mirror of
https://github.com/shadow-maint/shadow.git
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524 lines
16 KiB
XML
524 lines
16 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!--
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SPDX-FileCopyrightText: 1989 - 1994, Julianne Frances Haugh
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SPDX-FileCopyrightText: 2007 - 2011, Nicolas François
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SPDX-License-Identifier: BSD-3-Clause
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-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd" [
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<!ENTITY BCRYPT_MIN_ROUNDS SYSTEM "login.defs.d/BCRYPT_MIN_ROUNDS.xml">
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<!ENTITY ENCRYPT_METHOD SYSTEM "login.defs.d/ENCRYPT_METHOD.xml">
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<!ENTITY MD5_CRYPT_ENAB SYSTEM "login.defs.d/MD5_CRYPT_ENAB.xml">
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<!ENTITY OBSCURE_CHECKS_ENAB SYSTEM "login.defs.d/OBSCURE_CHECKS_ENAB.xml">
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<!ENTITY PASS_ALWAYS_WARN SYSTEM "login.defs.d/PASS_ALWAYS_WARN.xml">
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<!ENTITY PASS_CHANGE_TRIES SYSTEM "login.defs.d/PASS_CHANGE_TRIES.xml">
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<!ENTITY PASS_MAX_LEN SYSTEM "login.defs.d/PASS_MAX_LEN.xml">
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<!ENTITY SHA_CRYPT_MIN_ROUNDS SYSTEM "login.defs.d/SHA_CRYPT_MIN_ROUNDS.xml">
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<!ENTITY YESCRYPT_COST_FACTOR SYSTEM "login.defs.d/YESCRYPT_COST_FACTOR.xml">
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<!-- SHADOW-CONFIG-HERE -->
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]>
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<refentry id='passwd.1'>
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<!-- $Id$ -->
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<refentryinfo>
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<author>
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<firstname>Julianne Frances</firstname>
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<surname>Haugh</surname>
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<contrib>Creation, 1989</contrib>
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</author>
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<author>
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<firstname>Thomas</firstname>
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<surname>Kłoczko</surname>
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<email>kloczek@pld.org.pl</email>
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<contrib>shadow-utils maintainer, 2000 - 2007</contrib>
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</author>
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<author>
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<firstname>Nicolas</firstname>
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<surname>François</surname>
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<email>nicolas.francois@centraliens.net</email>
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<contrib>shadow-utils maintainer, 2007 - now</contrib>
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</author>
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</refentryinfo>
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<refmeta>
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<refentrytitle>passwd</refentrytitle>
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<manvolnum>1</manvolnum>
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<refmiscinfo class="sectdesc">User Commands</refmiscinfo>
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<refmiscinfo class="source">shadow-utils</refmiscinfo>
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<refmiscinfo class="version">&SHADOW_UTILS_VERSION;</refmiscinfo>
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</refmeta>
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<refnamediv id='name'>
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<refname>passwd</refname>
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<refpurpose>change user password</refpurpose>
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</refnamediv>
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<refsynopsisdiv id='synopsis'>
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<cmdsynopsis>
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<command>passwd</command>
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<arg choice='opt'>
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<replaceable>options</replaceable>
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</arg>
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<arg choice='opt'>
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<replaceable>LOGIN</replaceable>
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</arg>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1 id='description'>
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<title>DESCRIPTION</title>
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<para>
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The <command>passwd</command> command changes passwords for user accounts.
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A normal user may only change the password for their own account, while
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the superuser may change the password for any account.
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<command>passwd</command> also changes the account or associated
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password validity period.
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</para>
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<refsect2 id='password_changes'>
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<title>Password Changes</title>
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<para>
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The user is first prompted for their old password, if one is
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present. This password is then encrypted and compared against the
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stored password. The user has only one chance to enter the correct
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password. The superuser is permitted to bypass this step so that
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forgotten passwords may be changed.
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</para>
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<para>
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After the password has been entered, password aging information is
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checked to see if the user is permitted to change the password at
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this time. If not, <command>passwd</command> refuses to change the
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password and exits.
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</para>
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<para>
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The user is then prompted twice for a replacement password. The
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second entry is compared against the first and both are required to
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match in order for the password to be changed.
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</para>
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<para>
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Then, the password is tested for complexity.
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<command>passwd</command> will reject any password which is not
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suitably complex. Care must be taken not to include the system
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default erase or kill characters.
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</para>
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</refsect2>
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<refsect2 id='hints_for_user_passwords'>
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<title>Hints for user passwords</title>
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<para>
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The security of a password depends upon the strength of the
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encryption algorithm and the size of the key space. The legacy
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<emphasis>UNIX</emphasis> System encryption method is based on the
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NBS DES algorithm. More recent methods are now recommended (see
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<option>ENCRYPT_METHOD</option>). The size of the key space
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depends upon the randomness of the password which is selected.
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</para>
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<para>
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Compromises in password security normally result from careless
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password selection or handling. For this reason, you should not
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select a password which appears in a dictionary or which must be
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written down. The password should also not be a proper name, your
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license number, birth date, or street address. Any of these may be
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used as guesses to violate system security.
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</para>
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<para>
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As a general guideline, passwords should be long and random. It's
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fine to use simple character sets, such as passwords consisting
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only of lowercase letters, if that helps memorizing longer
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passwords. For a password consisting only of lowercase English
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letters randomly chosen, and a length of 32, there are 26^32
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(approximately 2^150) different possible combinations. Being an
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exponential equation, it's apparent that the exponent (the length)
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is more important than the base (the size of the character set).
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</para>
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<para>
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You can find advice on how to choose a strong password on
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http://en.wikipedia.org/wiki/Password_strength
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</para>
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</refsect2>
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</refsect1>
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<refsect1 id='options'>
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<title>OPTIONS</title>
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<para>
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The options which apply to the <command>passwd</command> command are:
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</para>
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<variablelist remap='IP'>
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<varlistentry>
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<term>
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<option>-a</option>, <option>--all</option>
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</term>
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<listitem>
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<para>
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This option can be used only with <option>-S</option> and causes show
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status for all users.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-d</option>, <option>--delete</option>
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</term>
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<listitem>
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<para>
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Delete a user's password (make it empty). This is a quick way
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to disable a password for an account. It will set the named
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account passwordless.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-e</option>, <option>--expire</option>
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</term>
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<listitem>
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<para>
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Immediately expire an account's password. This in effect can
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force a user to change their password at the user's next login.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><option>-h</option>, <option>--help</option></term>
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<listitem>
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<para>Display help message and exit.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-i</option>, <option>--inactive</option> <replaceable>INACTIVE</replaceable>
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</term>
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<listitem>
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<para>
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This option is used to disable an account after the password has
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been expired for a number of days. After a user account has had
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an expired password for <replaceable>INACTIVE</replaceable>
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days, the user may no longer sign on to the account.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-k</option>, <option>--keep-tokens</option>
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</term>
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<listitem>
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<para>
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Indicate password change should be performed only for expired
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authentication tokens (passwords). The user wishes to keep their
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non-expired tokens as before.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-l</option>, <option>--lock</option>
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</term>
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<listitem>
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<para>
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Lock the password of the named account. This option disables a
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password by changing it to a value which matches no possible
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encrypted value (it adds a ´!´ at the beginning of the
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password).
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</para>
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<para>
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Note that this does not disable the account. The user may
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still be able to login using another authentication token
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(e.g. an SSH key). To disable the account, administrators
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should use <command>usermod --expiredate 1</command> (this sets
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the account's expire date to Jan 2, 1970).
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</para>
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<para>
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Users with a locked password are not allowed to change their
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password.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-n</option>, <option>--mindays</option> <replaceable>MIN_DAYS</replaceable>
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</term>
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<listitem>
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<para>
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Set the minimum number of days between password changes to
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<replaceable>MIN_DAYS</replaceable>. A value of zero for this field
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indicates that the user may change their password at any time.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-q</option>, <option>--quiet</option>
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</term>
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<listitem>
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<para>
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Quiet mode.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-r</option>, <option>--repository</option> <replaceable>REPOSITORY</replaceable>
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</term>
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<listitem>
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<para>
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change password in <replaceable>REPOSITORY</replaceable> repository
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-R</option>, <option>--root</option> <replaceable>CHROOT_DIR</replaceable>
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</term>
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<listitem>
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<para>
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Apply changes in the <replaceable>CHROOT_DIR</replaceable>
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directory and use the configuration files from the
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<replaceable>CHROOT_DIR</replaceable> directory.
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Only absolute paths are supported.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-P</option>, <option>--prefix</option> <replaceable>PREFIX_DIR</replaceable>
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</term>
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<listitem>
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<para>
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Apply changes to configuration files under the root filesystem
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found under the directory <replaceable>PREFIX_DIR</replaceable>.
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This option does not chroot and is intended for preparing a cross-compilation
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target. Some limitations: NIS and LDAP users/groups are
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not verified. PAM authentication is using the host files.
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No SELINUX support.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-S</option>, <option>--status</option>
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</term>
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<listitem>
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<para>
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Display account status information. The status information
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consists of 7 fields. The first field is the user's login name.
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The second field indicates if the user account has a locked
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password (L),
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has no password (NP), or has a usable password (P). The third
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field gives the date of the last password change. The next four
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fields are the minimum age, maximum age, warning period, and
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inactivity period for the password. These ages are expressed in
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days.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-u</option>, <option>--unlock</option>
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</term>
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<listitem>
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<para>
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Unlock the password of the named account. This option
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re-enables a password by changing the password back to its
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previous value (to the value before using the
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<option>-l</option> option).
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-w</option>, <option>--warndays</option> <replaceable>WARN_DAYS</replaceable>
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</term>
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<listitem>
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<para>
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Set the number of days of warning before a password change is
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required. The <replaceable>WARN_DAYS</replaceable> option is
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the number of days prior to the password expiring that a user
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will be warned that their password is about to expire.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-x</option>, <option>--maxdays</option> <replaceable>MAX_DAYS</replaceable>
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</term>
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<listitem>
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<para>
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Set the maximum number of days a password remains valid. After
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<replaceable>MAX_DAYS</replaceable>, the password is required
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to be changed.
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</para>
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<para>
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Passing the number <emphasis remap='I'>-1</emphasis> as
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<replaceable>MAX_DAYS</replaceable> will remove checking a
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password's validity.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>
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<option>-s</option>, <option>--stdin</option>
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</term>
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<listitem>
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<para>
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This option is used to indicate that passwd should read the new password from standard
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input, which can be a pipe.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 id='caveats'>
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<title>CAVEATS</title>
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<para>
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Password complexity checking may
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vary from site to site. The user is urged to select a password as
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complex as he or she feels comfortable with.
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</para>
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<para>
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Users may not be able to
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change their password on a system if NIS is enabled and they are not
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logged into the NIS server.
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</para>
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<para condition="pam">
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<command>passwd</command> uses PAM to authenticate users and to
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change their passwords.
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</para>
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</refsect1>
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<refsect1 id='configuration' condition="no_pam">
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<title>CONFIGURATION</title>
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<para>
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The following configuration variables in
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<filename>/etc/login.defs</filename> change the behavior of this
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tool:
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</para>
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<variablelist>
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&ENCRYPT_METHOD;
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&MD5_CRYPT_ENAB;
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&OBSCURE_CHECKS_ENAB;
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&PASS_ALWAYS_WARN;
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&PASS_CHANGE_TRIES;
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&PASS_MAX_LEN; <!-- documents also PASS_MIN_LEN -->
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&SHA_CRYPT_MIN_ROUNDS;
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</variablelist>
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</refsect1>
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<refsect1 id='files'>
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<title>FILES</title>
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<variablelist>
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<varlistentry>
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<term><filename>/etc/passwd</filename></term>
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<listitem>
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<para>User account information.</para>
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||
</listitem>
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||
</varlistentry>
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||
<varlistentry>
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||
<term><filename>/etc/shadow</filename></term>
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||
<listitem>
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<para>Secure user account information.</para>
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||
</listitem>
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||
</varlistentry>
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||
<varlistentry condition="no_pam">
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||
<term><filename>/etc/login.defs</filename></term>
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||
<listitem>
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||
<para>Shadow password suite configuration.</para>
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||
</listitem>
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||
</varlistentry>
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||
<varlistentry condition="pam">
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||
<term><filename>/etc/pam.d/passwd</filename></term>
|
||
<listitem>
|
||
<para>PAM configuration for <command>passwd</command>.</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</refsect1>
|
||
|
||
<refsect1 id='exit_values'>
|
||
<title>EXIT VALUES</title>
|
||
<para>
|
||
The <command>passwd</command> command exits with the following values:
|
||
<variablelist>
|
||
<varlistentry>
|
||
<term><replaceable>0</replaceable></term>
|
||
<listitem>
|
||
<para>success</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term><replaceable>1</replaceable></term>
|
||
<listitem>
|
||
<para>permission denied</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term><replaceable>2</replaceable></term>
|
||
<listitem>
|
||
<para>invalid combination of options</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term><replaceable>3</replaceable></term>
|
||
<listitem>
|
||
<para>unexpected failure, nothing done</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term><replaceable>4</replaceable></term>
|
||
<listitem>
|
||
<para>unexpected failure, <filename>passwd</filename> file missing</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term><replaceable>5</replaceable></term>
|
||
<listitem>
|
||
<para><filename>passwd</filename> file busy, try again</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
<varlistentry>
|
||
<term><replaceable>6</replaceable></term>
|
||
<listitem>
|
||
<para>invalid argument to option</para>
|
||
</listitem>
|
||
</varlistentry>
|
||
</variablelist>
|
||
</para>
|
||
</refsect1>
|
||
|
||
<refsect1 id='see_also'>
|
||
<title>SEE ALSO</title>
|
||
<para>
|
||
<citerefentry>
|
||
<refentrytitle>chpasswd</refentrytitle><manvolnum>8</manvolnum>
|
||
</citerefentry>,
|
||
<citerefentry>
|
||
<refentrytitle>makepasswd</refentrytitle><manvolnum>1</manvolnum>
|
||
</citerefentry>,
|
||
<citerefentry>
|
||
<refentrytitle>passwd</refentrytitle><manvolnum>5</manvolnum>
|
||
</citerefentry>,
|
||
<citerefentry>
|
||
<refentrytitle>shadow</refentrytitle><manvolnum>5</manvolnum>
|
||
</citerefentry>,
|
||
<phrase condition="no_pam">
|
||
<citerefentry>
|
||
<refentrytitle>login.defs</refentrytitle><manvolnum>5</manvolnum>
|
||
</citerefentry>,
|
||
</phrase>
|
||
<citerefentry>
|
||
<refentrytitle>usermod</refentrytitle><manvolnum>8</manvolnum>
|
||
</citerefentry>.
|
||
</para>
|
||
|
||
<para>
|
||
The following web page comically (yet correctly) compares the
|
||
strength of two different methods for choosing a password:
|
||
"https://xkcd.com/936/"
|
||
</para>
|
||
</refsect1>
|
||
</refentry>
|