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All about hosts file

All about hosts file

Surjendu Pal's photo
Surjendu Pal
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4 min read

Table of contents

The concept of the hosts file dates back to the early days of networking. Before the advent of Domain Name System (DNS) in 1983, a centralized text file named HOSTS.TXT was maintained by the Network Information Center (NIC). This file mapped domain names to IP addresses, and every computer connected to ARPANET (the precursor to the modern internet) used it.

System administrators periodically downloaded and updated this file to ensure their systems could resolve domain names. While this system worked in the early days of the internet, it quickly became impractical as the number of connected devices and websites grew. DNS was introduced to automate and scale this process, but the hosts file persisted in modern operating systems as a local override for DNS.

Why do we need them?

The hosts file serves as a local mapping of domain names to IP addresses. When a computer attempts to connect to a domain, it checks the hosts file first before querying a DNS server. This prioritization allows users to control domain-to-IP mappings locally.

It is needed for several reasons:

  1. Control and Customization: The hosts file enables users to define how their system resolves specific domain names, bypassing DNS queries entirely.

  2. Security: It allows administrators to block malicious domains by mapping them to a non-routable IP address like 127.0.0.1.

  3. Development: Developers often use the hosts file to redirect domains to a local environment for testing.

  4. Offline Use: In environments where DNS is not accessible, the hosts file can provide an alternative method for name resolution.

Benefits of the Hosts File

The hosts file offers several advantages, including:

  1. Simplicity: The hosts file is a plain text file that can be edited using basic text editors like Notepad, making it easy to configure.

  2. Immediate Effect: Changes to the hosts file take effect immediately after saving, without needing a system reboot or additional tools.

  3. Offline Functionality: It works without relying on external DNS servers, ensuring reliable name resolution in isolated environments.

  4. Fine-Grained Control: Users can override DNS settings on a per-device basis, offering granular control over domain resolution.

  5. Increased Security: By blocking known malicious domains, the hosts file can serve as a lightweight ad blocker or safeguard against phishing attacks.

How to Locate and Edit the Hosts File in Windows

In Windows, the hosts file is located at:
C:\Windows\System32\drivers\etc\hosts

To edit the file:

  1. Open a text editor (e.g., Notepad) as an administrator.

  2. Navigate to the file location above.

  3. Open the hosts file and make the necessary changes.

  4. Save the file.

Note: Always back up the original file before making changes.

Use Cases of the Hosts File

1. Blocking Unwanted Websites

By mapping unwanted domains to 127.0.0.1, you can block access to those websites. For example:

127.0.0.1 facebook.com 127.0.0.1 twitter.com

This technique is often used for parental controls, productivity boosts, or simple ad blocking.

2. Testing and Development

Developers use the hosts file to redirect a domain name to a local server. For instance:

127.0.0.1 mywebsite.local

This enables testing websites locally before deploying them to production.

3. Redirecting Domains

The hosts file can be used to redirect one domain to another, which is useful for migration or accessing restricted resources:

192.168.1.100 intranet.company.com

4. Improving Network Performance

For frequently accessed domains, adding them to the hosts file can reduce DNS lookup time, especially in environments with unreliable DNS servers.

5. Overriding DNS for Specific Devices

The hosts file allows localized overrides, enabling different behavior on a particular device without affecting others on the network.

6. Securing Devices Against Malicious Domains

By mapping known malicious domains to 127.0.0.1, users can block phishing attempts and malware sites:

127.0.0.1 malicious-site.com

7. Customizing Access to Local Resources

Organizations often use the hosts file to provide employees with easy-to-remember names for internal servers:

192.168.1.10 fileserver 192.168.1.20 mailserver

Caveats and Best Practices

While the hosts file is powerful, it has limitations:

  1. Scalability: It is not suitable for large-scale environments with thousands of domain mappings.

  2. Maintenance: Manually managing the file can become cumbersome as the number of entries grows.

  3. Security Risks: Incorrectly configured hosts files can lead to connectivity issues or security vulnerabilities.

To ensure proper use:

  • Backup Regularly: Keep a backup of the original file before making changes.

  • Verify Changes: Double-check mappings to prevent errors.

  • Restrict Access: Limit access to the file to prevent unauthorized changes.

Conclusion

The hosts file remains a versatile tool for local domain resolution and network management. Its simplicity and immediate impact make it a go-to solution for developers, IT administrators, and even casual users who want to customize their internet experience. While DNS has largely automated name resolution, the hosts file provides a reliable fallback and fine-grained control for a variety of use cases.

Understanding how to use the hosts file effectively can enhance your productivity, improve security, and streamline development workflows. Whether you're blocking ads, testing a new website, or managing an isolated network, the hosts file is a handy resource to have in your technical toolkit.

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