Internet Filter/Blocklist Indicator Script

This Python script helps to identify potential internet filtering or blocklisting mechanisms active on your system or network. It checks for common indicators such as:

1. Hosts File Modifications: Scans your system's hosts file for entries that redirect domains to 0.0.0.0 or 127.0.0.1.
2. Configured DNS Servers: Attempts to identify your currently configured DNS servers and provides hints if known public DNS services (like Google, Cloudflare, AdGuard DNS) or private IP addresses (which might indicate a local resolver like Pi-hole or AdGuard Home) are in use.
3. DNS Resolution and HTTP(S) Accessibility of Test Domains:
   • It resolves a predefined list of test domains (typically ad/tracker domains or domains known to be affected by certain blocks).
   • Checks if DNS resolution points to common block IPs (0.0.0.0, 127.0.0.1).
   • If DNS resolution is normal, it attempts to access the domain via HTTP and HTTPS.
   • Specifically checks if a domain redirects to notice.cuii.info, which is a strong indicator of an ISP-level block by CUII (Clearingstelle Urheberrechtsverletzungen im Internet) in Germany.
   • Reports DNS resolution failures or HTTP request errors.
4. Summary of Findings: Provides a consolidated report of the checks.

Prerequisites

• Python 3.x
• The requests library (for HTTP(S) checks, including CUII redirect detection):

  pip install requests

  If requests is not installed, the script will still run DNS checks but will skip HTTP(S) accessibility tests and CUII redirect detection.

How to Use

1. Save the script as testFilter.py (or any other .py name).
2. Make sure you have Python installed and the requests library if you want full functionality.
3. Run the script from your terminal:

   python testFilter.py

Understanding the Output

The script will print information in several sections:

• [1] Hosts File Check:
  • Lists any domains found in your hosts file that are redirected to 0.0.0.0 or 127.0.0.1.
• [2] Configured DNS Servers:
  • Shows the DNS servers it could detect.
  • Provides hints based on the IP addresses (e.g., private IP, known public DNS).
• [3] DNS Resolution and HTTP(S) Accessibility of Test Domains:
  • For each domain in TEST_DOMAINS, it shows:
    • RESOLVED_OK: Domain resolved to a regular IP address, and no CUII redirect was detected.
    • BLOCKED_POINTS_TO_0.0.0.0 / BLOCKED_POINTS_TO_127.0.0.1: Domain resolves to a block IP (likely DNS-level block).
    • FAILED_TO_RESOLVE: DNS resolution failed (e.g., NXDOMAIN, server issue).
    • REDIRECTED_TO_CUII_NOTICE: Domain successfully resolved, but an HTTP(S) request resulted in a redirect to notice.cuii.info.
    • HTTP_REQUEST_ERROR: DNS resolved, but an error occurred during the HTTP(S) request (e.g., timeout).
    • RESOLUTION_ERROR: A general error occurred during the check.
  • A summary table shows how many domains fall into each status category.
  • An overall assessment is provided based on the findings.

Entries of the cuii list might result either in REDIRECTED_TO_CUII_NOTICE or NXDOMAIN. For NXDOMAIN-Entries, online URL-Scanners like URLScan.io might help verifying if the site is actually available.

Additionally, the script will generate a text file named filter_test_results_{hostname}_{timestamp}.txt in the same directory where the script is run. This file contains:

• Timestamp of the test.
• Basic system information (OS, hostname).
• A list of domains found in the hosts file that are redirected to 0.0.0.0 or 127.0.0.1.
• The configured DNS servers.
• A summary of the domain check results (currently a direct string representation of the internal summary dictionary).

Customization

You can modify the TEST_DOMAINS list within the script to include other domains you wish to test.

Limitations

• Not Exhaustive: This script checks for common indicators but cannot detect all possible filtering methods (e.g., deep packet inspection, browser extensions, some VPN-internal blocks).
• DNS Server Detection: Reliably detecting all configured DNS servers across all operating systems from within Python can be challenging. The script uses common methods but might not always be 100% accurate, especially on Windows for complex network configurations.
• requests Dependency: Full HTTP(S) checking, including CUII redirect detection, requires the requests library.
• Interpretation: The results are indicators. A "blocked" status for a tracking domain might be intentional (e.g., due to an ad blocker). A CUII redirect is a strong sign of an ISP block.
• Might differ from your Browser Experience: Even here a domain is flagged as unavailable, it might be accessiable in your browser or vice versa. Your browsers might use different DNS settings, then tested here.

CUII

The entries of the CUII blocking are listed at the inofficial cuiilist: https://cuiiliste.de/.

References

• https://www.heise.de/news/Erste-Websperren-seit-Jahren-6020210.html
• https://netzpolitik.org/2025/netzsperren-provider-verstecken-welche-domains-sie-sperren/

Disclaimer

This script is for informational and educational purposes. Use it responsibly.

Please be also aware, that your scans might be logged on your local dns chain e.g. a router or adguard-dns server.