PAM_FAIL_DELAY(3) Programmers' Manual PAM_FAIL_DELAY(3)
NAME
pam_fail_delay - request a delay on failure
SYNOPSIS
#include <security/pam_appl.h>
or,
#include <security/pam_modules.h>
int pam_fail_delay(pam_handle_t *pamh, unsigned int usec);
DESCRIPTION
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. 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; failuredelay;
delay; the usec argument. This function can be called by the service application and/or the authen-tication authentication
tication modules, 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 distrib-uted distributed
uted 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 value: zero.
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 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 VALUE
Following a successful call to pam_fail_delay(3), PAM_SUCCESS is returned. All other returns should
be considered serious failures.
ERRORS
May be translated to text with pam_strerror(3).
CONFORMING TO
Under consideration by the X/Open group for future inclusion in the PAM RFC. 1996/1/10
BUGS
none known.
SEE ALSO
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)
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