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
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 seconds.
However, the modules, invoked in the authentication process, may also
(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
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)