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
#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);
SSL_CTX_set_options() adds the options set via bit-mask in
options
to
ctx. 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.
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 a 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_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 connections 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).
- 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_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_COMPRESSION
- Do not use compression even if it is supported. This option
is set by default. To switch it off use SSL_clear_options().
- 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 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 a 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
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.
Connections and renegotiation are always permitted by OpenSSL implementations.
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.
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.
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.
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)
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 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>.