SSL_CTX_SET_OPTIONS(3SSL)

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version 4.0.3 · 14 versions

💖 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 — OpenSSL 3SSL manual 🦋

🏷️ 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

Bestie, the exact C spells:

#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

Okay so these functions are your options remote controls. Here's the whole squad:

SSL_CTX_set_options() — takes the bit-mask in options and adds those options to ctx. ctx MUST NOT be NULL. Options already set before are not cleared! 🚨
SSL_set_options() — takes the bit-mask in options and adds those options to ssl. Options already set before are not cleared!
SSL_CTX_clear_options() — clears the options set via bit-mask in options to ctx.
SSL_clear_options() — clears the options set via bit-mask in options to 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. Note, this is implemented via a macro. 🪄

📝 NOTES

The SSL library's behaviour is customizable by setting options. Options are coded as bit-masks and can be combined by a bitwise or operation (|). So yes, stack them like accessories. 💅

SSL_CTX_set_options() and SSL_set_options() affect the (external) protocol behaviour of the SSL library. The (internal) behaviour of the API can be changed by using the similar SSL_CTX_set_mode(3) and SSL_set_mode() functions.

During a handshake, the option settings of the SSL object are used. When a new SSL object is created from a context using SSL_new(), the current option setting is copied. Changes to ctx do not affect already created SSL objects. SSL_clear() does not affect the settings.

The following bug workaround options are available:

SSL_OP_CRYPTOPRO_TLSEXT_BUG — Adds the server-hello extension from the early version of the cryptopro draft when a GOST ciphersuite is negotiated. Required 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 cannot be handled by some broken SSL implementations. 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. This is needed as a workaround for some implementations.
SSL_OP_ALL — All of the above bug workarounds. The whole bundle. 🎁

It is usually safe to use SSL_OP_ALL to enable the bug workaround options if compatibility with somewhat broken implementations is desired.

The following modifying options are available:

SSL_OP_ALLOW_CLIENT_RENEGOTIATION — Client-initiated renegotiation is disabled by default. Use this option to enable it. 🔁
SSL_OP_ALLOW_NO_DHE_KEX — In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means that there will be no forward secrecy for the resumed session. Be so for real about that. 😭
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. See the SECURE RENEGOTIATION section for more details.
SSL_OP_SERVER_PREFERENCE — When choosing a cipher, signature, (TLS 1.2) curve or (TLS 1.3) group, use the server's preferences instead of the client preferences. When not set, the SSL server will always follow the clients preferences. When set, the SSL/TLS server will choose following its own preferences. Use of the old but somewhat misleading name SSL_OP_CIPHER_SERVER_PREFERENCE is still possible but discouraged. 👑
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 is overwritten by the next portion of data. When enabling 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. In order 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. See the SECURE RENEGOTIATION section for more details.
SSL_OP_LEGACY_EC_POINT_FORMATS — Enable use of the legacy point formats for elliptic curves. This option enables support for the deprecated ANSI X9.62 prime and char2 compressed point formats, in addition to the uncompressed format that is enabled by default.
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.

okay okay picking up right where we left off — the very next thing is that cert-compression option where:

If this option is set, the certificate compression extension will not be sent and compressed certificates will not be accepted from the peer. Like, it just ghosts the whole thing. 💅

SSL_OP_NO_COMPRESSION — do not use TLS record compression even if it's supported. This one is set by default, bestie. To switch it off use SSL_clear_options().

⚠️ Heads up: TLS record compression is not recommended and it's not available at security level 2 or above. From OpenSSL 3.2 the default security level is 2, so clearing this option will have no effect unless you also change the default security level. See SSL_CTX_set_security_level(3).

SSL_OP_NO_ENCRYPT_THEN_MAC — normally clients and servers will transparently attempt to negotiate the RFC7366 Encrypt-then-MAC option on TLS and DTLS connections. If this option is set, Encrypt-then-MAC is disabled. Clients will not propose it, and servers will not accept the extension. Periodt.
SSL_OP_NO_EXTENDED_MASTER_SECRET — normally clients and servers will transparently attempt to negotiate the RFC7627 Extended Master Secret option on TLS and DTLS connections. If this option is set, Extended Master Secret is disabled. Clients will not propose it, and servers will not accept the extension. 🚫
SSL_OP_NO_QUERY_MTU — do not query the MTU. Only affects DTLS connections, so if you're not doing DTLS this is giving nothing.
SSL_OP_NO_RENEGOTIATION — disable all renegotiation in (D)TLSv1.2 and earlier. Don't send HelloRequest messages, and ignore renegotiation requests via ClientHello. The whole concept? Retired. 💀
SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION — when performing renegotiation as a server, always start a new session (i.e., session resumption requests are only accepted in the initial handshake). This option is not needed for clients, so clients can just scroll past. 🙈
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 options 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.

🚩 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 — that's the main character way now.

SSL_OP_NO_TICKET — okay this one needs a whole TED talk because it's about session resumption, and SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless session tickets. 🌸 The way it works with session ids: a copy of the session information is cached on the server, and a unique id is sent to the client. When the client wishes to resume, it provides that unique id so the server can retrieve the session information from its cache. The way it works 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 wishes to resume, it sends the encrypted data 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, honestly. ✨ Now, the TLSv1.3 protocol only supports tickets and does not directly support session ids. But 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. Also, in TLSv1.3 it is 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). Not me knowing both of these. 😌
SSL_OP_PRIORITIZE_CHACHA — when SSL_OP_SERVER_PREFERENCE is set, temporarily reprioritize 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 (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere in the server cipher list; but still allows other clients to use AES and other ciphers. Requires SSL_OP_SERVER_PREFERENCE. No solo missions here.
SSL_OP_TLS_ROLLBACK_BUG — disable version rollback attack detection. 😬 During the client key exchange, the client must send the same information about acceptable SSL/TLS protocol levels as 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.) So basically you're turning off a safety check for misbehaving peers and it's giving unserious. Don't unless you have to.
SSL_OP_ECH_GREASE — if set, TLS ClientHello messages emitted by the client will include GREASE Encrypted ClientHello (ECH) extension values, if ECH is not really being attempted.
SSL_OP_ECH_TRIALDECRYPT — if set, servers will attempt to decrypt ECH extensions using all loaded ECH key pairs. By default, servers will only attempt decryption using an ECH key pair that matches the config_id in the ECH extension value received.

💀 Careful bestie: note that a server that has loaded many ECH configurations and that enables ECH trial decryption will attempt decryption with every ECH key when presented with a GREASEd ECH (see SSL_OP_ECH_GREASE) and with possibly that many even when presented with a real ECH. That could easily become an accidental denial of service.

Note also that the ECH specification recommends that servers that enable this option consider implementing some form of rate limiting mechanism to limit the potential damage caused in such scenarios. If trial decryption is enabled then decryption will be attempted with the ECH configurations in the order they were loaded. So, where it is possible to load the configuration most likely to be used first, that would improve efficiency. Efficiency slay. 🦋

SSL_OP_ECH_GREASE_RETRY_CONFIG — if set, servers will add GREASEy ECHConfig values to those sent to the client after the client GREASEd or the client tried and failed to use ECH.
SSL_OP_ECH_IGNORED_CID — if set, TLS ClientHello messages emitted by the client will ignore the ECHConfig config_id chosen by the server and use a random octet. Chaos, but make it protocol-compliant. 🎀

The following options no longer have any effect but their identifiers are retained for compatibility purposes: (they're basically retired legends, still in the group chat, contributing nothing 💅)

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 💖

okay bestie so OpenSSL is always attempting to use secure renegotiation, exactly as described in RFC5746. ✨ This is what counters the prefix attack from CVE-2009-3555 and other messy places.

This attack has far reaching consequences and application writers should lowkey be aware of it. In the explanation below, an implementation supporting secure renegotiation is called patched. A server not supporting secure renegotiation is called unpatched. no cap.

The next bits 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. so like, be aware bestie.

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 can not 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 👑

🔥 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.
As of OpenSSL 3.6, use of the SSL_OP_CIPHER_SERVER_PREFERENCE constant is discouraged and its semantics became available using the more aptly named SSL_OP_SERVER_PREFERENCE constant.

🫶 COPYRIGHT 🫶

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 thing bestie 💅 this page is from OpenSSL 4.0.3, dated 2026-09-29, in the SSL_CTX_SET_OPTIONS(3SSL) manual. go forth and configure responsibly ✨💖

📦 where this page comes from

ubuntu stonking: libssl-doc 4.0.3-1ubuntu1 · Apache-2.0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-11-03: a language model's rewrite of the original, under its terms