SSL_CTX_set_options.pod 12 KB

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  1. =pod
  2. =head1 NAME
  3. 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
  4. =head1 SYNOPSIS
  5. #include <openssl/ssl.h>
  6. long SSL_CTX_set_options(SSL_CTX *ctx, long options);
  7. long SSL_set_options(SSL *ssl, long options);
  8. long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
  9. long SSL_clear_options(SSL *ssl, long options);
  10. long SSL_CTX_get_options(SSL_CTX *ctx);
  11. long SSL_get_options(SSL *ssl);
  12. long SSL_get_secure_renegotiation_support(SSL *ssl);
  13. =head1 DESCRIPTION
  14. Note: all these functions are implemented using macros.
  15. SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
  16. Options already set before are not cleared!
  17. SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
  18. Options already set before are not cleared!
  19. SSL_CTX_clear_options() clears the options set via bitmask in B<options>
  20. to B<ctx>.
  21. SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.
  22. SSL_CTX_get_options() returns the options set for B<ctx>.
  23. SSL_get_options() returns the options set for B<ssl>.
  24. SSL_get_secure_renegotiation_support() indicates whether the peer supports
  25. secure renegotiation.
  26. =head1 NOTES
  27. The behaviour of the SSL library can be changed by setting several options.
  28. The options are coded as bitmasks and can be combined by a logical B<or>
  29. operation (|).
  30. SSL_CTX_set_options() and SSL_set_options() affect the (external)
  31. protocol behaviour of the SSL library. The (internal) behaviour of
  32. the API can be changed by using the similar
  33. L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> and SSL_set_mode() functions.
  34. During a handshake, the option settings of the SSL object are used. When
  35. a new SSL object is created from a context using SSL_new(), the current
  36. option setting is copied. Changes to B<ctx> do not affect already created
  37. SSL objects. SSL_clear() does not affect the settings.
  38. The following B<bug workaround> options are available:
  39. =over 4
  40. =item SSL_OP_MICROSOFT_SESS_ID_BUG
  41. www.microsoft.com - when talking SSLv2, if session-id reuse is
  42. performed, the session-id passed back in the server-finished message
  43. is different from the one decided upon.
  44. =item SSL_OP_NETSCAPE_CHALLENGE_BUG
  45. Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte
  46. challenge but then appears to only use 16 bytes when generating the
  47. encryption keys. Using 16 bytes is ok but it should be ok to use 32.
  48. According to the SSLv3 spec, one should use 32 bytes for the challenge
  49. when operating in SSLv2/v3 compatibility mode, but as mentioned above,
  50. this breaks this server so 16 bytes is the way to go.
  51. =item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
  52. As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.
  53. =item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
  54. ...
  55. =item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
  56. ...
  57. =item SSL_OP_MSIE_SSLV2_RSA_PADDING
  58. As of OpenSSL 0.9.7h and 0.9.8a, this option has no effect.
  59. =item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
  60. ...
  61. =item SSL_OP_TLS_D5_BUG
  62. ...
  63. =item SSL_OP_TLS_BLOCK_PADDING_BUG
  64. ...
  65. =item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
  66. Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
  67. vulnerability affecting CBC ciphers, which cannot be handled by some
  68. broken SSL implementations. This option has no effect for connections
  69. using other ciphers.
  70. =item SSL_OP_ALL
  71. All of the above bug workarounds.
  72. =back
  73. It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
  74. options if compatibility with somewhat broken implementations is
  75. desired.
  76. The following B<modifying> options are available:
  77. =over 4
  78. =item SSL_OP_TLS_ROLLBACK_BUG
  79. Disable version rollback attack detection.
  80. During the client key exchange, the client must send the same information
  81. about acceptable SSL/TLS protocol levels as during the first hello. Some
  82. clients violate this rule by adapting to the server's answer. (Example:
  83. the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
  84. only understands up to SSLv3. In this case the client must still use the
  85. same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
  86. to the server's answer and violate the version rollback protection.)
  87. =item SSL_OP_SINGLE_DH_USE
  88. Always create a new key when using temporary/ephemeral DH parameters
  89. (see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
  90. This option must be used to prevent small subgroup attacks, when
  91. the DH parameters were not generated using "strong" primes
  92. (e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>).
  93. If "strong" primes were used, it is not strictly necessary to generate
  94. a new DH key during each handshake but it is also recommended.
  95. B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever
  96. temporary/ephemeral DH parameters are used.
  97. =item SSL_OP_EPHEMERAL_RSA
  98. Always use ephemeral (temporary) RSA key when doing RSA operations
  99. (see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>).
  100. According to the specifications this is only done, when a RSA key
  101. can only be used for signature operations (namely under export ciphers
  102. with restricted RSA keylength). By setting this option, ephemeral
  103. RSA keys are always used. This option breaks compatibility with the
  104. SSL/TLS specifications and may lead to interoperability problems with
  105. clients and should therefore never be used. Ciphers with EDH (ephemeral
  106. Diffie-Hellman) key exchange should be used instead.
  107. =item SSL_OP_CIPHER_SERVER_PREFERENCE
  108. When choosing a cipher, use the server's preferences instead of the client
  109. preferences. When not set, the SSL server will always follow the clients
  110. preferences. When set, the SSLv3/TLSv1 server will choose following its
  111. own preferences. Because of the different protocol, for SSLv2 the server
  112. will send its list of preferences to the client and the client chooses.
  113. =item SSL_OP_PKCS1_CHECK_1
  114. ...
  115. =item SSL_OP_PKCS1_CHECK_2
  116. ...
  117. =item SSL_OP_NETSCAPE_CA_DN_BUG
  118. If we accept a netscape connection, demand a client cert, have a
  119. non-self-signed CA which does not have its CA in netscape, and the
  120. browser has a cert, it will crash/hang. Works for 3.x and 4.xbeta
  121. =item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
  122. ...
  123. =item SSL_OP_NO_SSLv2
  124. Do not use the SSLv2 protocol.
  125. =item SSL_OP_NO_SSLv3
  126. Do not use the SSLv3 protocol.
  127. =item SSL_OP_NO_TLSv1
  128. Do not use the TLSv1 protocol.
  129. =item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
  130. When performing renegotiation as a server, always start a new session
  131. (i.e., session resumption requests are only accepted in the initial
  132. handshake). This option is not needed for clients.
  133. =item SSL_OP_NO_TICKET
  134. Normally clients and servers will, where possible, transparently make use
  135. of RFC4507bis tickets for stateless session resumption.
  136. If this option is set this functionality is disabled and tickets will
  137. not be used by clients or servers.
  138. =item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
  139. Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
  140. servers. See the B<SECURE RENEGOTIATION> section for more details.
  141. =item SSL_OP_LEGACY_SERVER_CONNECT
  142. Allow legacy insecure renegotiation between OpenSSL and unpatched servers
  143. B<only>: this option is currently set by default. See the
  144. B<SECURE RENEGOTIATION> section for more details.
  145. =back
  146. =head1 SECURE RENEGOTIATION
  147. OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
  148. described in RFC5746. This counters the prefix attack described in
  149. CVE-2009-3555 and elsewhere.
  150. The deprecated and highly broken SSLv2 protocol does not support
  151. renegotiation at all: its use is B<strongly> discouraged.
  152. This attack has far reaching consequences which application writers should be
  153. aware of. In the description below an implementation supporting secure
  154. renegotiation is referred to as I<patched>. A server not supporting secure
  155. renegotiation is referred to as I<unpatched>.
  156. The following sections describe the operations permitted by OpenSSL's secure
  157. renegotiation implementation.
  158. =head2 Patched client and server
  159. Connections and renegotiation are always permitted by OpenSSL implementations.
  160. =head2 Unpatched client and patched OpenSSL server
  161. The initial connection suceeds but client renegotiation is denied by the
  162. server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
  163. B<handshake_failure> alert in SSL v3.0.
  164. If the patched OpenSSL server attempts to renegotiate a fatal
  165. B<handshake_failure> alert is sent. This is because the server code may be
  166. unaware of the unpatched nature of the client.
  167. If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
  168. renegotiation B<always> succeeds.
  169. B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are
  170. unpatched) will result in the connection hanging if it receives a
  171. B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard
  172. a B<no_renegotiation> alert as fatal and respond with a fatal
  173. B<handshake_failure> alert. This is because the OpenSSL API currently has
  174. no provision to indicate to an application that a renegotiation attempt
  175. was refused.
  176. =head2 Patched OpenSSL client and unpatched server.
  177. If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
  178. B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
  179. and renegotiation between patched OpenSSL clients and unpatched servers
  180. succeeds. If neither option is set then initial connections to unpatched
  181. servers will fail.
  182. The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
  183. though it has security implications: otherwise it would be impossible to
  184. connect to unpatched servers (i.e. all of them initially) and this is clearly
  185. not acceptable. Renegotiation is permitted because this does not add any
  186. additional security issues: during an attack clients do not see any
  187. renegotiations anyway.
  188. As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
  189. B<not> be set by default in a future version of OpenSSL.
  190. OpenSSL client applications wishing to ensure they can connect to unpatched
  191. servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
  192. OpenSSL client applications that want to ensure they can B<not> connect to
  193. unpatched servers (and thus avoid any security issues) should always B<clear>
  194. B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
  195. SSL_clear_options().
  196. The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
  197. B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
  198. B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
  199. renegotiation between OpenSSL clients and unpatched servers B<only>, while
  200. B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
  201. and renegotiation between OpenSSL and unpatched clients or servers.
  202. =head1 RETURN VALUES
  203. SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
  204. after adding B<options>.
  205. SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
  206. after clearing B<options>.
  207. SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
  208. SSL_get_secure_renegotiation_support() returns 1 is the peer supports
  209. secure renegotiation and 0 if it does not.
  210. =head1 SEE ALSO
  211. L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>,
  212. L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>,
  213. L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
  214. L<dhparam(1)|dhparam(1)>
  215. =head1 HISTORY
  216. B<SSL_OP_CIPHER_SERVER_PREFERENCE> and
  217. B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in
  218. OpenSSL 0.9.7.
  219. B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically
  220. enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL>
  221. and must be explicitly set.
  222. B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e.
  223. Versions up to OpenSSL 0.9.6c do not include the countermeasure that
  224. can be disabled with this option (in OpenSSL 0.9.6d, it was always
  225. enabled).
  226. SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL
  227. 0.9.8m.
  228. B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT>
  229. and the function SSL_get_secure_renegotiation_support() were first added in
  230. OpenSSL 0.9.8m.
  231. =cut