Merge branch 'vendor/OPENSSL'
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_generate_session_id.3
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129.\" ========================================================================
130.\"
131.IX Title "SSL_CTX_set_generate_session_id 3"
18ed9402 132.TH SSL_CTX_set_generate_session_id 3 "2008-09-27" "0.9.8i" "OpenSSL"
984263bc 133.SH "NAME"
a7d27d5a 134SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id \- manipulate generation of SSL session IDs (server only)
984263bc 135.SH "SYNOPSIS"
e056f0e0 136.IX Header "SYNOPSIS"
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137.Vb 1
138\& #include <openssl/ssl.h>
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139.Ve
140.PP
141.Vb 2
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142\& typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
143\& unsigned int *id_len);
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144.Ve
145.PP
146.Vb 4
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147\& int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
148\& int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
149\& int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
150\& unsigned int id_len);
151.Ve
152.SH "DESCRIPTION"
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153.IX Header "DESCRIPTION"
154\&\fISSL_CTX_set_generate_session_id()\fR sets the callback function for generating
155new session ids for \s-1SSL/TLS\s0 sessions for \fBctx\fR to be \fBcb\fR.
984263bc 156.PP
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157\&\fISSL_set_generate_session_id()\fR sets the callback function for generating
158new session ids for \s-1SSL/TLS\s0 sessions for \fBssl\fR to be \fBcb\fR.
984263bc 159.PP
e056f0e0 160\&\fISSL_has_matching_session_id()\fR checks, whether a session with id \fBid\fR
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161(of length \fBid_len\fR) is already contained in the internal session cache
162of the parent context of \fBssl\fR.
163.SH "NOTES"
e056f0e0 164.IX Header "NOTES"
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165When a new session is established between client and server, the server
166generates a session id. The session id is an arbitrary sequence of bytes.
167The length of the session id is 16 bytes for SSLv2 sessions and between
1681 and 32 bytes for SSLv3/TLSv1. The session id is not security critical
169but must be unique for the server. Additionally, the session id is
170transmitted in the clear when reusing the session so it must not contain
171sensitive information.
172.PP
173Without a callback being set, an OpenSSL server will generate a unique
174session id from pseudo random numbers of the maximum possible length.
175Using the callback function, the session id can be changed to contain
176additional information like e.g. a host id in order to improve load balancing
177or external caching techniques.
178.PP
179The callback function receives a pointer to the memory location to put
e056f0e0 180\&\fBid\fR into and a pointer to the maximum allowed length \fBid_len\fR. The
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181buffer at location \fBid\fR is only guaranteed to have the size \fBid_len\fR.
182The callback is only allowed to generate a shorter id and reduce \fBid_len\fR;
183the callback \fBmust never\fR increase \fBid_len\fR or write to the location
e056f0e0 184\&\fBid\fR exceeding the given limit.
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185.PP
186If a SSLv2 session id is generated and \fBid_len\fR is reduced, it will be
187restored after the callback has finished and the session id will be padded
188with 0x00. It is not recommended to change the \fBid_len\fR for SSLv2 sessions.
e056f0e0 189The callback can use the \fISSL_get_version\fR\|(3) function
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190to check, whether the session is of type SSLv2.
191.PP
192The location \fBid\fR is filled with 0x00 before the callback is called, so the
193callback may only fill part of the possible length and leave \fBid_len\fR
194untouched while maintaining reproducibility.
195.PP
196Since the sessions must be distinguished, session ids must be unique.
197Without the callback a random number is used, so that the probability
198of generating the same session id is extremely small (2^128 possible ids
199for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the
200uniqueness of the generated session id, the callback must call
e056f0e0 201\&\fISSL_has_matching_session_id()\fR and generate another id if a conflict occurs.
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202If an id conflict is not resolved, the handshake will fail.
203If the application codes e.g. a unique host id, a unique process number, and
204a unique sequence number into the session id, uniqueness could easily be
205achieved without randomness added (it should however be taken care that
206no confidential information is leaked this way). If the application can not
207guarantee uniqueness, it is recommended to use the maximum \fBid_len\fR and
208fill in the bytes not used to code special information with random data
209to avoid collisions.
210.PP
e056f0e0 211\&\fISSL_has_matching_session_id()\fR will only query the internal session cache,
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212not the external one. Since the session id is generated before the
213handshake is completed, it is not immediately added to the cache. If
214another thread is using the same internal session cache, a race condition
215can occur in that another thread generates the same session id.
216Collisions can also occur when using an external session cache, since
217the external cache is not tested with \fISSL_has_matching_session_id()\fR
218and the same race condition applies.
219.PP
220When calling \fISSL_has_matching_session_id()\fR for an SSLv2 session with
221reduced \fBid_len\fR, the match operation will be performed using the
222fixed length required and with a 0x00 padded id.
223.PP
224The callback must return 0 if it cannot generate a session id for whatever
225reason and return 1 on success.
226.SH "EXAMPLES"
e056f0e0 227.IX Header "EXAMPLES"
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228The callback function listed will generate a session id with the
229server id given, and will fill the rest with pseudo random bytes:
230.PP
231.Vb 1
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232\& const char session_id_prefix = "www-18";
233.Ve
234.PP
235.Vb 6
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236\& #define MAX_SESSION_ID_ATTEMPTS 10
237\& static int generate_session_id(const SSL *ssl, unsigned char *id,
238\& unsigned int *id_len)
239\& {
240\& unsigned int count = 0;
241\& const char *version;
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242.Ve
243.PP
244.Vb 3
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245\& version = SSL_get_version(ssl);
246\& if (!strcmp(version, "SSLv2"))
247\& /* we must not change id_len */;
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248.Ve
249.PP
250.Vb 17
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251\& do {
252\& RAND_pseudo_bytes(id, *id_len);
253\& /* Prefix the session_id with the required prefix. NB: If our
aac4ff6f 254\& * prefix is too long, clip it - but there will be worse effects
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255\& * anyway, eg. the server could only possibly create 1 session
256\& * ID (ie. the prefix!) so all future session negotiations will
257\& * fail due to conflicts. */
258\& memcpy(id, session_id_prefix,
259\& (strlen(session_id_prefix) < *id_len) ?
260\& strlen(session_id_prefix) : *id_len);
261\& }
262\& while(SSL_has_matching_session_id(ssl, id, *id_len) &&
263\& (++count < MAX_SESSION_ID_ATTEMPTS));
264\& if(count >= MAX_SESSION_ID_ATTEMPTS)
265\& return 0;
266\& return 1;
267\& }
268.Ve
269.SH "RETURN VALUES"
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270.IX Header "RETURN VALUES"
271\&\fISSL_CTX_set_generate_session_id()\fR and \fISSL_set_generate_session_id()\fR
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272always return 1.
273.PP
e056f0e0 274\&\fISSL_has_matching_session_id()\fR returns 1 if another session with the
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275same id is already in the cache.
276.SH "SEE ALSO"
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277.IX Header "SEE ALSO"
278\&\fIssl\fR\|(3), \fISSL_get_version\fR\|(3)
984263bc 279.SH "HISTORY"
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280.IX Header "HISTORY"
281\&\fISSL_CTX_set_generate_session_id()\fR, \fISSL_set_generate_session_id()\fR
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282and \fISSL_has_matching_session_id()\fR have been introduced in
283OpenSSL 0.9.7.