Description of problem:
When SSSD is authenticating an AD user, it calls out to libkrb5. Normally, the Kerberos protocol uses UDP with optional fail over to TCP in case the data from the KDC would be too big. However, in AD environments, the fail over happens pretty much all the time, because AD also attaches a PAC to the Kerberos tickets. The PAC contains authorization data, including all the SIDs of groups the user is a member of which makes it usually too big for UDP.

The reason this should be documented is that at the moment, SSSD enrolled to AD always uses UDP but falls back to TCP pretty much always which might be confusing. In the future (please see bug #1399262) we are considering defalting to TCP in the first place, because the switch to TCP would happen anyway.

I think our books alrady document that both UDP and TCP ports should be open, but it might be nice to say why this is needed. If we extend the books for the 7.4 release, it would be nice to check if the TCP-by-default (tracked in #1399262) was also implemented and document that at least for user authentication TCP is used by default.


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