SSL_CTX_set_generate_sessionOSSL_CTX_set_generate_session_id(3)





NAME
       SSL_CTX_set_generate_session_id, SSL_set_generate_ses-
       sion_id, SSL_has_matching_session_id - manipulate gener-
       ation of SSL session IDs (server only)

SYNOPSIS
        #include <openssl/ssl.h>

        typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id,
                                      unsigned int *id_len);

        int SSL_CTX_set_generate_session_id(SSL_CTX *ctx, GEN_SESSION_CB cb);
        int SSL_set_generate_session_id(SSL *ssl, GEN_SESSION_CB, cb);
        int SSL_has_matching_session_id(const SSL *ssl, const unsigned char *id,
                                        unsigned int id_len);

DESCRIPTION
       SSL_CTX_set_generate_session_id() sets the callback
       function for generating new session ids for SSL/TLS ses-
       sions for ctx to be cb.

       SSL_set_generate_session_id() sets the callback function
       for generating new session ids for SSL/TLS sessions for
       ssl to be cb.

       SSL_has_matching_session_id() checks, whether a session
       with id id (of length id_len) is already contained in
       the internal session cache of the parent context of ssl.

NOTES
       When a new session is established between client and
       server, the server generates a session id. The session
       id is an arbitrary sequence of bytes.  The length of the
       session id is 16 bytes for SSLv2 sessions and between 1
       and 32 bytes for SSLv3/TLSv1. The session id is not
       security critical but must be unique for the server.
       Additionally, the session id is transmitted in the clear
       when reusing the session so it must not contain sensi-
       tive information.

       Without a callback being set, an OpenSSL server will
       generate a unique session id from pseudo random numbers
       of the maximum possible length.  Using the callback
       function, the session id can be changed to contain addi-
       tional information like e.g. a host id in order to
       improve load balancing or external caching techniques.

       The callback function receives a pointer to the memory
       location to put id into and a pointer to the maximum
       allowed length id_len. The buffer at location id is only
       guaranteed to have the size id_len.  The callback is
       only allowed to generate a shorter id and reduce id_len;
       the callback must never increase id_len or write to the
       location id exceeding the given limit.

       If a SSLv2 session id is generated and id_len is
       reduced, it will be restored after the callback has fin-
       ished and the session id will be padded with 0x00. It is
       not recommended to change the id_len for SSLv2 sessions.
       The callback can use the SSL_get_version(3) function to
       check, whether the session is of type SSLv2.

       The location id is filled with 0x00 before the callback
       is called, so the callback may only fill part of the
       possible length and leave id_len untouched while main-
       taining reproducibility.

       Since the sessions must be distinguished, session ids
       must be unique.  Without the callback a random number is
       used, so that the probability of generating the same
       session id is extremely small (2^128 possible ids for an
       SSLv2 session, 2^256 for SSLv3/TLSv1). In order to
       assure the uniqueness of the generated session id, the
       callback must call SSL_has_matching_session_id() and
       generate another id if a conflict occurs.  If an id con-
       flict is not resolved, the handshake will fail.  If the
       application codes e.g. a unique host id, a unique
       process number, and a unique sequence number into the
       session id, uniqueness could easily be achieved without
       randomness added (it should however be taken care that
       no confidential information is leaked this way). If the
       application can not guarantee uniqueness, it is recom-
       mended to use the maximum id_len and fill in the bytes
       not used to code special information with random data to
       avoid collisions.

       SSL_has_matching_session_id() will only query the inter-
       nal session cache, not the external one. Since the ses-
       sion id is generated before the handshake is completed,
       it is not immediately added to the cache. If another
       thread is using the same internal session cache, a race
       condition can occur in that another thread generates the
       same session id.  Collisions can also occur when using
       an external session cache, since the external cache is
       not tested with SSL_has_matching_session_id() and the
       same race condition applies.

       When calling SSL_has_matching_session_id() for an SSLv2
       session with reduced id_len, the match operation will be
       performed using the fixed length required and with a
       0x00 padded id.

       The callback must return 0 if it cannot generate a ses-
       sion id for whatever reason and return 1 on success.

EXAMPLES
       The callback function listed will generate a session id
       with the server id given, and will fill the rest with
       pseudo random bytes:

        const char session_id_prefix = "www-18";

        #define MAX_SESSION_ID_ATTEMPTS 10
        static int generate_session_id(const SSL *ssl, unsigned char *id,
                                     unsigned int *id_len)
             {
             unsigned int count = 0;
             const char *version;

             version = SSL_get_version(ssl);
             if (!strcmp(version, "SSLv2"))
                 /* we must not change id_len */;












             do      {
                     RAND_pseudo_bytes(id, *id_len);
                     /* Prefix the session_id with the required prefix. NB: If our
                      * prefix is too long, clip it - but there will be worse effects
                      * anyway, eg. the server could only possibly create 1 session
                      * ID (ie. the prefix!) so all future session negotiations will
                      * fail due to conflicts. */
                     memcpy(id, session_id_prefix,
                             (strlen(session_id_prefix) < *id_len) ?
                             strlen(session_id_prefix) : *id_len);
                     }
             while(SSL_has_matching_session_id(ssl, id, *id_len) &&
                     (++count < MAX_SESSION_ID_ATTEMPTS));
             if(count >= MAX_SESSION_ID_ATTEMPTS)
                     return 0;
             return 1;
             }

RETURN VALUES
       SSL_CTX_set_generate_session_id() and SSL_set_gener-
       ate_session_id() always return 1.

       SSL_has_matching_session_id() returns 1 if another ses-
       sion with the same id is already in the cache.

SEE ALSO
       ssl(3), SSL_get_version(3)

HISTORY
       SSL_CTX_set_generate_session_id(), SSL_set_generate_ses-
       sion_id() and SSL_has_matching_session_id() have been
       introduced in OpenSSL 0.9.7.



0.9.7c                     20SSL_CTX_set_generate_session_id(3)
