/etc/login.defs 是设置用户帐号限制的文件。该文件里的配置对root用户无效。/etc/login.defs 文件用于在Linux创建用户时，对用户的一些基本属性做默认设置，例如指定用户 UID 和 GID 的范围，用户的过期时间，密码的最大长度，等等。
需要注意的是，该文件的用户默认配置对 root 用户无效。并且，当此文件中的配置与 /etc/passwd 和 /etc/shadow 文件中的用户信息有冲突时，系统会以/etc/passwd 和 /etc/shadow 为准。
如果/etc/shadow文件里有相同的选项，则以/etc/shadow里的设置为准，也就是说/etc/shadow的配置优先级高于/etc/login.defs
读者可自行使用 vim /etc/login.defs 命令查看该文件中的内容，表 1 中对文件中的各个选项做出了具体的解释。
root@linuxidc:/home/linuxidc/linuxidc.com# cat /etc/login.defs
#
# /etc/login.defs - Configuration control definitions for the login package.
#
# Three items must be defined: MAIL_DIR, ENV_SUPATH, and ENV_PATH.
# If unspecified, some arbitrary (and possibly incorrect) value will
# be assumed. All other items are optional - if not specified then
# the described action or option will be inhibited.
#
# Comment lines (lines beginning with "#") and blank lines are ignored.
#
# Modified for Linux. --marekm
# REQUIRED for useradd/userdel/usermod
# Directory where mailboxes reside, _or_ name of file, relative to the
# home directory. If you _do_ define MAIL_DIR and MAIL_FILE,
# MAIL_DIR takes precedence.
#
# Essentially:
# - MAIL_DIR defines the location of users mail spool files
# (for mbox use) by appending the username to MAIL_DIR as defined
# below.
# - MAIL_FILE defines the location of the users mail spool files as the
# fully-qualified filename obtained by prepending the user home
# directory before $MAIL_FILE
#
# NOTE: This is no more used for setting up users MAIL environment variable
# which is, starting from shadow 4.0.12-1 in Debian, entirely the
# job of the pam_mail PAM modules
# See default PAM configuration files provided for
# login, su, etc.
#
# This is a temporary situation: setting these variables will soon
# move to /etc/default/useradd and the variables will then be
# no more supported
MAIL_DIR /var/mail #创建用户时，要在目录/var/spool/mail中创建一个用户mail文件；
#MAIL_FILE .mail
#
# Enable logging and display of /var/log/faillog login failure info.
# This option conflicts with the pam_tally PAM module.
#
FAILLOG_ENAB yes #登录错误记录到日志
#
# Enable display of unknown usernames when login failures are recorded.
#
# WARNING: Unknown usernames may become world readable.
# See #290803 and #298773 for details about how this could become a security
# concern
LOG_UNKFAIL_ENAB no #在建立用户时是否创建邮箱
#
# Enable logging of successful logins
#
LOG_OK_LOGINS no
#
# Enable "syslog" logging of su activity - in addition to sulog file logging.
# SYSLOG_SG_ENAB does the same for newgrp and sg.
#
SYSLOG_SU_ENAB yes #当限定超级用户管理日志时使用。
SYSLOG_SG_ENAB yes #当限定超级用户组管理日志时使用
#
# If defined, all su activity is logged to this file.
#
#SULOG_FILE /var/log/sulog
#
# If defined, file which maps tty line to TERM environment parameter.
# Each line of the file is in a format something like "vt100 tty01".
#
#TTYTYPE_FILE /etc/ttytype
#
# If defined, login failures will be logged here in a utmp format
# last, when invoked as lastb, will read /var/log/btmp, so...
#
FTMP_FILE /var/log/btmp
#
# If defined, the command name to display when running "su -". For
# example, if this is defined as "su" then a "ps" will display the
# command is "-su". If not defined, then "ps" would display the
# name of the shell actually being run, e.g. something like "-sh".
#
SU_NAME su #使用户在使用su命令后用ps查看相关进程时显示的是-su而不是-sh。
#
# If defined, file which inhibits all the usual chatter during the login
# sequence. If a full pathname, then hushed mode will be enabled if the
# user's name or shell are found in the file. If not a full pathname, then
# hushed mode will be enabled if the file exists in the user's home directory.
#
HUSHLOGIN_FILE .hushlogin #打开该选项可以掩盖登录信息,也就是在login(登录)过程中不显示/etc/motd中的信息。
#HUSHLOGIN_FILE /etc/hushlogins
#
# *REQUIRED* The default PATH settings, for superuser and normal users.
#
# (they are minimal, add the rest in the shell startup files)
ENV_SUPATH PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
ENV_PATH PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games
#
# Terminal permissions
#
# TTYGROUP Login tty will be assigned this group ownership.
# TTYPERM Login tty will be set to this permission.
#
# If you have a "write" program which is "setgid" to a special group
# which owns the terminals, define TTYGROUP to the group number and
# TTYPERM to 0620. Otherwise leave TTYGROUP commented out and assign
# TTYPERM to either 622 or 600.
#
# In Debian /usr/bin/bsd-write or similar programs are setgid tty
# However, the default and recommended value for TTYPERM is still 0600
# to not allow anyone to write to anyone else console or terminal
# Users can still allow other people to write them by issuing
# the "mesg y" command.
TTYGROUP tty
TTYPERM 0600 #设置用户登陆时的tty权限。
#
# Login configuration initializations:
#
# ERASECHAR Terminal ERASE character ('\010' = backspace).
# KILLCHAR Terminal KILL character ('\025' = CTRL/U).
# UMASK Default "umask" value.
#
# The ERASECHAR and KILLCHAR are used only on System V machines.
#
# UMASK is the default umask value for pam_umask and is used by
# useradd and newusers to set the mode of the new home directories.
# 022 is the "historical" value in Debian for UMASK
# 027, or even 077, could be considered better for privacy
# There is no One True Answer here : each sysadmin must make up his/her
# mind.
#
# If USERGROUPS_ENAB is set to "yes", that will modify this UMASK default value
# for private user groups, i. e. the uid is the same as gid, and username is
# the same as the primary group name: for these, the user permissions will be
# used as group permissions, e. g. 022 will become 002.
#
# Prefix these values with "0" to get octal, "0x" to get hexadecimal.
#
ERASECHAR 0177
KILLCHAR 025
UMASK 022
#
# Password aging controls:
#
# PASS_MAX_DAYS Maximum number of days a password may be used.
# PASS_MIN_DAYS Minimum number of days allowed between password changes.
# PASS_WARN_AGE Number of days warning given before a password expires.
#
PASS_MAX_DAYS 99999 #用户的密码不过期最多的天数；
PASS_MIN_DAYS 0 #密码修改之间最小的天数；
PASS_WARN_AGE 7 #密码失效前多少天在用户登录时通知用户修改密码
#
# Min/max values for automatic uid selection in useradd
#
UID_MIN 1000 #最小UID为1000，也就是说添加用户时，UID是从1000开始的；
UID_MAX 60000 #最大UID为60000；
# System accounts
#SYS_UID_MIN 100
#SYS_UID_MAX 999
#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN 1000 #最小用户组ID号
GID_MAX 60000 #最大用户组ID
# System accounts
#SYS_GID_MIN 100
#SYS_GID_MAX 999
#
# Max number of login retries if password is bad. This will most likely be
# overriden by PAM, since the default pam_unix module has it's own built
# in of 3 retries. However, this is a safe fallback in case you are using
# an authentication module that does not enforce PAM_MAXTRIES.
#
LOGIN_RETRIES 5
#
# Max time in seconds for login
#
LOGIN_TIMEOUT 60
#
# Which fields may be changed by regular users using chfn - use
# any combination of letters "frwh" (full name, room number, work
# phone, home phone). If not defined, no changes are allowed.
# For backward compatibility, "yes" = "rwh" and "no" = "frwh".
#
CHFN_RESTRICT rwh
#
# Should login be allowed if we can't cd to the home directory?
# Default in no.
#
DEFAULT_HOME yes #是否创用户家目录，要求创建；
#
# If defined, this command is run when removing a user.
# It should remove any at/cron/print jobs etc. owned by
# the user to be removed (passed as the first argument).
#
#USERDEL_CMD /usr/sbin/userdel_local
#
# Enable setting of the umask group bits to be the same as owner bits
# (examples: 022 -> 002, 077 -> 007) for non-root users, if the uid is
# the same as gid, and username is the same as the primary group name.
#
# If set to yes, userdel will remove the user's group if it contains no
# more members, and useradd will create by default a group with the name
# of the user.
#
USERGROUPS_ENAB yes
#
# Instead of the real user shell, the program specified by this parameter
# will be launched, although its visible name (argv[0]) will be the shell's.
# The program may do whatever it wants (logging, additional authentification,
# banner, ...) before running the actual shell.
#
# FAKE_SHELL /bin/fakeshell
#
# If defined, either full pathname of a file containing device names or
# a ":" delimited list of device names. Root logins will be allowed only
# upon these devices.
#
# This variable is used by login and su.
#
#CONSOLE /etc/consoles
#CONSOLE console:tty01:tty02:tty03:tty04
#
# List of groups to add to the user's supplementary group set
# when logging in on the console (as determined by the CONSOLE
# setting). Default is none.
#
# Use with caution - it is possible for users to gain permanent
# access to these groups, even when not logged in on the console.
# How to do it is left as an exercise for the reader...
#
# This variable is used by login and su.
#
#CONSOLE_GROUPS floppy:audio:cdrom
#
# If set to "yes", new passwords will be encrypted using the MD5-based
# algorithm compatible with the one used by recent releases of FreeBSD.
# It supports passwords of unlimited length and longer salt strings.
# Set to "no" if you need to copy encrypted passwords to other systems
# which don't understand the new algorithm. Default is "no".
#
# This variable is deprecated. You should use ENCRYPT_METHOD.
#
#MD5_CRYPT_ENAB no
#
# If set to MD5 , MD5-based algorithm will be used for encrypting password
# If set to SHA256, SHA256-based algorithm will be used for encrypting password
# If set to SHA512, SHA512-based algorithm will be used for encrypting password
# If set to DES, DES-based algorithm will be used for encrypting password (default)
# Overrides the MD5_CRYPT_ENAB option
#
# Note: It is recommended to use a value consistent with
# the PAM modules configuration.
#
ENCRYPT_METHOD SHA512 #加密模式
#
# Only used if ENCRYPT_METHOD is set to SHA256 or SHA512.
#
# Define the number of SHA rounds.
# With a lot of rounds, it is more difficult to brute forcing the password.
# But note also that it more CPU resources will be needed to authenticate
# users.
#
# If not specified, the libc will choose the default number of rounds (5000).
# The values must be inside the 1000-999999999 range.
# If only one of the MIN or MAX values is set, then this value will be used.
# If MIN > MAX, the highest value will be used.
#
# SHA_CRYPT_MIN_ROUNDS 5000
# SHA_CRYPT_MAX_ROUNDS 5000
################# OBSOLETED BY PAM ##############
# #
# These options are now handled by PAM. Please #
# edit the appropriate file in /etc/pam.d/ to #
# enable the equivelants of them.
#
###############
#MOTD_FILE
#DIALUPS_CHECK_ENAB
#LASTLOG_ENAB
#MAIL_CHECK_ENAB
#OBSCURE_CHECKS_ENAB
#PORTTIME_CHECKS_ENAB
#SU_WHEEL_ONLY
#CRACKLIB_DICTPATH
#PASS_CHANGE_TRIES
#PASS_ALWAYS_WARN
#ENVIRON_FILE
#NOLOGINS_FILE
#ISSUE_FILE
#PASS_MIN_LEN
#PASS_MAX_LEN
#ULIMIT
#ENV_HZ
#CHFN_AUTH
#CHSH_AUTH
#FAIL_DELAY
################# OBSOLETED #######################
# #
# These options are no more handled by shadow. #
# #
# Shadow utilities will display a warning if they #
# still appear. #
# #
###################################################
# CLOSE_SESSIONS
# LOGIN_STRING
# NO_PASSWORD_CONSOLE
# QMAIL_DIR
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