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1 .\" $OpenBSD: SSL_shutdown.3,v 1.3 2016/12/06 12:24:33 schwarze Exp $
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52 .Dd $Mdocdate: December 6 2016 $
53 .Dt SSL_SHUTDOWN 3
54 .Os
55 .Sh NAME
56 .Nm SSL_shutdown
57 .Nd shut down a TLS/SSL connection
58 .Sh SYNOPSIS
59 .In openssl/ssl.h
60 .Ft int
61 .Fn SSL_shutdown "SSL *ssl"
62 .Sh DESCRIPTION
63 .Fn SSL_shutdown
64 shuts down an active TLS/SSL connection.
65 It sends the
66 .Dq close notify
67 shutdown alert to the peer.
68 .Pp
69 .Fn SSL_shutdown
70 tries to send the
71 .Dq close notify
72 shutdown alert to the peer.
73 Whether the operation succeeds or not, the
74 .Dv SSL_SENT_SHUTDOWN
75 flag is set and a currently open session is considered closed and good and will
76 be kept in the session cache for further reuse.
77 .Pp
78 The shutdown procedure consists of 2 steps: the sending of the
79 .Dq close notify
80 shutdown alert and the reception of the peer's
81 .Dq close notify
82 shutdown alert.
83 According to the TLS standard, it is acceptable for an application to only send
84 its shutdown alert and then close the underlying connection without waiting for
85 the peer's response (this way resources can be saved, as the process can
86 already terminate or serve another connection).
87 When the underlying connection shall be used for more communications,
88 the complete shutdown procedure (bidirectional
89 .Dq close notify
90 alerts) must be performed, so that the peers stay synchronized.
91 .Pp
92 .Fn SSL_shutdown
93 supports both uni- and bidirectional shutdown by its 2 step behavior.
94 .Pp
95 When the application is the first party to send the
96 .Dq close notify
97 alert,
98 .Fn SSL_shutdown
99 will only send the alert and then set the
100 .Dv SSL_SENT_SHUTDOWN
101 flag (so that the session is considered good and will be kept in cache).
102 .Fn SSL_shutdown
103 will then return 0.
104 If a unidirectional shutdown is enough
105 (the underlying connection shall be closed anyway), this first call to
106 .Fn SSL_shutdown
107 is sufficient.
108 In order to complete the bidirectional shutdown handshake,
109 .Fn SSL_shutdown
110 must be called again.
111 The second call will make
112 .Fn SSL_shutdown
113 wait for the peer's
114 .Dq close notify
115 shutdown alert.
116 On success, the second call to
117 .Fn SSL_shutdown
118 will return 1.
119 .Pp
120 If the peer already sent the
121 .Dq close notify
122 alert and it was already processed implicitly inside another function
123 .Pq Xr SSL_read 3 ,
124 the
125 .Dv SSL_RECEIVED_SHUTDOWN
126 flag is set.
127 .Fn SSL_shutdown
128 will send the
129 .Dq close notify
130 alert, set the
131 .Dv SSL_SENT_SHUTDOWN
132 flag and will immediately return with 1.
133 Whether
134 .Dv SSL_RECEIVED_SHUTDOWN
135 is already set can be checked using the
136 .Fn SSL_get_shutdown
137 (see also the
138 .Xr SSL_set_shutdown 3
139 call).
140 .Pp
141 It is therefore recommended to check the return value of
142 .Fn SSL_shutdown
143 and call
144 .Fn SSL_shutdown
145 again, if the bidirectional shutdown is not yet complete (return value of the
146 first call is 0).
147 .Pp
148 The behaviour of
149 .Fn SSL_shutdown
150 additionally depends on the underlying
151 .Vt BIO .
152 .Pp
153 If the underlying
154 .Vt BIO
155 is
156 .Em blocking ,
157 .Fn SSL_shutdown
158 will only return once the
159 handshake step has been finished or an error occurred.
160 .Pp
161 If the underlying
162 .Vt BIO
163 is
164 .Em non-blocking ,
165 .Fn SSL_shutdown
166 will also return when the underlying
167 .Vt BIO
168 could not satisfy the needs of
169 .Fn SSL_shutdown
170 to continue the handshake.
171 In this case a call to
172 .Xr SSL_get_error 3
173 with the
174 return value of
175 .Fn SSL_shutdown
176 will yield
177 .Dv SSL_ERROR_WANT_READ
178 or
179 .Dv SSL_ERROR_WANT_WRITE .
180 The calling process then must repeat the call after taking appropriate action
181 to satisfy the needs of
182 .Fn SSL_shutdown .
183 The action depends on the underlying
184 .Vt BIO .
185 When using a non-blocking socket, nothing is to be done, but
186 .Xr select 2
187 can be used to check for the required condition.
188 When using a buffering
189 .Vt BIO ,
190 like a
191 .Vt BIO
192 pair, data must be written into or retrieved out of the
193 .Vt BIO
194 before being able to continue.
195 .Pp
196 .Fn SSL_shutdown
197 can be modified to only set the connection to
198 .Dq shutdown
199 state but not actually send the
200 .Dq close notify
201 alert messages; see
202 .Xr SSL_CTX_set_quiet_shutdown 3 .
203 When
204 .Dq quiet shutdown
205 is enabled,
206 .Fn SSL_shutdown
207 will always succeed and return 1.
208 .Sh RETURN VALUES
209 The following return values can occur:
210 .Bl -tag -width Ds
211 .It 0
212 The shutdown is not yet finished.
213 Call
214 .Fn SSL_shutdown
215 for a second time, if a bidirectional shutdown shall be performed.
216 The output of
217 .Xr SSL_get_error 3
218 may be misleading, as an erroneous
219 .Dv SSL_ERROR_SYSCALL
220 may be flagged even though no error occurred.
221 .It 1
222 The shutdown was successfully completed.
223 The
224 .Dq close notify
225 alert was sent and the peer's
226 .Dq close notify
227 alert was received.
228 .It \(mi1
229 The shutdown was not successful because a fatal error occurred either
230 at the protocol level or a connection failure occurred.
231 It can also occur if action is need to continue the operation for non-blocking
232 .Vt BIO Ns
233 s.
234 Call
235 .Xr SSL_get_error 3
236 with the return value
237 .Fa ret
238 to find out the reason.
239 .El
240 .Sh SEE ALSO
241 .Xr BIO_new 3 ,
242 .Xr ssl 3 ,
243 .Xr SSL_accept 3 ,
244 .Xr SSL_clear 3 ,
245 .Xr SSL_connect 3 ,
246 .Xr SSL_CTX_set_quiet_shutdown 3 ,
247 .Xr SSL_free 3 ,
248 .Xr SSL_get_error 3 ,
249 .Xr SSL_set_shutdown 3