SSL_CTX_SET_MODE(3SSL) OpenSSL SSL_CTX_SET_MODE(3SSL)
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().
This mode cannot be enabled while in the middle of an incomplete
write operation.
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 2001-2023 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>.
3.5.7 2026-06-09 SSL_CTX_SET_MODE(3SSL)
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