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; the
usec argument. This function can be called by the service application
and/or the authentication 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 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 value: zero.
EXAMPLE
For example, a
login application may require a failure delay of roughly 3 seconds. It will
contain the following code:
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:
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" ". "
