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Samba4, like earlier versions of Samba, uses Heimdal Kerberos.
  
Samba4, like earlier versions of Samba, uses Heimdal Kerberos.
 
The Samba4 Port project proposes to enable Samba4 to use MIT kerberos
  
The Samba4 Port project proposes to enable Samba4 to use MIT kerberos
−
instead. The near-term goal is that mixed krb5+AD deployments could
 +
as an alternative. The near-term goal is that mixed krb5+AD deployments
−
use Samba4 to provide better interoperation between AD realms and krb5
 +
could use Samba4 to provide better interoperation between AD realms
−
realms.
 +
and MIT-krb5 realms.
   
 
Use case: For example, suppose a kerberos customer is deploying a network
 
Use case: For example, suppose a kerberos customer is deploying a network
Line 23: Line 23:
 
non-AD-style KDC, so as to access UNIX services securely;
  
non-AD-style KDC, so as to access UNIX services securely;
 
<li> A UNIX client's ticket will ''not'' carry a PAC, except when
  
<li> A UNIX client's ticket will ''not'' carry a PAC, except when
−
the UNIX client accesses a Windows server.
 +
the UNIX client accesses a Windows server
  +
([http://k5wiki.kerberos.org/wiki/Samba4:_Optional_PACs_for_Unix_clients '''Rationale'''])
  +
.
 
</li>
  
</li>
 
</ol>
  
</ol>
−  
   
 
The Samba4 team, the MIT Krb Consortium, RedHat, Ubuntu, and Sun all have
  
The Samba4 team, the MIT Krb Consortium, RedHat, Ubuntu, and Sun all have
−
shown some interest in this Samba4 Port project.
 +
shown some interest in this Samba4 Port project.
  +
[http://k5wiki.kerberos.org/wiki/Supported_platforms_for_Samba4_port '''Here''']
  +
is a table showing which OS platforms are supported by Samba4, Heimdal, and MIT kerberos.
  +
Summary: MIT-krb5 & Samba4 both run on Mac OS X, NetBSD, Debian, RedHat, Ubuntu, & Solaris.
   
−
==== Key to the asterisks in the Table of Contents ====
  
  +
----
−
<ol>
  
−
<li> No asterisks: Work that needs to be done. </li>
  
−
<li> '''*''': Some work to be done, some already done. </li>
  
−
<li> '''**''': Nothing much to do.
  
−
<li> '''***''': Can be done later, if at all.
  
−
</ol>
  
   
 
== Concise to-do list ==
  
== Concise to-do list ==
   
 
This is a condensed version of the
 
This is a condensed version of the
−
task-list offered by Samba4's Andrew Bartlett,
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_%28Andrew_Bartlett%29#Data-Abstraction_Layer_.28DAL.29 '''task-list'''] offered by Samba4's Andrew Bartlett,
 
containing only what hasn't yet been done already by MIT.
  
containing only what hasn't yet been done already by MIT.
   
 
The two big chunks of work are
 
The two big chunks of work are
−
[[#LDAP_driver | '''LDAP Driver''']] and
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#LDAP_driver '''LDAP Driver'''] and
−
[[#Data-Abstraction_Layer_.28DAL.29 | '''Replacing / improving MIT's DAL''']], but the DAL work may not be needed.
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Data-Abstraction_Layer_.28DAL.29 '''Replacing / improving MIT's DAL'''],
  +
but the DAL work may not be needed.
   
 
=== Replace the MIT KDC's LDAP driver ===
  
=== Replace the MIT KDC's LDAP driver ===
   
 
Samba4's LDAP driver for the MIT KDB needs to know how to do
  
Samba4's LDAP driver for the MIT KDB needs to know how to do
−
[[#LDAP_driver | '''AD's intricate naming''']]:
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#LDAP_driver '''AD's intricate naming''']:
 
<ol>
  
<ol>
 
<li> Canonicalization of server names,
  
<li> Canonicalization of server names,
Line 58: Line 51:
 
[[#Use_1.7.27s_AD-support_features | '''supports canonicalization''']].
  
[[#Use_1.7.27s_AD-support_features | '''supports canonicalization''']].
 
</li>
  
</li>
−
<li> AD-style aliases for HOST/ service names.
+
<li> AD-style aliases for
  +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Keytab-sharing_amongst_HOST.2F_service_names '''HOST/ service names'''].
 
</li>
  
</li>
−
<li> Implicit names for Win2k accounts.
 
  +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Implicit_names_for_Win2000_Accounts '''Implicit names''']
  +
for Win2k accounts.
 
</li>
  
</li>
−
<li> Principal "types": client / server / krbtgs
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Principal_.22types.22 '''Principal "types":'''] client / server / krbtgs
−
<li> [[#Flexible_server-naming | '''Flexible server-naming''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Flexible_server-naming '''Flexible server-naming''']
 
</li>
  
</li>
−
<li> [[#.2A_Keytabs_.26_Name-canonicalization | '''Keytabs & name-canonicalization''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A_Keytabs_.26_Name-canonicalization '''Keytabs & name-canonicalization''']
 
</li>
  
</li>
 
</ol>
  
</ol>
 
Most or all of Heimdal's LDAP driver code is in
 
Most or all of Heimdal's LDAP driver code is in
−
[[#LDAP_driver | '''three Samba4 source files''']],
+
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#LDAP_driver '''three Samba4 source files'''],
 
~1000 lines in all.
  
~1000 lines in all.
   
Line 75: Line 69:
   
 
=== Small changes ===
  
=== Small changes ===
  +
Of the things on this list, only NTLM support (bullet 2)
  +
is needed for the Samba4 KDC port.
  +
The other tasks are all application-library stuff,
  +
and arguably aren't needed at all, because Samba3
  +
already works well with MIT application libraries.
 
<ol>
  
<ol>
−
<li> [[#MIT_libraries | '''MIT library changes''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#MIT_libraries '''MIT library changes''']
 
</li>
  
</li>
−
<li> [[#NTLM_support | '''Samba4/AD libraries: NTLM support''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#NTLM_support '''Samba4/AD libraries: NTLM support''']. See also
  +
[http://k5wiki.kerberos.org/wiki/Samba4_Port:_NTLM_thread '''this Sept-2009 NTLM thread'''] (this implies to me that a GSS NTLM mech is not an immediate requirement - LH)
 
</li>
  
</li>
−
<li> [[#Key-handling_changes | '''Key-handling changes''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Key-handling_changes '''Key-handling changes''']]
 
</li>
  
</li>
−
<li> [[#.2A_Extra_krb_library_functions | '''Extra Krb library functions''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A_Extra_krb_library_functions '''Extra Krb library functions''']
 
</li>
  
</li>
−
<li> [[#Error-handling.2C_logging.2C_testing | '''Error-handling, logging, testing''']]
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Error-handling.2C_logging.2C_testing '''Error-handling, logging, testing''']
−
</li>
  
−
<li> [[#Host_PWs_vs_service-keys | '''Host PWs vs service-keys''']]
  
 
</li>
  
</li>
 
</ol>
  
</ol>
Line 93: Line 92:
 
This stuff should already just work:
  
This stuff should already just work:
 
<ol>
  
<ol>
−
<li> [[#.2A.2A_Turn_on_MIT-krb_1.7.27s_PAC_handling | '''PAC handling''']]; </li>
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A.2A_Turn_on_MIT-krb_1.7.27s_PAC_handling '''PAC handling''']; </li>
−
<li> [[#Name_Canonicalization | '''AD-style name canonicalization''']]; </li>
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Name_Canonicalization '''AD-style name canonicalization''']; </li>
−
<li> [[#Doubled_realm-names | '''NT-ENTERPRISE names''']],
+
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Doubled_realm-names '''NT-ENTERPRISE names'''],
−
which carry two realms-suffixes; </li>
 +
which carry two realm-suffixes; </li>
 
<li> CHECK_POLICY/AUDIT methods (needed for
 
<li> CHECK_POLICY/AUDIT methods (needed for
−
[[#.2A.2A.2A_Add_access-control_to_the_TGS | '''TGS access-control''']]); </li>
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A.2A.2A_Add_access-control_to_the_TGS '''TGS access-control''']); </li>
 
<li> DCE_STYLE Challenge/Response handshakes: see
 
<li> DCE_STYLE Challenge/Response handshakes: see
−
[[#.2A_Krb5_lib_.26_GSSAPI | '''Krb lib & GSSAPI''']]. </li>
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A_Krb5_lib_.26_GSSAPI '''Krb lib & GSSAPI''']. </li>
 
<li> Accept legacy Samba3 clients'
 
<li> Accept legacy Samba3 clients'
−
[[#.2A.2A_Legacy_Samba3_clients_.26_GSSAPI | '''bad GSSAPI checksums''']]; </li>
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A.2A_Legacy_Samba3_clients_.26_GSSAPI '''bad GSSAPI checksums''']; </li>
−
<li> [[#.2A_Extra_krb_library_functions | '''Principal-manipulation functions''']]; </li>
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A_Extra_krb_library_functions '''Principal-manipulation functions''']; </li>
−
<li> [[#.2A.2A_State-machine_safety_for_krb_libraries | '''State-machine safety''']]; </li>
 +
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A.2A_State-machine_safety_for_krb_libraries '''State-machine safety''']; </li>
 
</ol>
  
</ol>
   
Line 113: Line 112:
   
 
==== Maybe: Improve or replace MIT's DAL ====
  
==== Maybe: Improve or replace MIT's DAL ====
−
[[#Data-Abstraction_Layer_.28DAL.29 | '''Rewrite the MIT KDC's Data-Abstraction Layer (DAL)''']],
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Data-Abstraction_Layer_.28DAL.29 '''Rewrite the MIT KDC's Data-Abstraction Layer (DAL)'''],
 
mostly because the MIT KDC needs to see & manipulate
  
mostly because the MIT KDC needs to see & manipulate
 
more LDAP detail, on Samba4's behalf;
  
more LDAP detail, on Samba4's behalf;
   
−
==== ** [[Maybe not: Add a KDC-as-library API]] ====
 +
==== Maybe, or not: Add a KDC-as-library API ====
−
Samba4 currently runs as a single process, and Samba4 invokes the Heimdal
 +
Samba4 currently runs as a single process, and Samba4's smbd invokes the Heimdal KDC via a
−
KDC via a libkdc interface (KDC as library).
+
[http://k5wiki.kerberos.org/wiki/Samba4_port:_libkdc_Interface#krb5_kdc_update_time.28.29 '''libkdc interface'''] (KDC as library).
 
<ol>
  
<ol>
−
<li> Andrew Bartlett says this libkdc interface is [[#libkdc | '''"nice to have"''']],
 
  +
<li> Rationale:
−
but not essential.
  
  +
# smbd uses the libkdc interface to configure the KDC, both at startup & during runtime.
  +
# Samba4's build/test environment uses libkdc's socket-passing, to simulate network traffic.
  +
</li>
  +
<li> Andrew Bartlett says this libkdc interface is
  +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#libkdc '''"nice to have"'''],
  +
but not essential for getting the port to work.
 
</li>
  
</li>
 
<li> Tom Yu says adding a libkdc interface to MIT's code would be a lot
  
<li> Tom Yu says adding a libkdc interface to MIT's code would be a lot
Line 128: Line 132:
 
to do, anyway.
 
to do, anyway.
 
</li>
  
</li>
−
<li> If we build a libkdc interface for MIT's KDC,
+
<li> Sam Hartman says he needs the libkdc interface, too, for his work on PK-U2U (but not immediately).
  +
</li>
  +
<li>
  +
Another way, which Simo dismisses on Samba4's behalf:
  +
Samba can use
  +
[http://k5wiki.kerberos.org/wiki/Samba4_Port:_iptables_Remapping '''iptables remapping'''],
  +
but only for kdc packets, so that Samba acts as a router between the AD client and the KDC.
  +
This would work for MIT-krb & for Heimdal.
  +
</li>
  +
<li> If we do have to build a libkdc interface for MIT's KDC,
 
Samba4 will need the KDC to use
  
Samba4 will need the KDC to use
−
[[#.2A.2A_Samba4.27s_portable_socket_API | '''Samba's socket library''']]
+
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#.2A.2A_Samba4.27s_portable_socket_API '''Samba's socket library''']
 
correctly.
  
correctly.
 
</li>
  
</li>
 
</ol>
  
</ol>
   
−
==== *** [[Later: TGS access-control]] ====
 +
==== [[Later: TGS access-control]] ====
 
MIT krb will need to support these AD features, once Samba4 does.
  
MIT krb will need to support these AD features, once Samba4 does.
 
Alternatively, this could be seen as an opportunity for MIT-based
  
Alternatively, this could be seen as an opportunity for MIT-based
 
Samba4 to surpass Heimdal-based Samba.
  
Samba4 to surpass Heimdal-based Samba.
 
<ol>
  
<ol>
−
<li> [[#HBAC_for_the_TGS | '''Add HBAC to the TGS''']],
+
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#HBAC_for_the_TGS '''Add HBAC to the TGS'''],
 
so that Samba4 can refuse TGTs to kinit,
 
so that Samba4 can refuse TGTs to kinit,
 
based on time-of-day & IP-addr constraints;
  
based on time-of-day & IP-addr constraints;
Line 148: Line 152:
 
</li>
  
</li>
 
<li> TGS-HBAC is part of the rationale for
 
<li> TGS-HBAC is part of the rationale for
−
[[#Data-Abstraction_Layer_.28DAL.29 | '''rewriting the DAL''']].
 +
[http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Data-Abstraction_Layer_.28DAL.29 '''rewriting the DAL'''].
 
</li>
  
</li>
 
</ol>
  
</ol>
 
</li>
  
</li>
−
<li> [[#Failed_PW_lockouts | '''Failed-kinit counts''']]:
+
<li> [http://k5wiki.kerberos.org/wiki/Task-List_for_Samba4_Port_(Andrew_Bartlett)#Failed_PW_lockouts '''Failed-kinit counts''']:
 
Add a KDC heuristic for tracking intervals between kinits,
 
Add a KDC heuristic for tracking intervals between kinits,
 
so that Samba4 can enforce AD's unified account-lockout on kinit.
  
so that Samba4 can enforce AD's unified account-lockout on kinit.
Line 162: Line 166:
 
----
  
----
   
−
== Andrew Bartlett's Task-List (June '09) ==
  
  +
== Samba's use of Heimdal symbols, with MIT differences ==
−
This document was written by Andrew Bartlett (abartlet@samba.org)
  
−
and Don Davis (don@mit.edu), in June 2009. The original
 
−
document did not take into account MIT's past few years of work
 
−
on kerberos, because Samba4, like Samba3, was built to use
 
−
Heimdal for authentication. This version of the task-list
  
−
includes comments from Luke Howard and others, who know more
 
−
about the current state of interoperability obstacles between
 
−
Samba4 and MIT's kerberos.
  
   
−
<pre>
  
  +
Samba4 uses around
−
Copyright Andrew Bartlett <abartlet@samba.org> 2005-2009
  
  +
[http://k5wiki.kerberos.org/wiki/Samba%27s_use_of_Heimdal_symbols%2C_with_MIT_differences '''265 Heimdal symbols:''']
−
Copyright Donald T. Davis <don@mit.edu> 2009
  
  +
# 150 functions,
  +
# 45 structs & typedefs, and
  +
# 70 macros & enums.
   
−
Released under the GPLv3
  
  +
Of these, roughly half present problems for the port:
−
</pre>
  
  +
# 25 symbols have different definitions in the MIT & Heimdal trees.
−  
  +
# 110 symbols are missing from MIT's krb5 tree.
−
'''Title: Porting Samba4 to MIT-Krb'''
  
−  
−
IPA v3 will use a version of Samba4 built on top of MIT's kerberos implementation, instead of Heimdal's version of kerberos.
  
−  
−
Task list summary for porting changes needed, from Andrew Bartlett:
  
−
* Rewrite or extend the LDAP driver that MIT-KDC will use.
  
−
* MIT KDC changes: rewrite DAL, add TGS-KBAC, enable PACs,...
  
−
* Full thread-safety for MIT's library code,
  
−
* Many small library changes
  
−  
−
=== Introduction: ===
  
−  
−
This document should be read alongside
  
−
the Samba4 source code, as follows:
 
−
<ol>
  
−
<li> For [[DAL and KDC requirements]], please see Samba4's
 
−
'''source4/kdc/hdb-samba4.c''' in particular. This file
  
−
is an implementation against Heimdal's HDB abstraction
  
−
layer, and is the biggest part of the 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. </li>
  
−
<li> Samba4's [[PAC requirements]] are implemeneted in
 
−
'''source4/kdc/pac-glue.c''' </li>
  
−
<li> Both of the above two layers are [[Heimdal plugins]], and
 
−
both get loaded in '''source4/kdc/kdc.c''' </li>
  
−
<li> For [[GSSAPI requirements]], see auth/gensec/gensec_gssapi.c
 
−
(the consumer of GSSAPI in Samba4)</li>
  
−
<li> For [[Kerberos library requirements]], see
 
−
'''auth/kerberos/krb5_init_context.c''' .</li>
  
−
<li> [[Samba's credentials system]] wraps GSS creds,
 
−
just as GSS creds wrap around krb5 creds. For the
 
−
interaction between Samba4 credential system and GSSAPI
  
−
and Kerberos, see '''auth/credentials/credentials_krb5''' . </li>
  
−
</ol>
  
   
 
----
  
----
   
−
=== LDAP driver ===
  
  +
== Samba4 Interfaces with Heimdal ==
−
Rewrite or extend the LDAP driver that MIT-KDC will use.
  
−
Most or all of the Heimdal LDAP driver code is in three source
  
−
files, ~1000 lines in all. These files are in '''samba4/kdc''' :
  
−
* '''hdb-samba4.c''' (samba4-to-kdb glue-layer plugin)
  
−
* '''pac-glue.c''' (samba4's pac glue-layer plugin)
  
−
* '''kdc.c''' (loads the above two plugins).
  
   
−
==== Name Canonicalization ====
  
−
The LDAP driver for the Samba4 KDB needs to know how to do
  
−
Samba4's intricate canonicalization of server names,
  
−
user-names, and realm names.
 
−
<ol>
  
−
<li> [[For hostnames & usernames,]] alternate names appear in
  
−
LDAP as extra values in the multivalued "principal name"
 
−
attributes:
  
−
<ol>
  
−
<li> For a hostname, the alternate names (other than
  
−
the short name, implied from the CN), are stored in
 
−
the servicePrincipalName.
  
−
* LH: This is implied from samAccountName, I would think; CN is not used by the security subsystem."
  
−
</li>
  
−
<li> For a username, the alternate names are stored in
 
−
the userPrincipalName attribute, and can be long
 
−
email-address-like names, such as joe@microsoft.com
 
−
(see "Type 10 names," below). </li>
  
−
</ol></li>
  
−
<li> [[GSSAPI layer requirements]]: The MIT Krb5 libs (including
 
−
GSSAPI) do not enable the AS to send kinit a TGT containing
  
−
a different realm-name than what the client asked for,
  
−
even in U/L case differences. Heimdal has the same problem,
 
−
and this applies to the krb5 layer too, not just GSSAPI.
  
−
There are two kinds of name-canonicalization that can
 
−
occur on Windows:
  
−
<ol>
  
−
<li> Lower-to-upper case conversion, because Windows domain
 
−
names are usually in upper case; </li>
  
−
<li> 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.</li>
  
−
</ol>
  
−
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. </li>
  
−
</li>
  
−
<li> [[Principal Names, long and short names:]]
  
−
AD's KDC does not canonicalize servicePrincipalNames, except
 
−
for the realm in the KDC reply. That is, the client gets
 
−
back the principal it asked for, with the realm portion
 
−
'fixed' to uppercase, long form.
 
−
 
−
Samba4 does some canonicalization, though Heimdal doesn't
 
−
canonicalize names itself: 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 found. </li>
  
−
<li> [[Usernames]]: AndrewB says that Samba4 used to return
 
−
the canonicalized username exactly as retrieved from LDAP.
 
−
The reason Samba4 treated usernames differently was that
 
−
the user needs to present his own canonicalized username
 
−
to servers, for ACL-matching. For hostnames this isn't
 
−
necessary. </li>
  
−
<li> [[Realm-names]]: AD seems to accept a realm's short name
 
−
in krb-requests, at least for 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.
 
−
Luke H says:
  
−
<ol>
  
−
<li> "If you set the canonicalize flag you will be fine. </li>
 
−
<li> If not, then yes, the server returning a different
  
−
realm will cause a problem for the MIT client libraries. </li>
  
−
<li> Take a look at how '''verify_as_reply()''' selectively
  
−
enforces the KDC_OPT_CANONICALIZE flag, in
 
−
MIT's '''src/lib/krb5/krb/get_in_tkt.c'''." </li>
  
−
</ol>
  
−
[[Punchline]]: For bug-compatibility, we may need to
 
−
selectively or optionally disable the MIT-KDC's name-
  
−
canonicalization.
 
−
</li>
  
−
<li> [[Application-code:]]
  
−
Name-canonicalisation matters not only for the KDC, but
 
−
also for app-server-code that has to deal with keytabs.
  
−
<ol><li>
  
−
Luke H. says,
  
−
"MIT's canonicalization works with 1.7 as long as you don't
 
−
provide a server principal to '''krb5_rd_req()''';
 
−
'''rd_req()''' will iterate through the keytab, looking
 
−
for an entry that will decrypt the ticket."
  
−
</li></ol>
  
−
Further, with credential-caches, canonicalization can
  
−
lead to cache-misses, but then 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.
  
−
</li>
  
−
</ol>
  
−  
−
==== Doubled realm-names ====
  
−
Samba4's KDC also needs to handle type 10
 
−
names (NT-ENTERPRISE), which have a full principal name
 
−
in the principal field, unrelated to the realm. Thus,
 
−
the principal field contains both principal & realm names,
 
−
while the realm field contains a realm name, too, possibly
 
−
different.
 
−
This is used for AD's "email-address-like login-names."
 
−
For example, an NT-ENTERPRISE principal name might look like:
  
−
<pre>
 
−
joeblow@microsoft.com@NTDEV.MICROSOFT.COM ,
  
−
<--principal field-->|<----realm name--->|
  
−
</pre>
  
−
Where joe@microsoft.com is the leading portion, and
 
−
NTDEV.MICROSOFT.COM is the realm.
  
−  
−
Luke H. says,
 
−
* "MIT's client libraries and KDC support this (to the extent necessary).
 
−
* Most of the work needs to happen in Samba4's new LDAP backend.
  
−  
−
==== Keytab-sharing amongst HOST/ service names ====
  
−
AD keeps a list of service-prefixed aliases for the host's
  
−
principal name. The AD KDC reads & parses this list, so
 
−
as to allow the aliased services to share the HOST/ key.
  
−
This means that every ticket-request for a service-alias
 
−
gets a service-ticket encrypted in the HOST/ key.
  
−  
−
For example, this is how HTTP/ and CIFS/ can use the
 
−
HOST/ AD-LDAP entry, without any explicitly CIFS-prefixed
  
−
entry in the host's servicePrincipalName attribute. In the
 
−
app-server host's AD record, the servicePrincipalName says
  
−
only HOST/my.computer@MY.REALM , but the client asks
  
−
for CIFS/my.omputer@MY.REALM tickets. So, AD looks in
 
−
LDAP for both name-variants, and finds the HOST/ version,
  
−
In AD's reply, AD replaces the HOST/ prefix with CIFS/ .
  
−
We implement this in '''hdb-ldb'''.
  
−
[[(TBD: Andrew, is this correct?:)]]
  
−  
−
List of HOST/ aliases: Samba4 currently uses only a small
  
−
set of HOST/ aliases, called "sPNMappings:"
  
−  
−
<pre>
  
−
host=ldap,dns,cifs,http .
 
−
</pre>
 
−
BTW, dns's presence in this short list is a bug, somehow.
  
−  
−
AD's real sPNMappings list has 52 entries:
  
−
<pre>
  
−
host =
  
−
alerter eventsystem netlogon rpclocator tapisrv
 
−
appmgmt fax netman rpcss time
 
−
browser http nmagent rsvp trksvr
 
−
cifs ias oakley samss trkwks
 
−
cisvc iisadmin plugplay scardsvr ups
 
−
dcom mcsvc policyagent scesrv w3svc
 
−
dhcp messenger protectedsto schedule wins
 
−
dmserver msdtc rasman scm www
 
−
dns msiserver remoteaccess seclogon
 
−
dnscache netdde replicator snmp
 
−
eventlog netddedsm rpc spooler
 
−
</pre>
  
−
Domain members that expect the longer list will break in
 
−
Samba4, as of 6/09. AB says he'll try to fix this right
 
−
away. There is another post somewhere (ref lost for the
 
−
moment) that details where active directory stores this long
  
−
list of stored aliases for HOST/.
  
−  
−
==== Implicit names for Win2000 Accounts ====
  
−
AD keys its
 
−
server-records by CN or by servicePrincipalName, but a
  
−
win2k box's server-entry in LDAP doesn't include the
  
−
servicePrincipalName attribute, So, win2k server-accounts
 
−
are keyed by the CN attribute instead. Because AD's LDAP
 
−
doesn't have a servicePrincipalName for win2k servers'
  
−
entries, Samba4 has to have an implicit mapping from
 
−
host/computer.full.name and from host/computer, to the
 
−
computer's CN-keyed entry in the AD LDAP database, so as to
 
−
be able to find the win2k server's host name in the KDB.
  
−  
−
==== Principal "types" ====
  
−
Samba4 has 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 AD's LDAP
  
−
structure, which uses very different record-structures
 
−
for user-principals, trust principals & server-principals.
  
−
We currently define 3 classes:
  
−
* client (kinit)
  
−
* server (tgt)
  
−
* krbtgt the TGS's own ldap record
  
−
Samba4 also now specifies the kerberos principal as an
 
−
explicit parameter to LDB_fetch(), not an in/out value
 
−
on the struct hdb_entry parameter itself.
  
−  
−
Luke H says this isn't really necessary:
  
 
<ol>
  
<ol>
−
<li> I quite like the principal-classes idea, but it mightn't
 
  +
<li> Samba4's
−
be a good fit for the existing KDC code. </li>
  
  +
[http://k5wiki.kerberos.org/wiki/Samba4_Port:_hdb_%26_ldb_Interfaces '''Database Interfaces''']
−
<li> You can find out whether it is a TGS from the principal
 
  +
enable Heimdal to use Samba4's directory data,
−
name. </li>
  
  +
whether the directory is stored in LDAP or in local disk files.
−
<li> You can also find out fairly easily whether it is the
 
−
TGS from your local realm or not, or outgoing/incoming. </li>
 
−
<li> You can check the get_principal_ext flag
 
−
KRB5_KDB_FLAG_CLIENT_REFERRALS_ONLY to determine whether
 
−
you are looking up a client in the AS-REQ path. </li>
  
−
</ol>
  
−  
−
----
  
−  
−
=== * [[MIT KDC changes]] ===
  
−  
−
==== Data-Abstraction Layer (DAL) ====
  
−
It would be good to
  
−
rewrite or circumvent the MIT KDC's DAL, mostly because
 
−
the MIT KDC needs to see & manipulate more LDAP detail,
 
−
on Samba4's behalf.
 
−
<ol>
  
−
<li> MIT's DAL calls lack context parameters (as of 2006),
 
−
so presumably they rely instead on global storage, and
 
−
aren't fully thread-safe.
  
−
</li>
 
−
<li> 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 user's
  
−
name (like the addresses).
  
−
<ol>
  
−
<li> Luke H says, "The KDC has a different model here --
 
−
a separate authorization check.
  
−
</li>
  
−
<li> But I agree, this could be improved."
  
−
</li>
  
−
</ol>
  
−
</li>
  
−
<li> Here's why Samba4 needs more entry detail than the DAL
  
−
provides: The AS needs to have ACL rules that will allow
  
−
a TGT to a user only when the user logs in from the
  
−
right desktop addresses, and at the right times of day.
  
−
This coarse-granularity access-control could be enforced
 
−
directly by the KDC's LDAP driver, without Samba having
 
−
to see the entry's pertinent authZ attributes. But,
 
−
there's a notable exception: a user whose TGT has
 
−
expired, and who wants to change his password, should
 
−
be allowed a restricted-use TGT that gives him access
 
−
to the kpasswd service. This ACL-logic could be buried
 
−
in the LDAP driver, in the same way as the TGS ACL could
  
−
be enforced down there, but to do so would just be even
 
−
uglier than it was to put the TGS's ACL-logic in the driver.
  
−
<ol>
  
−
<li> Luke H says, "This works today, you just need to set
 
−
'''KRB5_KDB_PWCHANGE_SERVICE''' appropriately."
  
−
</li>
  
−
</ol>
  
−
</li>
  
−
<li> Yet another complaint is that the DAL always pulls an
 
−
entire LDAP entry, non-selectively. The current DAL
 
−
is OK for Samba4's purposes, because Samba4 only reads,
 
−
and doesn't write, the KDB. But this all-or-nothing
  
−
retrieval hurts the KDC's performance, and would do so
  
−
even more, if Samba had to use the DAL to change KDB
  
−
entries.
  
−
<ol>
  
−
<li> Luke H says, "You can have your backend retrieve
 
−
a subset of attributes based on the flags passed to
 
−
'''get_principal_ext()'''.
  
−
</li>
  
−
<li> "I don't understand the comment "DAL always pulls
 
−
an entire LDAP entry", because the DAL is just an
 
−
abstraction layer.
 
−
</li>
  
−
<li> "FWIW, I wouldn't use the Novell LDAP backend,
 
−
I would just port your existing backend to DAL."
  
−
</li>
  
−
</ol>
  
−
</li>
  
−
</ol>
  
−  
−
There's some confusion about whether the MIT DAL is OK:
  
−
<ol>
  
−
<li> AB says "The MIT DAL may serve well-enough, mostly as is,
 
 
</li>
  
</li>
−
<li> ...but Samba4 will need the
 
  +
<li> Heimdal's
−
[[#.2A_Appendix_2:_windc_KDC_Plugin_for_Account-AuthZ |'''private pointer''']]
  
  +
[http://k5wiki.kerberos.org/wiki/Samba4_port:_libkdc_Interface '''libkdc Interface''']
−
part of the KDC plugin API, or the
 
  +
gives Samba4 a direct subroutine interface to the Heimdal KDC,
−
[[#.2A.2A_Turn_on_MIT-krb_1.7.27s_PAC_handling | '''PAC generation''']]
 
  +
with the KDC running as part of the Samba4 process.
−
won't work."
  
−
</li>
  
−
<li> Luke H replies, "You can use e_data in '''krb5_db_entry()'''
  
−
to store some private data,"
  
−
</li>
  
−
<li> but DTD thinks Luke doesn't understand what AB means by
  
−
"[[#.2A_Appendix_2:_windc_KDC_Plugin_for_Account-AuthZ |'''private pointer''']]."
  
 
</li>
  
</li>
 
</ol>
  
</ol>
−  
−
==== *** [[Add access-control to the TGS]] ====
  
−
For AD compatibility, Samba4 has to be able to
 
−
refuse TGTs to kinit, based on time-of-day &
 
−
IP-address constraints. This is standard AD behavior
 
−
that Samba4 needs to get right,
 
−
whether Heimdal or MIT-krb is doing the ticket work.
  
−
[[Samba4 doesn't yet have these TGS access-controls with Heimdal.]]
  
−  
−
AB asks, "Is a DAL the layer we need for Samba4 HBAC?"
 
−
Looking at what we need to pass around, AB doesn't think
  
−
the DAL is the right layer; what we really want instead
  
−
is to create an account-authorization abstraction layer
 
−
(e.g., is this account permitted to login to this computer,
  
−
at this time?). Samba4 ended up doing account-authorization
  
−
inside Heimdal, via a specialized KDC plugin called '''windc'''.
 
−
For a summary description of the '''windc''' plugin API, see
 
−
[[#.2A_Appendix_2:_windc_KDC_Plugin_for_Account-AuthZ | '''Appendix 2''']].
  
−  
−
Luke H says, "Well, this exists today. See above."
  
−  
−
===== HBAC for the TGS =====
  
−
Samba4 doesn't yet have HBAC hooks in the Heimdal KDC.
  
−
<ol>
  
−
<li> If PADL doesn't publish their patch for this,
 
−
we'll need to write our own.
  
−
</li>
  
−
<li> LH: "use the '''KRB5_KDB_METHOD_CHECK_POLICY_TGS''' method.
 
−
* Samba4 has access to the complete request.
 
−
* See '''against_local_policy_tgs()''' in MIT's '''src/kdc/policy.c''' ."
  
−
</li>
  
−
<li> The '''windc''' plugin provides a function for the main
 
−
access control routines. A new '''windc''' function
  
−
should be added to increment the bad password counter
  
−
on failure.
  
−
</li>
  
−
<li>
  
−
[[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)
  
−
AB guesses that this is to support the userWorkstations
 
−
field (in user's AD record). The idea is to support
 
−
client-address HBAC restrictions, as was standard in NT:
  
−
The AD authentication server probably checks the netbios
 
−
address against this userWorkstations value (BTW, the
 
−
NetLogon server does this, too).
  
−
</li>
  
−
</ol>
  
−  
−
===== Failed PW lockouts =====
  
−
Samba4 needs to lockout accounts (eg, after 10 failed PW-attempts).
 
−
<ol>
  
−
<li> Tom Yu correctly points out that
  
−
no-one can tell how many failed PW-attempts the user
  
−
makes against kinit's PW-encrypted tickets.
 
−
</li>
  
−
<li> DTD: Maybe we can count rapidly-repeated kinit requests,
 
−
instead?
  
−
</li>
  
−
<li> Samba4 doesn't yet handle bad password counts (or good
 
−
password notification), so that a single policy can be
 
−
applied against all means of checking a password (NTLM,
 
−
Kerberos, LDAP Simple Bind, etc). Novell's original DAL
  
−
did not provide a way to update the PW counts information.
  
−
</li>
  
−
<li> Nevertheless, we know that this is very much required in
  
−
AD, because Samba3 + eDirectory goes to great lengths to
  
−
update this information. (This may have been addressed in
  
−
Simo's subsequent IPA-KDC design.)
  
−
</li>
  
−
<li> Luke H says this is straightforward:
  
−
* Use a '''KRB5_KDB_METHOD_AUDIT_AS''' method for this.
  
−
* See the end of '''log_as_req()''' in MIT's '''/src/kdc/kdc_util.c'''.
 
−
</li>
  
−
</ol>
  
−  
−
==== ** Turn on MIT-krb 1.7's [[PAC handling]] ====
  
−
In addition, Andrew Bartlett has 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.
  
   
 
----
  
----
−  
−
=== ** [[State-machine safety for krb 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.
 
−
[[MIT's code has this level of thread-safety now.]]
  
−
<ol>
  
−
<li>
  
−
Samba requires all of the libraries it uses to be "state
 
−
machine safe" in their use of internal data. This does not
 
−
necessarily mean "thread safe," and an application could be
 
−
thread safe, but not state machine safe (if it instead used
 
−
thread-local variables). so, if MIT's libraries were made
 
−
thread-safe only by inserting spinlock() code, then the MIT
  
−
libraries aren't yet "state machine safe."
  
−
</li>
  
−
<li>
  
−
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).
  
−
</li>
  
−
<li>
  
−
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.
  
−
</li>
  
−
<li>
  
−
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).
  
−
</li>
  
−
<li>
  
−
Heimdal has this 'state machine safety' in parts, and we have modified
  
−
Samba4's lorikeet branch to improve this behaviour, 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.
  
−
</li>
  
−
<li>
  
−
This tunnelling trick replaces an older scheme using the '''KRB5_CCACHE'''
 
−
environment variable to get the same job done. The tunnelling trick
 
−
enables a command-line app-client to run kinit tacitly, before running
 
−
GSSAPI for service-authentication. The tunnelling trick avoids the
 
−
more usual approach of keeping the ccache pointer in a global variable.
  
−
</li>
  
−
<li>
  
−
[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. (No longer true,
  
−
because the '''krb5_context''' global is gone now.)]
 
−
</li>
  
−
<li>
  
−
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.
 
−
</li>
  
−
<li>
  
−
At a more theoretical level (simply counting static and global
  
−
variables) Heimdal is not state machine safe for the GSSAPI layer.
  
−
(But 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. .
  
−
</li>
  
−
<li>
  
−
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.
 
−
</li>
  
−
<li>
  
−
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.
  
−
</li>
  
−
<li>
  
−
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.
  
−
</li>
  
−
<li>
  
−
In the short-term, we deal with blocking by taking over the network
  
−
send() and recv() functions, therefore making them 'semi-async'. This
  
−
doens'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.
  
−
This may require more thorough thread-safe-ing work on the MIT libraries.
  
−
</li>
  
−
</ol>
  
−  
−
----
  
−  
−
=== Many small library changes (~15) ===
  
−
==== MIT libraries ====
  
−
===== * Krb5 lib & GSSAPI =====
  
−
Some extensions to MIT's [[libkrb5 & GSSAPI libraries]], including
 
−
GSSAPI ticket-forwarding: This is a general list of the other
 
−
extensions Samba4 has made to / need from the kerberos libraries
  
−
<ol>
  
−
</li>
  
−
<li> '''[[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)]].
  
−
</li>
  
−
<li> '''[[gsskrb5_get_initiator_subkey()]]''' (return the exact key that
 
−
Samba3 has always asked for. '''gsskrb5_get_subkey()''' might
 
−
do what we need anyway). This routine is necessary, because
 
−
in some spots, Microsoft uses raw Kerberos keys, outside the
 
−
Kerberos protocols, as a direct input to MD5 and ARCFOUR,
 
−
without using the '''make_priv()''' or '''make_safe()'''
 
−
calls, and without GSSAPI wrappings etc.
  
−
<ol>
  
−
<li>
 
−
Luke H says, "'''gsskrb5_get_subkey()''' doesn't exist in MIT's krb.
 
−
</li>
  
−
<li> "But you can use '''GSS_C_INQ_SSPI_SESSION_KEY''' instead."
 
−
</li>
  
−
</ol>
  
−
[[So this should be easy enough]], according to LH.
  
−
</li>
  
−
<li> '''gsskrb5_acquire_creds()''' (takes keytab and/or ccache as input
  
−
parameters, see keytab and state machine discussion in
 
−
[[#.2A.2A_State-machine_safety_for_krb_libraries | '''previous section, bullet 8''']])
  
−
</li>
  
−
<li> '''gsskrb5_extract_authz_data_from_sec_context()''' (The new function
 
−
to handle the PAC fully).
 
−
<ol>
  
−
<li> AB: We'll need to test that MIT's PAC-handling
 
−
code checks the PAC's signature.
  
−
</li>
  
−
</li> Luke H says, "It does not, a future API might do that. </li>
  
−
<li> "in the meantime you need to extract it and verify it explicitly. </li>
  
−
<li> "You can use '''krb5_pac_verify()''' for this (same API as Heimdal)." </li>
  
−
</ol>
  
−
</li>
  
−
<li> '''gsskrb5_wrap_size()''' (find out how big the wrapped
 
−
packet will be, given input length).
  
−
<ol>
  
−
<li> AB says, "Samba still needs this one." </li>
  
−
<li> Luke says, "You can use '''gss_wrap_iov_length()''' for this, but... </li>
  
−
<li> "...it won't really help you for MIC tokens, </li>
  
−
<li> "You can take advantage of two facts:
  
−
* "MIC tokens are never bigger than wrap header tokens for Kerberos and NTLM,
 
−
* "MS RPC permits the authentication trailer value to have redundant bytes at the end."
  
−
</li>
  
−
</ol>
  
−
</ol>
  
−  
−
===== Some refitting in Samba4's use of the MIT libs =====
  
−
===== ** [[Legacy Samba3 clients & GSSAPI]] =====
  
−
MIT's GSSAPI code should support some legacy Samba3
 
−
clients that present incorrectly-calculated checksums.
  
−
(Luke H: [["This is already in MIT's v1.7.]])
  
−
<ol>
  
−
<li> 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
  
−
(GSSAPI) perfectly, so server-side krblib code has to
  
−
be flexible enough to accept these bent tokens.
 
−
</li>
  
−
<li> 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 (acceptable 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.
 
−
</li>
  
−
<li> The upshot is that old Samba3 clients send GSSAPI
 
−
tokens bearing incorrect checksums, which AD's
 
−
GSSAPI library 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.
  
−
</li>
  
−
<li> 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 allow
 
−
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.
  
−
</li>
  
−
<li> For the code that handles the incorrect & correct
  
−
checksums, see heimdal/lib/gssapi/krb5/accept_sec_context.c,
  
−
lines 390-450 or so.
  
−
</li>
  
−
<li> 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
  
−
</li>
  
−
<li> So, MIT's krb5lib needs to also support old clients!
  
−
</li>
  
−
</ol>
  
−  
−
==== Samba4 / AD libraries ====
  
−
===== NTLM support =====
  
−
For AD compatibility, Samba4 has to support NTLM authentication,
  
−
so the NTLM library has to be able to access the MIT KDB.
  
−
AB says we should be able to get an OSS NTLM authentication
  
−
library: AB says Likewise software probably will give us their
  
−
OSS "NTLM for MIT-krb" implementation.
  
−  
−
===== ** [[Samba4's portable socket API]] =====
  
−
(This section assumes that we'll build a [[#libkdc | '''libkdc''']],
 
−
which seems unlikely.) Make sure Samba4's portable socket API works:
  
−
<ol>
  
−
<li> An important detail in the use of libkdc is that samba4 uses its
 
−
own socket lib. This allows the KDC code to be as portable as
 
−
the rest of samba, but more importantly it ensures consistancy
 
−
in the handling of requests, binding to sockets etc.
  
−
</li>
  
−
<li> 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.
  
−
</li>
  
−
<li> 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.
  
−
</li>
  
−
<li> The interface we have defined for libkdc allows 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.
  
−
</li>
  
−
<li> 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 currently unsupported
 
−
in both Heimdal and MIT.
  
−
</li>
  
−
</ol>
  
−  
−
==== Key-handling changes ====
  
−
<ol>
  
−
<li> Samba4 app-server-host holds a [[UTF-16 PW]], plus a key bitstring; See
  
−
[[#Appendix_1:__Keytab_Requirements | '''Appendix 1, Keytab Requirements''']].
  
−
</li>
  
−
<li> In-memory-only credentials cache for forwarded tickets
  
−
Samba4 extracts forwarded tickets from the GSSAPI layer,
 
−
and puts them into the memory-based credentials cache.
 
−
We can then use them for proxy work. This needs to be
  
−
ported, if the MIT library doesn't do it yet.
  
−
</li>
  
−
<li> [[In-memory-only keytab]] (nice to have):
  
−
Heimdal used to offer "in-memory keytabs" for servers that use
 
−
passwords. These server-side passwords were held in a Samba LDB
 
−
database called secrets.ldb . The heimdal library would fetch
 
−
the server's password from the ldb file and would construct an
 
−
in-memory keytab struct containing the password, somewhat as if
 
−
the library had read an MIT-style keytab file. Unfortunately,
 
−
only later, at recv_auth() time, would the Heimdal library convert
 
−
the server-PW into a salted-&-hashed AES key, by hashing 10,000
 
−
times with SHA-1. Naturally, this is really too slow for recv_auth(),
 
−
which runs when an app-server authenticates a client's app-service-
  
−
request. So, nowadays, this password-based in-memory keytab is
 
−
falling into disuse.
  
−
</li>
  
−
</ol>
  
−  
−
==== * Extra krb library functions ====
  
−
<ol>
  
−
<li> Special Heimdal-specific functions; These functions didn't
 
−
exist in the MIT code, years ago, when Samba started.
 
−
* krb5_free_keyblock_contents()
  
−
AB will try to build a final list of these functions:
  
−
</li>
  
−
<li> Principal-manipulation functions: Samba makes extensive
  
−
use of the principal manipulation functions in Heimdal,
 
−
including the known structure behind '''krb5_principal''' and
  
−
'''krb5_realm''' (a char *). For example,
  
−
<ol>
  
−
<li> '''krb5_parse_name_flags'''(smb_krb5_context->krb5_context, name,
 
−
KRB5_PRINCIPAL_PARSE_REQUIRE_REALM, &principal);
  
−
</li>
  
−
<li> '''krb5_unparse_name_flags'''(smb_krb5_context->krb5_context, principal,
 
−
KRB5_PRINCIPAL_UNPARSE_NO_REALM, &new_princ);
  
−
</li>
  
−
<li> '''krb5_principal_get_realm'''()
  
−
</li>
  
−
<li> '''krb5_principal_set_realm'''()
  
−
</ol>
  
−
These are needed for juggling the AD
 
−
variant-structures for server names.
  
−
(Luke says [[the last two are in MIT's code now.]])
  
−
</li>
  
−
</ol>
  
−  
−
==== Error-handling, logging, testing ====
  
−
<ol>
  
−
<li> Special [[Short name rules]] check for misconfigured Samba4
  
−
hostnames; Samba is highly likely to be misconfigured, in
 
−
many weird and interesting ways. So, 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.
  
−
(These errors need to be caught in MIT's library.)
  
−
</li>
  
−
<li> Improved krb error-messages;
  
−
</li>
  
−
<li> Improved Kerberos logging support:
 
−
<ol>
  
−
<li> '''krb5_log_facility()''': Samba4 now uses this Heimdal function,
  
−
which allows us to redirect the warnings and status from
 
−
the KDC (and client/server Kerberos code) to Samba's DEBUG()
 
−
system. Samba uses this logging routine optionally in the
 
−
main code, but it's required for KDC errors.
 
−
</li>
  
−
<li> '''krb5_get_error_string'''(): This Heimdal-specific function
  
−
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
 
−
necessary for the port, but it's more useful than MIT's
 
−
default err-handling; Make sure this works for MIT-krb)
  
−
</li>
  
−
</ol>
  
−
</li>
  
−
<li> MS GSSMonger test-suite: Microsoft has released a krb-specific
 
−
testsuite called gssmonger, which tests interoperability. 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.
  
−
</li>
  
−
<li> Testsuite for kpasswd daemon: I have a partial kpasswd server
 
−
which needs finishing, and Samba4 needs 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 Samba4's kpasswd only works
  
−
for Heimdal, not MIT clients. This may be due to call-ordering
 
−
constraints.
  
−
</li>
  
−
</ol>
  
−  
−
----
  
−  
−
=== Appendix 1: Keytab Requirements ===
  
−  
−
Traditional 'MIT' keytab operation is very different from AD's
 
−
account-handling for application-servers:
  
−
==== Host PWs vs service-keys ====
  
−
<ol>
  
−
<li> Traditional 'MIT' behaviour is for the app-server to use a keytab
  
−
containing several named random-bitstring service-keys, created
 
−
by the KDC. An MIT-style keytab holds a different service-key
 
−
for every kerberized application-service that the server offers
 
−
to clients. Heimdal also implements this behaviour. MIT's model
 
−
doesn't use AD's UTF-16 'service password', and no salting is
 
−
necessary for service-keys, because each service-key is random
 
−
enough to withstand an exhaustive key-search attack.
  
−
</li>
  
−
<li> In the Windows model, the server key's construction is very
 
−
different: The app-server itself, not the KDC, generates a
 
−
random UTF-16 pseudo-textual password, and sends this password
 
−
to the KDC using SAMR, a DCE-RPC "domain-joining" protocol (but
 
−
for windows 7, see below). Then, the KDC shares this server-
  
−
password with every application service on the whole machine.
 
−
</li>
  
−
<li> Only when the app-server uses kerberos does the password get
 
−
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, and the member server must use the rules
 
−
described in Luke's mail, in Appendix 3, below). The salted-
  
−
and-hashed version of the server-host's PW gets stored in the
  
−
server-host's keytab.
  
−
</li>
  
−
<li> Samba file-servers can have many server-names simultaneously
 
−
(kind of like web servers' software-virtual-hosting), but since
 
−
these servers are running in AD, these names can be set up to
 
−
all share the same secret key. In AD, co-located 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. In
  
−
dealing with this model, Samba4 uses both the traditional file
  
−
keytab and an in-MEMORY keytabs.
 
−
</li>
  
−
<li> Pre-Windows7 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.
  
−
</li>
  
−
</ol>
  
−
==== Flexible server-naming ====
  
−  
−
The other big difference between AD's keytabs and MIT's is that
 
−
Windows offers a lot more flexibility about service-principals'
 
−
names. When the kerberos server-side library receives Windows-style
 
−
tickets from an app-client, MIT's krb library (or GSSAPI) must
 
−
accommodate Windows' flexibility about case-sensitivity and
 
−
canonicalization.
 
−
This means that an incoming application-request to a member server
  
−
may use a wide variety of service-principal names. These include:
  
−
<pre>
  
−
machine$@REALM (samba clients)
  
−
HOST/foo.bar@realm (win2k clients)
  
−
cifs/foo.bar@realm (winxp clients)
  
−
HOST/foo@realm (win2k clients, using netbios)
  
−
cifs/foo@realm (winxp clients, using netbios),
  
−
</pre>
  
−
as well as all upper/lower-case variations on the above.
  
−
</ol>
  
−
==== * [[Keytabs & Name-canonicalization]] ====
  
−
<ol>
  
−
<li> Heimdal's GSSAPI expects to to be called with 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 have to "game" the GSSAPI, by supplying
 
−
the server's known canonical name, with the one-key
 
−
keytab. This doesn't really affect IPA's port of
 
−
Samba4 to MIT-krb.
  
−
* Luke says, [["This is much easier in 1.7, because all keys are tried.]]"
  
−
</li>
  
−
<li> 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 specifiy the principal name to
 
−
GSSAPI, which avoids the need to store duplicate
 
−
principals.
  
−
</li>
  
−
<li> We use a 'private' keytab in our private dir,
 
−
referenced from the secrets.ldb by default.
  
−
</li>
  
−
</ol>
  
−  
−
----
  
−  
−
=== * Appendix 2: [[windc KDC Plugin for Account-AuthZ]] ===
  
−  
−
Samba4 does account-authorization in Heimdal with
  
−
the specialized KDC plugin '''windc'''.
 
−
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 the plugin's
 
−
AD-specific layer, not in the DB-agnostic KDC server.
  
−
For Heimdal, we created a separate KDC plugin, with this API:
  
−
<ol>
 
−
<li> '''pac_generate()''' creates an initial PAC from the user's AD record.
  
−
</li>
  
−
<li> '''pac_verify()''' checks that a AC is correctly signed,
 
−
adds additional groups (for cross-realm tickets)
 
−
and re-signs with the key of the target kerberos
 
−
service's account
  
−
</li>
  
−
<li> '''client_access()''' performs additional access checks, such as
 
−
allowedWorkstations and account expiry.
  
−
</li>
  
−
<ol>
  
−
<li> Luke H says, "Implement
 
−
[[#HBAC_for_the_TGS | '''KRB5_KDB_METHOD_CHECK_POLICY_TGS''']] &
  
−
'''KRB5_KDB_METHOD_CHECK_POLICY_AS'''."
  
−
</li>
  
−
</ol>
  
−
<li> Here are the main definitions, from '''samba4/heimdal/kdc/windc_plugin.h'''
  
−
* Luke H says, "[[There is similar stuff in 1.7.]]"
  
−
<pre>
  
−
typedef struct
 
−
hdb_entry_ex { void *ctx;
  
−
hdb_entry entry;
  
−
void (*free_entry)(krb5_context, struct hdb_entry_ex *);
  
−
} hdb_entry_ex;
  
−
</pre>
  
−
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.
 
−
<pre>
  
−
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 *);
  
−
</pre>
  
−
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.
  
−  
−
Luke H says, "Well, the pure Kerberos error seems to work most
 
−
of the time but yes, we should fix that. I think this will
 
−
require a change. Let's try not to break Novell's backend
 
−
(or keep them in the loop) when doing so."
  
−  
−
<pre>
  
−
typedef struct
  
−
krb5plugin_windc_ftable { int minor_version;
  
−
krb5_error_code (*init)(krb5_context, void **);
  
−
void (*fini)(void *);
  
−
krb5plugin_windc_pac_generate pac_generate;
  
−
krb5plugin_windc_pac_verify pac_verify;
  
−
krb5plugin_windc_client_access client_access;
  
−
} krb5plugin_windc_ftable;
  
−
</pre>
  
−
This API has some Heimdal-specific stuff, that'll
 
−
have to change when we port this KDC plugin to MIT krb.
  
−
</li>
  
−
<li> For example, to register this plugin, use the kdc's standard
 
−
plugin-system at Samba4's initialisation:
  
−
<pre>
  
−
/* 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) */
  
−
</pre>
  
−
</li>
  
−
<li> An alternative way to register the plugin is with a
 
−
config-file that names a DSO (Dynamically Shared Object).
  
−
</li>
  
−
----
  
−  
−
=== ** Appendix 3: [[Samba4 stuff that doesn't need to get ported.]] ===
  
−
==== Heimdal oddities ====
  
−
<ol>
  
−
<li> 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.
  
−
</li>
  
−
<li> 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. No MIT porting task here, as long as MIT kdc
  
−
doesn't refuse to do some LDAP lookups (eg, alias' realm-name looks
 
−
wrong).
  
−
</li>
  
−
<li> 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.
  
−
</li>
  
−
</ol>
  
−  
−
==== Not needed anymore, because MIT's code now handles PACs fully ====
  
−
<ol>
  
−
<li> 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).
  
−
</li>
  
−
<li> gsskrb5_extract_authtime_from_sec_context (get authtime from
  
−
kerberos ticket)
  
−
</li>
  
−
<li> gsskrb5_extract_authz_data_from_sec_context (get authdata from
  
−
ticket, ie the PAC. Must unwrap the data if in an AD-IFRELEVANT)]
  
−
</li>
  
−
</ol>
  
−  
−
==== 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-IFRELEVANT container. This doesn't need to be ported to MIT.
 
−
This should be obsoleted by MIT's new PAC code.
  
−  
−
==== libkdc ====
  
−
<ol>
  
−
<li> 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.
 
−
</li>
  
−
<li> But, 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.)
  
−
</li>
  
−
</ol>
  
−  
−
==== Returned Salt for PreAuthentication ====
  
−  
−
When the AD-KDC replies to 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.
  
−  
−
<pre>
  
−
>
  
−
> 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
  
−
</pre>
  

Latest revision as of 08:40, 18 September 2009

This is an early stage project for MIT Kerberos. It is being fleshed out by its proponents. Feel free to help flesh out the details of this project. After the project is ready, it will be presented for review and approval.


Introduction

Samba4 aims to provide a complete OSS replacement for Active Directory. Samba4, like earlier versions of Samba, uses Heimdal Kerberos. The Samba4 Port project proposes to enable Samba4 to use MIT kerberos as an alternative. The near-term goal is that mixed krb5+AD deployments could use Samba4 to provide better interoperation between AD realms and MIT-krb5 realms.

Use case: For example, suppose a kerberos customer is deploying a network with mixed operating systems using kerberos and would want to use one KDC for all of them. In this case, a single MIT Kerberos deployment should be able to support both traditonal UNIX clients and servers, intermixed with Windows clients and Samba servers:

  1. The Windows clients should be able to use the MIT KDC(s) as AD servers, so as to authenticate themselves to Samba file-servers and to Windows servers;
  2. A Windows client's tickets will carry PACs, as usual for AD;
  3. The UNIX clients should be able to access the KDC as a traditional non-AD-style KDC, so as to access UNIX services securely;
  4. A UNIX client's ticket will not carry a PAC, except when the UNIX client accesses a Windows server (Rationale) .

The Samba4 team, the MIT Krb Consortium, RedHat, Ubuntu, and Sun all have shown some interest in this Samba4 Port project. Here is a table showing which OS platforms are supported by Samba4, Heimdal, and MIT kerberos. Summary: MIT-krb5 & Samba4 both run on Mac OS X, NetBSD, Debian, RedHat, Ubuntu, & Solaris.

Concise to-do list

This is a condensed version of the task-list offered by Samba4's Andrew Bartlett, containing only what hasn't yet been done already by MIT.

The two big chunks of work are LDAP Driver and Replacing / improving MIT's DAL, but the DAL work may not be needed.

Replace the MIT KDC's LDAP driver

Samba4's LDAP driver for the MIT KDB needs to know how to do AD's intricate naming:

  1. Canonicalization of server names, user-names, and realm names. MIT 1.7 already supports canonicalization.
  2. AD-style aliases for HOST/ service names.
  3. Implicit names for Win2k accounts.
  4. Principal "types": client / server / krbtgs
  5. Flexible server-naming
  6. Keytabs & name-canonicalization

Most or all of Heimdal's LDAP driver code is in three Samba4 source files, ~1000 lines in all.

Small changes

Of the things on this list, only NTLM support (bullet 2) is needed for the Samba4 KDC port. The other tasks are all application-library stuff, and arguably aren't needed at all, because Samba3 already works well with MIT application libraries.

  1. MIT library changes
  2. Samba4/AD libraries: NTLM support. See also this Sept-2009 NTLM thread (this implies to me that a GSS NTLM mech is not an immediate requirement - LH)
  3. Key-handling changes]
  4. Extra Krb library functions
  5. Error-handling, logging, testing

Use 1.7's AD-support features

This stuff should already just work:

  1. PAC handling;
  2. AD-style name canonicalization;
  3. NT-ENTERPRISE names, which carry two realm-suffixes;
  4. CHECK_POLICY/AUDIT methods (needed for TGS access-control);
  5. DCE_STYLE Challenge/Response handshakes: see Krb lib & GSSAPI.
  6. Accept legacy Samba3 clients' bad GSSAPI checksums;
  7. Principal-manipulation functions;
  8. State-machine safety;

Controversial proposed changes for the port

Maybe: Improve or replace MIT's DAL

Rewrite the MIT KDC's Data-Abstraction Layer (DAL), mostly because the MIT KDC needs to see & manipulate more LDAP detail, on Samba4's behalf;

Maybe, or not: Add a KDC-as-library API

Samba4 currently runs as a single process, and Samba4's smbd invokes the Heimdal KDC via a libkdc interface (KDC as library).

  1. Rationale:
    1. smbd uses the libkdc interface to configure the KDC, both at startup & during runtime.
    2. Samba4's build/test environment uses libkdc's socket-passing, to simulate network traffic.
  2. Andrew Bartlett says this libkdc interface is "nice to have", but not essential for getting the port to work.
  3. Tom Yu says adding a libkdc interface to MIT's code would be a lot of work, but would tie naturally into code-cleanup work that MIT wants to do, anyway.
  4. Sam Hartman says he needs the libkdc interface, too, for his work on PK-U2U (but not immediately).
  5. Another way, which Simo dismisses on Samba4's behalf: Samba can use iptables remapping, but only for kdc packets, so that Samba acts as a router between the AD client and the KDC. This would work for MIT-krb & for Heimdal.
  6. If we do have to build a libkdc interface for MIT's KDC, Samba4 will need the KDC to use Samba's socket library correctly.

Later: TGS access-control

MIT krb will need to support these AD features, once Samba4 does. Alternatively, this could be seen as an opportunity for MIT-based Samba4 to surpass Heimdal-based Samba.

  1. Add HBAC to the TGS, so that Samba4 can refuse TGTs to kinit, based on time-of-day & IP-addr constraints;
    1. DTD: This is natural; the TGS should enforce its own access-control, as all other services do.
    2. TGS-HBAC is part of the rationale for rewriting the DAL.
  2. Failed-kinit counts: Add a KDC heuristic for tracking intervals between kinits, so that Samba4 can enforce AD's unified account-lockout on kinit. Samba4 already does lockouts for other PW-based authentication methods (NTLM, LDAP simple bind, etc).

Samba's use of Heimdal symbols, with MIT differences

Samba4 uses around 265 Heimdal symbols:

  1. 150 functions,
  2. 45 structs & typedefs, and
  3. 70 macros & enums.

Of these, roughly half present problems for the port:

  1. 25 symbols have different definitions in the MIT & Heimdal trees.
  2. 110 symbols are missing from MIT's krb5 tree.

Samba4 Interfaces with Heimdal

  1. Samba4's Database Interfaces enable Heimdal to use Samba4's directory data, whether the directory is stored in LDAP or in local disk files.
  2. Heimdal's libkdc Interface gives Samba4 a direct subroutine interface to the Heimdal KDC, with the KDC running as part of the Samba4 process.
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