Squid a proxy server and web cache daemon. It has a wide variety of uses, from speeding up a web server by caching repeated requests, to caching web, DNS and other computer network lookups for a group of people sharing network resources, to aiding security by filtering traffic. Squid is primarily used for HTTP and FTP and it includes limited support for several other protocols such as TLS, SSL, Internet Gopher and HTTPS and the development version of Squid includes IPv6 and ICAP support too.
In this article I’m not going to cover the installation process of Squid-cache. My focus will be on the access control based configuration of Squid-cache for various requirements and also I’ll be covering how to fine tune the other applications to work with Squid, such as the firewall. In other words I’m gonna talk about access-controls (ACLs) in squid.conf and some post configurations.
The “/etc/squid/squid.conf” file
The main Squid configuration file is squid.conf, and, like most Linux applications, Squid needs to be restarted for changes to the configuration file can take effect.
Squid will fail to start if you don’t give your server a hostname. You can set this with the visible_hostname parameter. Here, the hostname is set to the real name of the server ‘myhost’.
visible_hostname myhost
You can limit users’ ability to browse the Internet with access control lists (ACLs). Each ACL line defines a particular type of activity, such as an access time or source network, they are then linked to an http_access statement that tells Squid whether or not to deny or allow traffic that matches the ACL.
Squid matches each Web access request it receives by checking the http_access list from top to bottom. If it finds a match, it enforces the allow or deny statement and stops reading further. You have to be careful not to place a deny statement in the list that blocks a similar allow statement below it.
NOTE: The final http_access statement denies everything, so it is best to place new http_access statements above that statement.
Squid has a minimum required set of ACL statements in the ACCESS_CONTROL section of the squid.conf file. It is best to put new customized entries right after this list to improve the readability.
Restricting web access by time
You can create access control lists with time parameters. For example, you can allow only business hour access from the home network, while always restricting access to host 192.168.1.10.
#
# Add this to the bottom of the ACL section of squid.conf
#
acl home_network src 192.168.1.0/24
acl business_hours time M T W H F 9:00-17:00
acl RestrictedHost src 192.168.1.10
#
# Add this at the top of the http_access section of squid.conf
#
http_access deny RestrictedHost
http_access allow home_network business_hours
Or, you can allow morning access only:
#
# Add this to the bottom of the ACL section of squid.conf
#
acl morning_hours time 08:00-12:00
#
# Add this at the top of the http_access section of squid.conf
#
http_access allow morning_hours
Restricting access to specific URLs
Squid is also capable of reading files containing lists of web sites and/or domains for use in ACLs. In this example we create to lists in files named /etc/squid/allowed-sites.acl and /etc/squid/restricted-sites.acl
# File: /etc/squid/allowed-sites.acl
www.gnu.org
mysite.com
# File: /etc/squid/restricted-sites.acl
www.restricted.com
illegal.com
These can then be used to always block the restricted sites and permit the allowed sites during working hours. This can be illustrated by expanding our previous example slightly.
#
# Add this to the bottom of the ACL section of squid.conf
#
acl home_network src 192.168.1.0/24
acl business_hours time M T W H F 9:00-17:00
acl GoodSites dstdomain "/etc/allowed-sites.acl"
acl BadSites dstdomain "/etc/restricted-sites.acl"
#
# Add this at the top of the http_access section of squid.conf
#
http_access deny BadSites
http_access allow home_network business_hours GoodSites
Restricting web access by IP address
You can create an access control list that restricts web access to users on certain networks. In this case, it’s an ACL that defines a home network of 192.168.1.0.
#
# Add this to the bottom of the ACL section of squid.conf
#
acl home_network src 192.168.1.0/255.255.255.0
You also have to add a corresponding http_access statement that allows traffic that matches the ACL:
#
# Add this at the top of the http_access section of squid.conf
#
http_access allow home_network
Password based authentication using NCSA
You can configure Squid to prompt users for a username and password when they are browsing any URLs. Squid comes with a program called ncsa_auth that reads any NCSA-compliant encrypted password file. You can use the htpasswd program that comes installed with Apache to create your passwords. Here is how it’s done:
First you need to create the password file. Here the name of the password file should be /etc/squid/squid_passwd, and you need to make sure that it’s universally readable.
[root]# touch /etc/squid/squid_passwd
[root]# chmod o+r /etc/squid/squid_passwd
Then use the htpasswd program to add users to the password file. You can add users at anytime without having to restart Squid. In this case, you add a username called ‘test_user’:
[root]# htpasswd /etc/squid/squid_passwd test_user
New password:
Re-type new password:
Adding password for user test_user
Now you have to locate the ncsa_auth file.
[root]# locate ncsa_auth
/usr/lib/squid/ncsa_auth
Edit squid.conf; specifically, you need to define the authentication program in squid.conf, which is in this case ncsa_auth. Next, create an ACL named ncsa_users with the REQUIRED keyword that forces Squid to use the NCSA auth_param method you defined previously. Finally, create an http_access entry that allows traffic that matches the ncsa_users ACL entry. Here’s a simple user authentication example; the order of the statements are important:
#
# Add this to the auth_param section of squid.conf
#
auth_param basic program /usr/lib/squid/ncsa_auth /etc/squid/squid_passwd
#
# Add this to the bottom of the ACL section of squid.conf
#
acl ncsa_users proxy_auth REQUIRED
#
# Add this at the top of the http_access section of squid.conf
#
http_access allow ncsa_users
This will enable the password based authentication and allows access only during business hours. Once again, the order of the statements is important:
#
# Add this to the auth_param section of squid.conf
#
auth_param basic program /usr/lib/squid/ncsa_auth /etc/squid/squid_passwd
#
# Add this to the bottom of the ACL section of squid.conf
#
acl ncsa_users proxy_auth REQUIRED
acl business_hours time M T W H F 9:00-17:00
#
# Add this at the top of the http_access section of squid.conf
#
http_access allow ncsa_users business_hours
Remember to restart Squid for the changes to take effect.
Forcing users to use your Squid Server
If you are using access controls on Squid, you may also want to configure your firewall to allow only HTTP Internet access to only the Squid server. This forces your users to browse the Web through the Squid proxy. Also it is possible to limit HTTP Internet access to only the Squid server without having to modify the browser settings on your client PCs. This called a transparent proxy configuration. It is usually achieved by configuring a firewall between the client PCs and the WAN to redirect all HTTP (TCP port 80) traffic to the Squid server on TCP port 3128, which is the Squid server’s default TCP port.
Squid transparent proxy configuration
Your first step will be to modify your squid.conf to create a transparent proxy. The procedure is different depending on your version of Squid. In older versions of Squid ( <>squid.conf would be as follows:
httpd_accel_host virtual
httpd_accel_port 80
httpd_accel_with_proxy on
httpd_accel_uses_host_header on
Newer versions of Squid simply require you to add the word “transparent” to the default “http_port 3128″ statement. In this example, Squid not only listens on TCP port 3128 for proxy connections, but will also do so in transparent mode.
http_port 3128 transparent
Configuring iptables to support the Squid tansparent proxy
In this example, assuming the Squid server and firewall are in the same server, all HTTP traffic from the home network is redirecting to the firewall itself on the Squid port of 3128 and then only the firewall itself has access the Internet on port 80.
iptables -t nat -A PREROUTING -i eth1 -p tcp --dport 80 -j REDIRECT --to-port 3128
iptables -A INPUT -j ACCEPT -m state --state NEW,ESTABLISHED,RELATED -i eth1 -p tcp --dport 3128
iptables -A OUTPUT -j ACCEPT -m state --state ESTABLISHED,RELATED -o eth1 -p tcp --sport 80
Note: This example is specific to HTTP traffic. You won’t be able to adapt this example to support HTTPS web browsing on TCP port 443, as that protocol specifically doesn’t allow the insertion of a “man in the middle” server for security purposes. One solution is to add IP masquerading statements for port 443, or any other important traffic, immediately after the code snippet. This will allow non HTTP traffic to access the Internet without being cached by Squid.
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