Thanks for the reply. One strange thing is that when Windows is using AD
domain, sname doesn't have this format: host/win11client.mylab.com but
win11client$. I have no idea what makes Windows have this difference.
For PAC validation error, I also can't get more detailed information from
Windows logging what causes the validation failure.
-----Original Message-----
From: Greg Hudson <ghud...@mit.edu>
Sent: Friday, November 10, 2023 6:44 AM
To: JianJun Li <j...@rocketsoftware.com>; kerberos@mit.edu
Subject: Re: Question about Windows S4U support
EXTERNAL EMAIL
On 11/8/23 09:23, JianJun Li wrote:
> In fact, principle "host/win11client.mylab....@mylab.com" exists. By
> Wireshark I can see Windows sends "host/win11client.mylab....@mylab.com" as
> sname, KDC converts the sname to host\/win11client.mylab....@mylab.com.
> I have a look at the code but find no parameters or setting can change this
> behavior.
I can give a detailed but ultimately not very helpful answer:
As Ken explained in part, the wire representation of principals in Kerberos is
the ASN.1 DER encoding of a name-type and a sequence of strings. Microsoft
created a name type NT-ENTERPRISE which puts an email-address-like string in
the first string element. When you see "host\/..." in your log, that is the
MIT krb5 library's string representation of an NT-ENTERPRISE principal.
RFC 6806 section 5 describes this name type as conveying alias names, to be
used in the client field of an AS-REQ to a KDC with a directory service that
can map email addresses to canonical principal names.
However, Microsoft's implementation now also uses this type in server names
during under some circumstances, including some S4U operations.
[MS-KILE] 3.3.5.1.1 defines semantics for server name lookup of NT-ENTERPRISE
principals (in terms of underlying facilities specific to Active Directory);
[MS-SFU] unfortunately does not seem to say precisely when they are used. I
had thought they were only used for cross-realm S4U2Self operations where it is
necessary to communicate the requesting service's realm to the client realm,
but based on your log it sounds like they are also used for same-realm S4U2Self
requests made by Windows clients.
Although MIT krb5 has S4U2Self and S4U2Proxy logic in the KDC code, it does not
implement NT-ENTERPRISE lookup. The translation from NT-ENTERPRISE
{"host/win11client.mylab....@mylab.com"} to NT-PRINCIPAL {"host",
"win11client.mylab.com"} currently has to be done within the KDB layer, either
by using an encompassing piece of software with a KDB module (such as Samba),
or by setting up an explicit alias in the LDAP KDB module (the BDB and LMDB
modules do not support aliases). I believe the situation could be improved by
performing this translation within the KDC for TGS service lookups, but that
improvement, although simple in concept, would require careful testing.
> The digitally signed Privilege Attribute Certificate (PAC) that contains the
> authorization information for client user in realm MYLAB.COM could not be
> validated.
> This error is usually caused by domain trust failures; Contact your system
> administrator.
I don't know exactly what is causing this error on the Windows side, especially
if it only happens some of the time. I will note that when used with any of
the built-in KDB modules (BDB, LMDB, or LDAP), MIT krb5's KDC includes a
minimal PAC with no SID or group information.
Encompassing software such as Samba is required to supply a complete PAC within
issued tickets. This limitation may be unrelated to the error given that the
error does not always occur.
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