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.\" ========================================================================
.\"
.IX Title "SSL_CTX_set_options 3"
.TH SSL_CTX_set_options 3 "2019-12-20" "1.0.2u" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "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
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #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);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Note: all these functions are implemented using macros.
.PP
\&\fISSL_CTX_set_options()\fR adds the options set via bitmask in \fBoptions\fR to \fBctx\fR.
Options already set before are not cleared!
.PP
\&\fISSL_set_options()\fR adds the options set via bitmask in \fBoptions\fR to \fBssl\fR.
Options already set before are not cleared!
.PP
\&\fISSL_CTX_clear_options()\fR clears the options set via bitmask in \fBoptions\fR
to \fBctx\fR.
.PP
\&\fISSL_clear_options()\fR clears the options set via bitmask in \fBoptions\fR to \fBssl\fR.
.PP
\&\fISSL_CTX_get_options()\fR returns the options set for \fBctx\fR.
.PP
\&\fISSL_get_options()\fR returns the options set for \fBssl\fR.
.PP
\&\fISSL_get_secure_renegotiation_support()\fR indicates whether the peer supports
secure renegotiation.
.SH "NOTES"
.IX Header "NOTES"
The behaviour of the \s-1SSL\s0 library can be changed by setting several options.
The options are coded as bitmasks and can be combined by a logical \fBor\fR
operation (|).
.PP
\&\fISSL_CTX_set_options()\fR and \fISSL_set_options()\fR affect the (external)
protocol behaviour of the \s-1SSL\s0 library. The (internal) behaviour of
the \s-1API\s0 can be changed by using the similar
\&\fISSL_CTX_set_mode\fR\|(3) and \fISSL_set_mode()\fR functions.
.PP
During a handshake, the option settings of the \s-1SSL\s0 object are used. When
a new \s-1SSL\s0 object is created from a context using \fISSL_new()\fR, the current
option setting is copied. Changes to \fBctx\fR do not affect already created
\&\s-1SSL\s0 objects. \fISSL_clear()\fR does not affect the settings.
.PP
The following \fBbug workaround\fR options are available:
.IP "\s-1SSL_OP_MICROSOFT_SESS_ID_BUG\s0" 4
.IX Item "SSL_OP_MICROSOFT_SESS_ID_BUG"
www.microsoft.com \- when talking SSLv2, if session-id reuse is
performed, the session-id passed back in the server-finished message
is different from the one decided upon.
.IP "\s-1SSL_OP_NETSCAPE_CHALLENGE_BUG\s0" 4
.IX Item "SSL_OP_NETSCAPE_CHALLENGE_BUG"
Netscape\-Commerce/1.12, when talking SSLv2, accepts a 32 byte
challenge but then appears to only use 16 bytes when generating the
encryption keys.  Using 16 bytes is ok but it should be ok to use 32.
According to the SSLv3 spec, one should use 32 bytes for the challenge
when operating in SSLv2/v3 compatibility mode, but as mentioned above,
this breaks this server so 16 bytes is the way to go.
.IP "\s-1SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG\s0" 4
.IX Item "SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG"
As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.
.IP "\s-1SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG\s0" 4
.IX Item "SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG"
\&...
.IP "\s-1SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER\s0" 4
.IX Item "SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER"
\&...
.IP "\s-1SSL_OP_SAFARI_ECDHE_ECDSA_BUG\s0" 4
.IX Item "SSL_OP_SAFARI_ECDHE_ECDSA_BUG"
Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on \s-1OS X.
OS X 10.8..10.8.3\s0 has broken support for ECDHE-ECDSA ciphers.
.IP "\s-1SSL_OP_SSLEAY_080_CLIENT_DH_BUG\s0" 4
.IX Item "SSL_OP_SSLEAY_080_CLIENT_DH_BUG"
\&...
.IP "\s-1SSL_OP_TLS_D5_BUG\s0" 4
.IX Item "SSL_OP_TLS_D5_BUG"
\&...
.IP "\s-1SSL_OP_TLS_BLOCK_PADDING_BUG\s0" 4
.IX Item "SSL_OP_TLS_BLOCK_PADDING_BUG"
\&...
.IP "\s-1SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS\s0" 4
.IX Item "SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS"
Disables a countermeasure against a \s-1SSL 3.0/TLS 1.0\s0 protocol
vulnerability affecting \s-1CBC\s0 ciphers, which cannot be handled by some
broken \s-1SSL\s0 implementations.  This option has no effect for connections
using other ciphers.
.IP "\s-1SSL_OP_TLSEXT_PADDING\s0" 4
.IX Item "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.
.IP "\s-1SSL_OP_ALL\s0" 4
.IX Item "SSL_OP_ALL"
All of the above bug workarounds.
.PP
It is usually safe to use \fB\s-1SSL_OP_ALL\s0\fR to enable the bug workaround
options if compatibility with somewhat broken implementations is
desired.
.PP
The following \fBmodifying\fR options are available:
.IP "\s-1SSL_OP_TLS_ROLLBACK_BUG\s0" 4
.IX Item "SSL_OP_TLS_ROLLBACK_BUG"
Disable version rollback attack detection.
.Sp
During the client key exchange, the client must send the same information
about acceptable \s-1SSL/TLS\s0 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.)
.IP "\s-1SSL_OP_SINGLE_DH_USE\s0" 4
.IX Item "SSL_OP_SINGLE_DH_USE"
Always create a new key when using temporary/ephemeral \s-1DH\s0 parameters
(see \fISSL_CTX_set_tmp_dh_callback\fR\|(3)).
This option must be used to prevent small subgroup attacks, when
the \s-1DH\s0 parameters were not generated using \*(L"strong\*(R" primes
(e.g. when using DSA-parameters, see \fIdhparam\fR\|(1)).
If \*(L"strong\*(R" primes were used, it is not strictly necessary to generate
a new \s-1DH\s0 key during each handshake but it is also recommended.
\&\fB\s-1SSL_OP_SINGLE_DH_USE\s0\fR should therefore be enabled whenever
temporary/ephemeral \s-1DH\s0 parameters are used.
.IP "\s-1SSL_OP_EPHEMERAL_RSA\s0" 4
.IX Item "SSL_OP_EPHEMERAL_RSA"
This option is no longer implemented and is treated as no op.
.IP "\s-1SSL_OP_CIPHER_SERVER_PREFERENCE\s0" 4
.IX Item "SSL_OP_CIPHER_SERVER_PREFERENCE"
When choosing a cipher, use the server's preferences instead of the client
preferences. When not set, the \s-1SSL\s0 server will always follow the clients
preferences. When set, the SSLv3/TLSv1 server will choose following its
own preferences. Because of the different protocol, for SSLv2 the server
will send its list of preferences to the client and the client chooses.
.IP "\s-1SSL_OP_PKCS1_CHECK_1\s0" 4
.IX Item "SSL_OP_PKCS1_CHECK_1"
\&...
.IP "\s-1SSL_OP_PKCS1_CHECK_2\s0" 4
.IX Item "SSL_OP_PKCS1_CHECK_2"
\&...
.IP "\s-1SSL_OP_NETSCAPE_CA_DN_BUG\s0" 4
.IX Item "SSL_OP_NETSCAPE_CA_DN_BUG"
If we accept a netscape connection, demand a client cert, have a
non-self-signed \s-1CA\s0 which does not have its \s-1CA\s0 in netscape, and the
browser has a cert, it will crash/hang.  Works for 3.x and 4.xbeta
.IP "\s-1SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG\s0" 4
.IX Item "SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG"
\&...
.IP "SSL_OP_NO_SSLv2" 4
.IX Item "SSL_OP_NO_SSLv2"
Do not use the SSLv2 protocol.
As of OpenSSL 1.0.2g the \fBSSL_OP_NO_SSLv2\fR option is set by default.
.IP "SSL_OP_NO_SSLv3" 4
.IX Item "SSL_OP_NO_SSLv3"
Do not use the SSLv3 protocol.
It is recommended that applications should set this option.
.IP "SSL_OP_NO_TLSv1" 4
.IX Item "SSL_OP_NO_TLSv1"
Do not use the TLSv1 protocol.
.IP "SSL_OP_NO_TLSv1_1" 4
.IX Item "SSL_OP_NO_TLSv1_1"
Do not use the TLSv1.1 protocol.
.IP "SSL_OP_NO_TLSv1_2" 4
.IX Item "SSL_OP_NO_TLSv1_2"
Do not use the TLSv1.2 protocol.
.IP "\s-1SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION\s0" 4
.IX Item "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.
.IP "\s-1SSL_OP_NO_TICKET\s0" 4
.IX Item "SSL_OP_NO_TICKET"
Normally clients and servers will, where possible, transparently make use
of RFC4507bis tickets for stateless session resumption.
.Sp
If this option is set this functionality is disabled and tickets will
not be used by clients or servers.
.IP "\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0" 4
.IX Item "SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION"
Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
servers. See the \fB\s-1SECURE RENEGOTIATION\s0\fR section for more details.
.IP "\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0" 4
.IX Item "SSL_OP_LEGACY_SERVER_CONNECT"
Allow legacy insecure renegotiation between OpenSSL and unpatched servers
\&\fBonly\fR: this option is currently set by default. See the
\&\fB\s-1SECURE RENEGOTIATION\s0\fR section for more details.
.SH "SECURE RENEGOTIATION"
.IX Header "SECURE RENEGOTIATION"
OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
described in \s-1RFC5746.\s0 This counters the prefix attack described in
\&\s-1CVE\-2009\-3555\s0 and elsewhere.
.PP
The deprecated and highly broken SSLv2 protocol does not support
renegotiation at all: its use is \fBstrongly\fR discouraged.
.PP
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 \fIpatched\fR. A server not supporting secure
renegotiation is referred to as \fIunpatched\fR.
.PP
The following sections describe the operations permitted by OpenSSL's secure
renegotiation implementation.
.SS "Patched client and server"
.IX Subsection "Patched client and server"
Connections and renegotiation are always permitted by OpenSSL implementations.
.SS "Unpatched client and patched OpenSSL server"
.IX Subsection "Unpatched client and patched OpenSSL server"
The initial connection succeeds but client renegotiation is denied by the
server with a \fBno_renegotiation\fR warning alert if \s-1TLS\s0 v1.0 is used or a fatal
\&\fBhandshake_failure\fR alert in \s-1SSL\s0 v3.0.
.PP
If the patched OpenSSL server attempts to renegotiate a fatal
\&\fBhandshake_failure\fR alert is sent. This is because the server code may be
unaware of the unpatched nature of the client.
.PP
If the option \fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR is set then
renegotiation \fBalways\fR succeeds.
.PP
\&\fB\s-1NB:\s0\fR a bug in OpenSSL clients earlier than 0.9.8m (all of which are
unpatched) will result in the connection hanging if it receives a
\&\fBno_renegotiation\fR alert. OpenSSL versions 0.9.8m and later will regard
a \fBno_renegotiation\fR alert as fatal and respond with a fatal
\&\fBhandshake_failure\fR alert. This is because the OpenSSL \s-1API\s0 currently has
no provision to indicate to an application that a renegotiation attempt
was refused.
.SS "Patched OpenSSL client and unpatched server."
.IX Subsection "Patched OpenSSL client and unpatched server."
If the option \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR or
\&\fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR 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.
.PP
The option \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR 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.
.PP
As more servers become patched the option \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR will
\&\fBnot\fR be set by default in a future version of OpenSSL.
.PP
OpenSSL client applications wishing to ensure they can connect to unpatched
servers should always \fBset\fR \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR
.PP
OpenSSL client applications that want to ensure they can \fBnot\fR connect to
unpatched servers (and thus avoid any security issues) should always \fBclear\fR
\&\fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR using \fISSL_CTX_clear_options()\fR or
\&\fISSL_clear_options()\fR.
.PP
The difference between the \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR and
\&\fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR options is that
\&\fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR enables initial connections and secure
renegotiation between OpenSSL clients and unpatched servers \fBonly\fR, while
\&\fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR allows initial connections
and renegotiation between OpenSSL and unpatched clients or servers.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
\&\fISSL_CTX_set_options()\fR and \fISSL_set_options()\fR return the new options bitmask
after adding \fBoptions\fR.
.PP
\&\fISSL_CTX_clear_options()\fR and \fISSL_clear_options()\fR return the new options bitmask
after clearing \fBoptions\fR.
.PP
\&\fISSL_CTX_get_options()\fR and \fISSL_get_options()\fR return the current bitmask.
.PP
\&\fISSL_get_secure_renegotiation_support()\fR returns 1 is the peer supports
secure renegotiation and 0 if it does not.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fIssl\fR\|(3), \fISSL_new\fR\|(3), \fISSL_clear\fR\|(3),
\&\fISSL_CTX_set_tmp_dh_callback\fR\|(3),
\&\fISSL_CTX_set_tmp_rsa_callback\fR\|(3),
\&\fIdhparam\fR\|(1)
.SH "HISTORY"
.IX Header "HISTORY"
\&\fB\s-1SSL_OP_CIPHER_SERVER_PREFERENCE\s0\fR and
\&\fB\s-1SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION\s0\fR have been added in
OpenSSL 0.9.7.
.PP
\&\fB\s-1SSL_OP_TLS_ROLLBACK_BUG\s0\fR has been added in OpenSSL 0.9.6 and was automatically
enabled with \fB\s-1SSL_OP_ALL\s0\fR. As of 0.9.7, it is no longer included in \fB\s-1SSL_OP_ALL\s0\fR
and must be explicitly set.
.PP
\&\fB\s-1SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS\s0\fR has been added in OpenSSL 0.9.6e.
Versions up to OpenSSL 0.9.6c do not include the countermeasure that
can be disabled with this option (in OpenSSL 0.9.6d, it was always
enabled).
.PP
\&\fISSL_CTX_clear_options()\fR and \fISSL_clear_options()\fR were first added in OpenSSL
0.9.8m.
.PP
\&\fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR, \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR
and the function \fISSL_get_secure_renegotiation_support()\fR were first added in
OpenSSL 0.9.8m.