pam_fail_delay(3) — Linux manual page


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

NAME         top

       pam_fail_delay - request a delay on failure

SYNOPSIS         top

       #include <security/pam_appl.h>

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

DESCRIPTION         top

       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

       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

       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         top

       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

       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         top

       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         top

           Delay was successful adjusted.

           A NULL pointer was submitted as PAM handle.

SEE ALSO         top

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

STANDARDS         top

       The pam_fail_delay function is an Linux-PAM extension.

COLOPHON         top

       This page is part of the linux-pam (Pluggable Authentication
       Modules for Linux) project.  Information about the project can be
       found at ⟨⟩.  If you have a bug report
       for this manual page, see ⟨//⟩.  This page was
       obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At
       that time, the date of the most recent commit that was found in
       the repository was 2023-12-18.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

Linux-PAM Manual               12/22/2023              PAM_FAIL_DELAY(3)

Pages that refer to this page: pam_get_item(3)pam_set_item(3)pam_faildelay(8)