SSL_CTX_SET_OPTIONS(3) OpenSSL SSL_CTX_SET_OPTIONS(3)
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>
long SSL_CTX_set_options(SSL_CTX *ctx, long options);
long SSL_set_options(SSL *ssl, long options);
long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
long SSL_clear_options(SSL *ssl, long options);
long SSL_CTX_get_options(SSL_CTX *ctx);
long SSL_get_options(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. 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_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_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_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 plus SSL_OP_LEGACY_SERVER_CONNECT
as mentioned below.
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_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.)
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_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_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_COMPRESSION
Do not use compression even if it is supported.
SSL_OP_NO_QUERY_MTU
Do not query the MTU. Only affects DTLS connections.
SSL_OP_COOKIE_EXCHANGE
Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only
affects DTLS connections.
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_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_LEGACY_SERVER_CONNECT
Allow legacy insecure renegotiation between OpenSSL and unpatched
servers only: this option is currently set by default. See the
SECURE RENEGOTIATION section for more details.
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_RENEGOTIATION
Disable all renegotiation in TLSv1.2 and earlier. Do not send
HelloRequest messages, and ignore renegotiation requests via
ClientHello.
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_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_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_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
opton only. It is ignored by clients.
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
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.
The option SSL_OP_LEGACY_SERVER_CONNECT is currently set by default
even though it has security implications: otherwise it would be
impossible to connect to unpatched servers (i.e. all of them initially)
and this is clearly not acceptable. Renegotiation is permitted because
this does not add any additional security issues: during an attack
clients do not see any renegotiations anyway.
As more servers become patched the option SSL_OP_LEGACY_SERVER_CONNECT
will not be set by default in a future version of OpenSSL.
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.
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_CTX_set_tmp_dh_callback(3),
SSL_CTX_set_min_proto_version(3), 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.
COPYRIGHT
Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the OpenSSL license (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>;.
1.1.1k 2021-03-25 SSL_CTX_SET_OPTIONS(3)
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