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Red Hat Enterprise Linux 9 Essentials Book now available.
Purchase a copy of Red Hat Enterprise Linux 9 (RHEL 9) Essentials Red Hat Enterprise Linux 9 Essentials Print and eBook (PDF) editions contain 34 chapters and 298 pages
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Chapter 3. The Apache HTTP Server
"The Apache HTTP Server Project is an effort to develop and maintain an open-source HTTP server for modern operating systems including UNIX and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards".[]
In Red Hat Enterprise Linux, the httpd package provides the Apache HTTP Server. Run rpm -q httpd 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
3.1. 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:
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Run getenforce to confirm SELinux is running in enforcing mode:
$ getenforce
Enforcing
The getenforce command returns Enforcing when SELinux is running in enforcing mode.
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Run service httpd start as the root user to start httpd:
# service httpd start
Starting httpd: [ OK ]
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Run ps -eZ | grep httpd 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 can not write to them, even if Linux (DAC) permissions allow write access. Booleans must be turned on to allow certain behavior, such as allowing scripts network access, allowing httpd access to NFS and CIFS file systems, 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:
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Run service httpd status to confirm httpd is not running:
# service httpd status
httpd is stopped
If the output differs, run service httpd stop to stop the process:
# service httpd stop
Stopping httpd: [ OK ]
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Run semanage port -l | grep -w http_port_t 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
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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
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Run service httpd start 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
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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
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Run service httpd start again to start httpd and have it listen on the new port:
# service httpd start
Starting httpd: [ OK ]
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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.
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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...]
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