Update build for OpenSSL-0.9.8j upgrade.
[dragonfly.git] / secure / lib / libssl / man / SSL_CTX_set_generate_session_id.3
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132.\" ========================================================================
133.\"
134.IX Title "SSL_CTX_set_generate_session_id 3"
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135.TH SSL_CTX_set_generate_session_id 3 "2009-01-11" "0.9.8j" "OpenSSL"
136.\" For nroff, turn off justification. Always turn off hyphenation; it makes
137.\" way too many mistakes in technical documents.
138.if n .ad l
139.nh
984263bc 140.SH "NAME"
a7d27d5a 141SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id \- manipulate generation of SSL session IDs (server only)
984263bc 142.SH "SYNOPSIS"
e056f0e0 143.IX Header "SYNOPSIS"
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144.Vb 1
145\& #include <openssl/ssl.h>
e257b235 146\&
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147\& typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
148\& unsigned int *id_len);
e257b235 149\&
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150\& int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
151\& int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
152\& int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
153\& unsigned int id_len);
154.Ve
155.SH "DESCRIPTION"
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156.IX Header "DESCRIPTION"
157\&\fISSL_CTX_set_generate_session_id()\fR sets the callback function for generating
158new session ids for \s-1SSL/TLS\s0 sessions for \fBctx\fR to be \fBcb\fR.
984263bc 159.PP
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160\&\fISSL_set_generate_session_id()\fR sets the callback function for generating
161new session ids for \s-1SSL/TLS\s0 sessions for \fBssl\fR to be \fBcb\fR.
984263bc 162.PP
e056f0e0 163\&\fISSL_has_matching_session_id()\fR checks, whether a session with id \fBid\fR
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164(of length \fBid_len\fR) is already contained in the internal session cache
165of the parent context of \fBssl\fR.
166.SH "NOTES"
e056f0e0 167.IX Header "NOTES"
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168When a new session is established between client and server, the server
169generates a session id. The session id is an arbitrary sequence of bytes.
170The length of the session id is 16 bytes for SSLv2 sessions and between
1711 and 32 bytes for SSLv3/TLSv1. The session id is not security critical
172but must be unique for the server. Additionally, the session id is
173transmitted in the clear when reusing the session so it must not contain
174sensitive information.
175.PP
176Without a callback being set, an OpenSSL server will generate a unique
177session id from pseudo random numbers of the maximum possible length.
178Using the callback function, the session id can be changed to contain
179additional information like e.g. a host id in order to improve load balancing
180or external caching techniques.
181.PP
182The callback function receives a pointer to the memory location to put
e056f0e0 183\&\fBid\fR into and a pointer to the maximum allowed length \fBid_len\fR. The
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184buffer at location \fBid\fR is only guaranteed to have the size \fBid_len\fR.
185The callback is only allowed to generate a shorter id and reduce \fBid_len\fR;
186the callback \fBmust never\fR increase \fBid_len\fR or write to the location
e056f0e0 187\&\fBid\fR exceeding the given limit.
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188.PP
189If a SSLv2 session id is generated and \fBid_len\fR is reduced, it will be
190restored after the callback has finished and the session id will be padded
191with 0x00. It is not recommended to change the \fBid_len\fR for SSLv2 sessions.
e056f0e0 192The callback can use the \fISSL_get_version\fR\|(3) function
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193to check, whether the session is of type SSLv2.
194.PP
195The location \fBid\fR is filled with 0x00 before the callback is called, so the
196callback may only fill part of the possible length and leave \fBid_len\fR
197untouched while maintaining reproducibility.
198.PP
199Since the sessions must be distinguished, session ids must be unique.
200Without the callback a random number is used, so that the probability
201of generating the same session id is extremely small (2^128 possible ids
202for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the
203uniqueness of the generated session id, the callback must call
e056f0e0 204\&\fISSL_has_matching_session_id()\fR and generate another id if a conflict occurs.
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205If an id conflict is not resolved, the handshake will fail.
206If the application codes e.g. a unique host id, a unique process number, and
207a unique sequence number into the session id, uniqueness could easily be
208achieved without randomness added (it should however be taken care that
209no confidential information is leaked this way). If the application can not
210guarantee uniqueness, it is recommended to use the maximum \fBid_len\fR and
211fill in the bytes not used to code special information with random data
212to avoid collisions.
213.PP
e056f0e0 214\&\fISSL_has_matching_session_id()\fR will only query the internal session cache,
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215not the external one. Since the session id is generated before the
216handshake is completed, it is not immediately added to the cache. If
217another thread is using the same internal session cache, a race condition
218can occur in that another thread generates the same session id.
219Collisions can also occur when using an external session cache, since
220the external cache is not tested with \fISSL_has_matching_session_id()\fR
221and the same race condition applies.
222.PP
223When calling \fISSL_has_matching_session_id()\fR for an SSLv2 session with
224reduced \fBid_len\fR, the match operation will be performed using the
225fixed length required and with a 0x00 padded id.
226.PP
227The callback must return 0 if it cannot generate a session id for whatever
228reason and return 1 on success.
229.SH "EXAMPLES"
e056f0e0 230.IX Header "EXAMPLES"
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231The callback function listed will generate a session id with the
232server id given, and will fill the rest with pseudo random bytes:
233.PP
234.Vb 1
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235\& const char session_id_prefix = "www\-18";
236\&
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237\& #define MAX_SESSION_ID_ATTEMPTS 10
238\& static int generate_session_id(const SSL *ssl, unsigned char *id,
239\& unsigned int *id_len)
240\& {
241\& unsigned int count = 0;
242\& const char *version;
e257b235 243\&
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244\& version = SSL_get_version(ssl);
245\& if (!strcmp(version, "SSLv2"))
246\& /* we must not change id_len */;
e257b235 247\&
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248\& do {
249\& RAND_pseudo_bytes(id, *id_len);
250\& /* Prefix the session_id with the required prefix. NB: If our
e257b235 251\& * prefix is too long, clip it \- but there will be worse effects
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252\& * anyway, eg. the server could only possibly create 1 session
253\& * ID (ie. the prefix!) so all future session negotiations will
254\& * fail due to conflicts. */
255\& memcpy(id, session_id_prefix,
256\& (strlen(session_id_prefix) < *id_len) ?
257\& strlen(session_id_prefix) : *id_len);
258\& }
259\& while(SSL_has_matching_session_id(ssl, id, *id_len) &&
260\& (++count < MAX_SESSION_ID_ATTEMPTS));
261\& if(count >= MAX_SESSION_ID_ATTEMPTS)
262\& return 0;
263\& return 1;
264\& }
265.Ve
266.SH "RETURN VALUES"
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267.IX Header "RETURN VALUES"
268\&\fISSL_CTX_set_generate_session_id()\fR and \fISSL_set_generate_session_id()\fR
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269always return 1.
270.PP
e056f0e0 271\&\fISSL_has_matching_session_id()\fR returns 1 if another session with the
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272same id is already in the cache.
273.SH "SEE ALSO"
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274.IX Header "SEE ALSO"
275\&\fIssl\fR\|(3), \fISSL_get_version\fR\|(3)
984263bc 276.SH "HISTORY"
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277.IX Header "HISTORY"
278\&\fISSL_CTX_set_generate_session_id()\fR, \fISSL_set_generate_session_id()\fR
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279and \fISSL_has_matching_session_id()\fR have been introduced in
280OpenSSL 0.9.7.