Question about Windows S4U support

[Greg Hudson]

On 11/8/23 09:23, JianJun Li wrote:
> In fact,   principle "host/win11client.mylab.com at MYLAB.COM" exists.  By Wireshark I can see Windows sends "host/win11client.mylab.com at MYLAB.COM"  as sname, KDC converts the sname to host\/win11client.mylab.com at 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.com at 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.