Updated: 2021/Apr/14

SSL_CTX_set_mode(3)                 OpenSSL                SSL_CTX_set_mode(3)

       SSL_CTX_set_mode, SSL_CTX_clear_mode, SSL_set_mode, SSL_clear_mode,
       SSL_CTX_get_mode, SSL_get_mode - manipulate SSL engine mode

       libcrypto, -lcrypto

        #include <openssl/ssl.h>

        long SSL_CTX_set_mode(SSL_CTX *ctx, long mode);
        long SSL_CTX_clear_mode(SSL_CTX *ctx, long mode);
        long SSL_set_mode(SSL *ssl, long mode);
        long SSL_clear_mode(SSL *ssl, long mode);

        long SSL_CTX_get_mode(SSL_CTX *ctx);
        long SSL_get_mode(SSL *ssl);

       SSL_CTX_set_mode() adds the mode set via bit mask in mode to ctx.
       Options already set before are not cleared.  SSL_CTX_clear_mode()
       removes the mode set via bit mask in mode from ctx.

       SSL_set_mode() adds the mode set via bit mask in mode to ssl.  Options
       already set before are not cleared.  SSL_clear_mode() removes the mode
       set via bit mask in mode from ssl.

       SSL_CTX_get_mode() returns the mode set for ctx.

       SSL_get_mode() returns the mode set for ssl.

       The following mode changes are available:

           Allow SSL_write_ex(..., n, &r) to return with 0 < r < n (i.e.
           report success when just a single record has been written). This
           works in a similar way for SSL_write(). When not set (the default),
           SSL_write_ex() or SSL_write() will only report success once the
           complete chunk was written. Once SSL_write_ex() or SSL_write()
           returns successful, r bytes have been written and the next call to
           SSL_write_ex() or SSL_write() must only send the n-r bytes left,
           imitating the behaviour of write().

           Make it possible to retry SSL_write_ex() or SSL_write() with
           changed buffer location (the buffer contents must stay the same).
           This is not the default to avoid the misconception that nonblocking
           SSL_write() behaves like nonblocking write().

           During normal operations, non-application data records might need
           to be sent or received that the application is not aware of.  If a
           non-application data record was processed, SSL_read_ex(3) and
           SSL_read(3) can return with a failure and indicate the need to
           retry with SSL_ERROR_WANT_READ.  If such a non-application data
           record was processed, the flag SSL_MODE_AUTO_RETRY causes it to try
           to process the next record instead of returning.

           In a nonblocking environment applications must be prepared to
           handle incomplete read/write operations.  Setting
           SSL_MODE_AUTO_RETRY for a nonblocking BIO will process non-
           application data records until either no more data is available or
           an application data record has been processed.

           In a blocking environment, applications are not always prepared to
           deal with the functions returning intermediate reports such as
           retry requests, and setting the SSL_MODE_AUTO_RETRY flag will cause
           the functions to only return after successfully processing an
           application data record or a failure.

           Turning off SSL_MODE_AUTO_RETRY can be useful with blocking BIOs in
           case they are used in combination with something like select() or
           poll().  Otherwise the call to SSL_read() or SSL_read_ex() might
           hang when a non-application record was sent and no application data
           was sent.

           When we no longer need a read buffer or a write buffer for a given
           SSL, then release the memory we were using to hold it.  Using this
           flag can save around 34k per idle SSL connection.  This flag has no
           effect on SSL v2 connections, or on DTLS connections.

           Send TLS_FALLBACK_SCSV in the ClientHello.  To be set only by
           applications that reconnect with a downgraded protocol version; see
           draft-ietf-tls-downgrade-scsv-00 for details.

           DO NOT ENABLE THIS if your application attempts a normal handshake.
           Only use this in explicit fallback retries, following the guidance
           in draft-ietf-tls-downgrade-scsv-00.

           Enable asynchronous processing. TLS I/O operations may indicate a
           retry with SSL_ERROR_WANT_ASYNC with this mode set if an
           asynchronous capable engine is used to perform cryptographic
           operations. See SSL_get_error(3).

           Older versions of OpenSSL had a bug in the computation of the label
           length used for computing the endpoint-pair shared secret. The bug
           was that the terminating zero was included in the length of the
           label. Setting this option enables this behaviour to allow
           interoperability with such broken implementations. Please note that
           setting this option breaks interoperability with correct
           implementations. This option only applies to DTLS over SCTP.

       All modes are off by default except for SSL_MODE_AUTO_RETRY which is on
       by default since 1.1.1.

       SSL_CTX_set_mode() and SSL_set_mode() return the new mode bit mask
       after adding mode.

       SSL_CTX_get_mode() and SSL_get_mode() return the current bit mask.

       ssl(7), SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or SSL_write(3),

       SSL_MODE_ASYNC was added in OpenSSL 1.1.0.

       Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2020-12-10               SSL_CTX_set_mode(3)