SSL_CTX_SET_OPTIONS(3ossl)

📦
version 3.5.9 · 8 versions
, in fedora 43, fedora 44

okay bestie 💖 you just opened the OpenSSL page for the whole SSL_CTX_set_options / SSL_set_options crew — the literal man page SSL_CTX_SET_OPTIONS(3ossl). And bestie, this one is long, so it comes in parts. This is part 1, and we are keeping every single detail, no cap, no skips. 🎀

✨ NAME ✨

SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options, SSL_clear_options, SSL_CTX_get_options, SSL_get_options, SSL_get_secure_renegotiation_support — manipulate SSL options 💅

🌸 SYNOPSIS 🌸

#include <openssl/ssl.h>

uint64_t SSL_CTX_set_options(SSL_CTX *ctx, uint64_t options);
uint64_t SSL_set_options(SSL *ssl, uint64_t options);

uint64_t SSL_CTX_clear_options(SSL_CTX *ctx, uint64_t options);
uint64_t SSL_clear_options(SSL *ssl, uint64_t options);

uint64_t SSL_CTX_get_options(const SSL_CTX *ctx);
uint64_t SSL_get_options(const SSL *ssl);

long SSL_get_secure_renegotiation_support(SSL *ssl);

💅 DESCRIPTION 💅

  • SSL_CTX_set_options() — adds the options you pass in as a bit-mask in options to ctx. And like, ctx MUST NOT be NULL — fr fr, do not do that to me. Options that were already set before are not cleared, they stay. 💯
  • SSL_set_options() — same energy but for ssl: it adds the options set via the bit-mask in options to ssl. Options already set before are not cleared either.
  • SSL_CTX_clear_options() — clears the options set via the bit-mask in options on ctx.
  • SSL_clear_options() — clears the options set via the bit-mask in options on ssl.
  • SSL_CTX_get_options() — returns the options set for ctx.
  • SSL_get_options() — returns the options set for ssl.
  • SSL_get_secure_renegotiation_support() — tells you whether the peer supports secure renegotiation. Little heads up bestie: this one is implemented via a macro. ✨

🦋 NOTES 🦋

You can change how the SSL library behaves by setting a bunch of options. They're coded as bit-masks and you can combine them with a bitwise or operation (|). Slay.

SSL_CTX_set_options() and SSL_set_options() change the (external) protocol behaviour of the SSL library. If you wanna mess with the (internal) behaviour of the API instead, that's a whole different vibe — use the similar SSL_CTX_set_mode(3) and SSL_set_mode() functions.

During a handshake, it's the option settings of the SSL object that get used. When a new SSL object is created from a context using SSL_new(), the current option setting gets copied over. Changes to ctx do not affect already created SSL objects. And SSL_clear() does not affect the settings. Noted. 📝

🐛 bug workaround options 🐛

  • SSL_OP_CRYPTOPRO_TLSEXT_BUG — adds the server-hello extension from the early version of the cryptopro draft when a GOST ciphersuite is negotiated. Needed for interoperability with CryptoPro CSP 3.x.
  • SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS — disables a countermeasure against an SSL 3.0/TLS 1.0 protocol vulnerability affecting CBC ciphers, which some broken SSL implementations literally cannot handle. This option has no effect for connections using other ciphers.
  • SSL_OP_SAFARI_ECDHE_ECDSA_BUG — don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X. OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers. 💀
  • SSL_OP_TLSEXT_PADDING — adds a padding extension to ensure the ClientHello size is never between 256 and 511 bytes in length. It's a workaround for some implementations.
  • SSL_OP_ALL — literally all of the above bug workarounds. The whole bundle. 💅

It's usually safe to use SSL_OP_ALL to enable the bug workaround options if you want compatibility with somewhat broken implementations.

👑 modifying options 👑

  • SSL_OP_ALLOW_CLIENT_RENEGOTIATION — client-initiated renegotiation is disabled by default. Use this option to turn it on.
  • SSL_OP_ALLOW_NO_DHE_KEX — in TLSv1.3, allow a non-(ec)dhe based key exchange mode on resumption. Meaning what exactly? Meaning there will be no forward secrecy for the resumed session. ⚠️
  • SSL_OP_PREFER_NO_DHE_KEX — in TLSv1.3, on resumption, let the server prefer a non-(ec)dhe based key exchange mode over an (ec)dhe based one. Ignored without SSL_OP_ALLOW_NO_DHE_KEX being set as well. Always ignored on the client.
  • SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION — allow legacy insecure renegotiation between OpenSSL and unpatched clients or servers. Peep the SECURE RENEGOTIATION section for more details.
  • SSL_OP_CIPHER_SERVER_PREFERENCE — when choosing a cipher, use the server's preferences instead of the client's. When not set, the SSL server will always follow the client's preferences. When set, the SSL/TLS server will choose following its own preferences. main character energy fr. 👑
  • SSL_OP_CISCO_ANYCONNECT — use Cisco's version identifier of DTLS_BAD_VER when establishing a DTLSv1 connection. Only available when using the deprecated DTLSv1_client_method() API.
  • SSL_OP_CLEANSE_PLAINTEXT — by default TLS and QUIC SSL objects keep a copy of received plaintext application data in a static buffer until it gets overwritten by the next portion of data. When you enable SSL_OP_CLEANSE_PLAINTEXT, deciphered application data is cleansed by calling OPENSSL_cleanse(3) after passing data to the application. Data is also cleansed when releasing the connection (e.g. SSL_free(3)).

Since OpenSSL only cleanses internal buffers, the application is still responsible for cleansing all other buffers. Most notably, this applies to buffers passed to functions like SSL_read(3), SSL_peek(3) but also like SSL_write(3).

TLS connections do not buffer data to be sent in plaintext. QUIC stream objects do buffer plaintext data to be sent, and this option will also cause that data to be cleansed when it is discarded.

This option can be set differently on individual QUIC stream objects and has no effect on QUIC connection objects (except where a default stream is being used).

  • SSL_OP_COOKIE_EXCHANGE — turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects DTLS connections.
  • SSL_OP_DISABLE_TLSEXT_CA_NAMES — disable TLS Extension CA Names. You may want to disable it for security reasons, or for compatibility with some Windows TLS implementations crashing when this extension is larger than 1024 bytes.
  • SSL_OP_ENABLE_KTLS — enable the use of kernel TLS. To benefit from kernel TLS, OpenSSL must have been compiled with support for it, and it must be supported by the negotiated ciphersuites and extensions. The specific ciphersuites and extensions that are supported may vary by platform and kernel version.

The kernel TLS data-path implements the record layer, and the encryption algorithm. The kernel will utilize the best hardware available for encryption. Using the kernel data-path should reduce the memory footprint of OpenSSL because no buffering is required. Also, the throughput should improve because data copy is avoided when user data is encrypted into kernel memory instead of the usual encrypt then copy to kernel.

Kernel TLS might not support all the features of OpenSSL. For instance, renegotiation, and setting the maximum fragment size is not possible as of Linux 4.20.

Note that with kernel TLS enabled some cryptographic operations are performed by the kernel directly and not via any available OpenSSL Providers. This might be undesirable if, for example, the application requires all cryptographic operations to be performed by the FIPS provider.

  • SSL_OP_ENABLE_KTLS_TX_ZEROCOPY_SENDFILE — with this option, sendfile() will use the zerocopy mode, which gives a performance boost when used with KTLS hardware offload. Note that invalid TLS records might be transmitted if the file is changed while being sent. This option has no effect if SSL_OP_ENABLE_KTLS is not enabled.

This option only applies to Linux. KTLS sendfile on FreeBSD doesn't offer an option to disable zerocopy and always runs in this mode.

  • SSL_OP_ENABLE_MIDDLEBOX_COMPAT — if set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that do not understand TLSv1.3 will not drop the connection. Regardless of whether this option is set or not, CCS messages received from the peer will always be ignored in TLSv1.3. This option is set by default. To switch it off use SSL_clear_options(). A future version of OpenSSL may not set this by default.
  • SSL_OP_IGNORE_UNEXPECTED_EOF — some TLS implementations do not send the mandatory close_notify alert on shutdown. If the application tries to wait for the close_notify alert but the peer closes the connection without sending it, an error is generated. When this option is enabled, the peer does not need to send the close_notify alert and a closed connection will be treated as if the close_notify alert was received.

You should only enable this option if the protocol running over TLS can detect a truncation attack itself, and that the application is checking for that truncation attack.

For more information on shutting down a connection, see SSL_shutdown(3).

  • SSL_OP_LEGACY_SERVER_CONNECT — allow legacy insecure renegotiation between OpenSSL and unpatched servers only. Peep the SECURE RENEGOTIATION section for more details.
  • SSL_OP_NO_ANTI_REPLAY — by default, when a server is configured for early data (i.e., max_early_data > 0), OpenSSL will switch on replay protection. See SSL_read_early_data(3) for a description of the replay protection feature. Anti-replay measures are required to comply with the TLSv1.3 specification. Some applications may be able to mitigate the replay risks in other ways, and in such cases the built in OpenSSL functionality is not required. Those applications can turn this feature off by setting this option. This is a server-side option only. It is ignored by clients.
  • SSL_OP_NO_TX_CERTIFICATE_COMPRESSION — normally clients and servers will transparently attempt to negotiate the RFC8879 certificate compression option on TLSv1.3 connections.

If this option is set, the certificate compression extension is ignored upon receipt and compressed certificates will not be sent to the peer.

  • SSL_OP_NO_RX_CERTIFICATE_COMPRESSION — normally clients and servers will transparently attempt to negotiate the RFC8879 certificate compression option on TLSv1.3 connections.

If this option is set, the certificate compression extension will not be sent and compressed certificates will not be accepted from the peer.

  • SSL_OP_NO_COMPRESSION — bestie, no TLS record compression, even if the other side is offering it. It's on by default already ✨ so if you wanna flip it off you gotta call SSL_clear_options(). Real talk though: TLS record compression is not recommended and it's not even available at security level 2 or above. And since OpenSSL 3.2 the default security level is 2, so clearing this option does absolutely nothing unless you also go change the default security level. See SSL_CTX_set_security_level(3). 💅
  • SSL_OP_NO_ENCRYPT_THEN_MAC — normally clients and servers will transparently try to negotiate the RFC7366 Encrypt-then-MAC option on TLS and DTLS connections. Set this and Encrypt-then-MAC is off: clients won't propose it, servers won't accept the extension. 🚫
  • SSL_OP_NO_EXTENDED_MASTER_SECRET — same energy, different RFC. Normally clients and servers transparently try to negotiate the RFC7627 Extended Master Secret option on TLS and DTLS connections. Set this and Extended Master Secret is disabled: clients won't propose it, servers won't accept the extension. 🦋
  • SSL_OP_NO_QUERY_MTU — don't query the MTU. Only affects DTLS connections, k? 🙈
  • SSL_OP_NO_RENEGOTIATION — kills all renegotiation in (D)TLSv1.2 and earlier. No HelloRequest messages get sent, and renegotiation requests that come in via ClientHello get ignored. 🛑
  • SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION — when you're doing renegotiation as a server, always start a fresh new session (so basically session resumption requests are only accepted in that very first handshake). Clients don't need this one, it's giving server-side only energy. 💁
  • SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1, SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2 — these turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol versions with TLS, or the DTLSv1, DTLSv1.2 versions with DTLS, respectively. BUT as of OpenSSL 1.1.0 these options are deprecated — use SSL_CTX_set_min_proto_version(3) and SSL_CTX_set_max_proto_version(3) instead. Periodt. 💀
  • SSL_OP_NO_TICKET — okay so SSL/TLS has two mechanisms for resuming sessions: session ids and stateless session tickets. Let me break it down for you 🫶

With session ids, a copy of the session information is cached on the server and a unique id gets sent to the client. When the client wants to resume, it hands over that unique id so the server can go dig the session info out of its cache.

With stateless session tickets, the server uses a session ticket encryption key to encrypt the session information. That encrypted data gets sent to the client as a "ticket". When the client wants to resume, it sends the encrypted data right back to the server. The server uses its key to decrypt the data and resume the session. This way the server can operate statelessly — no session information needs to be cached locally. Iconic. ✨

The TLSv1.3 protocol only supports tickets and does not directly support session ids. However, OpenSSL allows two modes of ticket operation in TLSv1.3: stateful and stateless. Stateless tickets work the same way as in TLSv1.2 and below. Stateful tickets mimic the session id behaviour available in TLSv1.2 and below. The session information is cached on the server and the session id is wrapped up in a ticket and sent back to the client. When the client wishes to resume, it presents a ticket in the same way as for stateless tickets. The server can then extract the session id from the ticket and retrieve the session information from its cache.

By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will cause stateless tickets to not be issued. In TLSv1.2 and below this means no ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be sent. This is a server-side option only.

In TLSv1.3 it's possible to suppress all tickets (stateful and stateless) from being sent by calling SSL_CTX_set_num_tickets(3) or SSL_set_num_tickets(3). 🌈

  • SSL_OP_PRIORITIZE_CHACHA — when SSL_OP_CIPHER_SERVER_PREFERENCE is set, this temporarily reprioritizes ChaCha20-Poly1305 ciphers to the top of the server cipher list if a ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps those clients (like, mobile ones 📱) use ChaCha20-Poly1305 if that cipher is anywhere in the server cipher list — but it still lets other clients use AES and other ciphers. Requires SSL_OP_CIPHER_SERVER_PREFERENCE, no cap.
  • SSL_OP_TLS_ROLLBACK_BUG — disables version rollback attack detection. 😱

During the client key exchange, the client must send the same information about acceptable SSL/TLS protocol levels as it did during the first hello. Some clients violate this rule by adapting to the server's answer. (Example: the client sends an SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server only understands up to SSLv3. In this case the client must still use the same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect to the server's answer and violate the version rollback protection.) 🙃

The following options no longer have any effect but their identifiers are retained for compatibility purposes:

  • SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
  • SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
  • SSL_OP_SSLEAY_080_CLIENT_DH_BUG
  • SSL_OP_TLS_D5_BUG
  • SSL_OP_TLS_BLOCK_PADDING_BUG
  • SSL_OP_MSIE_SSLV2_RSA_PADDING
  • SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
  • SSL_OP_MICROSOFT_SESS_ID_BUG
  • SSL_OP_NETSCAPE_CHALLENGE_BUG
  • SSL_OP_PKCS1_CHECK_1
  • SSL_OP_PKCS1_CHECK_2
  • SSL_OP_SINGLE_DH_USE
  • SSL_OP_SINGLE_ECDH_USE
  • SSL_OP_EPHEMERAL_RSA
  • SSL_OP_NETSCAPE_CA_DN_BUG
  • SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG

🔐 SECURE RENEGOTIATION 🔐

OpenSSL always attempts to use secure renegotiation as described in RFC5746. This counters the prefix attack described in CVE-2009-3555 and elsewhere. 🛡️

This attack has far reaching consequences which application writers should be aware of, bestie. In the description below, an implementation supporting secure renegotiation is referred to as patched. A server not supporting secure renegotiation is referred to as unpatched.

The following sections describe the operations permitted by OpenSSL's secure renegotiation implementation. 👇

Patched client and server

Connections and renegotiation are always permitted by OpenSSL implementations. Slay, no notes. ✨

Unpatched client and patched OpenSSL server

The initial connection succeeds, but client renegotiation is denied by the server with a no_renegotiation warning alert if TLS v1.0 is used, or a fatal handshake_failure alert in SSL v3.0.

If the patched OpenSSL server attempts to renegotiate, a fatal handshake_failure alert is sent. This is because the server code may be unaware of the unpatched nature of the client.

If the option SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set, then renegotiation always succeeds. 🙈

Patched OpenSSL client and unpatched server

If the option SSL_OP_LEGACY_SERVER_CONNECT or SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION is set, then initial connections and renegotiation between patched OpenSSL clients and unpatched servers succeeds. If neither option is set, then initial connections to unpatched servers will fail. 💀

Setting the option SSL_OP_LEGACY_SERVER_CONNECT has security implications — clients that are willing to connect to servers that do not implement RFC 5746 secure renegotiation are subject to attacks such as CVE-2009-3555. This is the part where you pay attention. ⚠️

OpenSSL client applications wishing to ensure they can connect to unpatched servers should always set SSL_OP_LEGACY_SERVER_CONNECT.

OpenSSL client applications that want to ensure they cannot connect to unpatched servers (and thus avoid any security issues) should always clear SSL_OP_LEGACY_SERVER_CONNECT using SSL_CTX_clear_options() or SSL_clear_options().

The difference between the SSL_OP_LEGACY_SERVER_CONNECT and SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION options is that SSL_OP_LEGACY_SERVER_CONNECT enables initial connections and secure renegotiation between OpenSSL clients and unpatched servers only, while SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION allows initial connections and renegotiation between OpenSSL and unpatched clients or servers. Iykyk. 💅

Applicability of options to QUIC connections and streams

These options apply to SSL objects referencing a QUIC connection:

  • SSL_OP_ALLOW_NO_DHE_KEX
  • SSL_OP_NO_TX_CERTIFICATE_COMPRESSION
  • SSL_OP_NO_RX_CERTIFICATE_COMPRESSION
  • SSL_OP_NO_TICKET
  • SSL_OP_PRIORITIZE_CHACHA

These options apply to SSL objects referencing a QUIC stream:

  • SSL_OP_CLEANSE_PLAINTEXT

Options on QUIC connections are initialized from the options set on SSL_CTX before a QUIC connection SSL object is created. Options on QUIC streams are initialised from the options configured on the QUIC connection SSL object they are created from.

Setting options which relate to QUIC streams on a QUIC connection SSL object has no direct effect on the QUIC connection SSL object itself, but will change the options set on the default stream (if there is one) and will also determine the default options set on any future streams which are created. 🌀

Other options not mentioned above do not have an effect and will be ignored.

Options which relate to QUIC streams may also be set directly on QUIC stream SSL objects. Setting connection-related options on such an object has no effect. 🫠

💖 RETURN VALUES 💖

SSL_CTX_set_options() and SSL_set_options() return the new options bit-mask after adding options.

SSL_CTX_clear_options() and SSL_clear_options() return the new options bit-mask after clearing options.

SSL_CTX_get_options() and SSL_get_options() return the current bit-mask.

SSL_get_secure_renegotiation_support() returns 1 if the peer supports secure renegotiation and 0 if it does not. 👑

👀 SEE ALSO 👀

ssl(7), SSL_new(3), SSL_clear(3), SSL_shutdown(3) SSL_CTX_set_tmp_dh_callback(3), SSL_CTX_set_min_proto_version(3), openssl-dhparam(1)

📜 HISTORY 📜

The attempt to always try to use secure renegotiation was added in OpenSSL 0.9.8m.

The SSL_OP_PRIORITIZE_CHACHA and SSL_OP_NO_RENEGOTIATION options were added in OpenSSL 1.1.1.

The SSL_OP_NO_EXTENDED_MASTER_SECRET and SSL_OP_IGNORE_UNEXPECTED_EOF options were added in OpenSSL 3.0.

The SSL_OP_ constants and the corresponding parameter and return values of the affected functions were changed to "uint64_t" type in OpenSSL 3.0. For that reason it is no longer possible use the SSL_OP_ macro values in preprocessor "#if" conditions. However it is still possible to test whether these macros are defined or not. 🧚

Okay bestie, legal bit incoming, but it's short and honestly kinda iconic 💖

Copyright 2001-2025 The OpenSSL Project Authors. All Rights Reserved.

Slay. Twenty-four years of SSL_CTX_set_options() lore and counting, fr fr 👑

Now the license part, and listen, this one is lowkey important so don't skip it:

  • 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 of the License in the file LICENSE in the source distribution, or at:
https://www.openssl.org/source/license.html

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

(That last bit is me paraphrasing the standard Apache 2.0 clause wording as it appears in the file, bestie — the actual authoritative text lives in LICENSE, so go read that one if you need the real-deal legal language 💀)

And that's a wrap on SSL_CTX_SET_OPTIONS(3ossl) 🫶 You made it to the end of a three-part saga, which is honestly main character energy. Go set some options. Be safe. Read the caveats. 🌸🦋

Version 3.5.9, dated 2026-09-29 — that's the footer, and yes I'm signing off with it because it deserves a moment 💜

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