugin_windc_pac_verify		pac_verify;
            krb5plugin_windc_client_access	client_access;
       } krb5plugin_windc_ftable;
      This API has some heimdal-specific stuff, that'll have to change when we port the plugin to MIT krb.
   * 1st callback (pac_generate) creates an initial PAC from the user's AD record.
   * 2nd callback (pac_verify) check that a PAC is correctly signed, add additional groups (for cross-realm tickets) and re-sign with the key of the target kerberos service's account
   * 3rd callback (client_access) perform additional access checks, such as allowedWorkstations and account expiry.
   * for example, to register this plugin, use the kdc's standard 
     plugin-system at Samba4's initialisation:
        /* first, setup the table of callback pointers */
      	/* Registar WinDC hooks */
	ret = krb5_plugin_register(krb5_context, 
				   PLUGIN_TYPE_DATA, "windc",
				   &windc_plugin_table);
	/* once registered, the KDC will invoke the callbacks */
	/* while preparing each new ticket (TGT or app-tkt)   */
   * an alternate way to register the plugin is with a config-file that names
     a DSO (Dynamically Shared Object).

 
This plugin helps bridge an important gap:  The user's AD record is much 
richer than the Heimdal HDB format allows, so we do AD-specific access 
control checks in an AD-specific layer (ie, the plugin), not in the 
DB-agnostic KDC server.

In Novell's pure DAL approach, the DAL only read in the principalName as 
the key, so it had trouble performing access-control decisions on things 
other than the name (like the addresses).

There is another, currently unhandled challenge in this area - the need to handle
bad password counts (and good password notification), so that a single policy can
be applied against all means of checking a password (NTLM, Kerberos, LDAP Simple 
bind etc)

The Original work by Novell in creating a DAL did not seem to provide a way to 
update the PW counts information.  Nevertheless, we know that this is very much
required (and may have been addressed in Simo's subsequent IPA-KDC design), 
because in Samba3+eDirectory, great lengths are taken to update this information. 

GSSAPI layer requirements
-------------------------

Welcome to the wonderful world of canonicalisation

The MIT Krb5 libs (including GSSAPI) do not support kinit returning a different
realm to what the client asked for, even just in case differences.

Heimdal has the same problem, and this too applies to the krb5 layer, not
just gssapi.

there's two kinds of name-canonicalization that can occur:
   * lower-to-upper case conversion, because Windows domain names are
     usually in upper case;
   * an unrecognizable subsitution of names, such as might happen when 
     a user requests a ticket for a NetBIOS domain name, but gets back
     a ticket for the corresponging FQDN.

As developers, we should test if the AD KDC's name-canonicalisation 
can be turned off with the KDCOption flags in the AS-REQ or TGS-REQ;  
Windows clients always send the Canonicalize flags as KDCOption values.

Old Clients (samba3 and HPUX clients) use 'selfmade' gssapi/krb5 tokens
for use in the CIFS session setup.  these hand-crafted ASN.1 packets don't
follow rfc1964 perfectly, so server-side krblib code has to be flexible 
enough to accept these bent tokens.  
It turns out that Windows' GSSAPI server-side code is sloppy about checking
some GSSAPI tokens' checksums.  During initial work to implement an AD client,
it was easier to make an acceptable solution (to Windows servers) than to 
correctly implement the GSSAPI specification, particularly on top of the 
(inflexible) MIT Kerberos API.  It did not seem possible to write a correct, 
separate GSSAPI implementation on top of MIT Kerberos's public krb5lib API,
and at the time, the effort did not need to extend beyond what Windows would 
require.  

The upshot is that old Samba3 clients send GSSAPI tokens bearing incorrect
checksums, which AD's Krb5lib cheerfully accepts (but accepts the good checksums,
too).  Similarly, Samba4's heimdal krb5lib accepts these incorrect checksums.  
Accordingly, if MIT's krb5lib wants to interoperate with the old Samba3 clients, 
then MIT's library will have to do the same.

Because these old clients use krb5_mk_req()
the app-servers get a chksum field depending on the encryption type, but that's 
wrong for GSSAPI (see rfc 1964 section 1.1.1). The Checksum type 8003 should 
be used in the Authenticator of the AP-REQ! That (correct use of the 8003 type) 
would allows the channel bindings, the GCC_C_* req_flags and optional delegation
tickets to be passed from the client to the server.  However windows doesn't 
seem to care whether the checksum is of the wrong type, and for CIFS SessionSetups, 
it seems that the req_flags are just set to 0.
This deviant checksum can't work for LDAP connections with sign or seal, or 
for any DCERPC connection, because those connections do not require the 
negotiation of GSS-Wrap paraemters (signing or sealing of whole payloads).  
Note:  CIFS has an independent SMB signing mechanism, using the Kerberos key.

see heimdal/lib/gssapi/krb5/accept_sec_context.c, lines 390-450 or so.

This bug-compatibility is likely to be controversial in the kerberos community, 
but a similar need for bug-compatibility arose around MIT's & Heimdal's both 
failing to support TGS_SUBKEYs correctly, and there are numerous other cases.
see https://lists.anl.gov/pipermail/ietf-krb-wg/2009-May/007630.html

So MIT's krb5lib needs to also support old clients!

Principal Names, long and short names
-------------------------------------

As far as servicePrincipalNames are concerned, these are not
canonicalised by AD's KDC, except as regards the realm in the reply.
That is, the client gets back the principal it asked for, with 
the realm portion 'fixed' to uppercase, long form.  
Heimdal doesn't canonicalize names, but Samba4 does some canonicalization:
For hostnames and usernames, Samba4 canonicalizes the requested name only 
for the LDAP principal-lookup, but then Samba4 returns the retrieved LDAP 
record with the request's original, uncanonicalized hostname replacing the 
canonicalized name that actually was retrieved.
AB says that for usernames, Samba4 used to return the canonicalized username, 
as retrieved from LDAP.  The reason for the different treatment was that 
the user needs to present his own canonicalized username to servers, for
ACL-matching.  For hostnames this isn't necessary.
So, for bug-compatibility, we may need to optionally disable any
namne-canonicalization that MIT's KDC does.

The short name of the realm seems to be accepted for at least AS_REQ
operations, but the AD KDC always performs realm-canonicalisation, 
which converts the short realm-name to the canonical long form.  
So, this causes pain for current krb client libraries. 

The canonicalisation of names matters not only for the KDC, but also
for code that has to deal with keytabs.
With credential-caches, when canonicalization leads to cache-misses,
the client just asks for new credentials for the variant server-name.
This could happen, for example, if the user asks to access the server
twice, using different variants of the server-name.

We also need to handle type 10 names (NT-ENTERPRISE), which are a full
principal name in the principal field, unrelated to the realm.
The principal field contains both principal & realm names, while the 
realm field contains a realm name, too, possibly different.
For example, an NT-ENTERPRISE principal name might look like:
joeblow@microsoft.com@NTDEV.MICROSOFT.COM ,
<--principal field-->|<----realm name--->|

Where joe@microsoft.com is the leading portion, and NTDEV.MICROSOFT.COM is 
the realm.  This is used for the 'email address-like login-name' feature of AD.

HOST/ Aliases
-------------

There is another post somewhere (ref lost for the moment) that details
where in active directory the list of stored aliases for HOST/ is.
This list is read & parsed by the AD KDC, so as to allow any of these
aliased ticket-requests to use the HOST/ key.

Samba4 currently has set:
sPNMappings: host=ldap,dns,cifs,http  (but dns's presence is a bug, somehow)

AD actually has ~50 entries:

sPNMappings: host=alerter,appmgmt,cisvc,clipsrv,browser,dhcp,dnscache,replicat
 or,eventlog,eventsystem,policyagent,oakley,dmserver,dns,mcsvc,fax,msiserver,i
 as,messenger,netlogon,netman,netdde,netddedsm,nmagent,plugplay,protectedstora
 ge,rasman,rpclocator,rpc,rpcss,remoteaccess,rsvp,samss,scardsvr,scesrv,seclog
 on,scm,dcom,cifs,spooler,snmp,schedule,tapisrv,trksvr,trkwks,ups,time,wins,ww
 w,http,w3svc,iisadmin,msdtc
 
Domain members that expect the longer list will break in damb4, as of 6/09.
AB says he'll try to fix this right away.

For example, this is how HTTP/, and CIFS/ can use HOST/ without
any explicit entry in the servicePrincipalName attribute


For example, the application-server might have (on its AD record):
servicePrincipalName: HOST/my.computer@MY.REALM

but the client asks for a ticket to cifs/my.computer@MY.REALM
AD looks in LDAP for both name-variants
AD then transposes cifs -> host after performing the lookup in the 
directory (for the original name), then looks for host/my.computer@MY.REALM

for hostnames & usernames, alternate names appear as extra values in 
the multivalued "principal name" attributes:
 - For hostnames, the other names (other than it's short name, implied 
   from the CN), is stored in the servicePrincipalName
 - For usernames, the other names are stored in the userPrincipalName 
   attribute, and can be full e-mail address like names, such as 
   joe@microsoft.com (see above).
   
Jean-Baptiste.Marchand@hsc.fr reminds me:
> This is the SPNMappings attribute in Active Directory:
> http://msdn.microsoft.com/library/en-us/adschema/adschema/a_spnmappings.asp

We implement this in hdb-ldb.

Implicit names for Win2000 Accounts
-----------------------------------
AD's records for servers are keyed by CN or by servicePrincipalName,
but for win2k boxes, these records don't include servicePrincipalName, 
so, the CN attribute is used instead.
Despite not having a servicePrincipalName on accounts created 
by computers running win2000, it appears we are expected 
to have an implicit mapping from host/computer.full.name and
host/computer to the computer's entry in the AD LDAP database 
(ie, be able to obtain tickets for that host name in the KDC).

Returned Salt for PreAuthentication
-----------------------------------

When the KDC replies for pre-authentication, it returns the Salt,
which may be in the form of a principalName that is in no way
connected with the current names.  (ie, even if the userPrincipalName
and samAccountName are renamed, the old salt is returned).

This is the kerberos standard salt, kept in the 'Key'.  The
AD generation rules are found in a Mail from Luke Howard dated
10 Nov 2004.  The MIT glue layer doesn't really need to care about
these salt-handling details;  the samba4 code & the LDAP backend 
will conspire to make sure that MIT's KDC gets correct salts.


From: Luke Howard <lukeh@padl.com>
Organization: PADL Software Pty Ltd
To: lukeh@padl.com
Date: Wed, 10 Nov 2004 13:31:21 +1100
Cc: huaraz@moeller.plus.com, samba-technical@lists.samba.org
Subject: Re: Samba-3.0.7-1.3E Active Directory Issues
-------

Did some more testing, it appears the behaviour has another
explanation. It appears that the standard Kerberos password salt
algorithm is applied in Windows 2003, just that the source principal
name is different.

Here is what I've been able to deduce from creating a bunch of
different accounts:  
[SAM name in this mail means the AD attribute samAccountName .
 E.g., jbob for a user and jbcomputer$ for a computer.]

[UPN is the AD userPrincipalName attribute.  For example, jbob@mydomain.com]

Type of account		        Principal for Salting
========================================================================
Computer Account                host/<SAM-Name-Without-$>.realm@REALM
User Account Without UPN        <SAM-Name>@REALM
User Account With UPN           <LHS-Of-UPN>@REALM

Note that if the computer account's SAM account name does not include
the trailing '$', then the entire SAM account name is used as input to
the salting principal. Setting a UPN for a computer account has no
effect.

It seems to me odd that the RHS of the UPN is not used in the salting
principal. For example, a user with UPN foo@mydomain.com in the realm
MYREALM.COM would have a salt of MYREALM.COMfoo. Perhaps this is to
allow a user's UPN suffix to be changed without changing the salt. And
perhaps using the UPN for salting signifies a move away SAM names and
their associated constraints.

For more information on how UPNs relate to the Kerberos protocol,
see:

http://www.ietf.org/proceedings/01dec/I-D/draft-ietf-krb-wg-kerberos-referrals-02.txt

-- Luke



Heimdal oddities
----------------

Heimdal is built such that it should be able to serve multiple realms
at the same time.  This isn't relevant for Samba's use, but it shows
up in a lot of generalisations throughout the code.

Samba4's code originally tried internally to make it possible to use
Heimdal's multi-realms-per-KDC ability, but this was ill-conceived,
and AB has recently (6/09) ripped the last of that multi-realms
stuff out of samba4.  AB says that in AD, it's not really possible
to make this work;  several AD components structurally assume that
there's one realm per KDC.  However, we do use this to support
canonicalization of realm-names:  case variations, plus long-vs-short
variants of realm-names.

Other odd things:
 - Heimdal supports multiple passwords on a client account:  Samba4
   seems to call hdb_next_enctype2key() in the pre-authentication 
   routines to allow multiple passwords per account in krb5.  
   (I think this was intended to allow multiple salts).
   AD doesn't support this, so the MIT port shouldn't bother with 
   this.

State Machine safety when using Kerberos and GSSAPI libraries
-------------------------------------------------------------

Samba's client-side & app-server-side libraries are built on a giant 
state machine, and as such have very different
requirements to those traditionally expressed for kerberos and GSSAPI
libraries. 

Samba requires all of the libraries it uses to be state machine safe in
their use of internal data.  This does not mean thread safe, and an
application could be thread safe, but not state machine safe (if it
instead used thread-local variables).

So, what does it mean for a library to be state machine safe?  This is
mostly a question of context, and how the library manages whatever
internal state machines it has.  If the library uses a context
variable, passed in by the caller, which contains all the information
about the current state of the library, then it is safe.  An example
of this state is the sequence number and session keys for an ongoing
encrypted session).

The other issue affecting state machines is 'blocking' (waiting for a
read on a network socket).  Samba's non-blocking I/O doesn't like
waiting for libkrb5 to go away for awhile to talk to the KDC.

Samba4 provides a hook 'send_to_kdc', that allows Samba4 to take over the 
IO handling, and run other events in the meantime.  This uses a 
'nested event context' (which presents the challenges that the kerberos 
library might be called again, while still in the send_to_kdc hook).

Heimdal has this 'state machine safety' in parts, and we have modified
the lorikeet branch to improve this behviour, when using a new,
non-standard API to tunnelling a ccache (containing a set of tickets)
through the gssapi, by temporarily casting the ccache pointer to a 
gss credential pointer.  
This new API is Heimdal's samba4-requested gss_krb5_import_cred() fcn;
this will have to be rewritten or ported in the MIT port.

This replaces an older scheme using the KRB5_CCACHE
environment variable to get the same job done.  This tunnelling trick 
enables a command-line app-client to run kinit tacitly, before running 
GSSAPI for service-authentication.  This tunnelling trick avoids the 
more usual approach of keeping the ccache pointer in a global variable.

No longer true;  the krb5_context global is gone now:
[Heimdal uses a per-context variable for the 'krb5_auth_context', which
controls the ongoing encrypted connection, but does use global
variables for the ubiquitous krb5_context parameter.] 

The modification that has added most to 'state machine safety' of
GSSAPI is the addition of the gss_krb5_acquire_creds() function.  This
allows the caller to specify a keytab and ccache, for use by the
GSSAPI code.  Therefore there is no need to use global variables to
communicate this information about keytab & ccache. 

At a more theoritical level (simply counting static and global
variables) Heimdal is not state machine safe for the GSSAPI layer.
(Heimdal is now (6/09) much more nearly free of globals.)
The Krb5 layer alone is much closer, as far as I can tell, blocking
excepted. .


As an alternate to fixing MIT Kerberos for better safety in this area, 
a new design might be implemented in Samba, where blocking read/write 
is made to the KDC in another (fork()ed) child process, and the results 
passed back to the parent process for use in other non-blocking operations. 

To deal with blocking, we could have a fork()ed child per context,
using the 'GSSAPI export context' function to transfer
the GSSAPI state back into the main code for the wrap()/unwrap() part
of the operation.  This will still hit issues of static storage (one
gss_krb5_context per process, and multiple GSSAPI encrypted sessions
at a time) but these may not matter in practice.

This approach has long been controversial in the Samba team. 
An alternate way would be to be implement E_AGAIN in libkrb5:  similar
to the way to way read() works with incomplete operations.  to do this
in libkrb5 would be difficult, but valuable.

In the short-term, we deal with blocking by taking over the network
send() and recv() functions, therefore making them 'semi-async'.  This
doesn't apply to DNS yet.These thread-safety context-variables will
probably present porting problems, during the MIT port.  This will
probably be most of the work in the port to MIT.



GSSAPI and Kerberos extensions
------------------------------

This is a general list of the other extensions we have made to / need from
the kerberos libraries

 - DCE_STYLE : Microsoft's hard-coded 3-msg Challenge/Response handshake
   emulates DCE's preference for C/R.  Microsoft calls this DCE_STYLE.
   MIT already has this nowadays (6/09).

 - gsskrb5_get_initiator_subkey() (return the exact key that Samba3
   has always asked for.  gsskrb5_get_subkey() might do what we need
   anyway).  This is necessary, because in some spots, Microsoft uses
   raw Kerberos keys, outside the Kerberos protocls, and not using Kerberos 
   wrappings etc.  Ie, as a direct input to MD5 and ARCFOUR, without using 
   the make_priv() or make_safe() calls.

 - gsskrb5_acquire_creds() (takes keytab and/or ccache as input
   parameters, see keytab and state machine discussion in prev section)

Not needed anymore, because MIT's code now handles PACs fully:
 - gss_krb5_copy_service_keyblock() (get the key used to actually
   encrypt the ticket to the server, because the same key is used for
   the PAC validation).
 - gsskrb5_extract_authtime_from_sec_context (get authtime from
   kerberos ticket)
 - gsskrb5_extract_authz_data_from_sec_context (get authdata from
   ticket, ie the PAC.  Must unwrap the data if in an AD-IFRELEVENT)]
The new function to handle the PAC fully
 - gsskrb5_extract_authz_data_from_sec_context()

Samba still needs this one:
 - gsskrb5_wrap_size (find out how big the wrapped packet will be,
   given input length).

Keytab requirements
-------------------

Because windows machine account handling is very different to the
traditional 'MIT' keytab operation.
This starts when we look at the basics of the secrets handling:

Samba file-servers can have many server-name simultaneously (kindof
like web servers' software virtual hosting), but since these servers
are running in AD, these names are free to be set up to all share
the same secret key.  In AD, host-sharing server names almost always 
share a secret key like this.  In samba3, this key-sharing was optional, so
some samba3 hosts' keytabs did hold multiple keys.  samba4 abandons this
traditional "old MIT" style of keytab, and only supports one key per keytab,
and multiple server-names can use that keytab key in common.
Heimdal offered "in-memory keytabs" for servers that use passwords. 
These server-side passwords were held in a Samba LDB database called secrets.ldb,
and the heimdal library would be supplied the password from the ldb file and 
would construct an in-memory keytab struct containing the password, 
just as if the library had read an MIT-style keytab file.
Unfortunately, only later, at recv_auth() time, would the heimdal library
convert the PW into a salted-&-hashed AES key, by hashing 10,000 times with
SHA-1.  So, nowadays, this password-based in-memory keytab is seen as too
slow, and is falling into disuse.

Traditional 'MIT' behaviour is to use a keytab, containing salted key
data, extracted from the KDC.  (In this model, there is no 'service
password', instead the keys are often simply application of random
bytes).  Heimdal also implements this behaviour.

The windows model is very different - instead of sharing a keytab with
each member server, a random utf-16 pseudo-textual password is stored 
for the whole machine.  
The password is set with non-kerberos mechanisms (particularly SAMR, 
a DCE-RPC service) and when interacting on a kerberos basis, the
password is salted by the member server (ie, an AD server-host).  
(That is, no salt information appears to be conveyed from the AD KDC 
to the member server.  ie, the member server must use the rule's 
described in Luke's mail above).

pre-win7 AD and samba3/4 both use SAMR, an older protocol, to jumpstart
the member server's PW-sharing with AD (the "windows domain-join process").  
This PW-sharing transfers only the PW's utf-16 text, without any salting 
or hashing, so that non-krb security mechanisms can use the same utf-16
text PW.  for windows 7, this domain-joining uses LDAP for PW-setting.

In dealing with this model, we use both the traditional file
keytab and in-MEMORY keytabs.  

When dealing with a windows KDC, the behaviour regarding case
sensitivity and canonacolisation must be accomidated.  This means that
an incoming request to a member server may have a wide variety of
service principal names.  These include:

machine$@REALM (samba clients)
HOST/foo.bar@realm (win2k clients)
HOST/foo@realm (win2k clients, using netbios)
cifs/foo.bar@realm (winxp clients)
cifs/foo@realm (winxp clients, using netbios)

as well as all case variations on the above.  

Heimdal's GSSAPI expects to get a principal-name & a keytab, possibly containing
multiple principals' different keys.  However, AD has a different problem to
solve, which is that the client may know the member-server by a non-canonicalized
principal name, yet AD knows the keytab contains exactly one key, indexed by
the canonical name.  So, GSSAPI is unprepared to canonicalize the server-name
that the cliet requested, and is also overprepared to do an unnecessary search
through the keytab by principal-name.  So samba's server-side GSSAPI calls game 
the GSSAPI, by supplying the server's known canonical name, and the one-key keytab.
this doesn't really affect the port to mit-krb.

Because the number of U/L case combinations got 'too hard' to put into a keytab in the
traditional way (with the client to specify the name), we either
pre-compute the keys into a traditional keytab or make an in-MEMORY
keytab at run time.  In both cases we specify the principal name to
GSSAPI, which avoids the need to store duplicate principals.

We use a 'private' keytab in our private dir, referenced from the
secrets.ldb by default.

Extra Heimdal functions used
----------------------------
these fcns didn't exist in the MIT code, years ago, when samba started.
AB will try to build a final list of these fcns.

(an attempt to list some of the Heimdal-specific functions I know we use)

krb5_free_keyblock_contents()

also a raft of prinicpal manipulation functions:

Prncipal Manipulation
---------------------

Samba makes extensive use of the principal manipulation functions in
Heimdal, including the known structure behind krb_principal and
krb5_realm (a char *).  for example,
krb5_parse_name_flags(smb_krb5_context->krb5_context, name, 
                      KRB5_PRINCIPAL_PARSE_MUST_REALM, &principal);
krb5_princ_realm(smb_krb5_context->krb5_context, principal);
krb5_unparse_name_flags(smb_krb5_context->krb5_context, principal, 
                        KRB5_PRINCIPAL_UNPARSE_NO_REALM, &new_princ);
These are needed for juggling the AD variant-structures for server names.

Authz data extraction
---------------------

We use krb5_ticket_get_authorization_data_type(), and expect it to
return the correct authz data, even if wrapped in an AD-IFRELEVENT container.

KDC/hdb Extensions
--------------

We have modified Heimdal's 'hdb' interface to specify the 'class' of
Principal being requested.  This allows us to correctly behave with
the different 'classes' of Principal name.  This is necessary because
of the AD structure, which uses very different record-structures 
for user-principals, trust principals & server-principals.

We currently define 3 classes:
 - client (kinit)
 - server (tgt)
 - krbtgt (kinit, tgt) the kdc's own ldap record

I also now specify the kerberos principal as an explict parameter to LDB_fetch(), 
not an in/out value on the struct hdb_entry parameter itself.

Private Data pointer (and windc hooks) (see above):
 In addition, I have added a new interface hdb_fetch_ex(), which
 returns a structure including a private data-pointer, which may be used 
 by the windc plugin inferface functions.  The windc plugin provides 
 the hook for the PAC, as well as a function for the main access control routines.

 A new windc plugin function should be added to increment the bad password counter
 on failure.

libkdc (doesn't matter for IPA; Samba invokes the Heimdal kdc as a library call,
but this is just a convenience, and the MIT port can do otherwise w/o trouble.)
------

Samba4 needs to be built as a single binary (design requirement), and
this should include the KDC.  Samba also (and perhaps more
importantly) needs to control the configuration environment of the
KDC.  

The interface we have defined for libkdc allow for packet injection
into the post-socket layer, with a defined krb5_context and
kdb5_kdc_configuration structure.  These effectively redirect the
kerberos warnings, logging and database calls as we require.

Using our socket lib (para 3 does matter for the send_to_kdc() plugin).  
See also the discussion about state machine safety above)
--------------------

An important detail in the use of libkdc is that we use samba4's own socket
lib.  This allows the KDC code to be as portable as the rest of samba
(this cuts both ways), but far more importantly it ensures a
consistancy in the handling of requests, binding to sockets etc.

To handle TCP, we use of our socket layer in much the same way as
we deal with TCP for CIFS.  Tridge created a generic packet handling
layer for this.

For the client, samba4 likewise must take over the socket functions, 
so that our single thread smbd will not lock up talking to itself.  
(We allow processing while waiting for packets in our socket routines).
send_to_kdc() presents to its caller the samba-style socket interface,
but the MIT port will reimplement send_to_kdc(), and this routine will
use internally the same socket library that MIT-krb uses.

Kerberos logging support  (this will require porting attention)
------------------------

Samba4 now (optionally in the main code, required for the KDC) uses the
krb5_log_facility from Heimdal.  This allows us to redirect the
warnings and status from the KDC (and client/server kerberos code) to
Samba's DEBUG() system.

Similarly important is the Heimdal-specific krb5_get_error_string()
function, which does a lot to reduce the 'administrator pain' level,
by providing specific, english text-string error messages instead of
just error code translations.  (this isn't necessarty for the port,
but it's more useful than MIT's default err-handling;  make sure 
this works for MIT-krb)


Short name rules
----------------

Samba is highly likely to be misconfigured, in many weird and
interesting ways.  As such, we have a patch for Heimdal that avoids
DNS lookups on names without a . in them.  This should avoid some
delay and root server load.   (this may need to be ported to MIT.)

PAC Correctness
---------------

We now put the PAC into the TGT, not just the service ticket.  

Forwarded tickets
-----------------

We extract forwarded tickets from the GSSAPI layer, and put
them into the memory-based credentials cache.  
We can then use them for proxy work.


Kerberos TODO
=============

(Feel free to contribute to any of these tasks, or ask
abartlet@samba.org about them).

Lockout Control  (still undone in samba4 on heimdal)
--------------

We need to get (either if PADL publishes their patch, or write our
own) access control hooks in the Heimdal KDC.  We need to lockout
accounts (eg, after 10 failed PW-attemps), and perform other controls.
This is standard AD behavior, that samba4 needs to get right, whether
heimdal or MIT-krb is doing the ticket work.

Gssmonger
---------

Microsoft has released a krb-specific testsuite called gssmonger, 
which tests interop.  We should compile it against lorikeet-heimdal, 
MIT and see if we can build a 'Samba4' server for it.
GSSMonger wasn't intended to be Windows-specific.

Kpasswd server (kpasswd server is now finished, but not testsuite)
--------------

I have a partial kpasswd server which needs finishing, and a we need a
client testsuite written, either via the krb5 API or directly against
GENSEC and the ASN.1 routines.
Samba4 likes to test failure-modes, not just successful behavior.

Currently it only works for Heimdal, not MIT clients.  This may be due
to call ordering constraints.


Correct TCP support
-------------------

Samba4 socket-library's current TCP support does not send back 'too large' 
error messages if the high bit is set.  This is needed for a proposed extension
mechanism (SSL-armored kinit, by Leif Johansson <leifj@it.su.se>), 
but is likewise unsupported in both current Heimdal and MIT.

=========================================================================
AB says MIT's 1.7 announcement about AD support covers Luke Howard's 
changes.  It all should be easy for IPA to exploit/use during the port
of Samba4 to MIT.
AB says Likewise software will likely give us their freeware NTLM/MIT-krb
implementation.