7.2.2. How are Confined Services Running?

Services can be run in a variety of ways. To cater for this, you must tell SELinux how you are running services. This can be achieved via Booleans that allow parts of SELinux policy to be changed at runtime, without any knowledge of SELinux policy writing. This allows changes, such as allowing services access to NFS file systems, without reloading or recompiling SELinux policy. Also, running services on non-default port numbers requires policy configuration to be updated via the semanage command.

For example, to allow the Apache HTTP Server to communicate with MySQL, turn the httpd_can_network_connect_db Boolean on:

# /usr/sbin/setsebool -P httpd_can_network_connect_db on

If access is denied for a particular service, use the getsebool and grep commands to see if any Booleans are available to allow access. For example, use the getsebool -a | grep ftp command to search for FTP related Booleans:

$ /usr/sbin/getsebool -a | grep ftp
allow_ftpd_anon_write --> off
allow_ftpd_full_access --> off
allow_ftpd_use_cifs --> off
allow_ftpd_use_nfs --> off
ftp_home_dir --> off
httpd_enable_ftp_server --> off
tftp_anon_write --> off

For a list of Booleans and whether they are on or off, run the /usr/sbin/getsebool -a command. For a list of Booleans, an explanation of what each one is, and whether they are on or off, run the /usr/sbin/semanage boolean -l command as the Linux root user. Refer to Section 5.6, “Booleans” for information about listing and configuring Booleans.

Port Numbers

Depending on policy configuration, services may only be allowed to run on certain port numbers. Attempting to change the port a service runs on without changing policy may result in the service failing to start. For example, run the semanage port -l | grep http command as the Linux root user to list http related ports:

# /usr/sbin/semanage port -l | grep http
http_cache_port_t              tcp      3128, 8080, 8118
http_cache_port_t              udp      3130
http_port_t                    tcp      80, 443, 488, 8008, 8009, 8443
pegasus_http_port_t            tcp      5988
pegasus_https_port_t           tcp      5989

The http_port_t port type defines the ports Apache HTTP Server can listen on, which in this case, are TCP ports 80, 443, 488, 8008, 8009, and 8443. If an administrator configures httpd.conf so that httpd listens on port 9876 (Listen 9876), but policy is not updated to reflect this, the service httpd start command fails:

# /sbin/service httpd start
Starting httpd: (13)Permission denied: make_sock: could not bind to address [::]:9876
(13)Permission denied: make_sock: could not bind to address 0.0.0.0:9876
no listening sockets available, shutting down
Unable to open logs
						            [FAILED]

An SELinux denial similar to the following is logged to /var/log/audit/audit.log:

type=AVC msg=audit(1225948455.061:294): avc:  denied  { name_bind } for  pid=4997 comm="httpd" src=9876 scontext=unconfined_u:system_r:httpd_t:s0 tcontext=system_u:object_r:port_t:s0 tclass=tcp_socket

To allow httpd to listen on a port that is not listed for the http_port_t port type, run the semanage port command to add a port to policy configuration^[15]:

# /usr/sbin/semanage port -a -t http_port_t -p tcp 9876

The -a option adds a new record; the -t option defines a type; and the -p option defines a protocol. The last argument is the port number to add.

^[15] The semanage port -a command adds an entry to the /etc/selinux/targeted/modules/active/ports.local file. Note: by default, this file can only be viewed by the Linux root user.