# pam_fail_delay

## NAME



           pam_fail_delay - request a delay on failure
    


## 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 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 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



           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.
    


## EXAMPLE



           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



           PAM_SUCCESS
               Delay was successful adjusted.
    
           PAM_SYSTEM_ERR
               A NULL pointer was submitted as PAM handle.
    


## STANDARDS



           The **pam_fail_delay** function is an Linux-PAM extension.
    


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