As per the subject line:

        SSL_CTX_set_session_cache_mode(ctx, SSL_SESS_CACHE_OFF) 

does not seem to disable generation of stateless tickets with TLS 1.3:

        SSL_CTX_set_num_tickets(ctx, 0); 

is also required to prevent the tickets being generated.

There's no mention of this additional call on the 
SSL_CTX_set_session_cache_mode pages (but is documented elsewhere).

It really seems like SSL_SESS_CACHE_OFF should also disable TLS1.3 session 
tickets if the intent is for TLS 1.3 to mostly be a drop in replacement for TLS 
<= 1.2.  A user upgrades OpenSSL library and suddenly session resumption 
enabled where it wasn't before... that doesn't seem right.

In our case this broke our session_resumption control toggle, where 
'session_resumption = no' prevented our EAP server implementation from 
presenting tickets with TLS <= 1.2 but still allowed them if TLS 1.3 was 
negotiated.

Disabling session resumption is more important with EAP methods like EAP-TTLS 
and EAP-PEAP because it controls whether phase 2 runs or not - phase 2 being 
where the actual credential validation happens.

This was tested with current OpensSL master HEAD.  Can test with the 1.1.* 
branch if that'd help.

-Arran
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