Chapter 19. Squid Caching Proxy
Squid is a high-performance proxy caching server for web clients, supporting FTP, Gopher, and HTTP data objects. It reduces bandwidth and improves response times by caching and reusing frequently-requested web pages.[17]
In Red Hat Enterprise Linux, the squid package provides the Squid Caching Proxy. Enter the following command to see if the squid package is installed:
~]$ rpm -q squid
package squid is not installed
If it is not installed and you want to use squid, use the
yum
utility as root to install it:
~]# yum install squid
19.1. Squid Caching Proxy and SELinux
When SELinux is enabled, Squid 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 Squid processes running in their own domain. This example assumes the squid package is installed:
- Run the
getenforce
command to confirm SELinux is running in enforcing mode:~]$
getenforce
EnforcingThe command returnsEnforcing
when SELinux is running in enforcing mode. - Enter the following command as the root user to start the
squid
daemon:~]#
systemctl start squid.service
Confirm that the service is running. The output should include the information below (only the time stamp will differ):~]#
systemctl status squid.service
squid.service - Squid caching proxy Loaded: loaded (/usr/lib/systemd/system/squid.service; disabled) Active: active (running) since Mon 2013-08-05 14:45:53 CEST; 2s ago - Enter the following command to view the
squid
processes:~]$
ps -eZ | grep squid
system_u:system_r:squid_t:s0 27018 ? 00:00:00 squid system_u:system_r:squid_t:s0 27020 ? 00:00:00 log_file_daemonThe SELinux context associated with thesquid
processes issystem_u:system_r:squid_t:s0
. The second last part of the context,squid_t
, is the type. A type defines a domain for processes and a type for files. In this case, the Squid processes are running in thesquid_t
domain.
SELinux policy defines how processes running in confined domains, such as
squid_t
, interact with files, other processes, and the system in general. Files must be labeled correctly to allow squid access to them.
When the
/etc/squid/squid.conf
file is configured so squid
listens on a port other than the default TCP ports 3128, 3401 or 4827, the semanage port
command must be used to add the required port number to the SELinux policy configuration. The following example demonstrates configuring squid
to listen on a port that is not initially defined in SELinux policy configuration for it, and, as a consequence, the server failing to start. This example also demonstrates how to then configure the SELinux system to allow the daemon to successfully listen on a non-standard port that is not already defined in the policy. This example assumes the squid package is installed. Run each command in the example as the root user:
- Confirm the
squid
daemon is not running:~]#
systemctl status squid.service
squid.service - Squid caching proxy Loaded: loaded (/usr/lib/systemd/system/squid.service; disabled) Active: inactive (dead)If the output differs, stop the process:~]#
systemctl stop squid.service
- Enter the following command to view the ports SELinux allows
squid
to listen on:~]#
semanage port -l | grep -w -i squid_port_t
squid_port_t tcp 3401, 4827 squid_port_t udp 3401, 4827 - Edit
/etc/squid/squid.conf
as root. Configure thehttp_port
option so it lists a port that is not configured in SELinux policy configuration forsquid
. In this example, the daemon is configured to listen on port 10000:# Squid normally listens to port 3128 http_port 10000
- Run the
setsebool
command to make sure thesquid_connect_any
Boolean is set to off. This ensuressquid
is only permitted to operate on specific ports:~]#
setsebool -P squid_connect_any 0
- Start the
squid
daemon:~]#
systemctl start squid.service
Job for squid.service failed. See 'systemctl status squid.service' and 'journalctl -xn' for details.An SELinux denial message similar to the following is logged:localhost setroubleshoot: SELinux is preventing the squid (squid_t) from binding to port 10000. For complete SELinux messages. run sealert -l 97136444-4497-4fff-a7a7-c4d8442db982
- For SELinux to allow
squid
to listen on port 10000, as used in this example, the following command is required:~]#
semanage port -a -t squid_port_t -p tcp 10000
- Start
squid
again and have it listen on the new port:~]#
systemctl start squid.service
- Now that SELinux has been configured to allow Squid to listen on a non-standard port (TCP 10000 in this example), it starts successfully on this port.