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NAME | SYNOPSIS | DESCRIPTION | RATIONALE | EXAMPLE | RETURN VALUES | SEE ALSO | STANDARDS | COLOPHON |
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, if
PAM_FAIL_DELAY item is unset (or set to NULL), then no delay will be
performed.
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 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.
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
tarball Linux-PAM-1.3.0.tar.gz fetched from
⟨http://www.linux-pam.org/library/⟩ on 2017-07-05. 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 man-pages@man7.org
Linux-PAM Manual 04/01/2016 PAM_FAIL_DELAY(3)
Pages that refer to this page: pam_get_item(3), pam_set_item(3), pam_faildelay(8)