用户访问服务器的时候，服务器的某一个服务程序把用户的谁请求发送到PAM模块进行认证。对于不同的服务器应用程序所对应的PAM模块也是不同的。如果想查看某个程序是否支持PAM认证，可以用ldd命令进行检查：
例如：查看sshd是不是支持ＰＡＭ模块认证：
由于在程序模块里链接了libpam.so.0 =< /lib/libpam.so.0 ,说明此程序可以进行ＰＡＭ认证。
当一个服务器请求PAM模块的时候，ＰＡＭ本身是不提供服务验证的，它是调用其它的一群模块来进行服务器请求验证，这样的文件全放在了/lib/security中。具体到哪一个服务使用哪一种具体的模块，这是由具体的ＰＡＭ服务文件定义的（/etc/pam.d/）.
[root@localhost root]# ls /etc/pam.d/
authconfig neat redhat-config-network su
chfn other redhat-config-network-cmd sudo
chsh passwd redhat-config-network-druid system-auth
halt poweroff rhn_register up2date
internet-druid ppp setup up2date-config
kbdrate reboot smtp up2date-nox
login redhat-config-mouse sshd
PAM服务文件
１、# more /etc/pam.d/login
auth required pam_securetty.so
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_stack.so service=system-auth
session optional pam_console.so
PAM服务文件的格式（四部分）
Module-type control-flag module-path arguments
Module-type： auth、account、session、password
Control-flag：required、requisite、sufficient、optional
eg：
auth required pam_securety.so
auth required pam_stack.so service=system_auth
Module-type（属于认证里面的第一部分，主要是分配权限）
auth：认证、授权（检查用户的名字、密码正确与否）；
account：检查用户的帐户是否到期、禁用等。
session：控制会话
password：控制用户修改密码过程
Control-flag （属于认证里面的第二部分，控制标识位）
required：必须通过此认证，否则不再往下认证下去，直接退出；
requisite：必须通过认证，但以后还有机会，可以往下认证；
sufficient：一经通过，后面的不再认证（只要通过这个条件则直接通过）；
optional：可选的，通不通过均可。
常用的PAM服务文件
1）、login ---- /etc/pam.d/login 2）、ipop3d --- /etc/pam.d/pop
3）、ftp ---- /etc/pam.d/ftp 或 vsftpd -- /etc/pam.d/vsftpd
4）、sshd--- /etc/pam.d/sshd 5）、su --- /etc/pam.d/su 6）、imap--- /etc/pam.d/imcp
认证堆栈
①、auth required pam_securety.so
②、auth required pam_stack.so service=system-auth
③、auth required pam_nologin.so
如果①号认证结束，则在后面有个结束标志，转到下一个认证即②号认证,依次类推，相同类型的认证会放在一起进行。
其中pam_stack.so调用一个子模块服务，通过这个服务再调用一个第三方的模块进行认证授权。
常用PAM模块
1）、pam_access.so 控制访问者的地址与账号的名称
2）、pam_listfile.so 控制访问者的账号名称或登陆位置
3）、pam_limits.so 控制为用户分配的资源
4）、pam_rootok.so 对管理员（uid=0）无条件通过
5）、pam_userdb.so 设定独立用户账号数据库认证
如下：
[root@localhost root]# cd /etc/pam.d/
[root@localhost pam.d]# more login
#%PAM-1.0
auth required pam_securetty.so
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_stack.so service=system-auth
session optional pam_console.so
[root@localhost pam.d]# cd /usr/share/doc/pam-0.75/txts/
[root@localhost txts]# ls
pam_appl.txt README.pam_ftp README.pam_shells
pam_modules.txt README.pam_limits README.pam_stack
pam.txt README.pam_listfile README.pam_stress
README README.pam_localuser README.pam_tally
README.pam_access README.pam_mail README.pam_time
README.pam_chroot README.pam_nologin README.pam_timestamp
README.pam_console README.pam_permit README.pam_unix
README.pam_cracklib README.pam_pwdb README.pam_userdb
README.pam_deny README.pam_rhosts README.pam_warn
README.pam_env README.pam_rootok README.pam_wheel
README.pam_filter README.pam_securetty README.pam_xauth
[root@localhost txts]# more README.pam_securetty
pam_securetty:
Allows root logins only if the user is logging in on a
'secure' tty, as defined by the listing in /etc/securetty
Also checks to make sure that /etc/securetty is a plain
file and not world writable.
- Elliot Lee , Red Hat Software.
July 25, 1996.
[root@localhost txts]# more /etc/securetty
console
vc/1
vc/2
vc/3
vc/4
vc/5
vc/6
vc/7
vc/8
vc/9
vc/10
vc/11
tty1
tty2
tty3
tty4
tty5
tty6
tty7
tty8
tty9
tty10
tty11
[root@localhost txts]# more /etc/pam.d/system-auth
#%PAM-1.0
# This file is auto-generated.
# User changes will be destroyed the next time authconfig is run.
auth required /lib/security/$ISA/pam_env.so
auth sufficient /lib/security/$ISA/pam_unix.so likeauth nullok
auth required /lib/security/$ISA/pam_deny.so
account required /lib/security/$ISA/pam_unix.so
password required /lib/security/$ISA/pam_cracklib.so retry=3 type=
password sufficient /lib/security/$ISA/pam_unix.so nullok use_authtok md5 shadow
password required /lib/security/$ISA/pam_deny.so
session required /lib/security/$ISA/pam_limits.so
session required /lib/security/$ISA/pam_unix.so
[root@localhost txts]# pwd
/usr/share/doc/pam-0.75/txts
[root@localhost txts]# more README.pam_nologin
# $Id: README,v 1.1.1.1 2000/06/20 22:11:46 agmorgan Exp $
#
This module always lets root in; it lets other users in only if the file
/etc/nologin doesn't exist. In any case, if /etc/nologin exists, it's
contents are displayed to the user.
module services provided:
auth _authentication and _setcred (blank)
Michael K. Johnson
[root@localhost txts]# touch /etc/nologin
[root@localhost txts]# useradd leekwen
[root@localhost txts]# passwd leekwen
Changing password for user leekwen.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.
[root@localhost txts]# ssh leekwen@192.168.0.188
leekwen@192.168.0.188's password:
Permission denied, please try again.
leekwen@192.168.0.188's password:
Permission denied, please try again.
leekwen@192.168.0.188's password:
Permission denied (publickey,password,keyboard-interactive).
[root@localhost txts]# rm /etc/nologin
rm: remove regular empty file `/etc/nologin'? y
[root@localhost txts]# ssh leekwen@192.168.0.188
leekwen@192.168.0.188's password:
[leekwen@localhost leekwen]$ pwd
/home/leekwen
[leekwen@localhost leekwen]$ exit
logout
Connection to 192.168.0.188 closed.
[root@localhost txts]# cd /etc/pam.d/
[root@localhost pam.d]# more login
#%PAM-1.0
auth required pam_securetty.so
auth required pam_stack.so service=system-auth
auth required pam_nologin.so
account required pam_stack.so service=system-auth
password required pam_stack.so service=system-auth
session required pam_stack.so service=system-auth
session optional pam_console.so
[root@localhost pam.d]# tty
/dev/pts/2
[root@localhost pam.d]# ls /dev/tty1
/dev/tty1
