NAME
pam_fail_delay - request a delay on failureSYNOPSIS
#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 */
void (*delay_fn)(int retval, unsigned usec_delay, void *appdata_ptr);
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);
module #1: pam_fail_delay (pamh, 2000000); module #2: pam_fail_delay (pamh, 4000000);
RETURN VALUES
PAM_SUCCESSDelay 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.09/03/2021 | Linux-PAM Manual |