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| 132 | .\" ======================================================================== |
| 133 | .\" |
| 134 | .IX Title "SSL_CTX_set_generate_session_id 3" |
| 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 |
| 140 | .SH "NAME" |
| 141 | SSL_CTX_set_generate_session_id, SSL_set_generate_session_id, SSL_has_matching_session_id \- manipulate generation of SSL session IDs (server only) |
| 142 | .SH "SYNOPSIS" |
| 143 | .IX Header "SYNOPSIS" |
| 144 | .Vb 1 |
| 145 | \& #include <openssl/ssl.h> |
| 146 | \& |
| 147 | \& typedef int (*GEN_SESSION_CB)(const SSL *ssl, unsigned char *id, |
| 148 | \& unsigned int *id_len); |
| 149 | \& |
| 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" |
| 156 | .IX Header "DESCRIPTION" |
| 157 | \&\fISSL_CTX_set_generate_session_id()\fR sets the callback function for generating |
| 158 | new session ids for \s-1SSL/TLS\s0 sessions for \fBctx\fR to be \fBcb\fR. |
| 159 | .PP |
| 160 | \&\fISSL_set_generate_session_id()\fR sets the callback function for generating |
| 161 | new session ids for \s-1SSL/TLS\s0 sessions for \fBssl\fR to be \fBcb\fR. |
| 162 | .PP |
| 163 | \&\fISSL_has_matching_session_id()\fR checks, whether a session with id \fBid\fR |
| 164 | (of length \fBid_len\fR) is already contained in the internal session cache |
| 165 | of the parent context of \fBssl\fR. |
| 166 | .SH "NOTES" |
| 167 | .IX Header "NOTES" |
| 168 | When a new session is established between client and server, the server |
| 169 | generates a session id. The session id is an arbitrary sequence of bytes. |
| 170 | The length of the session id is 16 bytes for SSLv2 sessions and between |
| 171 | 1 and 32 bytes for SSLv3/TLSv1. The session id is not security critical |
| 172 | but must be unique for the server. Additionally, the session id is |
| 173 | transmitted in the clear when reusing the session so it must not contain |
| 174 | sensitive information. |
| 175 | .PP |
| 176 | Without a callback being set, an OpenSSL server will generate a unique |
| 177 | session id from pseudo random numbers of the maximum possible length. |
| 178 | Using the callback function, the session id can be changed to contain |
| 179 | additional information like e.g. a host id in order to improve load balancing |
| 180 | or external caching techniques. |
| 181 | .PP |
| 182 | The callback function receives a pointer to the memory location to put |
| 183 | \&\fBid\fR into and a pointer to the maximum allowed length \fBid_len\fR. The |
| 184 | buffer at location \fBid\fR is only guaranteed to have the size \fBid_len\fR. |
| 185 | The callback is only allowed to generate a shorter id and reduce \fBid_len\fR; |
| 186 | the callback \fBmust never\fR increase \fBid_len\fR or write to the location |
| 187 | \&\fBid\fR exceeding the given limit. |
| 188 | .PP |
| 189 | If a SSLv2 session id is generated and \fBid_len\fR is reduced, it will be |
| 190 | restored after the callback has finished and the session id will be padded |
| 191 | with 0x00. It is not recommended to change the \fBid_len\fR for SSLv2 sessions. |
| 192 | The callback can use the \fISSL_get_version\fR\|(3) function |
| 193 | to check, whether the session is of type SSLv2. |
| 194 | .PP |
| 195 | The location \fBid\fR is filled with 0x00 before the callback is called, so the |
| 196 | callback may only fill part of the possible length and leave \fBid_len\fR |
| 197 | untouched while maintaining reproducibility. |
| 198 | .PP |
| 199 | Since the sessions must be distinguished, session ids must be unique. |
| 200 | Without the callback a random number is used, so that the probability |
| 201 | of generating the same session id is extremely small (2^128 possible ids |
| 202 | for an SSLv2 session, 2^256 for SSLv3/TLSv1). In order to assure the |
| 203 | uniqueness of the generated session id, the callback must call |
| 204 | \&\fISSL_has_matching_session_id()\fR and generate another id if a conflict occurs. |
| 205 | If an id conflict is not resolved, the handshake will fail. |
| 206 | If the application codes e.g. a unique host id, a unique process number, and |
| 207 | a unique sequence number into the session id, uniqueness could easily be |
| 208 | achieved without randomness added (it should however be taken care that |
| 209 | no confidential information is leaked this way). If the application can not |
| 210 | guarantee uniqueness, it is recommended to use the maximum \fBid_len\fR and |
| 211 | fill in the bytes not used to code special information with random data |
| 212 | to avoid collisions. |
| 213 | .PP |
| 214 | \&\fISSL_has_matching_session_id()\fR will only query the internal session cache, |
| 215 | not the external one. Since the session id is generated before the |
| 216 | handshake is completed, it is not immediately added to the cache. If |
| 217 | another thread is using the same internal session cache, a race condition |
| 218 | can occur in that another thread generates the same session id. |
| 219 | Collisions can also occur when using an external session cache, since |
| 220 | the external cache is not tested with \fISSL_has_matching_session_id()\fR |
| 221 | and the same race condition applies. |
| 222 | .PP |
| 223 | When calling \fISSL_has_matching_session_id()\fR for an SSLv2 session with |
| 224 | reduced \fBid_len\fR, the match operation will be performed using the |
| 225 | fixed length required and with a 0x00 padded id. |
| 226 | .PP |
| 227 | The callback must return 0 if it cannot generate a session id for whatever |
| 228 | reason and return 1 on success. |
| 229 | .SH "EXAMPLES" |
| 230 | .IX Header "EXAMPLES" |
| 231 | The callback function listed will generate a session id with the |
| 232 | server id given, and will fill the rest with pseudo random bytes: |
| 233 | .PP |
| 234 | .Vb 1 |
| 235 | \& const char session_id_prefix = "www\-18"; |
| 236 | \& |
| 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; |
| 243 | \& |
| 244 | \& version = SSL_get_version(ssl); |
| 245 | \& if (!strcmp(version, "SSLv2")) |
| 246 | \& /* we must not change id_len */; |
| 247 | \& |
| 248 | \& do { |
| 249 | \& RAND_pseudo_bytes(id, *id_len); |
| 250 | \& /* Prefix the session_id with the required prefix. NB: If our |
| 251 | \& * prefix is too long, clip it \- but there will be worse effects |
| 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" |
| 267 | .IX Header "RETURN VALUES" |
| 268 | \&\fISSL_CTX_set_generate_session_id()\fR and \fISSL_set_generate_session_id()\fR |
| 269 | always return 1. |
| 270 | .PP |
| 271 | \&\fISSL_has_matching_session_id()\fR returns 1 if another session with the |
| 272 | same id is already in the cache. |
| 273 | .SH "SEE ALSO" |
| 274 | .IX Header "SEE ALSO" |
| 275 | \&\fIssl\fR\|(3), \fISSL_get_version\fR\|(3) |
| 276 | .SH "HISTORY" |
| 277 | .IX Header "HISTORY" |
| 278 | \&\fISSL_CTX_set_generate_session_id()\fR, \fISSL_set_generate_session_id()\fR |
| 279 | and \fISSL_has_matching_session_id()\fR have been introduced in |
| 280 | OpenSSL 0.9.7. |