NAME

       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

SYNOPSIS

        #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);

DESCRIPTION

       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.

NOTES

       The following mode changes are available:

       SSL_MODE_ENABLE_PARTIAL_WRITE
           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().

       SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER
           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().

       SSL_MODE_AUTO_RETRY
           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.

       SSL_MODE_RELEASE_BUFFERS
           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.

       SSL_MODE_SEND_FALLBACK_SCSV
           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.

       SSL_MODE_ASYNC
           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).

       SSL_MODE_DTLS_SCTP_LABEL_LENGTH_BUG
           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.

RETURN VALUES

       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.

SEE ALSO

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

HISTORY

       SSL_MODE_ASYNC was added in OpenSSL 1.1.0.

COPYRIGHT

       Copyright 2001-2021 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the Apache License 2.0 (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
       <https://www.openssl.org/source/license.html>.