1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" Copyright (c) Andrew G. Morgan 1997 <morgan@parc.power.net>
4 .TH PAM_FAIL_DELAY 3 "1997 Jan 12" "Linux-PAM 0.56" "Programmers' Manual"
7 pam_fail_delay \- request a delay on failure
10 .B #include <security/pam_appl.h>
14 .B #include <security/pam_modules.h>
16 .BI "int pam_fail_delay(pam_handle_t " "*pamh" ", unsigned int " "usec" ");"
20 It is often possible to attack an authentication scheme by exploiting
21 the time it takes the scheme to deny access to an applicant user. In
24 timeouts, it may prove possible to attempt a
26 dictionary attack -- with an automated process, the attacker tries all
27 possible passwords to gain access to the system. In other cases,
28 where individual failures can take measurable amounts of time
29 (indicating the nature of the failure), an attacker can obtain useful
30 information about the authentication process. These latter attacks
31 make use of procedural delays that constitute a
33 of useful information.
36 To minimize the effectiveness of such attacks, it is desirable to
37 introduce a random delay in a failed authentication process.
39 provides such a facility. The delay occurs upon failure of the
40 .BR pam_authenticate "(3) "
42 .BR pam_chauthtok "(3) "
45 all authentication modules have been called, but
47 control is returned to the service application.
51 .BR pam_fail_delay "(3),"
52 is used to specify a required minimum for the length of the
55 argument. This function can be called by the service application
56 and/or the authentication modules, both may have an interest in
57 delaying a reapplication for service by the user. The length of the
58 delay is computed at the time it is required. Its length is
59 pseudo-gausianly distributed about the
61 requested value; the resultant delay will differ by as much as 25% of
62 this maximum requested value (both up and down).
66 .BR pam_authenticate "(3) or " pam_chauthtok "(3),"
67 independent of success or failure, the new requested delay is reset to
68 its default value: zero.
74 application may require a failure delay of roughly 3 seconds. It will
75 contain the following code:
78 .B " pam_fail_delay(pamh, 3000000 /* micro-seconds */ );"
80 .B " pam_authenticate(pamh, 0);"
83 if the modules do not request a delay, the failure delay will be
84 between 2.25 and 3.75 seconds.
87 However, the modules, invoked in the authentication process, may
91 .RB " (module #1) " "pam_fail_delay(pamh, 2000000);"
94 .RB " (module #2) " "pam_fail_delay(pamh, 4000000);"
97 in this case, it is the largest requested value that is used to
98 compute the actual failed delay: here between 3 and 5 seconds.
101 Following a successful call to
102 .BR pam_fail_delay "(3), " PAM_SUCCESS
103 is returned. All other returns should be considered serious failures.
106 May be translated to text with
107 .BR pam_strerror "(3). "
110 Under consideration by the X/Open group for future inclusion in the
119 .BR pam_start "(3), "
120 .BR pam_get_item "(3) "
122 .BR pam_strerror "(3). "
127 .BR "System administrators" ", "
128 .BR "module developers" ", "
130 .BR "application developers" ". "