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 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, if
       PAM_FAIL_DELAY item is unset (or set to NULL), then no delay will be

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 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

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 2.25 and 3.75 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 3 and 5 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 tarball Linux-PAM-1.3.0.tar.gz
       fetched from ⟨⟩ on 2017-03-13.  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                 04/01/2016                PAM_FAIL_DELAY(3)