SSL_set_num_tickets, SSL_get_num_tickets, SSL_CTX_set_num_tickets, SSL_CTX_get_num_tickets, SSL_new_session_ticket - control the number of TLSv1.3 session tickets that are issued
#include <openssl/ssl.h> int SSL_set_num_tickets(SSL *s, size_t num_tickets); size_t SSL_get_num_tickets(SSL *s); int SSL_CTX_set_num_tickets(SSL_CTX *ctx, size_t num_tickets); size_t SSL_CTX_get_num_tickets(SSL_CTX *ctx); int SSL_new_session_ticket(SSL *s);
SSL_CTX_set_num_tickets() and SSL_set_num_tickets() can be called for a server application and set the number of TLSv1.3 session tickets that will be sent to the client after a full handshake. Set the desired value (which could be 0) in the num_tickets argument. Typically these functions should be called before the start of the handshake.
The default number of tickets is 2. Following a resumption the number of tickets issued will never be more than 1 regardless of the value set via SSL_set_num_tickets() or SSL_CTX_set_num_tickets(). If num_tickets is set to 0 then no tickets will be issued for either a normal connection or a resumption.
Tickets are also issued on receipt of a post-handshake certificate from the client following a request by the server using SSL_verify_client_post_handshake(3). These new tickets will be associated with the updated client identity (i.e. including their certificate and verification status). The number of tickets issued will normally be the same as was used for the initial handshake. If the initial handshake was a full handshake then SSL_set_num_tickets() can be called again prior to calling SSL_verify_client_post_handshake() to update the number of tickets that will be sent.
To issue tickets after other events (such as application-layer changes), SSL_new_session_ticket() is used by a server application to request that a new ticket be sent when it is safe to do so. New tickets are only allowed to be sent in this manner after the initial handshake has completed, and only for TLS 1.3 connections. By default, the ticket generation and transmission are delayed until the server is starting a new write operation, so that it is bundled with other application data being written and properly aligned to a record boundary. If the connection was at a record boundary when SSL_new_session_ticket() was called, the ticket can be sent immediately (without waiting for the next application write) by calling SSL_do_handshake(). SSL_new_session_ticket() can be called more than once to request additional tickets be sent; all such requests are queued and written together when it is safe to do so and triggered by SSL_write() or SSL_do_handshake(). Note that a successful return from SSL_new_session_ticket() indicates only that the request to send a ticket was processed, not that the ticket itself was sent. To be notified when the ticket itself is sent, a new-session callback can be registered with SSL_CTX_sess_set_new_cb(3) that will be invoked as the ticket or tickets are generated.
SSL_CTX_get_num_tickets() and SSL_get_num_tickets() return the number of tickets set by a previous call to SSL_CTX_set_num_tickets() or SSL_set_num_tickets(), or 2 if no such call has been made.
SSL_CTX_set_num_tickets(), SSL_set_num_tickets(), and SSL_new_session_ticket() return 1 on success or 0 on failure.
SSL_CTX_get_num_tickets() and SSL_get_num_tickets() return the number of tickets that have been previously set.
SSL_new_session_ticket() was added in OpenSSL 3.0.0. SSL_set_num_tickets(), SSL_get_num_tickets(), SSL_CTX_set_num_tickets(), and SSL_CTX_get_num_tickets() were added in OpenSSL 1.1.1.
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