mod_unique_id.c

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/*
 * mod_unique_id.c: generate a unique identifier for each request
 *
 * Original author: Dean Gaudet <[e-mail]>
 * UUencoding modified by: Alvaro Martinez Echevarria <[e-mail]>
 */

#include "httpd.h"
#include "http_config.h"
#include "http_log.h"
#include "multithread.h"

#ifdef MULTITHREAD
#error sorry this module does not support multithreaded servers yet
#endif

typedef struct {
    unsigned int stamp;
    unsigned int in_addr;
    unsigned int pid;
    unsigned short counter;
} unique_id_rec;

/* Comments:
 *
 * We want an identifier which is unique across all hits, everywhere.
 * "everywhere" includes multiple httpd instances on the same machine, or on
 * multiple machines.  Essentially "everywhere" should include all possible
 * httpds across all servers at a particular "site".  We make some assumptions
 * that if the site has a cluster of machines then their time is relatively
 * synchronized.  We also assume that the first address returned by a
 * gethostbyname (gethostname()) is unique across all the machines at the
 * "site".
 *
 * We also further assume that pids fit in 32-bits.  If something uses more
 * than 32-bits, the fix is trivial, but it requires the unrolled uuencoding
 * loop to be extended.  * A similar fix is needed to support multithreaded
 * servers, using a pid/tid combo.
 *
 * Together, the in_addr and pid are assumed to absolutely uniquely identify
 * this one child from all other currently running children on all servers
 * (including this physical server if it is running multiple httpds) from each
 * other.
 *
 * The stamp and counter are used to distinguish all hits for a particular
 * (in_addr,pid) pair.  The stamp is updated using r->request_time,
 * saving cpu cycles.  The counter is never reset, and is used to permit up to
 * 64k requests in a single second by a single child.
 *
 * The 112-bits of unique_id_rec are uuencoded using the alphabet
 * [A-Za-z0-9@-], resulting in 19 bytes of printable characters.  That is then
 * stuffed into the environment variable UNIQUE_ID so that it is available to
 * other modules.  The alphabet choice differs from normal base64 encoding
 * [A-Za-z0-9+/] because + and / are special characters in URLs and we want to
 * make it easy to use UNIQUE_ID in URLs.
 *
 * Note that UNIQUE_ID should be considered an opaque token by other
 * applications.  No attempt should be made to dissect its internal components.
 * It is an abstraction that may change in the future as the needs of this
 * module change.
 *
 * It is highly desirable that identifiers exist for "eternity".  But future
 * needs (such as much faster webservers, moving to 64-bit pids, or moving to a
 * multithreaded server) may dictate a need to change the contents of
 * unique_id_rec.  Such a future implementation should ensure that the first
 * field is still a time_t stamp.  By doing that, it is possible for a site to
 * have a "flag second" in which they stop all of their old-format servers,
 * wait one entire second, and then start all of their new-servers.  This
 * procedure will ensure that the new space of identifiers is completely unique
 * from the old space.  (Since the first four unencoded bytes always differ.)
 */
/*
 * Sun Jun  7 05:43:49 CEST 1998 -- Alvaro
 * More comments:
 * 1) The UUencoding prodecure is now done in a general way, avoiding the problems
 * with sizes and paddings that can arise depending on the architecture. Now the
 * offsets and sizes of the elements of the unique_id_rec structure are calculated
 * in unique_id_global_init; and then used to duplicate the structure without the
 * paddings that might exist. The multithreaded server fix should be now very easy:
 * just add a new "tid" field to the unique_id_rec structure, and increase by one
 * UNIQUE_ID_REC_MAX.
 * 2) unique_id_rec.stamp has been changed from "time_t" to "unsigned int", because
 * its size is 64bits on some platforms (linux/alpha), and this caused problems with
 * htonl/ntohl. Well, this shouldn't be a problem till year 2106.
 */

static unsigned global_in_addr;

static APACHE_TLS unique_id_rec cur_unique_id;

/*
 * Number of elements in the structure unique_id_rec.
 */
#define UNIQUE_ID_REC_MAX 4

static unsigned short unique_id_rec_offset[UNIQUE_ID_REC_MAX],
                      unique_id_rec_size[UNIQUE_ID_REC_MAX],
                      unique_id_rec_total_size,
                      unique_id_rec_size_uu;

static void unique_id_global_init(server_rec *s, pool *p)
{
#ifndef MAXHOSTNAMELEN
#define MAXHOSTNAMELEN 256
#endif
    char str[MAXHOSTNAMELEN + 1];
    struct hostent *hent;
#ifndef NO_GETTIMEOFDAY
    struct timeval tv;
#endif

    /*
     * Calculate the sizes and offsets in cur_unique_id.
     */
    unique_id_rec_offset[0] = XtOffsetOf(unique_id_rec, stamp);
    unique_id_rec_size[0] = sizeof(cur_unique_id.stamp);
    unique_id_rec_offset[1] = XtOffsetOf(unique_id_rec, in_addr);
    unique_id_rec_size[1] = sizeof(cur_unique_id.in_addr);
    unique_id_rec_offset[2] = XtOffsetOf(unique_id_rec, pid);
    unique_id_rec_size[2] = sizeof(cur_unique_id.pid);
    unique_id_rec_offset[3] = XtOffsetOf(unique_id_rec, counter);
    unique_id_rec_size[3] = sizeof(cur_unique_id.counter);
    unique_id_rec_total_size = unique_id_rec_size[0] + unique_id_rec_size[1] +
                               unique_id_rec_size[2] + unique_id_rec_size[3];

    /*
     * Calculate the size of the structure when uuencoded.
     */
    unique_id_rec_size_uu = (unique_id_rec_total_size*8+5)/6;

    /*
     * Now get the global in_addr.  Note that it is not sufficient to use one
     * of the addresses from the main_server, since those aren't as likely to
     * be unique as the physical address of the machine
     */
    if (gethostname(str, sizeof(str) - 1) != 0) {
        ap_log_error(APLOG_MARK, APLOG_NOERRNO|APLOG_ALERT, s,
          "gethostname: mod_unique_id requires the hostname of the server");
        exit(1);
    }
    str[sizeof(str) - 1] = '\0';

    if ((hent = gethostbyname(str)) == NULL) {
        ap_log_error(APLOG_MARK, APLOG_NOERRNO|APLOG_ALERT, s,