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1 | .\" $OpenBSD: SSL_read.3,v 1.8 2021/10/24 15:10:13 schwarze Exp $ | ||
2 | .\" full merge up to: OpenSSL 5a2443ae Nov 14 11:37:36 2016 +0000 | ||
3 | .\" partial merge up to: OpenSSL 24a535ea Sep 22 13:14:20 2020 +0100 | ||
4 | .\" | ||
5 | .\" This file was written by Lutz Jaenicke <jaenicke@openssl.org> and | ||
6 | .\" Matt Caswell <matt@openssl.org>. | ||
7 | .\" Copyright (c) 2000, 2001, 2008, 2016 The OpenSSL Project. | ||
8 | .\" All rights reserved. | ||
9 | .\" | ||
10 | .\" Redistribution and use in source and binary forms, with or without | ||
11 | .\" modification, are permitted provided that the following conditions | ||
12 | .\" are met: | ||
13 | .\" | ||
14 | .\" 1. Redistributions of source code must retain the above copyright | ||
15 | .\" notice, this list of conditions and the following disclaimer. | ||
16 | .\" | ||
17 | .\" 2. Redistributions in binary form must reproduce the above copyright | ||
18 | .\" notice, this list of conditions and the following disclaimer in | ||
19 | .\" the documentation and/or other materials provided with the | ||
20 | .\" distribution. | ||
21 | .\" | ||
22 | .\" 3. All advertising materials mentioning features or use of this | ||
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24 | .\" "This product includes software developed by the OpenSSL Project | ||
25 | .\" for use in the OpenSSL Toolkit. (http://www.openssl.org/)" | ||
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53 | .\" | ||
54 | .Dd $Mdocdate: October 24 2021 $ | ||
55 | .Dt SSL_READ 3 | ||
56 | .Os | ||
57 | .Sh NAME | ||
58 | .Nm SSL_read_ex , | ||
59 | .Nm SSL_read , | ||
60 | .Nm SSL_peek_ex , | ||
61 | .Nm SSL_peek | ||
62 | .Nd read bytes from a TLS connection | ||
63 | .Sh SYNOPSIS | ||
64 | .In openssl/ssl.h | ||
65 | .Ft int | ||
66 | .Fn SSL_read_ex "SSL *ssl" "void *buf" "size_t num" "size_t *readbytes" | ||
67 | .Ft int | ||
68 | .Fn SSL_read "SSL *ssl" "void *buf" "int num" | ||
69 | .Ft int | ||
70 | .Fn SSL_peek_ex "SSL *ssl" "void *buf" "size_t num" "size_t *readbytes" | ||
71 | .Ft int | ||
72 | .Fn SSL_peek "SSL *ssl" "void *buf" "int num" | ||
73 | .Sh DESCRIPTION | ||
74 | .Fn SSL_read_ex | ||
75 | and | ||
76 | .Fn SSL_read | ||
77 | try to read | ||
78 | .Fa num | ||
79 | bytes from the specified | ||
80 | .Fa ssl | ||
81 | into the buffer | ||
82 | .Fa buf . | ||
83 | On success | ||
84 | .Fn SSL_read_ex | ||
85 | stores the number of bytes actually read in | ||
86 | .Pf * Fa readbytes . | ||
87 | .Pp | ||
88 | .Fn SSL_peek_ex | ||
89 | and | ||
90 | .Fn SSL_peek | ||
91 | are identical to | ||
92 | .Fn SSL_read_ex | ||
93 | and | ||
94 | .Fn SSL_read , | ||
95 | respectively, | ||
96 | except that no bytes are removed from the underlying BIO during | ||
97 | the read, such that a subsequent call to | ||
98 | .Fn SSL_read_ex | ||
99 | or | ||
100 | .Fn SSL_read | ||
101 | will yield at least the same bytes once again. | ||
102 | .Pp | ||
103 | In the following, | ||
104 | .Fn SSL_read_ex , | ||
105 | .Fn SSL_read , | ||
106 | .Fn SSL_peek_ex , | ||
107 | and | ||
108 | .Fn SSL_peek | ||
109 | are called | ||
110 | .Dq read functions . | ||
111 | .Pp | ||
112 | If necessary, a read function will negotiate a TLS session, if | ||
113 | not already explicitly performed by | ||
114 | .Xr SSL_connect 3 | ||
115 | or | ||
116 | .Xr SSL_accept 3 . | ||
117 | If the peer requests a re-negotiation, it will be performed | ||
118 | transparently during the read function operation. | ||
119 | The behaviour of the read functions depends on the underlying | ||
120 | .Vt BIO . | ||
121 | .Pp | ||
122 | For the transparent negotiation to succeed, the | ||
123 | .Fa ssl | ||
124 | must have been initialized to client or server mode. | ||
125 | This is done by calling | ||
126 | .Xr SSL_set_connect_state 3 | ||
127 | or | ||
128 | .Xr SSL_set_accept_state 3 | ||
129 | before the first call to a read function. | ||
130 | .Pp | ||
131 | The read functions work based on the TLS records. | ||
132 | The data are received in records (with a maximum record size of 16kB). | ||
133 | Only when a record has been completely received, it can be processed | ||
134 | (decrypted and checked for integrity). | ||
135 | Therefore, data that was not retrieved at the last read call can | ||
136 | still be buffered inside the TLS layer and will be retrieved on the | ||
137 | next read call. | ||
138 | If | ||
139 | .Fa num | ||
140 | is higher than the number of bytes buffered, the read functions | ||
141 | will return with the bytes buffered. | ||
142 | If no more bytes are in the buffer, the read functions will trigger | ||
143 | the processing of the next record. | ||
144 | Only when the record has been received and processed completely | ||
145 | will the read functions return reporting success. | ||
146 | At most the contents of the record will be returned. | ||
147 | As the size of a TLS record may exceed the maximum packet size | ||
148 | of the underlying transport (e.g., TCP), it may be necessary to | ||
149 | read several packets from the transport layer before the record is | ||
150 | complete and the read call can succeed. | ||
151 | .Pp | ||
152 | If the underlying | ||
153 | .Vt BIO | ||
154 | is blocking, | ||
155 | a read function will only return once the read operation has been | ||
156 | finished or an error occurred, except when a renegotiation takes | ||
157 | place, in which case an | ||
158 | .Dv SSL_ERROR_WANT_READ | ||
159 | may occur. | ||
160 | This behavior can be controlled with the | ||
161 | .Dv SSL_MODE_AUTO_RETRY | ||
162 | flag of the | ||
163 | .Xr SSL_CTX_set_mode 3 | ||
164 | call. | ||
165 | .Pp | ||
166 | If the underlying | ||
167 | .Vt BIO | ||
168 | is non-blocking, a read function will also return when the underlying | ||
169 | .Vt BIO | ||
170 | could not satisfy the needs of the function to continue the operation. | ||
171 | In this case a call to | ||
172 | .Xr SSL_get_error 3 | ||
173 | with the return value of the read function will yield | ||
174 | .Dv SSL_ERROR_WANT_READ | ||
175 | or | ||
176 | .Dv SSL_ERROR_WANT_WRITE . | ||
177 | As at any time a re-negotiation is possible, a read function may | ||
178 | also cause write operations. | ||
179 | The calling process must then repeat the call after taking appropriate | ||
180 | action to satisfy the needs of the read function. | ||
181 | The action depends on the underlying | ||
182 | .Vt BIO . | ||
183 | When using a non-blocking socket, nothing is to be done, but | ||
184 | .Xr select 2 | ||
185 | can be used to check for the required condition. | ||
186 | When using a buffering | ||
187 | .Vt BIO , | ||
188 | like a | ||
189 | .Vt BIO | ||
190 | pair, data must be written into or retrieved out of the | ||
191 | .Vt BIO | ||
192 | before being able to continue. | ||
193 | .Pp | ||
194 | .Xr SSL_pending 3 | ||
195 | can be used to find out whether there are buffered bytes available for | ||
196 | immediate retrieval. | ||
197 | In this case a read function can be called without blocking or | ||
198 | actually receiving new data from the underlying socket. | ||
199 | .Pp | ||
200 | When a read function operation has to be repeated because of | ||
201 | .Dv SSL_ERROR_WANT_READ | ||
202 | or | ||
203 | .Dv SSL_ERROR_WANT_WRITE , | ||
204 | it must be repeated with the same arguments. | ||
205 | .Sh RETURN VALUES | ||
206 | .Fn SSL_read_ex | ||
207 | and | ||
208 | .Fn SSL_peek_ex | ||
209 | return 1 for success or 0 for failure. | ||
210 | Success means that one or more application data bytes | ||
211 | have been read from the SSL connection. | ||
212 | Failure means that no bytes could be read from the SSL connection. | ||
213 | Failures can be retryable (e.g. we are waiting for more bytes to be | ||
214 | delivered by the network) or non-retryable (e.g. a fatal network error). | ||
215 | In the event of a failure, call | ||
216 | .Xr SSL_get_error 3 | ||
217 | to find out the reason which indicates whether the call is retryable or not. | ||
218 | .Pp | ||
219 | For | ||
220 | .Fn SSL_read | ||
221 | and | ||
222 | .Fn SSL_peek , | ||
223 | the following return values can occur: | ||
224 | .Bl -tag -width Ds | ||
225 | .It >0 | ||
226 | The read operation was successful. | ||
227 | The return value is the number of bytes actually read from the | ||
228 | TLS connection. | ||
229 | .It 0 | ||
230 | The read operation was not successful. | ||
231 | The reason may either be a clean shutdown due to a | ||
232 | .Dq close notify | ||
233 | alert sent by the peer (in which case the | ||
234 | .Dv SSL_RECEIVED_SHUTDOWN | ||
235 | flag in the ssl shutdown state is set (see | ||
236 | .Xr SSL_shutdown 3 | ||
237 | and | ||
238 | .Xr SSL_set_shutdown 3 ) . | ||
239 | It is also possible that the peer simply shut down the underlying transport and | ||
240 | the shutdown is incomplete. | ||
241 | Call | ||
242 | .Xr SSL_get_error 3 | ||
243 | with the return value to find out whether an error occurred or the connection | ||
244 | was shut down cleanly | ||
245 | .Pq Dv SSL_ERROR_ZERO_RETURN . | ||
246 | .It <0 | ||
247 | The read operation was not successful, because either an error occurred or | ||
248 | action must be taken by the calling process. | ||
249 | Call | ||
250 | .Xr SSL_get_error 3 | ||
251 | with the return value to find out the reason. | ||
252 | .El | ||
253 | .Sh SEE ALSO | ||
254 | .Xr BIO_new 3 , | ||
255 | .Xr ssl 3 , | ||
256 | .Xr SSL_accept 3 , | ||
257 | .Xr SSL_connect 3 , | ||
258 | .Xr SSL_CTX_new 3 , | ||
259 | .Xr SSL_CTX_set_mode 3 , | ||
260 | .Xr SSL_get_error 3 , | ||
261 | .Xr SSL_pending 3 , | ||
262 | .Xr SSL_set_connect_state 3 , | ||
263 | .Xr SSL_set_shutdown 3 , | ||
264 | .Xr SSL_shutdown 3 , | ||
265 | .Xr SSL_write 3 | ||
266 | .Sh HISTORY | ||
267 | .Fn SSL_read | ||
268 | appeared in SSLeay 0.4 or earlier. | ||
269 | .Fn SSL_peek | ||
270 | first appeared in SSLeay 0.6.6. | ||
271 | Both functions have been available since | ||
272 | .Ox 2.4 . | ||
273 | .Pp | ||
274 | .Fn SSL_read_ex | ||
275 | and | ||
276 | .Fn SSL_peek_ex | ||
277 | first appeared in OpenSSL 1.1.1 and have been available since | ||
278 | .Ox 7.1 . | ||