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 procedural delays that constitute a covert channel of
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
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 applica-
tion and/or the authentication modules, both may have an interest in delaying a reapplica-
tion 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
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 */ );
if the modules do not request a delay, the failure delay will be between 2.25 and 3.75
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
pam_start(3), pam_get_item(3) and pam_strerror(3).
Also, see the three Linux-PAM Guides, for System administrators, module developers, and
Linux-PAM 0.56 1997 Jan 12 PAM_FAIL_DELAY(3)