1=pod
2
3=head1 NAME
4
5SSL_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
6
7=head1 SYNOPSIS
8
9 #include <openssl/ssl.h>
10
11 long SSL_CTX_set_options(SSL_CTX *ctx, long options);
12 long SSL_set_options(SSL *ssl, long options);
13
14 long SSL_CTX_clear_options(SSL_CTX *ctx, long options);
15 long SSL_clear_options(SSL *ssl, long options);
16
17 long SSL_CTX_get_options(SSL_CTX *ctx);
18 long SSL_get_options(SSL *ssl);
19
20 long SSL_get_secure_renegotiation_support(SSL *ssl);
21
22=head1 DESCRIPTION
23
24Note: all these functions are implemented using macros.
25
26SSL_CTX_set_options() adds the options set via bitmask in B<options> to B<ctx>.
27Options already set before are not cleared!
28
29SSL_set_options() adds the options set via bitmask in B<options> to B<ssl>.
30Options already set before are not cleared!
31
32SSL_CTX_clear_options() clears the options set via bitmask in B<options>
33to B<ctx>.
34
35SSL_clear_options() clears the options set via bitmask in B<options> to B<ssl>.
36
37SSL_CTX_get_options() returns the options set for B<ctx>.
38
39SSL_get_options() returns the options set for B<ssl>.
40
41SSL_get_secure_renegotiation_support() indicates whether the peer supports
42secure renegotiation.
43
44=head1 NOTES
45
46The behaviour of the SSL library can be changed by setting several options.
47The options are coded as bitmasks and can be combined by a logical B<or>
48operation (|).
49
50SSL_CTX_set_options() and SSL_set_options() affect the (external)
51protocol behaviour of the SSL library. The (internal) behaviour of
52the API can be changed by using the similar
53L<SSL_CTX_set_mode(3)|SSL_CTX_set_mode(3)> and SSL_set_mode() functions.
54
55During a handshake, the option settings of the SSL object are used. When
56a new SSL object is created from a context using SSL_new(), the current
57option setting is copied. Changes to B<ctx> do not affect already created
58SSL objects. SSL_clear() does not affect the settings.
59
60The following B<bug workaround> options are available:
61
62=over 4
63
64=item SSL_OP_MICROSOFT_SESS_ID_BUG
65
66www.microsoft.com - when talking SSLv2, if session-id reuse is
67performed, the session-id passed back in the server-finished message
68is different from the one decided upon.
69
70=item SSL_OP_NETSCAPE_CHALLENGE_BUG
71
72Netscape-Commerce/1.12, when talking SSLv2, accepts a 32 byte
73challenge but then appears to only use 16 bytes when generating the
74encryption keys.  Using 16 bytes is ok but it should be ok to use 32.
75According to the SSLv3 spec, one should use 32 bytes for the challenge
76when operating in SSLv2/v3 compatibility mode, but as mentioned above,
77this breaks this server so 16 bytes is the way to go.
78
79=item SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG
80
81As of OpenSSL 0.9.8q and 1.0.0c, this option has no effect.
82
83=item SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
84
85...
86
87=item SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
88
89...
90
91=item SSL_OP_SAFARI_ECDHE_ECDSA_BUG
92
93Don't prefer ECDHE-ECDSA ciphers when the client appears to be Safari on OS X.
94OS X 10.8..10.8.3 has broken support for ECDHE-ECDSA ciphers.
95
96=item SSL_OP_SSLEAY_080_CLIENT_DH_BUG
97
98...
99
100=item SSL_OP_TLS_D5_BUG
101
102...
103
104=item SSL_OP_TLS_BLOCK_PADDING_BUG
105
106...
107
108=item SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
109
110Disables a countermeasure against a SSL 3.0/TLS 1.0 protocol
111vulnerability affecting CBC ciphers, which cannot be handled by some
112broken SSL implementations.  This option has no effect for connections
113using other ciphers.
114
115=item SSL_OP_ALL
116
117All of the above bug workarounds.
118
119=back
120
121It is usually safe to use B<SSL_OP_ALL> to enable the bug workaround
122options if compatibility with somewhat broken implementations is
123desired.
124
125The following B<modifying> options are available:
126
127=over 4
128
129=item SSL_OP_TLS_ROLLBACK_BUG
130
131Disable version rollback attack detection.
132
133During the client key exchange, the client must send the same information
134about acceptable SSL/TLS protocol levels as during the first hello. Some
135clients violate this rule by adapting to the server's answer. (Example:
136the client sends a SSLv2 hello and accepts up to SSLv3.1=TLSv1, the server
137only understands up to SSLv3. In this case the client must still use the
138same SSLv3.1=TLSv1 announcement. Some clients step down to SSLv3 with respect
139to the server's answer and violate the version rollback protection.)
140
141=item SSL_OP_SINGLE_DH_USE
142
143Always create a new key when using temporary/ephemeral DH parameters
144(see L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>).
145This option must be used to prevent small subgroup attacks, when
146the DH parameters were not generated using "strong" primes
147(e.g. when using DSA-parameters, see L<dhparam(1)|dhparam(1)>).
148If "strong" primes were used, it is not strictly necessary to generate
149a new DH key during each handshake but it is also recommended.
150B<SSL_OP_SINGLE_DH_USE> should therefore be enabled whenever
151temporary/ephemeral DH parameters are used.
152
153=item SSL_OP_EPHEMERAL_RSA
154
155Always use ephemeral (temporary) RSA key when doing RSA operations
156(see L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>).
157According to the specifications this is only done, when a RSA key
158can only be used for signature operations (namely under export ciphers
159with restricted RSA keylength). By setting this option, ephemeral
160RSA keys are always used. This option breaks compatibility with the
161SSL/TLS specifications and may lead to interoperability problems with
162clients and should therefore never be used. Ciphers with EDH (ephemeral
163Diffie-Hellman) key exchange should be used instead.
164
165=item SSL_OP_CIPHER_SERVER_PREFERENCE
166
167When choosing a cipher, use the server's preferences instead of the client
168preferences. When not set, the SSL server will always follow the clients
169preferences. When set, the SSLv3/TLSv1 server will choose following its
170own preferences. Because of the different protocol, for SSLv2 the server
171will send its list of preferences to the client and the client chooses.
172
173=item SSL_OP_PKCS1_CHECK_1
174
175...
176
177=item SSL_OP_PKCS1_CHECK_2
178
179...
180
181=item SSL_OP_NETSCAPE_CA_DN_BUG
182
183If we accept a netscape connection, demand a client cert, have a
184non-self-signed CA which does not have its CA in netscape, and the
185browser has a cert, it will crash/hang.  Works for 3.x and 4.xbeta 
186
187=item SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG
188
189...
190
191=item SSL_OP_NO_SSLv2
192
193Do not use the SSLv2 protocol.
194
195=item SSL_OP_NO_SSLv3
196
197Do not use the SSLv3 protocol.
198
199=item SSL_OP_NO_TLSv1
200
201Do not use the TLSv1 protocol.
202
203=item SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
204
205When performing renegotiation as a server, always start a new session
206(i.e., session resumption requests are only accepted in the initial
207handshake). This option is not needed for clients.
208
209=item SSL_OP_NO_TICKET
210
211Normally clients and servers will, where possible, transparently make use
212of RFC4507bis tickets for stateless session resumption if extension support
213is explicitly set when OpenSSL is compiled.
214
215If this option is set this functionality is disabled and tickets will
216not be used by clients or servers.
217
218=item SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION
219
220Allow legacy insecure renegotiation between OpenSSL and unpatched clients or
221servers. See the B<SECURE RENEGOTIATION> section for more details.
222
223=item SSL_OP_LEGACY_SERVER_CONNECT
224
225Allow legacy insecure renegotiation between OpenSSL and unpatched servers
226B<only>: this option is currently set by default. See the
227B<SECURE RENEGOTIATION> section for more details.
228
229=back
230
231=head1 SECURE RENEGOTIATION
232
233OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
234described in RFC5746. This counters the prefix attack described in
235CVE-2009-3555 and elsewhere.
236
237The deprecated and highly broken SSLv2 protocol does not support
238renegotiation at all: its use is B<strongly> discouraged.
239
240This attack has far reaching consequences which application writers should be
241aware of. In the description below an implementation supporting secure
242renegotiation is referred to as I<patched>. A server not supporting secure
243renegotiation is referred to as I<unpatched>.
244
245The following sections describe the operations permitted by OpenSSL's secure
246renegotiation implementation.
247
248=head2 Patched client and server
249
250Connections and renegotiation are always permitted by OpenSSL implementations.
251
252=head2 Unpatched client and patched OpenSSL server
253
254The initial connection suceeds but client renegotiation is denied by the
255server with a B<no_renegotiation> warning alert if TLS v1.0 is used or a fatal
256B<handshake_failure> alert in SSL v3.0.
257
258If the patched OpenSSL server attempts to renegotiate a fatal
259B<handshake_failure> alert is sent. This is because the server code may be
260unaware of the unpatched nature of the client.
261
262If the option B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then
263renegotiation B<always> succeeds.
264
265B<NB:> a bug in OpenSSL clients earlier than 0.9.8m (all of which are
266unpatched) will result in the connection hanging if it receives a
267B<no_renegotiation> alert. OpenSSL versions 0.9.8m and later will regard
268a B<no_renegotiation> alert as fatal and respond with a fatal
269B<handshake_failure> alert. This is because the OpenSSL API currently has
270no provision to indicate to an application that a renegotiation attempt
271was refused.
272
273=head2 Patched OpenSSL client and unpatched server.
274
275If the option B<SSL_OP_LEGACY_SERVER_CONNECT> or
276B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> is set then initial connections
277and renegotiation between patched OpenSSL clients and unpatched servers
278succeeds. If neither option is set then initial connections to unpatched
279servers will fail.
280
281The option B<SSL_OP_LEGACY_SERVER_CONNECT> is currently set by default even
282though it has security implications: otherwise it would be impossible to
283connect to unpatched servers (i.e. all of them initially) and this is clearly
284not acceptable. Renegotiation is permitted because this does not add any
285additional security issues: during an attack clients do not see any
286renegotiations anyway.
287
288As more servers become patched the option B<SSL_OP_LEGACY_SERVER_CONNECT> will
289B<not> be set by default in a future version of OpenSSL.
290
291OpenSSL client applications wishing to ensure they can connect to unpatched
292servers should always B<set> B<SSL_OP_LEGACY_SERVER_CONNECT>
293
294OpenSSL client applications that want to ensure they can B<not> connect to
295unpatched servers (and thus avoid any security issues) should always B<clear>
296B<SSL_OP_LEGACY_SERVER_CONNECT> using SSL_CTX_clear_options() or
297SSL_clear_options().
298
299The difference between the B<SSL_OP_LEGACY_SERVER_CONNECT> and
300B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> options is that
301B<SSL_OP_LEGACY_SERVER_CONNECT> enables initial connections and secure
302renegotiation between OpenSSL clients and unpatched servers B<only>, while
303B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION> allows initial connections
304and renegotiation between OpenSSL and unpatched clients or servers.
305
306=head1 RETURN VALUES
307
308SSL_CTX_set_options() and SSL_set_options() return the new options bitmask
309after adding B<options>.
310
311SSL_CTX_clear_options() and SSL_clear_options() return the new options bitmask
312after clearing B<options>.
313
314SSL_CTX_get_options() and SSL_get_options() return the current bitmask.
315
316SSL_get_secure_renegotiation_support() returns 1 is the peer supports
317secure renegotiation and 0 if it does not.
318
319=head1 SEE ALSO
320
321L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>, L<SSL_clear(3)|SSL_clear(3)>,
322L<SSL_CTX_set_tmp_dh_callback(3)|SSL_CTX_set_tmp_dh_callback(3)>,
323L<SSL_CTX_set_tmp_rsa_callback(3)|SSL_CTX_set_tmp_rsa_callback(3)>,
324L<dhparam(1)|dhparam(1)>
325
326=head1 HISTORY
327
328B<SSL_OP_CIPHER_SERVER_PREFERENCE> and
329B<SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION> have been added in
330OpenSSL 0.9.7.
331
332B<SSL_OP_TLS_ROLLBACK_BUG> has been added in OpenSSL 0.9.6 and was automatically
333enabled with B<SSL_OP_ALL>. As of 0.9.7, it is no longer included in B<SSL_OP_ALL>
334and must be explicitly set.
335
336B<SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS> has been added in OpenSSL 0.9.6e.
337Versions up to OpenSSL 0.9.6c do not include the countermeasure that
338can be disabled with this option (in OpenSSL 0.9.6d, it was always
339enabled).
340
341SSL_CTX_clear_options() and SSL_clear_options() were first added in OpenSSL
3420.9.8m.
343
344B<SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION>, B<SSL_OP_LEGACY_SERVER_CONNECT>
345and the function SSL_get_secure_renegotiation_support() were first added in
346OpenSSL 0.9.8m.
347
348=cut
349