Chapter 8. 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.[6]
In Red Hat Enterprise Linux, the squid package provides the Squid Caching Proxy. Run the
rpm -q squid command to see if the squid package is installed. If it is not installed and you want to use squid, run the following command as the root user to install it:
~]# yum install squid8.1. 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
getenforcecommand to confirm SELinux is running in enforcing mode:~]$
getenforceEnforcingThegetenforcecommand returnsEnforcingwhen SELinux is running in enforcing mode. - Run the
service squid startcommand as the root user to startsquid:~]#
service squid startStarting squid: [ OK ] - Run the
ps -eZ | grep squidcommand to view thesquidprocesses:~]$
ps -eZ | grep squidunconfined_u:system_r:squid_t:s0 2522 ? 00:00:00 squid unconfined_u:system_r:squid_t:s0 2524 ? 00:00:00 squid unconfined_u:system_r:squid_t:s0 2526 ? 00:00:00 ncsa_auth unconfined_u:system_r:squid_t:s0 2527 ? 00:00:00 ncsa_auth unconfined_u:system_r:squid_t:s0 2528 ? 00:00:00 ncsa_auth unconfined_u:system_r:squid_t:s0 2529 ? 00:00:00 ncsa_auth unconfined_u:system_r:squid_t:s0 2530 ? 00:00:00 ncsa_auth unconfined_u:system_r:squid_t:s0 2531 ? 00:00:00 unlinkdThe SELinux context associated with thesquidprocesses isunconfined_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, thesquidprocesses are running in thesquid_tdomain.
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
/etc/squid/squid.conf 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 squid, and, as a consequence, squid failing to start. This example also demonstrates how to then configure the SELinux system to allow squid 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:
- Run the
service squid statuscommand to confirmsquidis not running:~]#
service squid statussquid is stoppedIf the output differs, run theservice squid stopcommand to stop the process:~]#
service squid stopStopping squid: [ OK ] - Run the
semanage port -l | grep -w squid_port_tcommand to view the ports SELinux allowssquidto listen on:~]#
semanage port -l | grep -w -i squid_port_tsquid_port_t tcp 3401, 4827 squid_port_t udp 3401, 4827 - Edit
/etc/squid/squid.confas the root user. Configure thehttp_portoption so it lists a port that is not configured in SELinux policy configuration forsquid. In this example,squidis configured to listen on port 10000:# Squid normally listens to port 3128 http_port 10000
- Run the
setseboolcommand to make sure thesquid_connect_anyBoolean is set to off. This ensures squid is only permitted to operate on specific ports:~]#
setsebool -P squid_connect_any 0 - Run the
service squid startcommand to startsquid:~]#
service squid startStarting squid: .................... [FAILED]An SELinux denial 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
squidto listen on port 10000, as used in this example, the following command is required:~]#
semanage port -a -t squid_port_t -p tcp 10000 - Run
service squid startagain to startsquidand have it listen on the new port:~]#
service squid startStarting squid: [ OK ] - Now that SELinux has been configured to allow
squidto listen on a non-standard port (TCP 10000 in this example),squidstarts successfully on this port.
