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1.\" $OpenBSD: SSL_CTX_set_options.3,v 1.16 2022/03/31 17:27:18 naddy Exp $
2.\" full merge up to: OpenSSL 7946ab33 Dec 6 17:56:41 2015 +0100
3.\" selective merge up to: OpenSSL edb79c3a Mar 29 10:07:14 2017 +1000
4.\"
5.\" This file was written by Lutz Jaenicke <jaenicke@openssl.org>,
6.\" Bodo Moeller <bodo@openssl.org>, and
7.\" Dr. Stephen Henson <steve@openssl.org>.
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55.Dd $Mdocdate: March 31 2022 $
56.Dt SSL_CTX_SET_OPTIONS 3
57.Os
58.Sh NAME
59.Nm SSL_CTX_set_options ,
60.Nm SSL_set_options ,
61.Nm SSL_CTX_clear_options ,
62.Nm SSL_clear_options ,
63.Nm SSL_CTX_get_options ,
64.Nm SSL_get_options ,
65.Nm SSL_get_secure_renegotiation_support
66.Nd manipulate SSL options
67.Sh SYNOPSIS
68.In openssl/ssl.h
69.Ft long
70.Fn SSL_CTX_set_options "SSL_CTX *ctx" "long options"
71.Ft long
72.Fn SSL_set_options "SSL *ssl" "long options"
73.Ft long
74.Fn SSL_CTX_clear_options "SSL_CTX *ctx" "long options"
75.Ft long
76.Fn SSL_clear_options "SSL *ssl" "long options"
77.Ft long
78.Fn SSL_CTX_get_options "SSL_CTX *ctx"
79.Ft long
80.Fn SSL_get_options "SSL *ssl"
81.Ft long
82.Fn SSL_get_secure_renegotiation_support "SSL *ssl"
83.Sh DESCRIPTION
84.Fn SSL_CTX_set_options
85adds the options set via bitmask in
86.Fa options
87to
88.Fa ctx .
89Options already set before are not cleared!
90.Pp
91.Fn SSL_set_options
92adds the options set via bitmask in
93.Fa options
94to
95.Fa ssl .
96Options already set before are not cleared!
97.Pp
98.Fn SSL_CTX_clear_options
99clears the options set via bitmask in
100.Fa options
101to
102.Fa ctx .
103.Pp
104.Fn SSL_clear_options
105clears the options set via bitmask in
106.Fa options
107to
108.Fa ssl .
109.Pp
110.Fn SSL_CTX_get_options
111returns the options set for
112.Fa ctx .
113.Pp
114.Fn SSL_get_options
115returns the options set for
116.Fa ssl .
117.Pp
118.Fn SSL_get_secure_renegotiation_support
119indicates whether the peer supports secure renegotiation.
120.Pp
121All these functions are implemented using macros.
122.Pp
123The behaviour of the SSL library can be changed by setting several options.
124The options are coded as bitmasks and can be combined by a bitwise OR
125operation (|).
126.Pp
127.Fn SSL_CTX_set_options
128and
129.Fn SSL_set_options
130affect the (external) protocol behaviour of the SSL library.
131The (internal) behaviour of the API can be changed by using the similar
132.Xr SSL_CTX_set_mode 3
133and
134.Xr SSL_set_mode 3
135functions.
136.Pp
137During a handshake, the option settings of the SSL object are used.
138When a new SSL object is created from a context using
139.Xr SSL_new 3 ,
140the current option setting is copied.
141Changes to
142.Fa ctx
143do not affect already created
144.Vt SSL
145objects.
146.Fn SSL_clear
147does not affect the settings.
148.Pp
149The following
150.Em bug workaround
151options are available:
152.Bl -tag -width Ds
153.It Dv SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
154Disables a countermeasure against a TLS 1.0 protocol vulnerability
155affecting CBC ciphers, which cannot be handled by some broken SSL
156implementations.
157This option has no effect for connections using other ciphers.
158.It Dv SSL_OP_ALL
159This is currently an alias for
160.Dv SSL_OP_LEGACY_SERVER_CONNECT .
161.El
162.Pp
163It is usually safe to use
164.Dv SSL_OP_ALL
165to enable the bug workaround options if compatibility with somewhat broken
166implementations is desired.
167.Pp
168The following
169.Em modifying
170options are available:
171.Bl -tag -width Ds
172.It Dv SSL_OP_CIPHER_SERVER_PREFERENCE
173When choosing a cipher, use the server's preferences instead of the client
174preferences.
175When not set, the server will always follow the client's preferences.
176When set, the server will choose following its own preferences.
177.It Dv SSL_OP_COOKIE_EXCHANGE
178Turn on Cookie Exchange as described in RFC 4347 Section 4.2.1.
179Only affects DTLS connections.
180.It Dv SSL_OP_LEGACY_SERVER_CONNECT
181Allow legacy insecure renegotiation between OpenSSL and unpatched servers
182.Em only :
183this option is currently set by default.
184See the
185.Sx SECURE RENEGOTIATION
186section for more details.
187.It Dv SSL_OP_NO_DTLSv1
188Do not use the DTLSv1 protocol.
189Deprecated; use
190.Xr SSL_CTX_set_min_proto_version 3
191instead.
192.It Dv SSL_OP_NO_DTLSv1_2
193Do not use the DTLSv1.2 protocol.
194Deprecated; use
195.Xr SSL_CTX_set_min_proto_version 3
196instead.
197.It Dv SSL_OP_NO_QUERY_MTU
198Do not query the MTU.
199Only affects DTLS connections.
200.It Dv SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION
201When performing renegotiation as a server, always start a new session (i.e.,
202session resumption requests are only accepted in the initial handshake).
203This option is not needed for clients.
204.It Dv SSL_OP_NO_TICKET
205Normally clients and servers using TLSv1.2 and earlier will, where possible,
206transparently make use of
207RFC 5077 tickets for stateless session resumption.
208.Pp
209If this option is set, this functionality is disabled and tickets will not be
210used by clients or servers.
211.It Dv SSL_OP_NO_TLSv1
212Do not use the TLSv1.0 protocol.
213Deprecated; use
214.Xr SSL_CTX_set_min_proto_version 3
215instead.
216.It Dv SSL_OP_NO_TLSv1_1
217Do not use the TLSv1.1 protocol.
218Deprecated; use
219.Xr SSL_CTX_set_min_proto_version 3
220instead.
221.It Dv SSL_OP_NO_TLSv1_2
222Do not use the TLSv1.2 protocol.
223Deprecated; use
224.Xr SSL_CTX_set_max_proto_version 3
225instead.
226.El
227.Pp
228The following options used to be supported at some point in the past
229and no longer have any effect:
230.Dv SSL_OP_ALLOW_UNSAFE_LEGACY_RENEGOTIATION ,
231.Dv SSL_OP_EPHEMERAL_RSA ,
232.Dv SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER ,
233.Dv SSL_OP_MICROSOFT_SESS_ID_BUG ,
234.Dv SSL_OP_NETSCAPE_CA_DN_BUG ,
235.Dv SSL_OP_NETSCAPE_CHALLENGE_BUG ,
236.Dv SSL_OP_NETSCAPE_DEMO_CIPHER_CHANGE_BUG ,
237.Dv SSL_OP_NETSCAPE_REUSE_CIPHER_CHANGE_BUG ,
238.Dv SSL_OP_NO_COMPRESSION ,
239.Dv SSL_OP_NO_SSLv2 ,
240.Dv SSL_OP_NO_SSLv3 ,
241.Dv SSL_OP_PKCS1_CHECK_1 ,
242.Dv SSL_OP_PKCS1_CHECK_2 ,
243.Dv SSL_OP_SAFARI_ECDHE_ECDSA_BUG ,
244.Dv SSL_OP_SINGLE_DH_USE ,
245.Dv SSL_OP_SINGLE_ECDH_USE ,
246.Dv SSL_OP_SSLEAY_080_CLIENT_DH_BUG ,
247.Dv SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG ,
248.Dv SSL_OP_TLS_BLOCK_PADDING_BUG ,
249.Dv SSL_OP_TLS_D5_BUG ,
250.Dv SSL_OP_TLS_ROLLBACK_BUG ,
251.Dv SSL_OP_TLSEXT_PADDING .
252.Sh SECURE RENEGOTIATION
253OpenSSL 0.9.8m and later always attempts to use secure renegotiation as
254described in RFC 5746.
255This counters the prefix attack described in CVE-2009-3555 and elsewhere.
256.Pp
257This attack has far-reaching consequences which application writers should be
258aware of.
259In the description below an implementation supporting secure renegotiation is
260referred to as
261.Dq patched .
262A server not supporting secure
263renegotiation is referred to as
264.Dq unpatched .
265.Pp
266The following sections describe the operations permitted by OpenSSL's secure
267renegotiation implementation.
268.Ss Patched client and server
269Connections and renegotiation are always permitted by OpenSSL implementations.
270.Ss Unpatched client and patched OpenSSL server
271The initial connection succeeds but client renegotiation is denied by the
272server with a
273.Em no_renegotiation
274warning alert.
275.Pp
276If the patched OpenSSL server attempts to renegotiate, a fatal
277.Em handshake_failure
278alert is sent.
279This is because the server code may be unaware of the unpatched nature of the
280client.
281.Pp
282Note that a bug in OpenSSL clients earlier than 0.9.8m (all of which
283are unpatched) will result in the connection hanging if it receives a
284.Em no_renegotiation
285alert.
286OpenSSL versions 0.9.8m and later will regard a
287.Em no_renegotiation
288alert as fatal and respond with a fatal
289.Em handshake_failure
290alert.
291This is because the OpenSSL API currently has no provision to indicate to an
292application that a renegotiation attempt was refused.
293.Ss Patched OpenSSL client and unpatched server
294If the option
295.Dv SSL_OP_LEGACY_SERVER_CONNECT
296is set then initial connections and renegotiation between patched OpenSSL
297clients and unpatched servers succeeds.
298If neither option is set then initial connections to unpatched servers will
299fail.
300.Pp
301The option
302.Dv SSL_OP_LEGACY_SERVER_CONNECT
303is currently set by default even though it has security implications:
304otherwise it would be impossible to connect to unpatched servers (i.e., all of
305them initially) and this is clearly not acceptable.
306Renegotiation is permitted because this does not add any additional security
307issues: during an attack clients do not see any renegotiations anyway.
308.Pp
309As more servers become patched, the option
310.Dv SSL_OP_LEGACY_SERVER_CONNECT
311will
312.Em not
313be set by default in a future version of OpenSSL.
314.Pp
315OpenSSL client applications wishing to ensure they can connect to unpatched
316servers should always
317.Em set
318.Dv SSL_OP_LEGACY_SERVER_CONNECT .
319.Pp
320OpenSSL client applications that want to ensure they can
321.Em not
322connect to unpatched servers (and thus avoid any security issues) should always
323.Em clear
324.Dv SSL_OP_LEGACY_SERVER_CONNECT
325using
326.Fn SSL_CTX_clear_options
327or
328.Fn SSL_clear_options .
329.Sh RETURN VALUES
330.Fn SSL_CTX_set_options
331and
332.Fn SSL_set_options
333return the new options bitmask after adding
334.Fa options .
335.Pp
336.Fn SSL_CTX_clear_options
337and
338.Fn SSL_clear_options
339return the new options bitmask after clearing
340.Fa options .
341.Pp
342.Fn SSL_CTX_get_options
343and
344.Fn SSL_get_options
345return the current bitmask.
346.Pp
347.Fn SSL_get_secure_renegotiation_support
348returns 1 is the peer supports secure renegotiation and 0 if it does not.
349.Sh SEE ALSO
350.Xr openssl 1 ,
351.Xr ssl 3 ,
352.Xr SSL_clear 3 ,
353.Xr SSL_CTX_ctrl 3 ,
354.Xr SSL_CTX_set_min_proto_version 3 ,
355.Xr SSL_new 3
356.Sh HISTORY
357.Fn SSL_CTX_set_options
358and
359.Fn SSL_set_options
360first appeared in SSLeay 0.9.0 and have been available since
361.Ox 2.4 .
362.Pp
363.Fn SSL_CTX_get_options
364and
365.Fn SSL_get_options
366first appeared in OpenSSL 0.9.2b and have been available since
367.Ox 2.6 .
368.Pp
369.Fn SSL_CTX_clear_options ,
370.Fn SSL_clear_options ,
371and
372.Fn SSL_get_secure_renegotiation_support
373first appeared in OpenSSL 0.9.8m and have been available since
374.Ox 4.9 .
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