Chapter 2. The Apache HTTP Server

The Apache HTTP Server provides an open-source HTTP server with the current HTTP standards.[3]
In Red Hat Enterprise Linux, the httpd package provides the Apache HTTP Server. Run the rpm -q httpd command to see if the httpd package is installed. If it is not installed and you want to use the Apache HTTP Server, run the following command as the root user to install it:
~]# yum install httpd

2.1. The Apache HTTP Server and SELinux

When SELinux is enabled, the Apache HTTP Server (httpd) runs confined by default. Confined processes run in their own domains, and are separated from other confined processes. If a confined process is compromised by an attacker, depending on SELinux policy configuration, an attacker's access to resources and the possible damage they can do is limited. The following example demonstrates the httpd processes running in their own domain. This example assumes the httpd, setroubleshoot, setroubleshoot-server and policycoreutils-python packages are installed:
  1. Run the getenforce command to confirm SELinux is running in enforcing mode:
    ~]$ getenforce
    Enforcing
    
    The getenforce command returns Enforcing when SELinux is running in enforcing mode.
  2. Run the service httpd start command as the root user to start httpd:
    ~]# service httpd start
    Starting httpd:                                            [  OK  ]
    
  3. Run the ps -eZ | grep httpd command to view the httpd processes:
    ~]$ ps -eZ | grep httpd
    unconfined_u:system_r:httpd_t:s0 2850 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2852 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2853 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2854 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2855 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2856 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2857 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2858 ?        00:00:00 httpd
    unconfined_u:system_r:httpd_t:s0 2859 ?        00:00:00 httpd
    
    The SELinux context associated with the httpd processes is unconfined_u:system_r:httpd_t:s0. The second last part of the context, httpd_t, is the type. A type defines a domain for processes and a type for files. In this case, the httpd processes are running in the httpd_t domain.
SELinux policy defines how processes running in confined domains (such as httpd_t) interact with files, other processes, and the system in general. Files must be labeled correctly to allow httpd access to them. For example, httpd can read files labeled with the httpd_sys_content_t type, but cannot write to them, even if Linux (DAC) permissions allow write access. Booleans must be enabled to allow certain behavior, such as allowing scripts network access, allowing httpd access to NFS and CIFS volumes, and httpd being allowed to execute Common Gateway Interface (CGI) scripts.
When /etc/httpd/conf/httpd.conf is configured so httpd listens on a port other than TCP ports 80, 443, 488, 8008, 8009, or 8443, the semanage port command must be used to add the new port number to SELinux policy configuration. The following example demonstrates configuring httpd to listen on a port that is not already defined in SELinux policy configuration for httpd, and, as a consequence, httpd failing to start. This example also demonstrates how to then configure the SELinux system to allow httpd to successfully listen on a non-standard port that is not already defined in the policy. This example assumes the httpd package is installed. Run each command in the example as the root user:
  1. Run the service httpd status command to confirm httpd is not running:
    ~]# service httpd status
    httpd is stopped
    
    If the output differs, run the service httpd stop command to stop the process:
    ~]# service httpd stop
    Stopping httpd:                                            [  OK  ]
    
  2. Run the semanage port -l | grep -w http_port_t command to view the ports SELinux allows httpd to listen on:
    ~]# semanage port -l | grep -w http_port_t
    http_port_t                    tcp      80, 443, 488, 8008, 8009, 8443
    
  3. Edit /etc/httpd/conf/httpd.conf as the root user. Configure the Listen option so it lists a port that is not configured in SELinux policy configuration for httpd. In this example, httpd is configured to listen on port 12345:
    # Change this to Listen on specific IP addresses as shown below to 
    # prevent Apache from glomming onto all bound IP addresses (0.0.0.0)
    #
    #Listen 12.34.56.78:80
    Listen 127.0.0.1:12345
    
  4. Run the service httpd start command to start httpd:
    ~]# service httpd start
    Starting httpd: (13)Permission denied: make_sock: could not bind to address 127.0.0.1:12345
    no listening sockets available, shutting down
    Unable to open logs					   [FAILED]
    
    An SELinux denial similar to the following is logged:
    setroubleshoot: SELinux is preventing the httpd (httpd_t) from binding to port 12345. For complete SELinux messages. run sealert -l f18bca99-db64-4c16-9719-1db89f0d8c77
    
  5. For SELinux to allow httpd to listen on port 12345, as used in this example, the following command is required:
    ~]# semanage port -a -t http_port_t -p tcp 12345
  6. Run service httpd start again to start httpd and have it listen on the new port:
    ~]# service httpd start
    Starting httpd:						   [  OK  ]
    
  7. Now that SELinux has been configured to allow httpd to listen on a non-standard port (TCP 12345 in this example), httpd starts successfully on this port.
  8. To prove that httpd is listening and communicating on TCP port 12345, open a telnet connection to the specified port and issue a HTTP GET command, as follows:
    ~]# telnet localhost 12345
    Trying 127.0.0.1...
    Connected to localhost.
    Escape character is '^]'.
    GET / HTTP/1.0
    
    HTTP/1.1 200 OK
    Date: Wed, 02 Dec 2009 14:36:34 GMT
    Server: Apache/2.2.13 (Red Hat)
    Accept-Ranges: bytes
    Content-Length: 3985
    Content-Type: text/html; charset=UTF-8
    [...continues...]
    


[3] Refer to the Apache HTTP Server Project page for more information.