1SSL_CTX_SET_GENERATE_SESSION_ID(3)  OpenSSL SSL_CTX_SET_GENERATE_SESSION_ID(3)
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NAME

6       SSL_CTX_set_generate_session_id, SSL_set_generate_session_id,
7       SSL_has_matching_session_id, GEN_SESSION_CB - manipulate generation of
8       SSL session IDs (server only)
9

SYNOPSIS

11        #include <openssl/ssl.h>
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13        typedef int (*GEN_SESSION_CB)(SSL *ssl, unsigned char *id,
14                                      unsigned int *id_len);
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16        int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
17        int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
18        int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
19                                        unsigned int id_len);
20

DESCRIPTION

22       SSL_CTX_set_generate_session_id() sets the callback function for
23       generating new session ids for SSL/TLS sessions for ctx to be cb.
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25       SSL_set_generate_session_id() sets the callback function for generating
26       new session ids for SSL/TLS sessions for ssl to be cb.
27
28       SSL_has_matching_session_id() checks, whether a session with id id (of
29       length id_len) is already contained in the internal session cache of
30       the parent context of ssl.
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NOTES

33       When a new session is established between client and server, the server
34       generates a session id. The session id is an arbitrary sequence of
35       bytes.  The length of the session id is between 1 and 32 bytes.  The
36       session id is not security critical but must be unique for the server.
37       Additionally, the session id is transmitted in the clear when reusing
38       the session so it must not contain sensitive information.
39
40       Without a callback being set, an OpenSSL server will generate a unique
41       session id from pseudo random numbers of the maximum possible length.
42       Using the callback function, the session id can be changed to contain
43       additional information like e.g. a host id in order to improve load
44       balancing or external caching techniques.
45
46       The callback function receives a pointer to the memory location to put
47       id into and a pointer to the maximum allowed length id_len. The buffer
48       at location id is only guaranteed to have the size id_len.  The
49       callback is only allowed to generate a shorter id and reduce id_len;
50       the callback must never increase id_len or write to the location id
51       exceeding the given limit.
52
53       The location id is filled with 0x00 before the callback is called, so
54       the callback may only fill part of the possible length and leave id_len
55       untouched while maintaining reproducibility.
56
57       Since the sessions must be distinguished, session ids must be unique.
58       Without the callback a random number is used, so that the probability
59       of generating the same session id is extremely small (2^256 for
60       SSLv3/TLSv1).  In order to assure the uniqueness of the generated
61       session id, the callback must call SSL_has_matching_session_id() and
62       generate another id if a conflict occurs.  If an id conflict is not
63       resolved, the handshake will fail.  If the application codes e.g. a
64       unique host id, a unique process number, and a unique sequence number
65       into the session id, uniqueness could easily be achieved without
66       randomness added (it should however be taken care that no confidential
67       information is leaked this way). If the application can not guarantee
68       uniqueness, it is recommended to use the maximum id_len and fill in the
69       bytes not used to code special information with random data to avoid
70       collisions.
71
72       SSL_has_matching_session_id() will only query the internal session
73       cache, not the external one. Since the session id is generated before
74       the handshake is completed, it is not immediately added to the cache.
75       If another thread is using the same internal session cache, a race
76       condition can occur in that another thread generates the same session
77       id.  Collisions can also occur when using an external session cache,
78       since the external cache is not tested with
79       SSL_has_matching_session_id() and the same race condition applies.
80
81       The callback must return 0 if it cannot generate a session id for
82       whatever reason and return 1 on success.
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RETURN VALUES

85       SSL_CTX_set_generate_session_id() and SSL_set_generate_session_id()
86       always return 1.
87
88       SSL_has_matching_session_id() returns 1 if another session with the
89       same id is already in the cache.
90

EXAMPLES

92       The callback function listed will generate a session id with the server
93       id given, and will fill the rest with pseudo random bytes:
94
95        const char session_id_prefix = "www-18";
96
97        #define MAX_SESSION_ID_ATTEMPTS 10
98        static int generate_session_id(SSL *ssl, unsigned char *id,
99                                       unsigned int *id_len)
100        {
101            unsigned int count = 0;
102
103            do {
104                RAND_pseudo_bytes(id, *id_len);
105                /*
106                 * Prefix the session_id with the required prefix. NB: If our
107                 * prefix is too long, clip it - but there will be worse effects
108                 * anyway, e.g. the server could only possibly create 1 session
109                 * ID (i.e. the prefix!) so all future session negotiations will
110                 * fail due to conflicts.
111                 */
112                memcpy(id, session_id_prefix, strlen(session_id_prefix) < *id_len ?
113                                              strlen(session_id_prefix) : *id_len);
114            } while (SSL_has_matching_session_id(ssl, id, *id_len)
115                      && ++count < MAX_SESSION_ID_ATTEMPTS);
116            if (count >= MAX_SESSION_ID_ATTEMPTS)
117                return 0;
118            return 1;
119        }
120

SEE ALSO

122       ssl(7), SSL_get_version(3)
123
125       Copyright 2001-2020 The OpenSSL Project Authors. All Rights Reserved.
126
127       Licensed under the OpenSSL license (the "License").  You may not use
128       this file except in compliance with the License.  You can obtain a copy
129       in the file LICENSE in the source distribution or at
130       <https://www.openssl.org/source/license.html>.
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1341.1.1i                            2021-01-26SSL_CTX_SET_GENERATE_SESSION_ID(3)
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