---
title: "How to Build Your Own Ad-Blocker"
description: "A dive into how DNS level content blockers work."
date: 2026-03-19
tags: ['ad-blocker', 'DNS', 'Linux', 'dnsmasq']
authors:
  - "Patrick Jaroszewski"
slug: "build-your-own-adblocker"
---
import DnsLookupDiagramWrapper from '../../../components/DnsLookupDiagramWrapper.astro';
import AdBlockDiagramWrapper from '../../../components/AdBlockDiagramWrapper.astro';

# Build Your Own Ad-Blocker: A Beginner's Guide to DNS-Level Blocking

The web in its unfiltered state is barely usable. Ads cover half the screen, trackers follow you from site to site, and in the worst cases, ad networks have been used to distribute malware to unsuspecting visitors. You've probably already installed an extension like uBlock Origin, which does a solid job. But it only protects that one browser, on that one device. Your phone, your smart TV, and every other device on your network are still wide open.

What if you could block ads and trackers across your entire network, on every device, with no installation required on any of them?

That's what we'll build in this guide. There are already a few [ready-made solutions](#part-11-where-to-go-from-here---pi-hole-and-adguard-home), but building a simple system ourselves will help us understand how the underlying logic actually works. By the end, you'll have a working network-level ad-blocker running on a Debian machine, and a solid understanding of the DNS system that makes it all possible.

> [!IMPORTANT]
> **Prerequisites**: A machine running Debian (this guide uses Debian 13 Trixie), basic comfort with the terminal, and a home network you control. You could also follow along up to Part 5 on Windows or Mac.

---

## Part 1: What Is DNS and Why Does It Matter?

Every device on the internet is identified by an **IP address**, a string of numbers like `142.250.80.46`. When computers talk to each other, they use these addresses. But humans don't type IP addresses into their browsers; they type domain names like `google.com`. Something has to bridge that gap.

That's  what the **Domain Name System**, or DNS, is for. It's a distributed, globally-operated system that translates domain names into IP addresses. When you navigate to `google.com`, your computer asks a DNS server: "What's the IP address for google.com?" The DNS server responds with an answer, and your browser uses that IP address to connect.

The classic analogy is a phone book: knowing someone's name isn't enough to call them, you need their number. DNS is the phone book that maps domain names to IP addresses.

Here's what makes DNS interesting from an ad-blocking perspective: this lookup doesn't just happen once when you load a page. It happens for *every domain* your browser contacts. A typical news website might load content from its own domain, plus a dozen or more third-party domains: ad networks, analytics services, tracking pixels, comment widgets, and more. Each of those requires its own DNS lookup.

That's our opening. If we can intercept those DNS lookups and return a dead-end answer for known ad-serving domains, those requests never go anywhere. The ads simply don't load.

---

## Part 2: How a DNS Lookup Works

Before we start intercepting DNS queries, it helps to understand the journey a query takes from your browser to a final answer. The interactive diagram below walks through each step. Click through to see how a DNS lookup flows from your browser all the way to a server and back.

<DnsLookupDiagramWrapper />

### Step 1: The Browser Cache

The first place your browser looks is its own internal cache. If you visited `google.com` five minutes ago and it resolved to `142.250.80.46`, the browser probably still remembers that. DNS responses include a **TTL (Time To Live)** value that tells the browser how long to cache the answer. If the cache entry is still valid, the lookup stops here and no network request is made.

### Step 2: The Hosts File and OS Cache

If the browser doesn't have a cached answer, it asks the operating system. The OS checks two things:

1. **The hosts file** (`/etc/hosts` on Linux), a static text file where you can manually map domain names to IP addresses. If a domain appears there, that mapping is used immediately. This is the mechanism we'll exploit for ad-blocking: by mapping ad-serving domains to `0.0.0.0`, we can block them before any network request is made.

2. **The OS DNS cache**, a temporary cache of previous DNS lookups, similar to the browser's cache.

If the domain is found in either place, the lookup stops here.

### Step 3: Upstream DNS Servers

If neither the hosts file nor the OS cache has an answer, the query leaves your machine and travels to an upstream DNS server.

Your device knows where to send it because of **DHCP**, the same protocol that assigns your IP address when you join a network. Along with an IP address, DHCP also hands your device a DNS server address to use. Depending on your setup, that might be your router's IP, your ISP's DNS server, or a public resolver like Cloudflare (`1.1.1.1`) or Google (`8.8.8.8`).

Here's the key insight: **DNS queries often pass through a chain of servers**. Your device sends the query to its configured DNS server. That server either answers from its own cache, or forwards the query to *its* upstream server. This can repeat multiple times. Your router might forward to your ISP, which might forward to another server, until the query reaches a **recursive resolver** that actually does the work of tracking down the answer.

Each server in this chain caches the responses it receives. This means a popular domain like `google.com` is probably already cached at your router or ISP, so the query never needs to travel all the way to the recursive resolver. The closer a cached answer is to your device, the faster the lookup.

A recursive resolver is the end of the chain. It takes full responsibility for finding the IP address, querying root servers, TLD servers, and authoritative nameservers as needed. From your device's perspective, all of this is invisible: you ask once, and eventually an answer comes back.

This chain of forwarders is exactly what we'll exploit. By inserting our own DNS server into the chain, we can intercept queries and block them before they ever reach the outside world.

---

## Part 3: The Hosts File

The hosts file is one of the oldest mechanisms in networking, predating DNS itself. It's a plain text file that the OS reads every time it needs to resolve a domain name. Entries in the hosts file take absolute priority over any DNS server.

**Location by operating system:**
- Linux / macOS: `/etc/hosts`
- Windows: `C:\Windows\System32\drivers\etc\hosts`

Open it on your Debian machine:

```bash
sudo nano /etc/hosts
```

You'll see something like this:

```
127.0.0.1   localhost
127.0.1.1   myhostname
::1         localhost ip6-localhost ip6-loopback
```

The format is simple: an IP address, followed by one or more hostnames. That first line maps `localhost` to `127.0.0.1`, the **loopback address** (your own machine).

### Blocking a Domain

To block a domain, you map it to an address that goes nowhere. For example the **null route** `0.0.0.0`:

Add a line like this to your hosts file:

```
0.0.0.0   facebook.com
```

> [!NOTE]
> If your network uses IPv6, you may also need to add the line `:: facebook.com` to block the IPv6 address. `::` is the IPv6 equivalent of `0.0.0.0`. We'll assume we're on an IPv4 network for the remainder of this post.

Save the file and try it:

```bash
curl -I facebook.com
```

You should see a "connection refused" error, which is exactly what we want. The address resolves to `0.0.0.0`, nothing is listening there, and the connection fails. Alternatively, try visiting `facebook.com` in your browser — you'll get an error page instead of the site.

> [!WARNING]
> This same mechanism is what makes DNS poisoning attacks possible. If an attacker can modify your hosts file or intercept your DNS queries, they can redirect `facebook.com` to a malicious server that looks identical to the real thing. This is one reason why HTTPS and certificate validation matter: even if DNS is compromised, a valid TLS certificate is hard to fake.

Remove the `facebook.com` line when you're done experimenting.

> [!NOTE]
> You may notice that pinging `0.0.0.0` actually reaches the loopback address `127.0.0.1`. This is because RFC 1122 specifies that `0.0.0.0` is only valid as a source address. On most Unix-like systems, using it as a destination causes the OS to treat it as the local loopback instead. Keep this in mind if you have a local web server running, as it may respond to requests directed at these blocked domains.
---

## Part 4: Using the Hosts File to Block Ads

Now that we understand how the hosts file works, let's use it for something practical. The key insight is that ads on a webpage aren't usually served from the same domain as the page itself. They come from separate ad-serving domains like `doubleclick.net`, `pagead2.googlesyndication.com`, or `ads.facebook.com`.

You can see this yourself. Open any ad-heavy website in your browser, press **F12**, and go to the **Network** tab. Reload the page and watch the requests come in. You'll see calls going out to dozens of domains you've never heard of. That's the ad and tracking ecosystem at work.

The scale here matters. A single news article might trigger dozens of requests to different ad and tracker domains. Each of those requests requires a full round-trip: DNS lookup, TCP connection, TLS handshake, HTTP request, and response. For unblocked domains, your DNS query travels out to the internet, bounces through resolvers, and waits for an answer before the browser can even *start* connecting to the server. Multiply that by dozens of ad domains, and you're looking at hundreds of network operations just to load tracking scripts and display banners.

When those domains are blocked at the DNS level the query never leaves your network. Your local DNS server instantly responds with `0.0.0.0`, the browser sees a dead end, and the request stops there — no connection attempt, no data transferred, no waiting. The cumulative effect of blocking these domains is substantial: faster page loads, less bandwidth consumed, and a browser that's not busy juggling connections to ad servers.

If we block those domains in our hosts file, the browser's requests to them will fail silently, and the ads won't load.

The interactive diagram below shows this in action. Toggle the ad-blocker on and off to see how DNS-level blocking prevents ad content from ever being fetched.

<AdBlockDiagramWrapper />

### Adding Block Entries Manually

Open your hosts file:

```bash
sudo nano /etc/hosts
```

Add entries for known ad-serving domains:

```
0.0.0.0   doubleclick.net
0.0.0.0   pagead2.googlesyndication.com
0.0.0.0   ads.facebook.com
0.0.0.0   analytics.google.com
```

Save and reload an ad-heavy page. You should notice fewer ads loading.

### The Scaling Problem

This approach has two obvious limitations:

1. **There are hundreds of thousands of ad-serving, tracking, and malware domains on the internet.** No one is going to maintain that list by hand. We'll need community-maintained block lists.

2. **This only protects the single machine you've configured.** Your phone, your TV, and every other device on your network still use their default DNS servers.

To address these problems, we need to move up a level: instead of editing hosts files on individual machines, we'll run a DNS server that applies block lists to every device on the network.

---

## Part 5: Community Block Lists

Maintaining a list of ad-serving domains by hand isn't realistic. Fortunately, the community has already done this work. Several projects maintain comprehensive, regularly-updated hosts files containing hundreds of thousands of known ad, tracker, and malware domains.

Some reliable sources:

| List | URL | Coverage |
|------|-----|----------|
| Steven Black's hosts | https://github.com/StevenBlack/hosts | Ads, malware, and various optional extensions |
| OISD | https://oisd.nl | Balanced, well-maintained, available in multiple sizes |
| Hagezi's lists | https://github.com/hagezi/dns-blocklists | Multiple tiers from light to aggressive |

Let's download OISD's "basic" list. It's a good balance of coverage without being overly aggressive:

```bash
# Download the list
sudo curl -o /etc/hosts.blocklist https://hosts.oisd.nl/

# Verify it looks right (you should see lines like: 0.0.0.0 some-ad-domain.com)
head /etc/hosts.blocklist
```

This file now contains tens of thousands of blocked domains. In the next section, we'll configure a DNS server to use it.

---

## Part 6: Taking Your Blocker to the Network Level with dnsmasq

This is where the project gets genuinely useful. Instead of modifying the hosts file on every device individually, we'll run a DNS server on our Debian machine. Every other device on the network will be pointed to use it. When a device asks "what's the IP for doubleclick.net?", our server will answer with a dead end, and that device never needs to know why.

The tool we'll use is **dnsmasq**, a lightweight DNS (and DHCP) server that's well-suited for exactly this kind of home-network use case. It reads your hosts file, applies it to all incoming DNS queries, and forwards anything it doesn't have a local answer for to an upstream resolver like `1.1.1.1`.

### Installing dnsmasq

```bash
sudo apt update
sudo apt install dnsmasq
```

Once installed, dnsmasq starts automatically and begins listening for DNS queries on port 53. It works out of the box with sensible defaults, but we'll customize it to point at our block list.

### Configuring dnsmasq

The main configuration file is `/etc/dnsmasq.conf`, which is heavily commented by default. Rather than editing it in place, it's cleaner to create a focused config file in the drop-in directory:

```bash
sudo nano /etc/dnsmasq.d/adblocker.conf
```

Add the following:

```ini
# Upstream DNS servers (queries not answered locally are forwarded here)
server=1.1.1.1
server=8.8.8.8

# Listen only on the local network interface (replace eth0 with your interface name)
# Run `ip link` to find your interface name
interface=eth0

# Don't forward queries for plain hostnames (no dots) to upstream
domain-needed

# Don't forward queries with private IP ranges to upstream
bogus-priv

# Use our block list as an additional hosts file
addn-hosts=/etc/hosts.blocklist
```

> [!TIP]
> **Find your interface name** with `ip link`. It's usually `eth0` for wired connections, but modern Debian systems often use names like `enp3s0` or `ens3`.

### Restarting dnsmasq

Since dnsmasq is already running, restart it to pick up the new configuration:

```bash
sudo systemctl restart dnsmasq.service
```

Check that it restarted cleanly:

```bash
sudo systemctl status dnsmasq.service
```

### Testing Your DNS Server

From the Debian machine itself, query your new DNS server directly:

```bash
# Install dig if it's not already present
sudo apt install dnsutils

# Query for a legitimate domain (should return a real IP)
dig @127.0.0.1 google.com

# Query for a blocked domain (should return 0.0.0.0)
dig @127.0.0.1 doubleclick.net
```

If `doubleclick.net` returns `0.0.0.0`, your server is working correctly.

---

## Part 7: Pointing Your Network at Your DNS Server

A working DNS server isn't much use if nothing is talking to it. To make every device on your network use it, you need to understand what we're actually replacing.

Right now, every device on your network has a DNS server address that was handed to it automatically via DHCP, either your router's IP or an upstream resolver address your router advertises. That's the address your devices send DNS queries to. We want to change it to your Debian machine's IP instead. When a device asks "what's the IP for doubleclick.net?", the query will go to dnsmasq first, get answered with `0.0.0.0` from your block list, and the ad request dies there. For everything not on the block list, dnsmasq forwards the query upstream to `1.1.1.1` or `8.8.8.8` as normal.

You *could* manually change the DNS server on each device individually, but that's tedious and doesn't scale. The right approach is to change it once in your router's DHCP settings.

### Configuring Your Router's DHCP

When a device joins your network and requests an IP address via DHCP, the router tells it which DNS server to use. If you set that to your Debian machine's IP, every device on your network gets ad-blocking automatically, with no configuration needed on the devices themselves.

This setting is usually found in your router's admin interface under **LAN settings**, **DHCP settings**, or **DNS settings**. The exact location depends on your router's firmware. Look for a field labelled something like "Primary DNS Server" and set it to your Debian machine's IP.

### Give Your Debian Machine a Static IP

For this to work reliably, your Debian machine needs a **static IP address**, one that doesn't change when it reboots or when the DHCP lease renews. If its IP changes, every device on the network will be pointing at a DNS server that no longer exists.

The simplest approach for a home network is to set a **DHCP reservation** in your router: find your Debian machine's MAC address and tell the router to always assign it the same IP. Most home routers support this under **DHCP reservations** or **Address reservation**. This is less intrusive than configuring a static IP directly on the machine.

---

## Part 8: What You Get

With your DNS-level ad-blocker running, here's what changes across your network:

**Faster page loads.** Ads are often the heaviest elements on a page — video ads, animated banners, and tracking scripts all take time to download and execute. When those requests never happen, pages load noticeably faster. The effect is especially pronounced on ad-heavy news sites and free-tier services.

**Reduced bandwidth.** Blocked requests mean fewer bytes transferred. On metered connections or slower networks, this adds up. Some users report 10-20% reductions in overall bandwidth usage. This is especially helpful on a metered connection.

**Privacy improvements.** Tracker domains are blocked at the network level, so those companies never see your IP address or get to set cookies. This applies to every device on your network, including ones where you can't install a browser extension.

**Protection from malvertising.** Ad networks have historically been vectors for malware distribution. By blocking ad-serving domains entirely, you reduce your exposure to compromised ads — even if you'd never click on them intentionally.

**Cleaner browsing experience.** No more banner ads, pop-ups, or "recommended content" widgets cluttering every page. The web looks more like it did before advertising took over.

**Works on everything.** Smart TVs, game consoles, IoT devices, guest phones — anything that connects to your network gets ad-blocking automatically, with no per-device configuration.

---

## Part 9: Keeping Your Block List Updated

### Automating Updates

Block lists go stale quickly as ad networks register new domains. Set up a weekly cron job to refresh your list automatically:

```bash
sudo crontab -e
```

Add this line to run every Sunday at 2 AM:

```
0 2 * * 0 curl -s -o /etc/hosts.blocklist https://hosts.oisd.nl/ && systemctl restart dnsmasq
```

### A Note on List Aggressiveness

Bigger lists are not always better. The most aggressive lists block domains used by some legitimate services and can break things in unexpected ways: a game that won't authenticate, a smart TV feature that stops working, a login form that won't submit. Start with a balanced list like OISD basic. If you want stricter coverage, layer in additional lists gradually and keep track of what you've added so you can diagnose issues.

---

## Part 10: Limitations and Caveats

It's worth being honest about what this setup does and doesn't do.

### Hardcoded DNS Server

Some devices, particularly IoT devices, have DNS servers hardcoded in their firmware and will ignore any DNS settings pushed by your router. These devices will bypass your DNS server entirely. The solution is to add transparent DNS proxy rule on your router or firewall that intercepts all outbound port 53 traffic (both UDP and TCP) and redirects it to your local DNS server, regardless of the destination the device specified. I'll write a short blog post about this soon.

### DNS-over-HTTPS (DoH) Bypasses Your Blocker
Redirecting port 53 traffic solves the hardcoded DNS problem, but some software bypasses local DNS entirely through a different mechanism: DNS-over-HTTPS (DoH).
Some browsers (notably Firefox and Chrome) use **DNS-over-HTTPS** by default, where DNS queries are sent encrypted to a specific provider (like Cloudflare or Google) over HTTPS. When DoH is active, the browser completely bypasses your local DNS server. Your DNS server never sees those queries, and your block lists have no effect.

You can disable it in Firefox by navigating to **Settings → Privacy & Security → DNS over HTTPS**. For a network-wide fix, you'd need to block DoH traffic at the firewall level — which is outside the scope of this guide, but worth knowing about.

### Block Lists Are Reactive

Lists only contain domains that someone has already identified as problematic. New ad networks, freshly-registered tracker domains, and novel malware distribution servers slip through until they're discovered and added to a list. This approach will catch the vast majority of ads, but it's not impenetrable.

### First-Party Serving Can't Be Blocked

Some platforms serve ads from their own domain. **YouTube** is the most notable example: Google serves YouTube ads from `youtube.com` itself, so you can't block the ads without blocking the whole site. DNS-level blocking fundamentally can't help here; a browser extension is the only practical solution for these cases.

### Privacy: Real Benefits, Real Limits

DNS-level blocking does provide meaningful privacy improvements. When you block tracker domains, those requests never happen — the tracking company never sees your IP address, never sets a cookie, and never logs your visit. Multiply that across hundreds of blocked domains and thousands of page loads, and you've significantly reduced your digital footprint.

That said, this setup doesn't make you invisible. Your DNS queries to the upstream servers (`1.1.1.1`, `8.8.8.8`) are still sent unencrypted. Anyone on the network path between your machine and those servers — your ISP, for example — can see what domains you're looking up (though not which specific pages you visit).

DNS-over-TLS (DoT) or DNS-over-HTTPS (DoH) to your upstream resolver would address this, which we'll look at in a future post.

---

## Part 11: Where to Go From Here - Pi-hole and AdGuard Home

If you've followed this guide, you've essentially built a simplified version of the two most popular self-hosted ad-blockers: **Pi-hole** and **AdGuard Home**. Both operate on exactly the same principle — a local DNS server that applies block lists before forwarding queries upstream.

The point of this guide was to teach you how DNS-level blocking works. For actual day-to-day use, you should probably run one of these production-grade tools instead of a hand-rolled dnsmasq setup. They handle edge cases, provide better diagnostics, and are actively maintained by communities that deal with the constantly-shifting ad ecosystem.

**[Pi-hole](https://pi-hole.net)** is the original project in this space, built specifically to run on a Raspberry Pi (though it works on any Linux machine). Under the hood, it uses a fork of dnsmasq called FTL (Faster Than Light), and adds a web dashboard where you can see real-time query logs, top blocked domains, per-device query statistics, and a UI for managing block lists. It also handles DHCP if you want to use it for that. I've been running Pi-hole on my home network for over a decade now, and it's one of the first things I set up on any new network. Highly recommended.

**[AdGuard Home](https://github.com/AdguardTeam/AdGuardHome)** is a newer alternative with a similar feature set but a more modern interface. It has native support for DNS-over-HTTPS and DNS-over-TLS as both client (for upstream queries) and server (for devices that support encrypted DNS), which addresses some of the privacy limitations mentioned earlier.


### Not Ready to Self-Host?

If running your own DNS server feels like too much for now, you can get many of the same benefits by pointing your devices (or your router) at a public DNS resolver that blocks ads for you:

- **[AdGuard DNS](https://adguard-dns.io)** (`94.140.14.14` / `94.140.15.15`) — blocks ads, trackers, and malware domains. This is the hosted version of AdGuard Home, run by the same team.
- **[NextDNS](https://nextdns.io)** — highly customizable, lets you choose which block lists to apply, and provides a dashboard showing what's being blocked. Free tier available.
- **[Quad9](https://quad9.net)** (`9.9.9.9`) — focuses on security rather than ads, blocking known malware and phishing domains. A good complement if privacy and security are your main concerns.

Just change your DNS server setting to one of these addresses, and you're done. You won't get the same level of control or visibility as a self-hosted solution, but it's a zero-maintenance way to get network-wide blocking.

---

## Conclusion

We started with a simple question — can we block ads for an entire network without touching each device? — and worked our way through the full answer.

DNS is the lookup system that translates domain names into IP addresses, and it's consulted for every domain a browser contacts, including ad servers and trackers. The hosts file lets you intercept those lookups locally and return a dead-end address. dnsmasq extends that to your whole network by acting as a DNS server that every device can use. Community block lists bring comprehensive coverage without manual maintenance. And all of this is the same foundation that mature tools like Pi-hole and AdGuard Home are built on.

If you move to Pi-hole, one of the most eye-opening features is the query log — you can watch in real-time as devices on your network try to phone home to ad networks, analytics services, and telemetry endpoints. The volume and variety might surprise you.

---