431 lines
18 KiB
Groff
431 lines
18 KiB
Groff
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.\" ========================================================================
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.\"
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.IX Title "SSL_CTX_set_options 3"
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.TH SSL_CTX_set_options 3 "2013-02-11" "1.0.1e" "OpenSSL"
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.\" For nroff, turn off justification. Always turn off hyphenation; it makes
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.\" way too many mistakes in technical documents.
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.if n .ad l
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.nh
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.SH "NAME"
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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
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.SH "SYNOPSIS"
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.IX Header "SYNOPSIS"
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.Vb 1
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\& #include <openssl/ssl.h>
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\&
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\& long SSL_CTX_set_options(SSL_CTX *ctx, long options);
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\& long SSL_set_options(SSL *ssl, long options);
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\&
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\& long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
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\& long SSL_clear_options(SSL *ssl, long options);
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\&
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\& long SSL_CTX_get_options(SSL_CTX *ctx);
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\& long SSL_get_options(SSL *ssl);
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\&
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\& long SSL_get_secure_renegotiation_support(SSL *ssl);
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.Ve
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.SH "DESCRIPTION"
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.IX Header "DESCRIPTION"
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Note: all these functions are implemented using macros.
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.PP
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\&\fISSL_CTX_set_options()\fR adds the options set via bitmask in \fBoptions\fR to \fBctx\fR.
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Options already set before are not cleared!
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.PP
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\&\fISSL_set_options()\fR adds the options set via bitmask in \fBoptions\fR to \fBssl\fR.
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Options already set before are not cleared!
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.PP
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\&\fISSL_CTX_clear_options()\fR clears the options set via bitmask in \fBoptions\fR
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to \fBctx\fR.
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.PP
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\&\fISSL_clear_options()\fR clears the options set via bitmask in \fBoptions\fR to \fBssl\fR.
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.PP
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\&\fISSL_CTX_get_options()\fR returns the options set for \fBctx\fR.
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.PP
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\&\fISSL_get_options()\fR returns the options set for \fBssl\fR.
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.PP
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\&\fISSL_get_secure_renegotiation_support()\fR indicates whether the peer supports
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secure renegotiation.
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.SH "NOTES"
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.IX Header "NOTES"
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The behaviour of the \s-1SSL\s0 library can be changed by setting several options.
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The options are coded as bitmasks and can be combined by a logical \fBor\fR
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operation (|).
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.PP
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\&\fISSL_CTX_set_options()\fR and \fISSL_set_options()\fR affect the (external)
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protocol behaviour of the \s-1SSL\s0 library. The (internal) behaviour of
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the \s-1API\s0 can be changed by using the similar
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\&\fISSL_CTX_set_mode\fR\|(3) and \fISSL_set_mode()\fR functions.
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.PP
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During a handshake, the option settings of the \s-1SSL\s0 object are used. When
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a new \s-1SSL\s0 object is created from a context using \fISSL_new()\fR, the current
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option setting is copied. Changes to \fBctx\fR do not affect already created
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\&\s-1SSL\s0 objects. \fISSL_clear()\fR does not affect the settings.
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.PP
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The following \fBbug workaround\fR options are available:
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.IP "\s-1SSL_OP_MICROSOFT_SESS_ID_BUG\s0" 4
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.IX Item "SSL_OP_MICROSOFT_SESS_ID_BUG"
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www.microsoft.com \- when talking SSLv2, if session-id reuse is
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performed, the session-id passed back in the server-finished message
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is different from the one decided upon.
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.IP "\s-1SSL_OP_NETSCAPE_CHALLENGE_BUG\s0" 4
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.IX Item "SSL_OP_NETSCAPE_CHALLENGE_BUG"
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Netscape\-Commerce/1.12, when talking SSLv2, accepts a 32 byte
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challenge but then appears to only use 16 bytes when generating the
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encryption keys. Using 16 bytes is ok but it should be ok to use 32.
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According to the SSLv3 spec, one should use 32 bytes for the challenge
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when operating in SSLv2/v3 compatibility mode, but as mentioned above,
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this breaks this server so 16 bytes is the way to go.
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.IP "\s-1SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG\s0" 4
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.IX Item "SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG"
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As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.
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.IP "\s-1SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG\s0" 4
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.IX Item "SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG"
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\&...
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.IP "\s-1SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER\s0" 4
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.IX Item "SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER"
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\&...
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.IP "\s-1SSL_OP_MSIE_SSLV2_RSA_PADDING\s0" 4
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.IX Item "SSL_OP_MSIE_SSLV2_RSA_PADDING"
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As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.
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.IP "\s-1SSL_OP_SSLEAY_080_CLIENT_DH_BUG\s0" 4
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.IX Item "SSL_OP_SSLEAY_080_CLIENT_DH_BUG"
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\&...
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.IP "\s-1SSL_OP_TLS_D5_BUG\s0" 4
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.IX Item "SSL_OP_TLS_D5_BUG"
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\&...
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.IP "\s-1SSL_OP_TLS_BLOCK_PADDING_BUG\s0" 4
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.IX Item "SSL_OP_TLS_BLOCK_PADDING_BUG"
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\&...
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.IP "\s-1SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS\s0" 4
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.IX Item "SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS"
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Disables a countermeasure against a \s-1SSL\s0 3.0/TLS 1.0 protocol
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vulnerability affecting \s-1CBC\s0 ciphers, which cannot be handled by some
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broken \s-1SSL\s0 implementations. This option has no effect for connections
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using other ciphers.
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.IP "\s-1SSL_OP_ALL\s0" 4
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.IX Item "SSL_OP_ALL"
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All of the above bug workarounds.
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.PP
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It is usually safe to use \fB\s-1SSL_OP_ALL\s0\fR to enable the bug workaround
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options if compatibility with somewhat broken implementations is
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desired.
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.PP
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The following \fBmodifying\fR options are available:
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.IP "\s-1SSL_OP_TLS_ROLLBACK_BUG\s0" 4
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.IX Item "SSL_OP_TLS_ROLLBACK_BUG"
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Disable version rollback attack detection.
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.Sp
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During the client key exchange, the client must send the same information
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about acceptable \s-1SSL/TLS\s0 protocol levels as during the first hello. Some
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clients violate this rule by adapting to the server's answer. (Example:
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the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
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only understands up to SSLv3. In this case the client must still use the
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same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
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to the server's answer and violate the version rollback protection.)
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.IP "\s-1SSL_OP_SINGLE_DH_USE\s0" 4
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.IX Item "SSL_OP_SINGLE_DH_USE"
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Always create a new key when using temporary/ephemeral \s-1DH\s0 parameters
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(see \fISSL_CTX_set_tmp_dh_callback\fR\|(3)).
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This option must be used to prevent small subgroup attacks, when
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the \s-1DH\s0 parameters were not generated using \*(L"strong\*(R" primes
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(e.g. when using DSA-parameters, see \fIdhparam\fR\|(1)).
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If \*(L"strong\*(R" primes were used, it is not strictly necessary to generate
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a new \s-1DH\s0 key during each handshake but it is also recommended.
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\&\fB\s-1SSL_OP_SINGLE_DH_USE\s0\fR should therefore be enabled whenever
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temporary/ephemeral \s-1DH\s0 parameters are used.
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.IP "\s-1SSL_OP_EPHEMERAL_RSA\s0" 4
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.IX Item "SSL_OP_EPHEMERAL_RSA"
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Always use ephemeral (temporary) \s-1RSA\s0 key when doing \s-1RSA\s0 operations
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(see \fISSL_CTX_set_tmp_rsa_callback\fR\|(3)).
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According to the specifications this is only done, when a \s-1RSA\s0 key
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can only be used for signature operations (namely under export ciphers
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with restricted \s-1RSA\s0 keylength). By setting this option, ephemeral
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\&\s-1RSA\s0 keys are always used. This option breaks compatibility with the
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\&\s-1SSL/TLS\s0 specifications and may lead to interoperability problems with
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clients and should therefore never be used. Ciphers with \s-1EDH\s0 (ephemeral
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Diffie-Hellman) key exchange should be used instead.
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.IP "\s-1SSL_OP_CIPHER_SERVER_PREFERENCE\s0" 4
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.IX Item "SSL_OP_CIPHER_SERVER_PREFERENCE"
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When choosing a cipher, use the server's preferences instead of the client
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preferences. When not set, the \s-1SSL\s0 server will always follow the clients
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preferences. When set, the SSLv3/TLSv1 server will choose following its
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own preferences. Because of the different protocol, for SSLv2 the server
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will send its list of preferences to the client and the client chooses.
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.IP "\s-1SSL_OP_PKCS1_CHECK_1\s0" 4
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.IX Item "SSL_OP_PKCS1_CHECK_1"
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\&...
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.IP "\s-1SSL_OP_PKCS1_CHECK_2\s0" 4
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.IX Item "SSL_OP_PKCS1_CHECK_2"
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\&...
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.IP "\s-1SSL_OP_NETSCAPE_CA_DN_BUG\s0" 4
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.IX Item "SSL_OP_NETSCAPE_CA_DN_BUG"
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If we accept a netscape connection, demand a client cert, have a
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non-self-signed \s-1CA\s0 which does not have its \s-1CA\s0 in netscape, and the
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browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta
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.IP "\s-1SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG\s0" 4
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.IX Item "SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG"
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\&...
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.IP "SSL_OP_NO_SSLv2" 4
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.IX Item "SSL_OP_NO_SSLv2"
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Do not use the SSLv2 protocol.
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.IP "SSL_OP_NO_SSLv3" 4
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.IX Item "SSL_OP_NO_SSLv3"
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Do not use the SSLv3 protocol.
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.IP "SSL_OP_NO_TLSv1" 4
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.IX Item "SSL_OP_NO_TLSv1"
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Do not use the TLSv1 protocol.
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.IP "\s-1SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION\s0" 4
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.IX Item "SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION"
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When performing renegotiation as a server, always start a new session
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(i.e., session resumption requests are only accepted in the initial
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handshake). This option is not needed for clients.
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.IP "\s-1SSL_OP_NO_TICKET\s0" 4
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.IX Item "SSL_OP_NO_TICKET"
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Normally clients and servers will, where possible, transparently make use
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of RFC4507bis tickets for stateless session resumption.
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.Sp
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If this option is set this functionality is disabled and tickets will
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not be used by clients or servers.
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.IP "\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0" 4
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.IX Item "SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION"
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Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
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servers. See the \fB\s-1SECURE\s0 \s-1RENEGOTIATION\s0\fR section for more details.
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.IP "\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0" 4
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.IX Item "SSL_OP_LEGACY_SERVER_CONNECT"
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Allow legacy insecure renegotiation between OpenSSL and unpatched servers
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\&\fBonly\fR: this option is currently set by default. See the
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\&\fB\s-1SECURE\s0 \s-1RENEGOTIATION\s0\fR section for more details.
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.SH "SECURE RENEGOTIATION"
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.IX Header "SECURE RENEGOTIATION"
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OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
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described in \s-1RFC5746\s0. This counters the prefix attack described in
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\&\s-1CVE\-2009\-3555\s0 and elsewhere.
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.PP
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The deprecated and highly broken SSLv2 protocol does not support
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renegotiation at all: its use is \fBstrongly\fR discouraged.
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.PP
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This attack has far reaching consequences which application writers should be
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aware of. In the description below an implementation supporting secure
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renegotiation is referred to as \fIpatched\fR. A server not supporting secure
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renegotiation is referred to as \fIunpatched\fR.
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.PP
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The following sections describe the operations permitted by OpenSSL's secure
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renegotiation implementation.
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.SS "Patched client and server"
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.IX Subsection "Patched client and server"
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Connections and renegotiation are always permitted by OpenSSL implementations.
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.SS "Unpatched client and patched OpenSSL server"
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.IX Subsection "Unpatched client and patched OpenSSL server"
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The initial connection suceeds but client renegotiation is denied by the
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server with a \fBno_renegotiation\fR warning alert if \s-1TLS\s0 v1.0 is used or a fatal
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\&\fBhandshake_failure\fR alert in \s-1SSL\s0 v3.0.
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.PP
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If the patched OpenSSL server attempts to renegotiate a fatal
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\&\fBhandshake_failure\fR alert is sent. This is because the server code may be
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unaware of the unpatched nature of the client.
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.PP
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If the option \fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR is set then
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renegotiation \fBalways\fR succeeds.
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.PP
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\&\fB\s-1NB:\s0\fR a bug in OpenSSL clients earlier than 0.9.8m (all of which are
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unpatched) will result in the connection hanging if it receives a
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\&\fBno_renegotiation\fR alert. OpenSSL versions 0.9.8m and later will regard
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a \fBno_renegotiation\fR alert as fatal and respond with a fatal
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\&\fBhandshake_failure\fR alert. This is because the OpenSSL \s-1API\s0 currently has
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no provision to indicate to an application that a renegotiation attempt
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was refused.
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.SS "Patched OpenSSL client and unpatched server."
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.IX Subsection "Patched OpenSSL client and unpatched server."
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If the option \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR or
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\&\fB\s-1SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION\s0\fR is set then initial connections
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and renegotiation between patched OpenSSL clients and unpatched servers
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|
succeeds. If neither option is set then initial connections to unpatched
|
|
servers will fail.
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.PP
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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.
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.PP
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As more servers become patched the option \fB\s-1SSL_OP_LEGACY_SERVER_CONNECT\s0\fR will
|
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\&\fBnot\fR be set by default in a future version of OpenSSL.
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.PP
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|
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.
|