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Mozilla LDAP C SDK Programmer's Guide

Chapter 13 - Using SASL Authentication

This chapter describes the process of using a SASL mechanism to authenticate an LDAP client to an LDAP server.

The chapter includes the following sections:

Understanding SASL

Simple Authentication and Security Layer (SASL) is described in RFC 2222, which you can find at this location:

SASL provides the means to use mechanisms other than simple authentication and SSL to authenticate to the Mozilla LDAP C SDK.

The ability to authenticate to an LDAP server using a SASL mechanism is a feature new to LDAPv3 (LDAPv2 servers do not support this method of authentication).

Determining the SASL Mechanisms Supported

To determine the SASL mechanisms supported by an LDAPv3 server, get the root DSE of the server, and check the supportedSASLMechanisms attribute. The values of this attribute are the names of the SASL mechanisms supported by the server.

If the root DSE does not have a supportedSASLMechanisms attribute, the server does not support any SASL mechanisms.

For information on getting the root DSE, see "Getting the Root DSE".

Authenticating Using a SASL Mechanism

To authenticate to the server using a SASL mechanism, call one of the following functions:

For more information about the difference between synchronous and asynchronous functions, see "Calling Synchronous and Asynchronous Functions".

If you call the asynchronous function ldap_sasl_bind(), you need to call the ldap_result() and ldap_parse_sasl_bind_result() functions to get the result of the SASL bind operation.

Authentication using a SASL mechanism may take one or more round trips between your LDAP client and the server. (The server may send a number of "challenges" to the client.) You may need to call ldap_sasl_bind_s() several times (or ldap_sasl_bind(), ldap_result(), and ldap_parse_sasl_bind_result() several times) in order to respond to each server challenge.

Before calling the function to perform a SASL bind operation, make sure to specify that your client is LDAPv3 compliant. If you do not, an LDAP_NOT_SUPPORTED result code is returned. For details, see "Specifying the LDAP Version of Your Client".

Performing a Synchronous SASL Bind Operation

If you want to wait for the results of the SASL bind operation to complete before continuing, call the synchronous ldap_sasl_bind_s() function. This function sends a SASL bind request to the server and blocks until the server sends the results of the operation back to your client.

The ldap_sasl_bind_s() function returns one of the following values:

  • LDAP_SUCCESS if your client has successfully authenticated.
  • LDAP_SASL_BIND_IN_PROGRESS if the server sends a challenge to your client. If you receive this result code, check the servercredp argument for the berval structure containing the server's challenge. Call the ldap_sasl_bind_s() function again to send a response to that challenge.

  • An LDAP error code, if a problem occurred or if authentication failed.

See the documentation on the ldap_sasl_bind_s() function for a list of the possible result codes.

Performing an Asynchronous SASL Bind Operation

If you want to perform other work (in parallel) while waiting the SASL bind operation to complete, do the following:

  1. To send an LDAP SASL bind request, call the asynchronous ldap_sasl_bind() function.

    This function returns an LDAP_SUCCESS result code if the request was successfully sent (or an LDAP result code if an error occurred while sending the request). The function also sets the msgidp argument to point to a message ID identifying the SASL bind operation.
  2. To determine whether the server sent a response for this operation to your client, call the ldap_result() function and pass in this message ID.

    The ldap_result() function uses the message ID to determine if the server sent a SASL bind response. The function passes back the response in an LDAPMessage structure.
  3. Call the ldap_parse_sasl_bind_result() function to parse the LDAPMessage structure and retrieve information from the server's response.

    If the server sent a challenge to your client, the challenge is specified in the berval structure passed back as the servercredp argument.
  4. Call the ldap_get_lderrno() function to get the LDAP result code for the operation. The function can return one of the following values:

    • LDAP_SUCCESS if your client successfully authenticated to the server.
    • LDAP_SASL_BIND_IN_PROGRESS if the server sent a challenge to your client.
    • An LDAP error code, if a problem occurred or if authentication failed.
    See the documentation on the return values of the ldap_sasl_bind_s() function for a list of result codes that the server can return for this operation.

    If the server returned an LDAP_SASL_BIND_IN_PROGRESS result code, check the servercredp argument for the berval structure containing the server's challenge.
  5. If the result code is LDAP_SASL_BIND_IN_PROGRESS and the server passed back another challenge, determine the response to that challenge and call the ldap_sasl_bind() function again to send that response to the server.

    You can call ldap_result() and ldap_parse_sasl_bind_result() again to get the next challenge sent from the server, if the result is LDAP_SASL_BIND_IN_PROGRESS again.

The following example is an LDAP client that authenticates using the SASL mechanism named babsmechanism.

Code Example 13-1 - Authenticating over SASL

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include "ldap.h"

main( int argc, char **argv )
    LDAP        *ld;
    LDAPMod     mod0;
    LDAPMod     mod1;
    LDAPMod     *mods[ 3 ];
    char        *vals0[ 2 ];
    char        *vals1[ 2 ];
    time_t      now;
    char        buf[ 128 ];
    struct berval   cred;
    struct berval   *servcred;
    int         version;
    /* get a handle to an LDAP connection */
    if ( (ld = ldap_init( "localhost", 389 )) == NULL ) {
  perror( "ldap_init" );
  return( 1 );
    /* Set the LDAP protocol version supported by the client
       to 3. (By default, this is set to 2. SASL authentication
       is part of version 3 of the LDAP protocol.) */
    version = LDAP_VERSION3;
    ldap_set_option( ld, LDAP_OPT_PROTOCOL_VERSION, &version );
    /* authenticate */
    cred.bv_val = "magic";
    cred.bv_len = sizeof( "magic" ) - 1;
    if ( ldap_sasl_bind_s( ld, "uid=bjensen,ou=people,dc=example,dc=com", \ 
      "babsmechanism", &cred, NULL, NULL, &servcred ) != LDAP_SUCCESS ) {
  ldap_perror( ld, "ldap_sasl_bind_s" );
  return( 1 );
    /* get and print the credentials returned by the server */
    printf( "Server credentials: %s\n", servcred->bv_val );
    /* construct the list of modifications to make */
    mod0.mod_op = LDAP_MOD_REPLACE;
    mod0.mod_type = "mail";
    vals0[0] = "";
    vals0[1] = NULL;
    mod0.mod_values = vals0;
    mod1.mod_op = LDAP_MOD_ADD;
    mod1.mod_type = "description";
    time( &now );
    sprintf( buf, "This entry was modified with the modattrs program on %s",
      ctime( &now ));
    /* Get rid of \n which ctime put on the end of the time string */
    if ( buf[ strlen( buf ) - 1 ] == '\n' ) {
  buf[ strlen( buf ) - 1 ] = '\0';
    vals1[ 0 ] = buf;
    vals1[ 1 ] = NULL;
    mod1.mod_values = vals1;
    mods[ 0 ] = &mod0;
    mods[ 1 ] = &mod1;
    mods[ 2 ] = NULL;
    /* make the change */
    if ( ldap_modify_s( ld, "uid=bjensen,ou=people,dc=example,dc=com", mods )
      != LDAP_SUCCESS ) {
  ldap_perror( ld, "ldap_modify_s" );
  return( 1 );
    ldap_unbind( ld );
    printf( "modification was successful\n" );
    return( 0 );