Squid is a full-featured web proxy cache server application which provides proxy and cache services for Hyper Text Transport Protocol (HTTP), File Transfer Protocol (FTP), and other popular network protocols. Squid can implement caching and proxying of Secure Sockets Layer (SSL) requests and caching of Domain Name Server (DNS) lookups, and perform transparent caching. Squid also supports a wide variety of caching protocols, such as Internet Cache Protocol (ICP), the Hyper Text Caching Protocol (HTCP), the Cache Array Routing Protocol (CARP), and the Web Cache Coordination Protocol (WCCP).

The Squid proxy cache server is an excellent solution to a variety of proxy and caching server needs, and scales from the branch office to enterprise level networks while providing extensive, granular access control mechanisms, and monitoring of critical parameters via the Simple Network Management Protocol (SNMP). When selecting a computer system for use as a dedicated Squid caching proxy server for many users ensure it is configured with a large amount of physical memory as Squid maintains an in-memory cache for increased performance.

Installation
Configuration
References

Installation

At a terminal prompt, enter the following command to install the Squid server:

sudo apt install squid

Configuration

Squid is configured by editing the directives contained within the /etc/squid/squid.conf configuration file. The following examples illustrate some of the directives which may be modified to affect the behavior of the Squid server. For more in-depth configuration of Squid, see the References section.

Prior to editing the configuration file, you should make a copy of the original file and protect it from writing so you will have the original settings as a reference, and to re-use as necessary. Make this copy and protect it from writing using the following commands:

sudo cp /etc/squid/squid.conf /etc/squid/squid.conf.original
sudo chmod a-w /etc/squid/squid.conf.original

To set your Squid server to listen on TCP port 8888 instead of the default TCP port 3128, change the http_port directive as such:

http_port 8888

Change the visible_hostname directive in order to give the Squid server a specific hostname. This hostname does not necessarily need to be the computer’s hostname. In this example it is set to weezie

visible_hostname weezie

Using Squid’s access control, you may configure use of Internet services proxied by Squid to be available only users with certain Internet Protocol (IP) addresses. For example, we will illustrate access by users of the 192.168.42.0/24 subnetwork only:

Add the following to the bottom of the ACL section of your /etc/squid/squid.conf file:

acl fortytwo_network src 192.168.42.0/24

Then, add the following to the top of the http_access section of your /etc/squid/squid.conf file:

http_access allow fortytwo_network

Using the excellent access control features of Squid, you may configure use of Internet services proxied by Squid to be available only during normal business hours. For example, we’ll illustrate access by employees of a business which is operating between 9:00AM and 5:00PM, Monday through Friday, and which uses the 10.1.42.0/24 subnetwork:

Add the following to the bottom of the ACL section of your /etc/squid/squid.conf file:

acl biz_network src 10.1.42.0/24
acl biz_hours time M T W T F 9:00-17:00

Then, add the following to the top of the http_access section of your /etc/squid/squid.conf file:

http_access allow biz_network biz_hours

After making changes to the /etc/squid/squid.conf file, save the file and restart the squid server application to effect the changes using the following command entered at a terminal prompt:

sudo systemctl restart squid.service

If formerly a customized squid3 was used that set up the spool at /var/log/squid3 to be a mountpoint, but otherwise kept the default configuration the upgrade will fail. The upgrade tries to rename/move files as needed, but it can’t do so for an active mountpoint. In that case please either adapt the mountpoint or the config in /etc/squid/squid.conf so that they match.

The same applies if the include config statement was used to pull in more files from the old path at /etc/squid3/. In those cases you should move and adapt your configuration accordingly.