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.\" Hey Emacs! This file is -*- nroff -*- source.
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.\" $Id: pam_fail_delay.3,v 1.4 2002/09/21 18:19:19 hartmans Exp $
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.\" Copyright (c) Andrew G. Morgan 1997 <morgan@parc.power.net>
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.TH PAM_FAIL_DELAY 3 "1997 Jan 12" "Linux-PAM 0.56" "Programmers' Manual"
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pam_fail_delay \- request a delay on failure
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.B #include <security/pam_appl.h>
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.B #include <security/pam_modules.h>
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.BI "int pam_fail_delay(pam_handle_t " "*pamh" ", unsigned int " "usec" ");"
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It is often possible to attack an authentication scheme by exploiting
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the time it takes the scheme to deny access to an applicant user. In
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timeouts, it may prove possible to attempt a
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dictionary attack -- with an automated process, the attacker tries all
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possible passwords to gain access to the system. In other cases,
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where individual failures can take measurable amounts of time
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(indicating the nature of the failure), an attacker can obtain useful
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information about the authentication process. These latter attacks
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make use of procedural delays that constitute a
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of useful information.
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To minimize the effectiveness of such attacks, it is desirable to
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introduce a random delay in a failed authentication process.
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provides such a facility. The delay occurs upon failure of the
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.BR pam_authenticate "(3) "
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.BR pam_chauthtok "(3) "
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all authentication modules have been called, but
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control is returned to the service application.
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.BR pam_fail_delay "(3),"
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is used to specify a required minimum for the length of the
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argument. This function can be called by the service application
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and/or the authentication modules, both may have an interest in
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delaying a reapplication for service by the user. The length of the
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delay is computed at the time it is required. Its length is
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pseudo-gausianly distributed about the
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requested value; the resultant delay will differ by as much as 25% of
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this maximum requested value (both up and down).
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.BR pam_authenticate "(3) or " pam_chauthtok "(3),"
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independent of success or failure, the new requested delay is reset to
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its default value: zero.
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application may require a failure delay of roughly 3 seconds. It will
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contain the following code:
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.B " pam_fail_delay(pamh, 3000000 /* micro-seconds */ );"
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.B " pam_authenticate(pamh, 0);"
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if the modules do not request a delay, the failure delay will be
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between 2.25 and 3.75 seconds.
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However, the modules, invoked in the authentication process, may
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.RB " (module #1) " "pam_fail_delay(pamh, 2000000);"
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.RB " (module #2) " "pam_fail_delay(pamh, 4000000);"
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in this case, it is the largest requested value that is used to
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compute the actual failed delay: here between 3 and 5 seconds.
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Following a successful call to
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.BR pam_fail_delay "(3), " PAM_SUCCESS
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is returned. All other returns should be considered serious failures.
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May be translated to text with
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.BR pam_strerror "(3). "
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Under consideration by the X/Open group for future inclusion in the
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.BR pam_start "(3), "
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.BR pam_get_item "(3) "
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.BR pam_strerror "(3). "
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.BR "System administrators" ", "
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.BR "module developers" ", "
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.BR "application developers" ". "