SELinux就是Security-Enhanced Linux的简称，安全加强的linux。传统的linux权限是对文件和目录的owner, group和other的rwx进行控制，而SELinux采用的是委任式访问控制，也就是控制一个进程对具体文件系统上面的文件和目录的访问，SELinux规定了很多的规则，来决定哪个进程可以访问哪些文件和目录。
SELinux是根据进程或文件的security context来决定进程是否有权限访问文件系统的，security context由Identify:role:type三部分组成，当selinux的类型为SELINUXTYPE=targeted的时候，只有security context的type是有用的。selinux的配置文件在这里：/etc/selinux/config，内容如下：
# This file controls the state of SELinux on the system.
# SELINUX= can take one of these three values:
# enforcing - SELinux security policy is enforced.
# permissive - SELinux prints warnings instead of enforcing.
# disabled - SELinux is fully disabled.
SELINUX=enforcing
# SELINUXTYPE= type of policy in use. Possible values are:
# targeted - Only targeted network daemons are protected.
# strict - Full SELinux protection.
SELINUXTYPE=targeted
需要安装必要的SELinux的相关工具：
yum install policycoreutils-python setools-console setroubleshoot setroubleshoot-server查看当前SELinux的状态：
[root@centos ~]# getenforce
Enforcing状态为Enforcing就代表打开了SELinux，为disabled或permissive则需要使用下面的命令开启（permissive代表如果不符合SELinux的权限控制，则会出现warning提示信息，不会真的block进程的访问）：
设置SELinux为Enforcing:
[root@centos ~]# setenforce 1之后，重启电脑，并等待SELinux相关文件建立完成。
查看进程的security context：
[root@centos ~]# ps aux -Z | grep httpd
unconfined_u:system_r:httpd_t:s0 root 6056 0.0 0.3 11672 3504 ? Ss 15:31 0:03 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6061 0.0 0.2 11804 2664 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6062 0.0 0.2 11672 2132 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6063 0.0 0.2 11804 2664 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6064 0.0 0.2 11804 2780 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6065 0.0 0.2 11672 2132 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6066 0.0 0.2 11672 2132 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6067 0.0 0.2 11672 2132 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:system_r:httpd_t:s0 apache 6068 0.0 0.2 11672 2132 ? S 15:31 0:00 /usr/sbin/httpd
unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 root 6845 0.0 0.0 4340 752 pts/0 S+ 18:16 0:00 grep httpd查看目录的security context：
[root@centos ~]# ls -dZ /var/www/
drwxr-xr-x. root root system_u:object_r:httpd_sys_content_t:s0 /var/www/列出SELinux的统计信息，包括security context中的user，role和type，还有规则（Booleans）：
[root@centos ~]# seinfo
Statistics for policy file: /etc/selinux/targeted/policy/policy.24
Policy Version & Type: v.24 (binary, mls)
Classes: 81 Permissions: 235
Sensitivities: 1 Categories: 1024
Types: 3488 Attributes: 273
Users: 9 Roles: 12
Booleans: 187 Cond. Expr.: 222
Allow: 273920 Neverallow: 0
Auditallow: 96 Dontaudit: 199904
Type_trans: 23469 Type_change: 38
Type_member: 48 Role allow: 20
Role_trans: 291 Range_trans: 3993
Constraints: 87 Validatetrans: 0
Initial SIDs: 27 Fs_use: 22
Genfscon: 81 Portcon: 426
Netifcon: 0 Nodecon: 0
Permissives: 59 Polcap: 2
查看所有的规则名字：
[root@centos ~]# seinfo -b
Conditional Booleans: 187
allow_domain_fd_use
allow_ftpd_full_access
allow_sysadm_exec_content
allow_user_exec_content
allow_zebra_write_config
cdrecord_read_content
fcron_crond
httpd_manage_ipa
httpd_use_openstack
mmap_low_allowed
samba_share_fusefs
sepgsql_enable_users_ddl
abrt_handle_event
allow_ftpd_use_cifs
allow_httpd_mod_auth_pam
allow_java_execstack
cron_can_relabel
.......
查看一个规则是否打开：
[root@centos ~]# getsebool httpd_enable_homedirs
httpd_enable_homedirs --> off
打开一个规则：
[root@centos ~]# setsebool -P httpd_enable_homedirs=1
[root@centos ~]# getsebool httpd_enable_homedirs
httpd_enable_homedirs --> on
查看一个规则的具体信息(也就是allow或者deny某个进程的security context的type访问某个文件系统的security context的type）：
[root@centos ~]# sesearch -b httpd_enable_homedirs --all
ERROR: Cannot get avrules: Neverallow rules requested but not available
Found 46 semantic av rules:
allow httpd_sys_script_t home_root_t : dir { getattr search open } ;
allow httpd_sys_script_t home_root_t : lnk_file { read getattr } ;
allow httpd_suexec_t user_home_dir_t : dir { getattr search open } ;
allow httpd_suexec_t user_home_dir_t : lnk_file { read getattr } ;
allow httpd_suexec_t autofs_t : dir { ioctl read getattr lock search open } ;
allow httpd_suexec_t cifs_t : file { ioctl read getattr lock execute execute_no_trans open } ;
allow httpd_suexec_t cifs_t : dir { ioctl read getattr lock search open } ;
allow httpd_suexec_t cifs_t : lnk_file { read getattr } ;
allow httpd_suexec_t nfs_t : file { ioctl read getattr lock execute execute_no_trans open } ;
allow httpd_suexec_t nfs_t : dir { ioctl read getattr lock search open } ;
allow httpd_suexec_t nfs_t : lnk_file { read getattr } ;
allow httpd_t user_home_t : file { ioctl read getattr lock open } ;
.............
selinux在文件系统中是存在很多的默认设置的，可以通过semanage来查看系统中所有目录的默认security context:
[root@centos ~]# semanage fcontext -l
SELinux fcontext type Context
/ directory system_u:object_r:root_t:s0
/.* all files system_u:object_r:default_t:s0
/[^/]+ regular file system_u:object_r:etc_runtime_t:s0
/\.autofsck regular file system_u:object_r:etc_runtime_t:s0
/\.autorelabel regular file system_u:object_r:etc_runtime_t:s0
/\.journal all files <>
/\.suspended regular file system_u:object_r:etc_runtime_t:s0
/a?quota\.(user|group) regular file system_u:object_r:quota_db_t:s0
/afs directory system_u:object_r:mnt_t:s0
/bin directory system_u:object_r:bin_t:s0
/bin/.* all files system_u:object_r:bin_t:s0
/bin/alsaunmute regular file system_u:object_r:alsa_exec_t:s0
/bin/bash regular file system_u:object_r:shell_exec_t:s0
...............
增加一个目录的默认security context：
[root@centos ~]# semanage fcontext -a -t public_content_t "/srv/test(/.*)?"
上面这个命令将/srv/test的默认的security context的type设置为public_content_t
改变一个文件的security context：
[root@centos ~]# chcon -t var_t /var/www/html/index.html
[root@centos ~]# ll -Z /var/www/html/index.html
-rw-r--r--. root root unconfined_u:object_r:var_t:s0 /var/www/html/index.html
上面的命令将/var/www/html/index.html的type改为var_t
可以使用restorecon命令将档案恢复为所在目录的默认security context：
先查看一下/var/www的默认context是什么：
[root@centos ~]# semanage fcontext -l | grep /var/www
/var/www(/.*)? all files system_u:object_r:httpd_sys_content_t:s0
/var/www/[^/]*/cgi-bin(/.*)? all files system_u:object_r:httpd_sys_script_exec_t:s0
/var/www/apcupsd/multimon\.cgi regular file system_u:object_r:httpd_apcupsd_cgi_script_exec_t:s0
/var/www/apcupsd/upsfstats\.cgi regular file system_u:object_r:httpd_apcupsd_cgi_script_exec_t:s0
.............
使用restorecon恢复/var/www的所有档案和目录为默认：
[root@centos ~]# restorecon -Rv /var/www
restorecon reset /var/www/html/index.html context unconfined_u:object_r:var_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
——————————————————————————————
selinux是提供日志文件来记录错误信息的，错误信息记录在/var/log/messages 和 /var/log/setroubleshoot/* 里头，需要重启auditd服务来开启selinux的log服务：
[root@centos ~]# /etc/init.d/auditd restart
查看selinux的错误信息：
[root@centos ~]# cat /var/log/messages | grep setroubleshoot
Aug 9 17:46:47 centos yum[6590]: Installed: setroubleshoot-plugins-3.0.40-1.el6.noarch
Aug 9 17:46:50 centos yum[6590]: Installed: setroubleshoot-server-3.0.47-3.el6_3.i686
Aug 9 17:46:54 centos yum[6590]: Installed: setroubleshoot-3.0.47-3.el6_3.i686
Aug 9 17:58:57 centos setroubleshoot: SELinux is preventing /usr/sbin/httpd from getattr access on the file /var/www/html/index.html. For complete SELinux messages. run sealert -l c7a436a1-a114-4659-91a9-4155b1003dd7
Aug 9 17:58:58 centos setroubleshoot: SELinux is preventing /usr/sbin/httpd from getattr access on the file /var/www/html/index.html. For complete SELinux messages. run sealert -l c7a436a1-a114-4659-91a9-4155b1003dd7
Aug 9 18:00:35 centos setroubleshoot: SELinux is preventing /usr/sbin/httpd from getattr access on the file /var/www/html/index.html. For complete SELinux messages. run sealert -l c7a436a1-a114-4659-91a9-4155b1003dd7
Aug 9 18:00:36 centos setroubleshoot: SELinux is preventing /usr/sbin/httpd from getattr access on the file /var/www/html/index.html. For complete SELinux messages. run sealert -l c7a436a1-a114-4659-91a9-4155b1003dd7
运行sealert查看具体解决方法：
[root@centos ~]# sealert -l c7a436a1-a114-4659-91a9-4155b1003dd7
SELinux is preventing /usr/sbin/httpd from getattr access on the file /var/www/html/index.html.
***** Plugin restorecon (99.5 confidence) suggests *************************
If you want to fix the label.
/var/www/html/index.html default label should be httpd_sys_content_t.
Then you can run restorecon.
Do
# /sbin/restorecon -v /var/www/html/index.html
***** Plugin catchall (1.49 confidence) suggests ***************************
If you believe that httpd should be allowed getattr access on the index.html file by default.
Then you should report this as a bug.
You can generate a local policy module to allow this access.
Do
allow this access for now by executing:
# grep httpd /var/log/audit/audit.log | audit2allow -M mypol
# semodule -i mypol.pp
上面的信息已经给出了具体解决方法，就是使用restorecon将index.html恢复为默认。
——————————————————————————————
下面用httpd这个www服务简单实验一下：
没装httpd的就先装吧。。
[root@centos ~]# yum install httpd
Loaded plugins: fastestmirror, refresh-packagekit, security
Loading mirror speeds from cached hostfile
* base: mirrors.ta139.com
* extras: mirrors.ta139.com
* updates: mirrors.ta139.com
Setting up Install Process
Package httpd-2.2.15-15.el6.centos.1.i686 already installed and latest version
Nothing to do
启动httpd service：
[root@centos ~]# /etc/init.d/httpd restart
Stopping httpd: [ OK ]
Starting httpd: [ OK ]
查看系统是否启动了80端口监听:
[root@centos ~]# netstat -tupln | grep httpd
tcp 0 0 :::80 :::* LISTEN 9587/httpd
在root的家目录创建index.html文件：
[root@centos ~]# echo "Test for selinux" > index.html
查看它的context:
[root@centos ~]# ls -Z index.html
-rw-r--r--. root root unconfined_u:object_r:admin_home_t:s0 index.html
可以看到它的type为admin_home_t，是继承自/root这个目录的security context
把它copy到/var/www/html中：
[root@centos ~]# cp -a index.html /var/www/html/
注意，我们使用的是-a这个option来copy，也就是我们保留了源文件的security context:
[root@centos ~]# ll -Z /var/www/html/index.html
-rw-r--r--. root root unconfined_u:object_r:admin_home_t:s0 /var/www/html/index.html
使用links访问一下这个page:
[root@centos ~]# links -dump
Forbidden
You don't have permission to access /index.html on this server.
--------------------------------------------------------------------------
Apache/2.2.15 (CentOS) Server at localhost Port 80
意料之中，出现permission deny了，这个时候是可以直接去看/var/log/messages来解决问题的。
我们这里先查看一下进程httpd的context：
[root@centos ~]# ps aux -Z | grep httpd
unconfined_u:system_r:httpd_t:s0 apache 9590 0.0 0.2 11804 2852 ? S 10:38 0:00 /usr/sbin/httpd
再查看一下有没有允许httpd访问type为admin_home_t的文件：
[root@centos ~]# sesearch --all | grep "allow httpd_t admin_home_t"
ERROR: Cannot get avrules: Neverallow rules requested but not available
查询结果显示，没有这个rule，所以httpd进程被block。
查看有没有允许httpd访问type为httpd_sys_content_t的文件：
root@centos ~]# sesearch --all | grep "allow httpd_t httpd_sys_content_t"
ERROR: Cannot get avrules: Neverallow rules requested but not available
allow httpd_t httpd_sys_content_t : file { ioctl read getattr lock open } ;
allow httpd_t httpd_sys_content_t : dir { ioctl read getattr lock search open } ;
allow httpd_t httpd_sys_content_t : lnk_file { read getattr } ;
allow httpd_t httpd_sys_content_t : file { ioctl read getattr lock open } ;
allow httpd_t httpd_sys_content_t : dir { ioctl read getattr lock search open } ;
allow httpd_t httpd_sys_content_t : dir { ioctl read write getattr lock add_name remove_name search open } ;
allow httpd_t httpd_sys_content_t : lnk_file { read getattr } ;
好了，恢复/var/www/html/index.html吧：
[root@centos ~]# restorecon -Rv /var/www/html/
restorecon reset /var/www/html/index.html context unconfined_u:object_r:admin_home_t:s0->unconfined_u:object_r:httpd_sys_content_t:s0
在访问一次：
[root@centos ~]# links -dump
Test for selinux
OK了。
