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
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
additional 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 finished 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 maintaining 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 conflict 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 recommended 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 internal session cache,
not the external one. Since the session 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 session id for whatever
reason and return 1 on success.
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 */;
/* 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. */
(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)