pam_fail_delay man page on Archlinux

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

       pam_fail_delay - request a delay on failure

       #include <security/pam_appl.h>

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

       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 25%) 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

	   #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 performed.

       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.

       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.

	   Delay was successful adjusted.

	   A NULL pointer was submitted as PAM handle.

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

       The pam_fail_delay function is an Linux-PAM extension.

Linux-PAM Manual		  09/19/2013		     PAM_FAIL_DELAY(3)

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