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>.