The /etc/hosts file is used to map hostnames to IP addresses as a local manual substitution or supplement to DNS. It is checked first when the system connects to the hostname and can be used for local development, website blocking, and DNS change testing. 1. It achieves local control over domain name resolution by taking priority over DNS in the system search order; 2. Typical uses include pointing localhost to 127.0.0.1 or pointing custom domain names to local IP; 3. Common application scenarios include local development and testing, blocking specific websites and rehearsing DNS changes; 4. Editing requires administrator permissions and should follow format specifications to avoid errors.

The /etc/hosts file is a simple text file used by operating systems to map hostnames to IP addresses. It acts as a local, manual alternative or supplement to DNS (Domain Name System). When you type a domain name like example.com into your browser, your system checks the /etc/hosts file first before querying external DNS servers. This means you can use it to override where a domain resolves — useful for testing, development, or blocking certain websites.

How Does the /etc/hosts File Work?

When your computer needs to connect to a hostname, it follows a lookup order defined by the system. On Unix-like systems (including Linux and macOS), this is often controlled by the nsswitch.conf file, which typically lists "files" (ie, /etc/hosts ) before "dns". So the system will check /etc/hosts first, then go to DNS if no match is found.

A typical line in /etc/hosts looks like this:

 127.0.0.1 localhost

This tells the system that when you try to access localhost , it should point to the loopback address 127.0.0.1 .

You can add custom entries like:

 192.168.1.100 mylocalserver

Now typing mylocalserver in a terminal or browser will point to that local machine on your network.

Common Uses of /etc/hosts

There are several practical reasons people edit the /etc/hosts file:

• Local Development : Web developers often map domains like local.dev or mysite.test to 127.0.0.1 so they can test websites locally without needing a live server.
• Blocking Sites : You can redirect unwanted domains (like ad servers or distracting sites) to 127.0.0.1 or 0.0.0.0 . For example:

   0.0.0.0 badsite.com

• Testing DNS Changes : Before switching DNS records live, you can simulate how things will behave by temporarily editing hosts.

It's important to note that changes to this file affect only the local machine — not other devices on your network.

How to Edit the /etc/hosts File

Editing /etc/hosts usually require administrative privileges because it's a system-level configuration.

On Linux/macOS , you can open it with a text editor like nano or vim using sudo:

 sudo nano /etc/hosts

On Windows , it's located at C:\Windows\System32\drivers\etc\hosts , and you'll need to open your text editor as an administrator to make changes.

Some basic tips when editing:

• Always back up the original file before making changes
• Use plain text editors — avoid rich-text formats
• Each entry must be on its own line
• Formatting matters: put the IP address first, then one or more spaces or tabs, then the hostname(s)

If something breaks after editing the file, just restore the backup or remove the line causing issues.

That's the basics of what the /etc/hosts file does and why it matters. It's a small file but surprisingly powerful when used correctly. Just remember — it's easy to mess up if you're not careful, especially on production systems.

The above is the detailed content of What is the purpose of the /etc/hosts file?. For more information, please follow other related articles on the PHP Chinese website!