phpman > man > pam_fail_delay(3)

PAM_FAIL_DELAY(3)                         Linux-PAM Manual                         PAM_FAIL_DELAY(3)



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.



Linux-PAM Manual                             06/08/2020                            PAM_FAIL_DELAY(3)