df75c2bfcd
While stereotyped repetitions are frowned upon in literature, they serve a useful purpose in manual pages, because it is easier for the user to find certain information if it is always presented in the same way. For that reason, this commit harmonizes the varying formulations in the HISTORY section about which functions, flags, etc. were added in which OpenSSL version. It also attempts to make the pod files more grep friendly by avoiding to insert line breaks between the symbol names and the corresponding version number in which they were introduced (wherever possible). Some punctuation and typographical errors were fixed on the way. Reviewed-by: Tim Hudson <tjh@openssl.org> (Merged from https://github.com/openssl/openssl/pull/7854)
378 lines
14 KiB
Text
378 lines
14 KiB
Text
=pod
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=head1 NAME
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SSL_CTX_set_options, SSL_set_options, SSL_CTX_clear_options,
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SSL_clear_options, SSL_CTX_get_options, SSL_get_options,
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SSL_get_secure_renegotiation_support - manipulate SSL options
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=head1 SYNOPSIS
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#include <openssl/ssl.h>
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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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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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long SSL_CTX_get_options(SSL_CTX *ctx);
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long SSL_get_options(SSL *ssl);
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long SSL_get_secure_renegotiation_support(SSL *ssl);
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=head1 DESCRIPTION
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SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
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Options already set before are not cleared!
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SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
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Options already set before are not cleared!
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SSL_CTX_clear_options() clears the options set via bitmask in B<options>
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to B<ctx>.
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SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.
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SSL_CTX_get_options() returns the options set for B<ctx>.
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SSL_get_options() returns the options set for B<ssl>.
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SSL_get_secure_renegotiation_support() indicates whether the peer supports
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secure renegotiation.
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Note, this is implemented via a macro.
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=head1 NOTES
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The behaviour of the SSL library can be changed by setting several options.
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The options are coded as bitmasks and can be combined by a bitwise B<or>
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operation (|).
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SSL_CTX_set_options() and SSL_set_options() affect the (external)
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protocol behaviour of the SSL library. The (internal) behaviour of
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the API can be changed by using the similar
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L<SSL_CTX_set_mode(3)> and SSL_set_mode() functions.
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During a handshake, the option settings of the SSL object are used. When
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a new SSL object is created from a context using SSL_new(), the current
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option setting is copied. Changes to B<ctx> do not affect already created
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SSL objects. SSL_clear() does not affect the settings.
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The following B<bug workaround> options are available:
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=over 4
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=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG
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Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
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OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
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=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
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Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
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vulnerability affecting CBC ciphers, which cannot be handled by some
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broken SSL implementations. This option has no effect for connections
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using other ciphers.
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=item SSL_OP_TLSEXT_PADDING
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Adds a padding extension to ensure the ClientHello size is never between
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256 and 511 bytes in length. This is needed as a workaround for some
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implementations.
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=item SSL_OP_ALL
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All of the above bug workarounds plus B<SSL_OP_LEGACY_SERVER_CONNECT> as
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mentioned below.
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=back
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It is usually safe to use B<SSL_OP_ALL> 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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The following B<modifying> options are available:
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=over 4
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=item SSL_OP_TLS_ROLLBACK_BUG
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Disable version rollback attack detection.
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During the client key exchange, the client must send the same information
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about acceptable SSL/TLS 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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=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 SSL server will always follow the clients
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preferences. When set, the SSL/TLS server will choose following its
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own preferences.
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=item SSL_OP_NO_SSLv3, SSL_OP_NO_TLSv1, SSL_OP_NO_TLSv1_1,
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SSL_OP_NO_TLSv1_2, SSL_OP_NO_TLSv1_3, SSL_OP_NO_DTLSv1, SSL_OP_NO_DTLSv1_2
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These options turn off the SSLv3, TLSv1, TLSv1.1, TLSv1.2 or TLSv1.3 protocol
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versions with TLS or the DTLSv1, DTLSv1.2 versions with DTLS,
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respectively.
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As of OpenSSL 1.1.0, these options are deprecated, use
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L<SSL_CTX_set_min_proto_version(3)> and
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L<SSL_CTX_set_max_proto_version(3)> instead.
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=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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=item SSL_OP_NO_COMPRESSION
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Do not use compression even if it is supported.
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=item SSL_OP_NO_QUERY_MTU
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Do not query the MTU. Only affects DTLS connections.
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=item SSL_OP_COOKIE_EXCHANGE
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Turn on Cookie Exchange as described in RFC4347 Section 4.2.1. Only affects
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DTLS connections.
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=item SSL_OP_NO_TICKET
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SSL/TLS supports two mechanisms for resuming sessions: session ids and stateless
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session tickets.
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When using session ids a copy of the session information is
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cached on the server and a unique id is sent to the client. When the client
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wishes to resume it provides the unique id so that the server can retrieve the
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session information from its cache.
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When using stateless session tickets the server uses a session ticket encryption
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key to encrypt the session information. This encrypted data is sent to the
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client as a "ticket". When the client wishes to resume it sends the encrypted
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data back to the server. The server uses its key to decrypt the data and resume
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the session. In this way the server can operate statelessly - no session
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information needs to be cached locally.
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The TLSv1.3 protocol only supports tickets and does not directly support session
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ids. However OpenSSL allows two modes of ticket operation in TLSv1.3: stateful
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and stateless. Stateless tickets work the same way as in TLSv1.2 and below.
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Stateful tickets mimic the session id behaviour available in TLSv1.2 and below.
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The session information is cached on the server and the session id is wrapped up
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in a ticket and sent back to the client. When the client wishes to resume, it
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presents a ticket in the same way as for stateless tickets. The server can then
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extract the session id from the ticket and retrieve the session information from
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its cache.
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By default OpenSSL will use stateless tickets. The SSL_OP_NO_TICKET option will
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cause stateless tickets to not be issued. In TLSv1.2 and below this means no
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ticket gets sent to the client at all. In TLSv1.3 a stateful ticket will be
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sent. This is a server-side option only.
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In TLSv1.3 it is possible to suppress all tickets (stateful and stateless) from
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being sent by calling L<SSL_CTX_set_num_tickets(3)> or
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L<SSL_set_num_tickets(3)>.
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=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 B<SECURE RENEGOTIATION> section for more details.
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=item SSL_OP_LEGACY_SERVER_CONNECT
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Allow legacy insecure renegotiation between OpenSSL and unpatched servers
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B<only>: this option is currently set by default. See the
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B<SECURE RENEGOTIATION> section for more details.
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=item SSL_OP_NO_ENCRYPT_THEN_MAC
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Normally clients and servers will transparently attempt to negotiate the
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RFC7366 Encrypt-then-MAC option on TLS and DTLS connection.
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If this option is set, Encrypt-then-MAC is disabled. Clients will not
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propose, and servers will not accept the extension.
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=item SSL_OP_NO_RENEGOTIATION
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Disable all renegotiation in TLSv1.2 and earlier. Do not send HelloRequest
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messages, and ignore renegotiation requests via ClientHello.
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=item SSL_OP_ALLOW_NO_DHE_KEX
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In TLSv1.3 allow a non-(ec)dhe based key exchange mode on resumption. This means
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that there will be no forward secrecy for the resumed session.
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=item SSL_OP_PRIORITIZE_CHACHA
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When SSL_OP_CIPHER_SERVER_PREFERENCE is set, temporarily reprioritize
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ChaCha20-Poly1305 ciphers to the top of the server cipher list if a
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ChaCha20-Poly1305 cipher is at the top of the client cipher list. This helps
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those clients (e.g. mobile) use ChaCha20-Poly1305 if that cipher is anywhere
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in the server cipher list; but still allows other clients to use AES and other
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ciphers. Requires B<SSL_OP_CIPHER_SERVER_PREFERENCE>.
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=item SSL_OP_ENABLE_MIDDLEBOX_COMPAT
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If set then dummy Change Cipher Spec (CCS) messages are sent in TLSv1.3. This
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has the effect of making TLSv1.3 look more like TLSv1.2 so that middleboxes that
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do not understand TLSv1.3 will not drop the connection. Regardless of whether
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this option is set or not CCS messages received from the peer will always be
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ignored in TLSv1.3. This option is set by default. To switch it off use
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SSL_clear_options(). A future version of OpenSSL may not set this by default.
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=item SSL_OP_NO_ANTI_REPLAY
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By default, when a server is configured for early data (i.e., max_early_data > 0),
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OpenSSL will switch on replay protection. See L<SSL_read_early_data(3)> for a
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description of the replay protection feature. Anti-replay measures are required
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to comply with the TLSv1.3 specification. Some applications may be able to
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mitigate the replay risks in other ways and in such cases the built in OpenSSL
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functionality is not required. Those applications can turn this feature off by
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setting this option. This is a server-side opton only. It is ignored by
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clients.
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=back
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The following options no longer have any effect but their identifiers are
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retained for compatibility purposes:
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=over 4
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=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
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=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
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=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
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=item SSL_OP_TLS_D5_BUG
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=item SSL_OP_TLS_BLOCK_PADDING_BUG
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=item SSL_OP_MSIE_SSLV2_RSA_PADDING
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=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
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=item SSL_OP_MICROSOFT_SESS_ID_BUG
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=item SSL_OP_NETSCAPE_CHALLENGE_BUG
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=item SSL_OP_PKCS1_CHECK_1
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=item SSL_OP_PKCS1_CHECK_2
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=item SSL_OP_SINGLE_DH_USE
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=item SSL_OP_SINGLE_ECDH_USE
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=item SSL_OP_EPHEMERAL_RSA
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=back
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=head1 SECURE RENEGOTIATION
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OpenSSL always attempts to use secure renegotiation as
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described in RFC5746. This counters the prefix attack described in
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CVE-2009-3555 and elsewhere.
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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 I<patched>. A server not supporting secure
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renegotiation is referred to as I<unpatched>.
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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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=head2 Patched client and server
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Connections and renegotiation are always permitted by OpenSSL implementations.
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=head2 Unpatched client and patched OpenSSL server
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The initial connection succeeds but client renegotiation is denied by the
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server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
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B<handshake_failure> alert in SSL v3.0.
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If the patched OpenSSL server attempts to renegotiate a fatal
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B<handshake_failure> 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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If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
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renegotiation B<always> succeeds.
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=head2 Patched OpenSSL client and unpatched server.
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If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> 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
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servers will fail.
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The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
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though it has security implications: otherwise it would be impossible to
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connect to unpatched servers (i.e. all of them initially) and this is clearly
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not acceptable. Renegotiation is permitted because this does not add any
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additional security issues: during an attack clients do not see any
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renegotiations anyway.
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As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
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B<not> be set by default in a future version of OpenSSL.
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OpenSSL client applications wishing to ensure they can connect to unpatched
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servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
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OpenSSL client applications that want to ensure they can B<not> connect to
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unpatched servers (and thus avoid any security issues) should always B<clear>
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B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
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SSL_clear_options().
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The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
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B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
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renegotiation between OpenSSL clients and unpatched servers B<only>, while
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B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
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and renegotiation between OpenSSL and unpatched clients or servers.
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=head1 RETURN VALUES
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SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
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after adding B<options>.
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SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
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after clearing B<options>.
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SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
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SSL_get_secure_renegotiation_support() returns 1 is the peer supports
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secure renegotiation and 0 if it does not.
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=head1 SEE ALSO
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L<ssl(7)>, L<SSL_new(3)>, L<SSL_clear(3)>,
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L<SSL_CTX_set_tmp_dh_callback(3)>,
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L<SSL_CTX_set_min_proto_version(3)>,
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L<dhparam(1)>
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=head1 HISTORY
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The attempt to always try to use secure renegotiation was added in
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OpenSSL 0.9.8m.
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The B<SSL_OP_PRIORITIZE_CHACHA> and B<SSL_OP_NO_RENEGOTIATION> options
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were added in OpenSSL 1.1.1.
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=head1 COPYRIGHT
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Copyright 2001-2018 The OpenSSL Project Authors. All Rights Reserved.
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Licensed under the OpenSSL license (the "License"). You may not use
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this file except in compliance with the License. You can obtain a copy
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in the file LICENSE in the source distribution or at
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L<https://www.openssl.org/source/license.html>.
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=cut
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