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    Command:

    pam_fail_delay

    
    
    

    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 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
           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 performed.
    
    
    

    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
    
                   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

           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 03/02/2009 PAM_FAIL_DELAY(3)

    
    
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