DNS Filtering and Content Blocking
Learn how DNS-based filtering blocks malware, ads, and unwanted content at the network level using tools like Pi-hole and NextDNS.
How DNS Filtering Works
DNS filtering intercepts DNS queries and blocks requests for domains on a blocklist. Instead of returning the real IP address, the filtering DNS server returns a blocked response (NXDOMAIN, 0.0.0.0, or a block page IP):
Normal: browser -> DNS -> ad-server.com = 198.51.100.50 -> ads load
Filtered: browser -> DNS -> ad-server.com = 0.0.0.0 -> ads blocked
Because DNS is the first step in every connection, filtering at this level is efficient and affects all applications on the network -- not just browsers.
Use Cases
| Purpose | What Gets Blocked |
|---|---|
| Ad blocking | Ad networks, tracking domains |
| Malware protection | Known malicious domains, phishing sites |
| Parental controls | Adult content, gambling, social media |
| Corporate policy | Time-wasting sites, shadow IT services |
| Privacy | Telemetry domains, analytics trackers |
Pi-hole: Self-Hosted DNS Filtering
Pi-hole is an open-source DNS sinkhole that runs on a Raspberry Pi or any Linux machine. It acts as a DNS server for your network, blocking queries for domains on its blocklists.
# Install Pi-hole
curl -sSL https://install.pi-hole.net | bash
# After installation:
# 1. Set your router's DNS to the Pi-hole IP
# 2. All devices on the network are now filtered
# 3. Access the dashboard at http://pi.hole/admin
Pi-hole features: - Blocks 100K+ ad and tracking domains out of the box. - Web dashboard with query logs and statistics. - Custom blocklists and whitelists. - DHCP server functionality. - Works for all devices on the network (no per-device setup).
Cloud DNS Filtering Services
For users who do not want to self-host, cloud-based DNS filtering provides similar protection:
| Service | Free Tier | Features |
|---|---|---|
| NextDNS | 300K queries/month | Custom blocklists, analytics, per-device profiles |
| Cloudflare 1.1.1.3 | Unlimited | Malware + adult content blocking |
| Quad9 (9.9.9.9) | Unlimited | Threat intelligence-based blocking |
| OpenDNS (208.67.222.222) | Unlimited | Customizable categories |
| AdGuard DNS | Unlimited | Ad and tracker blocking |
Enterprise DNS Security
Organizations use more sophisticated DNS filtering:
- DNS firewalls (RPZ) -- Response Policy Zones allow custom rules for blocking, redirecting, or logging DNS queries.
- Threat intelligence feeds -- Continuously updated lists of malicious domains from security vendors.
- TLS inspection -- Decrypting DNS-over-HTTPS to maintain visibility into encrypted queries.
- SIEM integration -- DNS logs fed into security monitoring platforms for threat detection.
Limitations of DNS Filtering
- Easily bypassed -- Users can change their DNS settings to 8.8.8.8 or use DNS-over-HTTPS to bypass network-level filtering.
- No content inspection -- DNS filtering blocks entire domains, not specific pages. You cannot block a single YouTube video via DNS.
- False positives -- Legitimate services may share domains with ad networks (CDN-hosted content).
- Encrypted DNS -- DNS-over-HTTPS (DoH) and DNS-over-TLS (DoT) encrypt queries, preventing network-level filtering unless the endpoint is controlled.