If you upgrade a running system to Red Hat Enterprise Linux (RHEL) 6.6 or CentOS 6.6, the Security Enhanced Linux (SELinux) security permissions that apply to NGINX are relabelled to a much stricter posture. Although the permissions are adequate for the default configuration of NGINX, configuration for additional features can be blocked and you need to permit them explicitly in SELinux. This article describes the possible issues and recommended ways to resolve them.

Overview of SELinux

SELinux is enabled by default on RHEL and CentOS servers. Each operating system object (process, file descriptor, file, etc.) is associated with an SELinux context that defines the permissions and operations the object can perform. During an upgrade to RHEL 6.6 or CentOS 6.6, NGINX’s association ic changed to the httpd_t context:

# ps auZ | grep nginx
unconfined_u:system_r:httpd_t:s0 3234 ? Ss 0:00 nginx: master process /usr/sbin/nginx 
                                                              -c /etc/nginx/nginx.conf
unconfined_u:system_r:httpd_t:s0 3236 ? Ss 0:00 nginx: worker process

The httpd_t context permits NGINX to listen on common web server ports, to access configuration files in /etc/nginx, and to access content in the standard docroot location (/usr/share/nginx). It does not permit many other operations, such as proxying to upstream locations or communicating with other processes through sockets.

SELinux Modes

SELinux can be run in enforcing, permissive, or disabled mode. When you make a configuration change that might breach the current permissions, you can move SELinux from enforcing to permissive mode, on your test environment (if available) or on production. In permissive mode, SELinux permits all operations, but logs operations that would have breached the security policy in enforcing mode.

To add httpd_t to the list of permissive domains, run:

# semanage permissive -a httpd_t

To delete httpd_t from the list of permissive domains, run:

# semanage permissive -d httpd_t

To set the mode globally to permissive, run:

# setenforce 0

To set the mode globally to enforcing, run:

# setenforce 1

Checking for SELinux Exceptions

In permissive mode, security exceptions are logged to /var/log/audit/audit.log. If you encounter a problem that is only present when NGINX is in enforcing mode, review the exceptions that are logged in permissive mode and update the security policy to permit them.

Example 1: Proxy Connection is Forbidden

By default, the SELinux configuration does not allow NGINX to connect to a remote web, fastCGI, or other server, as indicated by an audit log message like the following:

type=AVC msg=audit(1415714880.156:29): avc:  denied  { name_connect } for  pid=1349 
  comm="nginx" dest=8080 scontext=unconfined_u:system_r:httpd_t:s0 
  tcontext=system_u:object_r:http_cache_port_t:s0 tclass=tcp_socket
type=SYSCALL msg=audit(1415714880.156:29): arch=c000003e syscall=42 success=no 
  exit=-115 a0=b a1=16125f8 a2=10 a3=7fffc2bab440 items=0 ppid=1347 pid=1349 
  auid=1000 uid=497 gid=496 euid=497 suid=497 fsuid=497 egid=496 sgid=496 fsgid=496 
  tty=(none) ses=1 comm="nginx" exe="/usr/sbin/nginx" 
  subj=unconfined_u:system_r:httpd_t:s0 key=(null)

To interpret the message code (1415714880.156:29), run the audit2why command:

# grep 1415714880.156:29 /var/log/audit/audit.log | audit2why
type=AVC msg=audit(1415714880.156:29): avc:  denied  { name_connect } for  pid=1349 
  comm="nginx" dest=8080 scontext=unconfined_u:system_r:httpd_t:s0 
  tcontext=system_u:object_r:http_cache_port_t:s0 tclass=tcp_socket

        Was caused by:
        One of the following booleans was set incorrectly.
        Description:
        Allow httpd to act as a relay

        Allow access by executing:
        # setsebool -P httpd_can_network_relay 1
        Description:
        Allow HTTPD scripts and modules to connect to the network using TCP.

        Allow access by executing:
        # setsebool -P httpd_can_network_connect 1

The output from audit2why recommends setting one or more boolean options. To permit the proxy connect operation, you can enable these boolean options, either temporarily or permanently (add the -P switch).

Understanding Boolean Options

If you install the setools package (yum install setools), you can run the sesearch command to get more information about the boolean options.

The httpd_can_network_relay Boolean Option

# sesearch -A -s httpd_t -b httpd_can_network_relay
Found 10 semantic av rules:
   allow httpd_t gopher_port_t : tcp_socket name_connect ;
   allow httpd_t http_cache_client_packet_t : packet { send recv } ;
   allow httpd_t ftp_port_t : tcp_socket name_connect ;
   allow httpd_t ftp_client_packet_t : packet { send recv } ;
   allow httpd_t http_client_packet_t : packet { send recv } ;
   allow httpd_t squid_port_t : tcp_socket name_connect ;
   allow httpd_t http_cache_port_t : tcp_socket name_connect ;
   allow httpd_t http_port_t : tcp_socket name_connect ;
   allow httpd_t gopher_client_packet_t : packet { send recv } ;
   allow httpd_t memcache_port_t : tcp_socket name_connect ;

This output indicates that httpd_can_network_relay permits connection to ports of various types, including typehttp_port_t:

# semanage port -l | grep http_port_t
http_port_t                    tcp      80, 81, 443, 488, 8008, 8009, 8443, 9000

To add more ports to the set (in this case, 8082), run:

# semanage port -a -t http_port_t -p tcp 8082

If a message indicates that a port is already defined, as in the following example, it means the port is included in another set. Do not reassign it, because other services might be negatively affected.

# semanage port -a -t http_port_t -p tcp 8080
/usr/sbin/semanage: Port tcp/8080 already defined
# semanage port -l | grep 8080
http_cache_port_t              tcp      3128, 8080, 8118, 8123, 10001-10010

The httpd_can_network_connect Boolean Option

# sesearch -A -s httpd_t -b httpd_can_network_connect
Found 1 semantic av rules:
   allow httpd_t port_type : tcp_socket name_connect ;

The httpd_can_network_connect option allows httpd_t to connect to all TCP socket types that have the port_typeattribute. To list them, run:

# seinfo -aport_type -x

Example 2: File Access is Forbidden

By default, the SELinux configuration does not allow NGINX to access files outside of well-known authorized locations, as indicated by an audit log message like the following:

type=AVC msg=audit(1415715270.766:31): avc:  denied  { getattr } for  pid=1380 
  comm="nginx" path="/www/t.txt" dev=vda1 ino=1084 
  scontext=unconfined_u:system_r:httpd_t:s0 
  tcontext=unconfined_u:object_r:default_t:s0 tclass=file

To interpret the message code (1415715270.766:31), run the audit2why command:

# grep 1415715270.766:31 /var/log/audit/audit.log | audit2why
type=AVC msg=audit(1415715270.766:31): avc:  denied  { getattr } for  pid=1380 
  comm="nginx" path="/www/t.txt" dev=vda1 ino=1084 
  scontext=unconfined_u:system_r:httpd_t:s0 
  tcontext=unconfined_u:object_r:default_t:s0 tclass=file

    Was caused by:
        Missing type enforcement (TE) allow rule.

        You can use audit2allow to generate a loadable module to allow this access.

When file access is forbidden, you have two options.

Option 1: Modify the File Label

Modify the file label so that the httpd_t domain can access the file:

# chcon -v --type=httpd_sys_content_t /www/t.txt

By default, this modification is deleted when the file system is relabelled. To make the change permanent, run:

# semanage fcontext -a -t httpd_sys_content_t /www/t.txt
# restorecon -v /www/t.txt

To modify file labels for groups of files, run:

# semanage fcontext -a -t httpd_sys_content_t /www(/.*)?
# restorecon -Rv /www

Option 2: Extend the httpd_t Domain Permissions

Extend the httpd_t policy to allow access to additional file locations:

# grep nginx /var/log/audit/audit.log | audit2allow -m nginx > nginx.te
# cat nginx.te

module nginx 1.0;

require {
        type httpd_t;
        type default_t;
        type http_cache_port_t;
        class tcp_socket name_connect;
        class file { read getattr open };
}

#============= httpd_t ==============
allow httpd_t default_t:file { read getattr open };

#!!!! This avc can be allowed using one of the these booleans:
#     httpd_can_network_relay, httpd_can_network_connect
allow httpd_t http_cache_port_t:tcp_socket name_connect;

To create a compiled policy, include the -M switch:

# grep nginx /var/log/audit/audit.log | audit2allow -M nginx

To load the policy, run semodule -i, then verify success with semodule -l:

# semodule -i nginx.pp
# semodule -l | grep nginx
nginx 1.0

This change persists across reboots.

Additional Resources

SELinux is a complex and powerful facility for managing operating system permissions. Additional information is available at the following locations.

• https://www.nsa.gov/research/selinux/docs.shtml
• http://docs.fedoraproject.org/en-US/Fedora/13/html/Security-Enhanced_Linux/index.html
• https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Security-Enhanced_Linux
• http://selinuxproject.org
• http://wiki.centos.org/HowTos/SELinux