Using EXTERNAL mechanism with Cyrus SASL

Question

Does the Cyrus SASL api not support the EXTERNAL mechanism? I'm trying to use it as a client, but it returns SASL_NOMECH when asked.

% cat cyrus_sal_ex.c
/* cyrus_sasl_ex.c: Example of using the Cyrus SASL api */
#include <stdio.h>      /* for printf() */
#include <sasl/sasl.h>  /* for sasl_client_*(), SASL_*, sasl_*_t */

static char const * SASL_return_code(int const code) 
{
  switch(code) 
  {
    /* ... */
    case SASL_OK:     return "SASL_OK[0]: successful result";
    /* ... */
    case SASL_NOMECH: return "SASL_NOMECH[-4]: mechanism not supported";
    /* ... */
  }
  return "unrecognized";
}

int main()
{
  char const *  output = NULL;
  unsigned      outlen = 0;
  char const *  mechanism = NULL;
  sasl_conn_t * conn;

# define PRINT_RESULT( x ) do\
  {\
    int const __result = (x);\
    printf("%s == %d\n\t%s\n", #x, __result, SASL_return_code(__result));\
    if (__result < 0) goto done;\
  }\
  while (0)

  PRINT_RESULT( sasl_client_init( NULL ) );
  PRINT_RESULT( sasl_client_new( "fake", "fakey.mcfaker.ton", "127.0.0.1", "127.255.255.1", NULL, 0, &conn) );
  PRINT_RESULT( sasl_client_start( conn, "EXTERNAL", NULL, &output, &outlen, &mechanism) );

done:
# undef PRINT_RESULT
  printf("output: [%d bytes] : %s\n", outlen, (output ? output : "NULL") );
  printf("mechanism: %s\n", (mechanism ? mechanism : "NULL"));

  return 0;
}
% gcc -I/sw/include -L/sw/lib -lsasl2 cyrus_sasl_ex.c -o cyrus_sasl_ex # your header/library locations may vary
% ./cyrus_sasl_ex
sasl_client_init( NULL ) == 0
        SASL_OK[0]: successful result
sasl_client_new( "fake", "fakey.mcfaker.ton", "127.0.0.1", "127.255.255.1", NULL, 0, &conn) == 0
        SASL_OK[0]: successful result
sasl_client_start( conn, "EXTERNAL", NULL, &output, &outlen, &mechanism) == -4
        SASL_NOMECH[-4]: mechanism not supported
output: [0 bytes] : NULL
mechanism: EXTERNAL
%

I browsed through the source though, and it looks like all the clients should support the EXTERNAL mechanism:

cyrus-sasl-2.1.22/lib/client.c:
196 int sasl_client_init(const sasl_callback_t *callbacks)
197 {
...
227
228   sasl_client_add_plugin("EXTERNAL", &external_client_plug_init);
229

So I'm guessing I'm doing something wrong here. I tried adding all the sasl_callback_ts I could think of to sasl_client_*(), but none of them even got called. Is there some argument I should pass that asserts that EXTERNAL is an acceptable mechanism? Or is SASL_NOMECH always returned for EXTERNAL - b/c that doesn't seem right.

Can anyone help me out?

Answer 1

Ok, I found the left out step.

According to sasl/sasl.h, I needed to set the SASL_AUTH_EXTERNAL property for my sasl_conn_t first:

/* set property in SASL connection state
 * returns:
 *  SASL_OK       -- value set
 *  SASL_BADPARAM -- invalid property or value
 */
LIBSASL_API int sasl_setprop(sasl_conn_t *conn,
           int propnum,
           const void *value);
#define SASL_SSF_EXTERNAL  100  /* external SSF active (sasl_ssf_t *) */
#define SASL_SEC_PROPS     101  /* sasl_security_properties_t */
#define SASL_AUTH_EXTERNAL 102  /* external authentication ID (const char *) */

/* If the SASL_AUTH_EXTERNAL value is non-NULL, then a special version of the
 * EXTERNAL mechanism is enabled (one for server-embedded EXTERNAL mechanisms).
 * Otherwise, the EXTERNAL mechanism will be absent unless a plug-in
 * including EXTERNAL is present.
 */

Once I did that, the rest worked out:

% cat cyrus_sasl_ex.c
/* Example of using the Cyrus SASL api */
#include <stdio.h>    /* for printf() */
#include <sasl/sasl.h>  /* for sasl_client_*(), SASL_*, sasl_*_t */

int main()
{
    char const * output = NULL;
    unsigned   outlen = 0;
    char const * mechanism = NULL;
    sasl_conn_t * conn;

#   define PRINT_RESULT( x ) do\
    {\
     int const __result = (x);\
     printf("%s == %d\n\t%s\n", #x, __result, sasl_errstring(__result,NULL,NULL));\
     if (__result < 0) goto done;\
    }\
    while (0)

    PRINT_RESULT( sasl_client_init( NULL ) );
    PRINT_RESULT( sasl_client_new( "fake", "fakey.mcfaker.ton", "127.0.0.1", "127.255.255.1", NULL, 0, &conn) );
    PRINT_RESULT( sasl_setprop( conn, SASL_AUTH_EXTERNAL, "fake authority" ) );
    PRINT_RESULT( sasl_client_start( conn, "EXTERNAL", NULL, &output, &outlen, &mechanism) );

done:
#   undef PRINT_RESULT
    printf("output: [%d bytes] : %s\n", outlen, (output ? output : "NULL") );
    printf("mechanism: %s\n", (mechanism ? mechanism : "NULL"));

    return 0;
}
% gcc -I/sw/include -L/sw/lib -lsasl2 cyrus_sasl_ex.c -o cyrus_sasl_ex
% ./cyrus_sasl_ex
sasl_client_init( NULL ) == 0
        successful result
sasl_client_new( "fake", "fakey.mcfaker.ton", "127.0.0.1", "127.255.255.1", NULL, 0, &conn) == 0
        successful result
sasl_setprop( conn, SASL_AUTH_EXTERNAL, "fake authority" ) == 0
        successful result
sasl_client_start( conn, "EXTERNAL", NULL, &output, &outlen, &mechanism) == 0
        successful result
output: [0 bytes] :
mechanism: EXTERNAL

However, since the version of Cyrus SASL that comes pre-installed on OS X 10.5 has a bug in it that makes the external plugin require a SASL_CB_USER callback and passes it a NULL pointer to store its return value in, this still means I'll have to update Cyrus SASL on all those machines.

Or maybe I'll just code around the bug.