Block Ads: Validate /etc/hosts Files

TL;DR

Using a third-party provider to block ads with your /etc/hosts file is effective, but you need to regularly check the list hasn’t been compromised. This guide shows how to validate those lists and keep your system safe.

1. Understand the Risk

Your /etc/hosts file maps domain names to IP addresses. Ad-blocking providers give you a list of domains (ad servers, trackers) that redirect to 127.0.0.1 (your own computer) or 0.0.0.0 (nowhere). If this list is tampered with, it could:

• Allow ads through
• Redirect you to malicious websites
• Cause network issues

2. Choose a Validation Method

There are several ways to validate your /etc/hosts file:

• Manual Inspection: Open the file and check for unexpected entries. This is time-consuming and error-prone.
• Checksum Comparison: If the provider offers a checksum (like SHA256), compare it to your local file.
• Automated Scripts: Use scripts to download the latest list and compare it with your current one. This is the most reliable method.

3. Checksum Validation (If Available)

Some providers publish a checksum for their /etc/hosts file. Here’s how to use it:

1. Download the Checksum: Get the latest checksum from the provider’s website.
2. Calculate Your File’s Checksum: Use a command-line tool like sha256sum.

sha256sum /etc/hosts

4. Automated Script Validation (Recommended)

This method uses a script to automatically download and compare lists.

1. Download the Latest List: Get the URL for the latest /etc/hosts file from your provider.
2. Create a Script: Here’s an example Bash script (save it as, e.g., check_hosts.sh):

   #!/bin/bash
   
   # Configuration - CHANGE THESE VALUES!
   HOSTS_FILE="/etc/hosts"
   PROVIDER_URL="https://example.com/hosts.txt" # Replace with your provider's URL
   BACKUP_DIR="/var/tmp/hosts_backups"
   
   # Create backup directory if it doesn't exist
   mkdir -p "$BACKUP_DIR"
   
   # Backup current hosts file
   cp "$HOSTS_FILE" "$BACKUP_DIR/hosts.backup.$(date +%Y%m%d)"
   
   # Download the latest hosts file
   wget -q -O /tmp/new_hosts "$PROVIDER_URL"
   
   # Compare the files
   diff "$HOSTS_FILE" /tmp/new_hosts > /dev/null
   
   if [ $? -eq 0 ]; then
   echo "Hosts file is up to date."
   else
   echo "Hosts file has changes. Replacing..."
   cp /tmp/new_hosts "$HOSTS_FILE"
   fi
   
   rm /tmp/new_hosts
   

3. Make the Script Executable:

   chmod +x check_hosts.sh

4. Run the Script Regularly: Use cron to schedule it (e.g., daily).

5. Edit your crontab:

   crontab -e

6. Add a line like this (runs at 3 AM every day):

   0 3 * * * /path/to/check_hosts.sh >/dev/null 2>&1

5. Important Considerations

• Backup Your File: Always back up your original /etc/hosts file before making changes. The script above includes a backup step.
• Provider Reliability: Choose a reputable provider with a good track record.
• Root Privileges: Modifying /etc/hosts requires root privileges (use sudo).
• Network Issues: Incorrect entries in /etc/hosts can cause network problems. Test thoroughly after making changes.