デフォルトのCentOS7では簡単なパスワードを設定しようとすると警告的なメッセージは出るものの、無視して設定することは可能となっています。例えば test / test といういかにもありがちなユーザアカウントとパスワードを設定しようとしたとき。
# useradd test # passwd test ユーザー test のパスワードを変更。 新しいパスワード: よくないパスワード: このパスワードは 8 未満の文字列です。 新しいパスワードを再入力してください: passwd: すべての認証トークンが正しく更新できました。 |
設定できちゃいました。これをIPアドレス等のアクセス制限なく外部インターネットに公開したらあっという間にアカウントを取られてしまいそうです。
こういった安易なパスワードを設定させないために、パスワードを設定する際には英字でそれぞれ大文字、小文字をそれぞれ1文字以上、数字も1文字以上、全体で何文字以上といったポリシーを設定しておき、それにあったものでないと設定できないよう強制することが可能となっております。
設定ファイルは/etc/pam.d/system-authとなります。このファイルの15行目あたりを編集します。
# vi /etc/pam.d/system-auth
<省略>
password requisite pam_pwquality.so try_first_pass local_users_only retry=3 authtok_type= \
minlen=8 dcredit=-1 ucredit=-1 lcredit=-1 ocredit=0 enforce_for_root
<省略> |
上のそれぞれの意味は下記の通りです。
| 項目 | 説明 |
| minlen | パスワード全体でN文字以上でなければならない |
| dcredit | パスワードに数字が最低含まれていなければならない数を指定。 minlenと併用の場合は-をつける |
| ucredit | パスワードに大文字の英字が最低含まれていなければならない数 を指定。minlenと併用の場合は-をつける |
| lcredit | パスワードに小文字の英字が最低含まれていなければならない数 を指定。minlenと併用の場合は-をつける |
| ocredit | パスワードに記号が最低含まれていなければならない数を指定。 minlenと併用の場合は-をつける |
| enforce_for_root | rootでパスワード変更する場合においても上で設定した条件を満 たさないと変更できないようにする |
上記の設定例では全体で8文字以上でなおかつ英数字 ( 大文字/小文字をそれぞれ含む ) のパスワードでなければ設定できないということになります。一般ユーザでパスワード変更するときはもちろん、rootで変更する場合も条件を満たさないと変更できないようになっています。但し、enforce_for_rootオプションを指定しなければrootであれば他のオプションを指定していても任意のパスワードに変更できるということになります。
その他にも指定できるオプションは上記以外にもありますが、ここでは割愛しています。man pam_pwqualityで参照可能となっております。
# man pam_pwquality
PAM_PWQUALITY(8) System Manager's Manual PAM_PWQUALITY(8)
NAME
pam_pwquality - PAM module to perform password quality checking
SYNOPSIS
pam_pwquality.so [...]
DESCRIPTION
This module can be plugged into the password stack of a given service to provide
some plug-in strength-checking for passwords. The code was originally based on
pam_cracklib module and the module is backwards compatible with its options.
The action of this module is to prompt the user for a password and check its
strength against a system dictionary and a set of rules for identifying poor
choices.
The first action is to prompt for a single password, check its strength and
then, if it is considered strong, prompt for the password a second time (to ver‐
ify that it was typed correctly on the first occasion). All being well, the
password is passed on to subsequent modules to be installed as the new authenti‐
cation token.
The strength checks works in the following manner: at first the Cracklib routine
is called to check if the password is part of a dictionary; if this is not the
case an additional set of strength checks is done. These checks are:
Palindrome
Is the new password a palindrome?
Case Change Only
Is the new password the the old one with only a change of case?
Similar
Is the new password too much like the old one? This is primarily controlled
by one argument, difok which is a number of character changes (inserts,
removals, or replacements) between the old and new password that are enough
to accept the new password. This defaults to 5 changes.
Simple
Is the new password too small? This is controlled by 6 arguments minlen,
maxclassrepeat, dcredit, ucredit, lcredit, and ocredit. See the section on
the arguments for the details of how these work and there defaults.
Rotated
Is the new password a rotated version of the old password?
Same consecutive characters
Optional check for same consecutive characters.
Too long monotonic character sequence
Optional check for too long monotonic character sequence.
Contains user name
Optional check whether the password contains the user's name in some form.
These checks are configurable either by use of the module arguments or by modi‐
fying the /etc/security/pwquality.conf configuration file.
OPTIONS
debug
This option makes the module write information to syslog(3) indicating the
behavior of the module (this option does not write password information to
the log file).
authtok_type=XXX
The default action is for the module to use the following prompts when
requesting passwords: "New UNIX password: " and "Retype UNIX password: ".
The example word UNIX can be replaced with this option, by default it is
empty.
retry=N
Prompt user at most N times before returning with error. The default is 1.
difok=N
This argument will change the default of 5 for the number of changes in the
new password from the old password.
minlen=N
The minimum acceptable size for the new password (plus one if credits are
not disabled which is the default). In addition to the number of characters
in the new password, credit (of +1 in length) is given for each different
kind of character (other, upper, lower and digit). The default for this
parameter is 9 . Note that there is a pair of length limits also in Crack‐
lib, which is used for dictionary checking, a "way too short" limit of 4
which is hard coded in and a build time defined limit (6) that will be
checked without reference to minlen.
dcredit=N
(N >= 0) This is the maximum credit for having digits in the new password.
If you have less than or N digits, each digit will count +1 towards meeting
the current minlen value. The default for dcredit is 1 which is the recom‐
mended value for minlen less than 10.
(N < 0) This is the minimum number of digits that must be met for a new
password.
ucredit=N
(N >= 0) This is the maximum credit for having upper case letters in the new
password. If you have less than or N upper case letters each letter will
count +1 towards meeting the current minlen value. The default for ucredit
is 1 which is the recommended value for minlen less than 10.
(N < 0) This is the minimum number of upper case letters that must be met
for a new password.
lcredit=N
(N >= 0) This is the maximum credit for having lower case letters in the new
password. If you have less than or N lower case letters, each letter will
count +1 towards meeting the current minlen value. The default for lcredit
is 1 which is the recommended value for minlen less than 10.
(N < 0) This is the minimum number of lower case letters that must be met
for a new password.
ocredit=N
(N >= 0) This is the maximum credit for having other characters in the new
password. If you have less than or N other characters, each character will
count +1 towards meeting the current minlen value. The default for ocredit
is 1 which is the recommended value for minlen less than 10.
(N < 0) This is the minimum number of other characters that must be met for
a new password.
minclass=N
The minimum number of required classes of characters for the new password.
The default number is zero. The four classes are digits, upper and lower
letters and other characters. The difference to the credit check is that a
specific class if of characters is not required. Instead N out of four of
the classes are required.
maxrepeat=N
Reject passwords which contain more than N same consecutive characters. The
default is 0 which means that this check is disabled.
maxsequence=N
Reject passwords which contain monotonic character sequences longer than N.
The default is 0 which means that this check is disabled. Examples of such
sequence are '12345' or 'fedcb'. Note that most such passwords will not pass
the simplicity check unless the sequence is only a minor part of the pass‐
word.
maxclassrepeat=N
Reject passwords which contain more than N consecutive characters of the
same class. The default is 0 which means that this check is disabled.
gecoscheck=N
If nonzero, check whether the individual words longer than 3 characters from
the passwd GECOS field of the user are contained in the new password. The
default is 0 which means that this check is disabled.
badwords=<list of words>
The words more than 3 characters long from this space separated list are
individually searched for and forbidden in the new password. By default the
list is empty which means that this check is disabled.
enforce_for_root
The module will return error on failed check even if the user changing the
password is root. This option is off by default which means that just the
message about the failed check is printed but root can change the password
anyway. Note that root is not asked for an old password so the checks that
compare the old and new password are not performed.
local_users_only
The module will not test the password quality for users that are not present
in the /etc/passwd file. The module still asks for the password so the fol‐
lowing modules in the stack can use the use_authtok option. This option is
off by default.
use_authtok
This argument is used to force the module to not prompt the user for a new
password but use the one provided by the previously stacked password module.
dictpath=/path/to/dict
Path to the cracklib dictionaries.
MODULE TYPES PROVIDED
Only the password module type is provided.
RETURN VALUES
PAM_SUCCESS
The new password passes all checks.
PAM_AUTHTOK_ERR
No new password was entered, the username could not be determined or the new
password fails the strength checks.
PAM_AUTHTOK_RECOVERY_ERR
The old password was not supplied by a previous stacked module or got not
requested from the user. The first error can happen if use_authtok is speci‐
fied.
PAM_SERVICE_ERR
A internal error occurred.
EXAMPLES
For an example of the use of this module, we show how it may be stacked with the
password component of pam_unix(8)
#
# These lines stack two password type modules. In this example the
# user is given 3 opportunities to enter a strong password. The
# "use_authtok" argument ensures that the pam_unix module does not
# prompt for a password, but instead uses the one provided by
# pam_pwquality.
#
passwd password required pam_pwquality.so retry=3
passwd password required pam_unix.so use_authtok
Another example (in the /etc/pam.d/passwd format) is for the case that you want
to use md5 password encryption:
#%PAM-1.0
#
# These lines allow a md5 systems to support passwords of at least 14
# bytes with extra credit of 2 for digits and 2 for others the new
# password must have at least three bytes that are not present in the
# old password
#
password required pam_pwquality.so \
difok=3 minlen=15 dcredit= 2 ocredit=2
password required pam_unix.so use_authtok nullok md5
And here is another example in case you don´t want to use credits:
#%PAM-1.0
#
# These lines require the user to select a password with a minimum
# length of 8 and with at least 1 digit number, 1 upper case letter,
# and 1 other character
#
password required pam_pwquality.so \
dcredit=-1 ucredit=-1 ocredit=-1 lcredit=0 minlen=8
password required pam_unix.so use_authtok nullok md5
SEE ALSO
pwscore(1), pwquality.conf(5), pam_pwquality(8), pam.conf(5), PAM(8)
AUTHORS
Tomas Mraz <tmraz@redhat.com>
Original author of pam_cracklib module Cristian Gafton <gafton@redhat.com>
Red Hat, Inc. 10 Nov 2011 PAM_PWQUALITY(8)
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