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NAME | SYNOPSIS | DESCRIPTION | RATIONALE | EXAMPLE | RETURN VALUES | SEE ALSO | STANDARDS | COLOPHON |
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PAM_FAIL_DELAY(3) Linux-PAM Manual PAM_FAIL_DELAY(3)
pam_fail_delay - request a delay on failure
#include <security/pam_appl.h>
int pam_fail_delay(pam_handle_t *pamh, unsigned int usec);
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.
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.
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.
PAM_SUCCESS
Delay was successful adjusted.
PAM_SYSTEM_ERR
A NULL pointer was submitted as PAM handle.
pam_start(3), pam_get_item(3), pam_strerror(3)
The pam_fail_delay function is an Linux-PAM extension.
This page is part of the linux-pam (Pluggable Authentication
Modules for Linux) project. Information about the project can be
found at ⟨http://www.linux-pam.org/⟩. If you have a bug report
for this manual page, see ⟨//www.linux-pam.org/⟩. This page was
obtained from the project's upstream Git repository
⟨https://github.com/linux-pam/linux-pam.git⟩ on 2023-12-22. (At
that time, the date of the most recent commit that was found in
the repository was 2023-12-18.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
Linux-PAM Manual 12/22/2023 PAM_FAIL_DELAY(3)
Pages that refer to this page: pam_get_item(3), pam_set_item(3), pam_faildelay(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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