Linux新建使用者組態檔 /etc/login.defs 詳解

/etc/login.defs 是設定使用者帳號限制的檔案。該檔案裡的設定對root使用者無效。/etc/login.defs 檔案用於在Linux建立使用者時，對使用者的一些基本屬性做預設設定，例如指定使用者 UID 和 GID 的範圍，使用者的過期時間，密碼的最大長度，等等。

需要注意的是，該檔案的使用者預設設定對 root 使用者無效。並且，當此檔案中的設定與 /etc/passwd 和 /etc/shadow 檔案中的使用者資訊有衝突時，系統會以/etc/passwd 和 /etc/shadow 為準。

如果/etc/shadow檔案裡有相同的選項，則以/etc/shadow裡的設定為準，也就是說/etc/shadow的設定優先順序高於/etc/login.defs

讀者可自行使用 vim /etc/login.defs 命令檢視該檔案中的內容，表 1 中對檔案中的各個選項做出了具體的解釋。

表 1 /etc/login.defs檔案內容

設定項	含義
MAIL_DIR /var/spool/mail	建立使用者時，系統會在目錄 /var/spool/mail 中建立一個使用者郵箱，比如 lamp 使用者的郵箱是 /var/spool/mail/lamp。
PASS_MAX_DAYS 99999	密碼有效期，99999 是自 1970 年 1 月 1 日起密碼有效的天數，相當於 273 年，可理解為密碼始終有效。
PASS_MIN_DAYS 0	表示自上次修改密碼以來，最少隔多少天後使用者才能再次修改密碼，預設值是 0。
PASS_MIN_LEN 5	指定密碼的最小長度，預設不小於 5 位，但是現在使用者登入時驗證已經被 PAM 模組取代，所以這個選項並不生效。
PASS_WARN_AGE 7	指定在密碼到期前多少天，系統就開始通過使用者密碼即將到期，預設為 7 天。
UID_MIN 500	指定最小 UID 為 500，也就是說，新增使用者時，預設 UID 從 500 開始。注意，如果手工指定了一個使用者的 UID 是 550，那麼下一個建立的使用者的 UID 就會從 551 開始，哪怕 500~549 之間的 UID 沒有使用。
UID_MAX 60000	指定使用者最大的 UID 為 60000。
GID_MIN 500	指定最小 GID 為 500，也就是在新增組時，組的 GID 從 500 開始。
GID_MAX 60000	使用者 GID 最大為 60000。
CREATE_HOME yes	指定在建立使用者時，是否同時建立使用者主目錄，yes 表示建立，no 則不建立，預設是 yes。
UMASK 077	使用者主目錄的許可權預設設定為 077。
USERGROUPS_ENAB yes	指定刪除使用者的時候是否同時刪除使用者組，準備地說，這裡指的是刪除使用者的初始組，此項的預設值為 yes。
ENCRYPT_METHOD SHA512	指定使用者密碼採用的加密規則，預設採用 SHA512，這是新的密碼加密模式，原先的 Linux 只能用 DES 或 MD5 加密。

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 #建立使用者時，要在目錄/var/spool/mail中建立一個使用者mail檔案；
#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 #登入錯誤記錄到紀錄檔

#
# 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 #在建立使用者時是否建立郵箱

#
# 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 #當限定超級使用者管理紀錄檔時使用。
SYSLOG_SG_ENAB  yes #當限定超級使用者組管理紀錄檔時使用

#
# 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 #使使用者在使用su命令後用ps檢視相關進程時顯示的是-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 #開啟該選項可以掩蓋登入資訊,也就是在login(登入)過程中不顯示/etc/motd中的資訊。
#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  #設定使用者登陸時的tty許可權。

#
# Login configuration initializations:
#
# ERASECHAR Terminal ERASE character ('10' = backspace).
# KILLCHAR Terminal KILL character ('25' = 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 #使用者的密碼不過期最多的天數；
PASS_MIN_DAYS 0 #密碼修改之間最小的天數；
PASS_WARN_AGE 7 #密碼失效前多少天在使用者登入時通知使用者修改密碼

#
# Min/max values for automatic uid selection in useradd
#
UID_MIN    1000 #最小UID為1000，也就是說新增使用者時，UID是從1000開始的；
UID_MAX   60000 #最大UID為60000；
# System accounts
#SYS_UID_MIN    100
#SYS_UID_MAX    999

#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN    1000 #最小使用者組ID號
GID_MAX   60000 #最大使用者組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 #是否創使用者家目錄，要求建立；

#
# 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