.\" $OpenBSD: SSL_CTX_set_options.3,v 1.15 2021/06/12 11:02:20 tb Exp $ .\" full merge up to: OpenSSL 7946ab33 Dec 6 17:56:41 2015 +0100 .\" selective merge up to: OpenSSL edb79c3a Mar 29 10:07:14 2017 +1000 .\" .\" This file was written by Lutz Jaenicke , .\" Bodo Moeller , and .\" Dr. Stephen Henson . .\" Copyright (c) 2001-2003, 2005, 2007, 2009, 2010, 2013-2015 .\" The OpenSSL Project. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in .\" the documentation and/or other materials provided with the .\" distribution. .\" .\" 3. All advertising materials mentioning features or use of this .\" software must display the following acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" .\" .\" 4. The names "OpenSSL Toolkit" and "OpenSSL Project" must not be used to .\" endorse or promote products derived from this software without .\" prior written permission. For written permission, please contact .\" openssl-core@openssl.org. .\" .\" 5. Products derived from this software may not be called "OpenSSL" .\" nor may "OpenSSL" appear in their names without prior written .\" permission of the OpenSSL Project. .\" .\" 6. Redistributions of any form whatsoever must retain the following .\" acknowledgment: .\" "This product includes software developed by the OpenSSL Project .\" for use in the OpenSSL Toolkit (http://www.openssl.org/)" .\" .\" THIS SOFTWARE IS PROVIDED BY THE OpenSSL PROJECT ``AS IS'' AND ANY .\" EXPRESSED OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: June 12 2021 $ .Dt SSL_CTX_SET_OPTIONS 3 .Os .Sh NAME .Nm SSL_CTX_set_options , .Nm SSL_set_options , .Nm SSL_CTX_clear_options , .Nm SSL_clear_options , .Nm SSL_CTX_get_options , .Nm SSL_get_options , .Nm SSL_get_secure_renegotiation_support .Nd manipulate SSL options .Sh SYNOPSIS .In openssl/ssl.h .Ft long .Fn SSL_CTX_set_options "SSL_CTX *ctx" "long options" .Ft long .Fn SSL_set_options "SSL *ssl" "long options" .Ft long .Fn SSL_CTX_clear_options "SSL_CTX *ctx" "long options" .Ft long .Fn SSL_clear_options "SSL *ssl" "long options" .Ft long .Fn SSL_CTX_get_options "SSL_CTX *ctx" .Ft long .Fn SSL_get_options "SSL *ssl" .Ft long .Fn SSL_get_secure_renegotiation_support "SSL *ssl" .Sh DESCRIPTION .Fn SSL_CTX_set_options adds the options set via bitmask in .Fa options to .Fa ctx . Options already set before are not cleared! .Pp .Fn SSL_set_options adds the options set via bitmask in .Fa options to .Fa ssl . Options already set before are not cleared! .Pp .Fn SSL_CTX_clear_options clears the options set via bitmask in .Fa options to .Fa ctx . .Pp .Fn SSL_clear_options clears the options set via bitmask in .Fa options to .Fa ssl . .Pp .Fn SSL_CTX_get_options returns the options set for .Fa ctx . .Pp .Fn SSL_get_options returns the options set for .Fa ssl . .Pp .Fn SSL_get_secure_renegotiation_support indicates whether the peer supports secure renegotiation. .Pp All these functions are implemented using macros. .Pp The behaviour of the SSL library can be changed by setting several options. The options are coded as bitmasks and can be combined by a bitwise OR operation (|). .Pp .Fn SSL_CTX_set_options and .Fn 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 .Xr SSL_CTX_set_mode 3 and .Xr SSL_set_mode 3 functions. .Pp During a handshake, the option settings of the SSL object are used. When a new SSL object is created from a context using .Xr SSL_new 3 , the current option setting is copied. Changes to .Fa ctx do not affect already created .Vt SSL objects. .Fn SSL_clear does not affect the settings. .Pp The following .Em bug workaround options are available: .Bl -tag -width Ds .It Dv SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS Disables a countermeasure against a 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. .It Dv SSL_OP_ALL This is currently an alias for .Dv SSL_OP_LEGACY_SERVER_CONNECT . .El .Pp It is usually safe to use .Dv SSL_OP_ALL to enable the bug workaround options if compatibility with somewhat broken implementations is desired. .Pp The following .Em modifying options are available: .Bl -tag -width Ds .It Dv SSL_OP_CIPHER_SERVER_PREFERENCE When choosing a cipher, use the server's preferences instead of the client preferences. When not set, the server will always follow the client's preferences. When set, the server will choose following its own preferences. .It Dv SSL_OP_COOKIE_EXCHANGE Turn on Cookie Exchange as described in RFC 4347 Section 4.2.1. Only affects DTLS connections. .It Dv SSL_OP_LEGACY_SERVER_CONNECT Allow legacy insecure renegotiation between OpenSSL and unpatched servers .Em only : this option is currently set by default. See the .Sx SECURE RENEGOTIATION section for more details. .It Dv SSL_OP_NO_DTLSv1 Do not use the DTLSv1 protocol. Deprecated; use .Xr SSL_CTX_set_min_proto_version 3 instead. .It Dv SSL_OP_NO_DTLSv1_2 Do not use the DTLSv1.2 protocol. Deprecated; use .Xr SSL_CTX_set_min_proto_version 3 instead. .It Dv SSL_OP_NO_QUERY_MTU Do not query the MTU. Only affects DTLS connections. .It Dv 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. .It Dv SSL_OP_NO_TICKET Normally clients and servers using TLSv1.2 and earlier will, where possible, transparently make use of RFC 5077 tickets for stateless session resumption. .Pp If this option is set this functionality is disabled and tickets will not be used by clients or servers. .It Dv SSL_OP_NO_TLSv1 Do not use the TLSv1.0 protocol. Deprecated; use .Xr SSL_CTX_set_min_proto_version 3 instead. .It Dv SSL_OP_NO_TLSv1_1 Do not use the TLSv1.1 protocol. Deprecated; use .Xr SSL_CTX_set_min_proto_version 3 instead. .It Dv SSL_OP_NO_TLSv1_2 Do not use the TLSv1.2 protocol. Deprecated; use .Xr SSL_CTX_set_max_proto_version 3 instead. .El .Pp The following options used to be supported at some point in the past and no longer have any effect: .Dv SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION , .Dv SSL_OP_EPHEMERAL_RSA , .Dv SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER , .Dv SSL_OP_MICROSOFT_SESS_ID_BUG , .Dv SSL_OP_NETSCAPE_CA_DN_BUG , .Dv SSL_OP_NETSCAPE_CHALLENGE_BUG , .Dv SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG , .Dv SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG , .Dv SSL_OP_NO_COMPRESSION , .Dv SSL_OP_NO_SSLv2 , .Dv SSL_OP_NO_SSLv3 , .Dv SSL_OP_PKCS1_CHECK_1 , .Dv SSL_OP_PKCS1_CHECK_2 , .Dv SSL_OP_SAFARI_ECDHE_ECDSA_BUG , .Dv SSL_OP_SINGLE_DH_USE , .Dv SSL_OP_SINGLE_ECDH_USE , .Dv SSL_OP_SSLEAY_080_CLIENT_DH_BUG , .Dv SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG , .Dv SSL_OP_TLS_BLOCK_PADDING_BUG , .Dv SSL_OP_TLS_D5_BUG , .Dv SSL_OP_TLS_ROLLBACK_BUG , .Dv SSL_OP_TLSEXT_PADDING . .Sh SECURE RENEGOTIATION OpenSSL 0.9.8m and later always attempts to use secure renegotiation as described in RFC 5746. This counters the prefix attack described in CVE-2009-3555 and elsewhere. .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 .Dq patched . A server not supporting secure renegotiation is referred to as .Dq unpatched . .Pp The following sections describe the operations permitted by OpenSSL's secure renegotiation implementation. .Ss Patched client and server Connections and renegotiation are always permitted by OpenSSL implementations. .Ss Unpatched client and patched OpenSSL server The initial connection succeeds but client renegotiation is denied by the server with a .Em no_renegotiation warning alert. .Pp If the patched OpenSSL server attempts to renegotiate a fatal .Em handshake_failure alert is sent. This is because the server code may be unaware of the unpatched nature of the client. .Pp Note that 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 .Em no_renegotiation alert. OpenSSL versions 0.9.8m and later will regard a .Em no_renegotiation alert as fatal and respond with a fatal .Em handshake_failure alert. This is because the OpenSSL API currently has no provision to indicate to an application that a renegotiation attempt was refused. .Ss Patched OpenSSL client and unpatched server If the option .Dv SSL_OP_LEGACY_SERVER_CONNECT 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 .Dv 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. .Pp As more servers become patched the option .Dv SSL_OP_LEGACY_SERVER_CONNECT will .Em not 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 .Em set .Dv SSL_OP_LEGACY_SERVER_CONNECT . .Pp OpenSSL client applications that want to ensure they can .Em not connect to unpatched servers (and thus avoid any security issues) should always .Em clear .Dv SSL_OP_LEGACY_SERVER_CONNECT using .Fn SSL_CTX_clear_options or .Fn SSL_clear_options . .Sh RETURN VALUES .Fn SSL_CTX_set_options and .Fn SSL_set_options return the new options bitmask after adding .Fa options . .Pp .Fn SSL_CTX_clear_options and .Fn SSL_clear_options return the new options bitmask after clearing .Fa options . .Pp .Fn SSL_CTX_get_options and .Fn SSL_get_options return the current bitmask. .Pp .Fn SSL_get_secure_renegotiation_support returns 1 is the peer supports secure renegotiation and 0 if it does not. .Sh SEE ALSO .Xr openssl 1 , .Xr ssl 3 , .Xr SSL_clear 3 , .Xr SSL_CTX_ctrl 3 , .Xr SSL_CTX_set_min_proto_version 3 , .Xr SSL_new 3 .Sh HISTORY .Fn SSL_CTX_set_options and .Fn SSL_set_options first appeared in SSLeay 0.9.0 and have been available since .Ox 2.4 . .Pp .Fn SSL_CTX_get_options and .Fn SSL_get_options first appeared in OpenSSL 0.9.2b and have been available since .Ox 2.6 . .Pp .Fn SSL_CTX_clear_options , .Fn SSL_clear_options , and .Fn SSL_get_secure_renegotiation_support first appeared in OpenSSL 0.9.8m and have been available since .Ox 4.9 .