A proxy server is an intermediary application that sits between clients and servers. Instead of connecting directly to a website or service, a client sends requests to the proxy, which then forwards them to the destination server.

Proxies are commonly used to:

Control user access
Monitor and log activities
Improve performance through caching
Filter content
Convert between different network protocols

There are proxies for many protocols, including:

SMTP (Email)
DNS
HTTP (Web)
IRC / IM (Chat)

This article focuses on web proxies, particularly using Squid.

Understanding Web Objects and Caching

What Are Web Objects?

When discussing web proxies and caching, we often refer to objects.

A web object can be:

An HTML page
A JPEG image
A PDF document
A Flash animation
A CSS or JavaScript file

Each object is identified by a URL (Uniform Resource Locator).

Note: A “web page” typically consists of multiple objects — the HTML file plus images, scripts, and stylesheets.

How Caching Works

Requests and Responses

Here’s what happens in a typical HTTP transaction:

A browser sends an HTTP request.
The origin server sends back an HTTP response (the object).
The proxy may cache the response for future reuse.

However, not all responses are cacheable.

Factors Affecting Cacheability

Caching depends on:

Request headers
Response headers
Authentication requirements
Cache-Control directives
TTL (Time-To-Live)

For example:

Authenticated content is usually not cacheable.
Servers may define TTL values for freshness control.

Fresh vs Stale Content

A cached object can be:

Fresh

The object is within its defined TTL and can be served immediately.

Stale

The TTL has expired. The proxy must validate the object using a conditional request before serving it.

This validation ensures users receive updated content when necessary.

Squid Proxy Server

Introduction to Squid

Squid is a free, open-source proxy server primarily designed for UNIX systems, though it can also run on Windows.

Squid:

Speaks HTTP to browsers
Is not an email proxy
Is not an IRC/IM proxy
Acts partially as an FTP proxy

It is widely used for:

Web caching
Access control
Bandwidth optimization
Logging and monitoring

Installing Squid on Ubuntu

The easiest way to install Squid is via your OS package manager.

Install via APT:

sudo apt-get install squid

Alternatively, install from source:

tar xzvf squid-version.tar.gz
cd squid-version
./configure
make
make install

Understanding squid.conf

The main configuration file is located at:

/etc/squid/squid.conf

To view only active (non-commented) lines:

grep -v ^# squid.conf | grep . | less

Important Configuration Directives

Port Configuration

http_port 3128

Default listening port.

Cache Directory

cache_dir ufs /var/spool/squid 1000 16 256

Defines storage location and size.

Memory Cache

cache_mem 8 MB

Specifies memory allocated for caching.

Effective User

cache_effective_user proxy

Runs Squid under a secure system user.

Example ACL Rule

acl QUERY urlpath_regex cgi-bin \?
no_cache deny QUERY

Prevents caching of dynamic CGI requests.

Starting and Managing Squid

Start / Stop / Restart

/etc/init.d/squid start
/etc/init.d/squid stop
/etc/init.d/squid restart
/etc/init.d/squid reload

Start at Boot

Enable:

sudo update-rc.d squid defaults

Disable:

sudo update-rc.d -f squid remove

Squid Log Files

Squid maintains several important logs:

cache.log

/var/log/squid/cache.log

Runtime status, warnings, and errors.

access.log

/var/log/squid/access.log

Client requests, URLs, transfer size, status codes.

store.log

/var/log/squid/store.log

Tracks cached objects.

To monitor logs live:

tail -f /var/log/squid/cache.log
tail -f /var/log/squid/access.log

Squid Access Control Lists (ACLs)

Why ACLs Are Critical

An improperly configured proxy becomes an open proxy, which can be abused for:

Illegal downloads
Fraudulent transactions
Spam and malicious activities

All misuse will be traced back to your server’s IP address.

Proper ACL configuration is essential for security.

Understanding ACL Syntax

ACLs define conditions that are later referenced in access rules.

General ACL Format:

acl aclname acltype value

Example:

acl allowed_clients src 192.168.1.0/255.255.255.0

This defines trusted local clients.

HTTP Access Rules

After defining ACLs, you use them in http_access rules.

General Format:

http_access allow|deny [!]aclname