NAME

       pam_fail_delay - request a delay on failure

SYNOPSIS

       #include <security/pam_appl.h>

       int pam_fail_delay(pam_handle_t *pamh, unsigned int usec);

DESCRIPTION

       The pam_fail_delay function provides a mechanism by which an application or module can
       suggest a minimum delay of usec micro-seconds. The function keeps a record of the longest
       time requested with this function. Should pam_authenticate(3) fail, the failing return to
       the application is delayed by an amount of time randomly distributed (by up to 50%) about
       this longest value.

       Independent of success, the delay time is reset to its zero default value when the PAM
       service module returns control to the application. The delay occurs after all
       authentication modules have been called, but before control is returned to the service
       application.

       When using this function the programmer should check if it is available with:

           #ifdef HAVE_PAM_FAIL_DELAY
               ....
           #endif /* HAVE_PAM_FAIL_DELAY */

       For applications written with a single thread that are event driven in nature, generating
       this delay may be undesirable. Instead, the application may want to register the delay in
       some other way. For example, in a single threaded server that serves multiple
       authentication requests from a single event loop, the application might want to simply
       mark a given connection as blocked until an application timer expires. For this reason the
       delay function can be changed with the PAM_FAIL_DELAY item. It can be queried and set with
       pam_get_item(3) and pam_set_item(3) respectively. The value used to set it should be a
       function pointer of the following prototype:

           void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);

       The arguments being the retval return code of the module stack, the usec_delay
       micro-second delay that libpam is requesting and the appdata_ptr that the application has
       associated with the current pamh. This last value was set by the application when it
       called pam_start(3) or explicitly with pam_set_item(3).

       Note that the PAM_FAIL_DELAY item is set to NULL by default. This indicates that PAM
       should perform a random delay as described above when authentication fails and a delay has
       been suggested. If an application does not want the PAM library to perform any delay on
       authentication failure, then the application must define a custom delay function that
       executes no statements and set the PAM_FAIL_DELAY item to point to this function.

RATIONALE

       It is often possible to attack an authentication scheme by exploiting the time it takes
       the scheme to deny access to an applicant user. In cases of short timeouts, it may prove
       possible to attempt a brute force dictionary attack -- with an automated process, the
       attacker tries all possible passwords to gain access to the system. In other cases, where
       individual failures can take measurable amounts of time (indicating the nature of the
       failure), an attacker can obtain useful information about the authentication process.
       These latter attacks make use of procedural delays that constitute a covert channel of
       useful information.

       To minimize the effectiveness of such attacks, it is desirable to introduce a random delay
       in a failed authentication process. Preferable this value should be set by the application
       or a special PAM module. Standard PAM modules should not modify the delay unconditional.

EXAMPLE

       For example, a login application may require a failure delay of roughly 3 seconds. It will
       contain the following code:

               pam_fail_delay (pamh, 3000000 /* micro-seconds */ );
               pam_authenticate (pamh, 0);

       if the modules do not request a delay, the failure delay will be between 1.5 and 4.5
       seconds.

       However, the modules, invoked in the authentication process, may also request delays:

           module #1:    pam_fail_delay (pamh, 2000000);
           module #2:    pam_fail_delay (pamh, 4000000);

       in this case, it is the largest requested value that is used to compute the actual failed
       delay: here between 2 and 6 seconds.

RETURN VALUES

       PAM_SUCCESS
           Delay was successful adjusted.

       PAM_SYSTEM_ERR
           A NULL pointer was submitted as PAM handle.

SEE ALSO

       pam_start(3), pam_get_item(3), pam_strerror(3)

STANDARDS

       The pam_fail_delay function is an Linux-PAM extension.