SSL_CTX_SET_MODE(3ossl)

💖 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 💖

okay bestie, welcome to SSL_CTX_SET_MODE(3ossl), the section-3ossl OpenSSL page where we fiddle with the SSL engine's mode bits. Think of a mode as a bunch of little switches 🎚️ you flip on an SSL object or an SSL_CTX — and every switch changes how the engine behaves. Let's get into it. ✨

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🎀 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);

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🦋 DESCRIPTION 🦋

SSL_CTX_set_mode() takes the mode set you pass in the bit-mask mode and adds it to ctx. Key detail: options that were already set before are not cleared. It's additive, not a reset. 💅

SSL_CTX_clear_mode() does the opposite — it removes the mode set given in the bit-mask mode from ctx.

SSL_set_mode() adds the mode set via the bit-mask mode to ssl, and again, options already set before are not cleared.

SSL_clear_mode() removes the mode set via the bit-mask mode from ssl.

SSL_CTX_get_mode() just hands you back the mode that's currently set for ctx.

SSL_get_mode() hands you back the mode that's currently set for ssl.

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✨ NOTES ✨

Here's the whole menu of mode changes you can flip, bestie. Each one's a whole vibe of its own:

• SSL_MODE_ENABLE_PARTIAL_WRITE — lets SSL_write_ex(..., n, &r) come back with 0 < r < n, meaning it reports success when just a single record has been written. SSL_write() works in a similar way. When this is not set (and that's 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(). 🫶

You cannot enable this mode while you're in the middle of an incomplete write operation. No mid-write glow-ups, sorry.

• SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER — makes it possible to retry SSL_write_ex() or SSL_write() with a changed buffer location (the buffer contents must stay the same, okay?). This is not the default, and the reason is very deliberate: it avoids 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 isn't even 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, we release the memory we were using to hold it. Using this flag can save around 34k per idle SSL connection. Note the limits though: this flag has no effect on SSL v2 connections, or on DTLS connections. 🌸
• SSL_MODE_SEND_FALLBACK_SCSV — sends 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. Like, genuinely, don't.

• 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. That one's the main character, fr fr. ⭐

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🌈 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.

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🧚 SEE ALSO 🧚

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

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🍓 HISTORY 🍓

SSL_MODE_ASYNC was added in OpenSSL 1.1.0.

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💜 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>.

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and that's the whole page, bestie 🫶 go forth and flip those mode bits responsibly — this was version 3.5.9, dated 2026-09-29, and it's the SSL_CTX_SET_MODE(3ossl) page signing off 💅✨