SSL_CTX_SET_OPTIONS(3SSL) OpenSSL SSL_CTX_SET_OPTIONS(3SSL)
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 set via bit-mask in options to
ctx. ctx MUST NOT be NULL. Options already set before are not cleared!
SSL_set_options() adds the options set via bit-mask in 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() indicates whether the peer
supports secure renegotiation. Note, this is implemented via a macro.
NOTES
The behaviour of the SSL library can be changed by setting several
options. The options are coded as bit-masks and can be combined by a
bitwise or operation (|).
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
Add server-hello extension from the early version of cryptopro draft
when 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.
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.
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_CIPHER_SERVER_PREFERENCE
When choosing a cipher, 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.
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_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
Do not use TLS record compression even if it is supported. This
option is set by default. To switch it off use SSL_clear_options().
Note that TLS record compression is not recommended and is 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
without also changing 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 connection.
If this option is set, Encrypt-then-MAC is disabled. Clients will
not propose, and servers will not accept the extension.
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
connection.
If this option is set, Extended Master Secret is disabled. Clients
will not propose, and servers will not accept the extension.
SSL_OP_NO_QUERY_MTU
Do not query the MTU. Only affects DTLS connections.
SSL_OP_NO_RENEGOTIATION
Disable all renegotiation in (D)TLSv1.2 and earlier. Do not send
HelloRequest messages, and ignore renegotiation requests via
ClientHello.
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.
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.
SSL_OP_NO_TICKET
SSL/TLS supports two mechanisms for resuming sessions: session ids
and stateless session tickets.
When using 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 the unique id so that the server can
retrieve the session information from its cache.
When using stateless session tickets the server uses a session
ticket encryption key to encrypt the session information. This
encrypted data is 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. In
this way the server can operate statelessly - no session information
needs to be cached locally.
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 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).
SSL_OP_PRIORITIZE_CHACHA
When SSL_OP_CIPHER_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_CIPHER_SERVER_PREFERENCE.
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.)
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. 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.
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.
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.
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 is 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.
COPYRIGHT
Copyright 2001-2025 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_OPTIONS(3SSL)
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