kdc: history of request_anonymous vs cname-in-addl-tkt confusion

kdc: history of request_anonymous vs cname-in-addl-tkt confusion

Drafts 0 through 10 of the Kerberos anonymity internet draft,
https://tools.ietf.org/html/draft-ietf-krb-wg-anon, specified the
TicketFlags.anonymous flag as bit 14 and the KDCOptions.anonymous
flag as bit 14.  These were changed to bit 16 by MIT after it was
discovered that Microsoft used KDCOptions bit 14 for S4U2Proxy
cname-in-addl-tkt.

(Feb 2007) Heimdal added constrained delegation support prior to
1.0 but named the KDCOptions flag constrained_delegation instead of
cname-in-addl-tkt as per MS-SFU.  It also assigned bit 16 instead
of bit 14.  Perhaps this was done in the hope that the conflict
with Microsoft would be resolved in favor of the IETF internet
draft instead of the proprietary protocol extension.

adf912182266321b754ed1cb5a705ba2103e139a ("Add PA-ClientCanonicalized
and friends.") introduced the KDCOptions.constrained_delegation flag
as bit 16.

(June 2007) In order to make Heimdal's constrained delegation work
with Microsoft's implementation Heimdal began to set both KDCOptions
bits 14 and 16 when requesting constrained delegation.

d5bb7a7c566841d52662b230248f06522bfa64ad ("(krb5_get_creds): if
KRB5_GC_CONSTRAINED_DELEGATION is set, set both") set both the
anonymous and constrained_delegation TicketFlags when issuing a
S4U2Proxy request.

(June 2010) MIT reassigned the KDCOption.anonymous and
TicketFlags.anonymous flags to bit 16.  draft-ietf-krb-anon-11
was published with this change.

(July 2014) After the release of Heimdal 1.5.0 and prior to 1.5.1
it was noticed that Heimdal's anonymous TGT support did not
interoperate with MIT.

86554f5a7f81da1efa2849fa6961ca71ad3b8e90 ("Use correct value for
anonymous flags") swapped the bit assignments for request_anonymous
and constrained_delegation but failed to remove the setting of
KDCOptions bit 16 ("anonymous") when requesting constrained
delegation.

(May 2019) Prior to the 7.6 release many corrections to Heimdal's
anonymity support were introduced to bring it into compliance
with RFC8062.  This included support for requesting anonymous
tickets via the TGS service.  Because not all KDC can satisfy
anonymous requests the client must verify if the response was
anonymized.  This check wasn't added until after 7.6 was
released.

014e318d6bdefd8ecfcb99ca9928921f6a49d721 ("krb5: check KDC
supports anonymous if requested").

The combination of setting KDCOption.anonymous when requesting
constrained delegation and the anonymized ticket validation
broke S4U2Proxy requests to Windows KDCs.  Windows KDCs ignore
the KDCOption.anonymous flag when processing a TGS request
with KDCOption.cname-in-addl-tkt set.

ea7615ade3af28843f358e715703226b760db73b ("Do not set
anonymous flag in S4U2Proxy request") removed the behavior
of setting the KDCOption.anonymous flag that should have
been removed in July 2014.

(June 2019) The Heimdal KDC includes fallback logic to handle
Heimdal clients from 1.0 to 1.5.0, inclusive, that set the
KDCOptions.anonymous flag as bit 14.  Prior to the 7.7 release
this logic only handled AS request but failed to handle the
constrained delegation request case where both bits 14 and 16
were set in the TGS request.

cdd0b70d37d87026e8618ff44b8d636c0bf9cb6c ("kdc: don't misidentify
constrained delegation requests as anonymous") added the TGS
request validation to distinguish anonymous requests from
constrained delegation requests.

This change documents the history in the commit message and
updates some in-tree comments.

Change-Id: I625cd012e2e6c263c71948c6021cc2fad4d2e53a