PAM_FAIL_DELAY(3) Linux-PAM Manual PAM_FAIL_DELAY(3)
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.
SEE ALSO
pam_start(3), pam_get_item(3), pam_strerror(3)
STANDARDS
The pam_fail_delay function is an Linux-PAM extension.
Linux-PAM 06/29/2025 PAM_FAIL_DELAY(3)
Generated by dwww version 1.16 on Sat Oct 3 06:01:30 CEST 2026.