X-Git-Url: https://git.openssl.org/gitweb/?p=openssl.git;a=blobdiff_plain;f=doc%2Fssl%2FSSL_CTX_set_options.pod;h=1abda2898e6a2c4a6f73d215b1115562d6110ea2;hp=5eb376eda88fd51e3afbcf910138024460f9a533;hb=c2c49969e236058090c081e591272ea325ca49b6;hpb=52d160d85d2a4c57729540d409747071f6b952f7 diff --git a/doc/ssl/SSL_CTX_set_options.pod b/doc/ssl/SSL_CTX_set_options.pod index 5eb376eda8..1abda2898e 100644 --- a/doc/ssl/SSL_CTX_set_options.pod +++ b/doc/ssl/SSL_CTX_set_options.pod @@ -2,7 +2,7 @@ =head1 NAME -SSL_CTX_set_options, SSL_set_options, SSL_CTX_get_options, SSL_get_options - manipulate SSL engine options +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 =head1 SYNOPSIS @@ -11,28 +11,48 @@ SSL_CTX_set_options, SSL_set_options, SSL_CTX_get_options, SSL_get_options - man 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); + =head1 DESCRIPTION +Note: all these functions are implemented using macros. + SSL_CTX_set_options() adds the options set via bitmask in B to B. -Options already set before are not cleared. +Options already set before are not cleared! SSL_set_options() adds the options set via bitmask in B to B. -Options already set before are not cleared. +Options already set before are not cleared! + +SSL_CTX_clear_options() clears the options set via bitmask in B +to B. + +SSL_clear_options() clears the options set via bitmask in B to B. SSL_CTX_get_options() returns the options set for B. SSL_get_options() returns the options set for B. +SSL_get_secure_renegotiation_support() indicates whether the peer supports +secure renegotiation. + =head1 NOTES 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 logical B -operation (|). Options can only be added but can never be reset. +operation (|). -During a handshake, the option settings of the SSL object used. When +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 +L 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 B do not affect already created SSL objects. SSL_clear() does not affect the settings. @@ -81,7 +101,7 @@ doing a re-connect, always takes the first cipher in the cipher list. =item SSL_OP_MSIE_SSLV2_RSA_PADDING -... +As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect. =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG @@ -95,17 +115,12 @@ doing a re-connect, always takes the first cipher in the cipher list. ... -=item SSL_OP_TLS_ROLLBACK_BUG - -Disable version rollback attack detection. +=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS -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.) +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. =item SSL_OP_ALL @@ -113,20 +128,49 @@ All of the above bug workarounds. =back -It is save and recommended to use SSL_OP_ALL to enable the bug workaround -options. +It is usually safe to use B to enable the bug workaround +options if compatibility with somewhat broken implementations is +desired. The following B options are available: =over 4 +=item 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.) + =item SSL_OP_SINGLE_DH_USE -Always create a new key when using temporary DH parameters. +Always create a new key when using temporary/ephemeral DH parameters +(see L). +This option must be used to prevent small subgroup attacks, when +the DH parameters were not generated using "strong" primes +(e.g. when using DSA-parameters, see L). +If "strong" primes were used, it is not strictly necessary to generate +a new DH key during each handshake but it is also recommended. +B should therefore be enabled whenever +temporary/ephemeral DH parameters are used. =item SSL_OP_EPHEMERAL_RSA -Also use the temporary RSA key when doing RSA operations. +Always use ephemeral (temporary) RSA key when doing RSA operations +(see L). +According to the specifications this is only done, when a RSA key +can only be used for signature operations (namely under export ciphers +with restricted RSA keylength). By setting this option, ephemeral +RSA keys are always used. This option breaks compatibility with the +SSL/TLS specifications and may lead to interoperability problems with +clients and should therefore never be used. Ciphers with EDH (ephemeral +Diffie-Hellman) key exchange should be used instead. =item SSL_OP_CIPHER_SERVER_PREFERENCE @@ -134,7 +178,7 @@ 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 SSLv3/TLSv1 server will choose following its own preferences. Because of the different protocol, for SSLv2 the server -will send his list of preferences to the client and the client chooses. +will send its list of preferences to the client and the client chooses. =item SSL_OP_PKCS1_CHECK_1 @@ -147,14 +191,9 @@ will send his list of preferences to the client and the client chooses. =item SSL_OP_NETSCAPE_CA_DN_BUG If we accept a netscape connection, demand a client cert, have a -non-self-sighed CA which does not have it's CA in netscape, and the +non-self-signed CA which does not have its CA in netscape, and the browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta -=item SSL_OP_NON_EXPORT_FIRST - -On servers try to use non-export (stronger) ciphers first. This option does -not work under all circumstances (in the code it is declared "broken"). - =item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG ... @@ -171,23 +210,136 @@ Do not use the SSLv3 protocol. Do not use the TLSv1 protocol. +=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. + +=item SSL_OP_NO_TICKET + +Normally clients and servers will, where possible, transparently make use +of RFC4507bis tickets for stateless session resumption. + +If this option is set this functionality is disabled and tickets will +not be used by clients or servers. + +=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION, SSL_OP_LEGACY_SERVER_CONNECT + +See the B section for a discussion of the purpose of +these options. + =back +=head1 SECURE RENEGOTIATION + +OpenSSL 0.9.8m and later always attempts to use secure renegotiation as +described in RFC5746. This counters the prefix attack described in +CVE-2009-3555 and elsewhere. + +The deprecated and highly broken SSLv2 protocol does not support +renegotiation at all: its use is B discouraged. + +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 I. A server not supporting secure +renegotiation is referred to as I. + +The following sections describe the operations permitted by OpenSSL's secure +renegotiation implementation. + +=head2 Patched client and server + +Connections and renegotiation are always permitted by OpenSSL implementations. + +=head2 Unpatched client and patched OpenSSL server + +The initial connection suceeds but client renegotiation is denied by the +server with a B warning alert if TLS v1.0 is used or a fatal +B alert in SSL v3.0. + +If the patched OpenSSL server attempts to renegotiate a fatal +B alert is sent. This is because the server code may be +unaware of the unpatched nature of the client. + +If the option B is set then +renegotiation B succeeds. + +B 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 +B alert. OpenSSL versions 0.9.8m and later will regard +a B alert as fatal and respond with a fatal +B alert. This is because the OpenSSL API currently has +no provision to indicate to an application that a renegotiation attempt +was refused. + +=head2 Patched OpenSSL client and unpatched server. + +If the option B is set then initial connections +and renegotiation between patched OpenSSL clients and unpatched servers +succeeds. + +This option 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 B will +B be set by default in a future version of OpenSSL. + +OpenSSL client applications wishing to ensure they can connect to unpatched +servers should always B B + +OpenSSL client applications that want to ensure they can B connect to +unpatched servers (and thus avoid any security issues) should always B +B using SSL_CTX_clear_options() or +SSL_clear_options(). + +As in the previous case if the option +B is set then renegotiation +B succeeds. + =head1 RETURN VALUES SSL_CTX_set_options() and SSL_set_options() return the new options bitmask after adding B. +SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask +after clearing B. + SSL_CTX_get_options() and SSL_get_options() return the current bitmask. +SSL_get_secure_renegotiation_support() returns 1 is the peer supports +secure renegotiation and 0 if it does not. + =head1 SEE ALSO -L, L, L +L, L, L, +L, +L, +L =head1 HISTORY -SSL_OP_CIPHER_SERVER_PREFERENCE has been added in OpenSSL 0.9.7. +B and +B have been added in +OpenSSL 0.9.7. + +B has been added in OpenSSL 0.9.6 and was automatically +enabled with B. As of 0.9.7, it is no longer included in B +and must be explicitly set. + +B 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). + +SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL +0.9.8m. -SSL_OP_TLS_ROLLBACK_BUG has been added in OpenSSL 0.9.6. +B, B +and the function SSL_get_secure_renegotiation_support() were first added in +OpenSSL 0.9.8m. =cut