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PAM_FAIL_DELAY(3)	      Programmers Manual	    PAM_FAIL_DELAY(3)

       pam_fail_delay - request a delay on failure


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

       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  procedu
       ral 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.  Linux-PAM
       provides  such  a  facility.   The  delay  occurs  upon	failure of the
       pam_authenticate(3) and pam_chauthtok(3) functions.   It  occurs  after
       all  authentication  modules  have  been  called, but before control is
       returned to the service application.

       The function, pam_fail_delay(3), is used to specify a required  minimum
       for  the length of the failure-delay; the usec argument.  This function
       can be called by the service application and/or the authentication mod
       ules, both may have an interest in delaying a reapplication for service
       by the user.  The length of the delay is computed at  the  time	it  is
       required.  Its length is pseudo-gausianly distributed about the maximum
       requested value; the resultant delay will differ by as much as  25%  of
       this maximum requested value (both up and down).

       On  return from pam_authenticate(3) or pam_chauthtok(3), independent of
       success or failure, the new requested delay is  reset  to  its  default
       value: zero.

       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.

       Following  a  successful  call  to  pam_fail_delay(3),  PAM_SUCCESS  is
       returned.  All other returns should be considered serious failures.

       May be translated to text with pam_strerror(3).

       Under consideration by the X/Open group for future inclusion in the PAM
       RFC. 1996/1/10

       none known.

       pam_start(3), pam_get_item(3) and pam_strerror(3).

       Also, see the three Linux-PAM Guides, for System administrators, module
       developers, and application developers.

Linux-PAM 0.56			  1997 Jan 12		     PAM_FAIL_DELAY(3)

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