SSL_CTX_SET_MODE(3ossl)

📦 version 3.5.0, in opensuse-leap 16.0

🌸 SSL_CTX_SET_MODE(3ossl) — the "manipulate SSL engine mode" era 🌸

okay so bestie, this page is from the 3ossl manual and it's literally all about flipping mode bits on your SSL engine. it's giving "small file, huge power" and honestly? iconic. let's get into it 💅

✨ NAME ✨

SSL_CTX_set_mode, SSL_CTX_clear_mode, SSL_set_mode, SSL_clear_mode, SSL_CTX_get_mode, SSL_get_mode — the whole girl gang for manipulating 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 💖

so there are basically two flavors of everything here: the SSL_CTX version (your context, the group chat) and the SSL version (one specific connection, your bestie's DMs). periodt.

  • SSL_CTX_set_mode() adds the mode set via bit-mask in mode to ctx. options that were already set before? not cleared, bestie, they stay. 💅
  • 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. again, whatever was already set before is 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.

like, set = OR it in, clear = AND-NOT it out, get = "what's the vibe rn". yw 🫶

🌈 NOTES 🌈

here's the menu of mode changes available, and yes you should read all of them (there's a quiz later, no cap):

🔥 SSL_MODE_ENABLE_PARTIAL_WRITE

lets SSL_write_ex(..., n, &r) come back with 0 < r < n — i.e. it reports success when just a single record has been written. works in a similar way for SSL_write(). when this is not set (which is the default), SSL_write_ex() or SSL_write() will only report success once the complete chunk was written. and once SSL_write_ex() or SSL_write() returns successful, r bytes have been written, so 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 you're in the middle of an incomplete write operation. don't try it, it's not the moment.

🔥 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, though — location change yes, personality change no). this is not the default, specifically to avoid the misconception that nonblocking SSL_write() behaves like nonblocking write(). it doesn't. don't be that girl.

🔥 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. lowkey a lifesaver.

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're 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. 😭 not the hang.

🔥 SSL_MODE_RELEASE_BUFFERS

when we no longer need a read buffer or a write buffer for a given SSL, 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. free real estate, we love to see it 💸

🔥 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. i'm serious, bestie, this is not a vibe-check-optional situation.

🔥 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). multitasking queen behavior ✨

🔥 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. but pls note: setting this option breaks interoperability with correct implementations. this option only applies to DTLS over SCTP. it's giving "backwards compatibility but make it toxic" 🙈

all modes are off by default except for SSL_MODE_AUTO_RETRY, which is on by default since 1.1.1. the one main character that's already on the guest list 🎟️

👑 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.

that's it, that's the tea 🍓

🧚 SEE ALSO 🧚

ssl(7), SSL_read_ex(3), SSL_read(3), SSL_write_ex(3) or SSL_write(3), SSL_get_error(3) — go read these next, i'm literally begging you 🫶

⭐ HISTORY ⭐

SSL_MODE_ASYNC was added in OpenSSL 1.1.0. a whole era 💜

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>.

and that's the whole page, bestie 💖 this one's version 3.5.0, dated 2026-09-25, and it's the SSL_CTX_SET_MODE(3ossl) manual — go forth and set those modes responsibly, mwah 🎀🦋✨

📦 where this page comes from