DNS and Hosts File Explained: Complete Guide for Developers

Key DNS Components

• DNS Resolver: Your computer's DNS client (usually provided by ISP or public DNS like 8.8.8.8)
• Root Nameservers: 13 sets of servers that know where TLD servers are
• TLD Nameservers: Handle top-level domains (.com, .org, .net, .dev, etc.)
• Authoritative Nameservers: Store actual DNS records for specific domains
• DNS Cache: Temporary storage at multiple levels (browser, OS, resolver, ISP)

Why Use Custom Domains?

• Professional URLs: myapp.local vs localhost:3000
• Subdomain Testing: Test api.myapp.local, admin.myapp.local
• Cookie Isolation: Separate cookies between projects
• Multi-Project Setup: Run multiple projects simultaneously without port conflicts in URLs
• Production Parity: URLs match production structure (except TLD)
• OAuth/SAML Testing: Some auth providers require specific URLs

Scenario: React + Express App with API Subdomain

Step 1: Edit Hosts File

127.0.0.1       myapp.local
127.0.0.1       api.myapp.local

Step 2: Flush DNS Cache

ipconfig /flushdns   # Windows
sudo killall -HUP mDNSResponder   # macOS

Step 3: Configure Your Development Servers

React (package.json):

{
  "scripts": {
    "start": "HOST=myapp.local PORT=3000 react-scripts start"
  }
}

Express API (server.js):

const express = require('express');
const app = express();

app.listen(4000, () => {
  console.log('API running at http://api.myapp.local:4000');
});

Step 4: Configure Reverse Proxy (Optional but Recommended)

Use Nginx or Apache to remove port numbers:

# /etc/nginx/sites-available/myapp.local
server {
    listen 80;
    server_name myapp.local;

    location / {
        proxy_pass http://localhost:3000;
    }
}

server {
    listen 80;
    server_name api.myapp.local;

    location / {
        proxy_pass http://localhost:4000;
    }
}

Step 5: Access Your App

• Frontend: http://myapp.local (if using reverse proxy) or http://myapp.local:3000
• API: http://api.myapp.local (if using reverse proxy) or http://api.myapp.local:4000

Multiple DNS Cache Layers

1. Browser Cache - Cached in your web browser (60-300 seconds typically)
2. OS Cache - Operating system DNS cache (varies by OS)
3. Router Cache - Your home/office router caches DNS (usually several hours)
4. ISP Cache - Your Internet Service Provider caches DNS (hours to days)

Implication: Flushing your OS cache won't clear upstream caches. DNS propagation can take time due to these multiple layers.

1. Domain Not Resolving (NXDOMAIN)

Error: "This site can't be reached" or "Server not found"

Causes:

• Domain doesn't exist or DNS records not set up
• Typo in domain name
• DNS propagation delay (new domain or recent changes)

Solutions:

# Check if domain exists
nslookup yourdomain.com

# Query authoritative nameserver directly
dig @ns1.yourdomain.com yourdomain.com

# Check domain registration
whois yourdomain.com

2. Wrong IP Address Returned

Error: Domain resolves but to wrong server

Causes:

• Stale DNS cache
• Incorrect hosts file entry
• DNS propagation in progress

Solutions:

# 1. Check hosts file first
cat /etc/hosts   # Mac/Linux
type C:\Windows\System32\drivers\etc\hosts   # Windows

# 2. Flush all DNS caches
ipconfig /flushdns   # Windows
sudo killall -HUP mDNSResponder   # macOS

# 3. Query public DNS to check propagation
dig @8.8.8.8 yourdomain.com
dig @1.1.1.1 yourdomain.com

# 4. Clear browser cache
# chrome://net-internals/#dns → Clear host cache

3. DNS Server Not Responding

Error: Timeout or "DNS server not responding"

Causes:

• ISP DNS server down
• Firewall blocking port 53
• Network connectivity issue

Solutions:

# Check current DNS server
ipconfig /all   # Windows - look for "DNS Servers"
cat /etc/resolv.conf   # Linux/Mac

# Switch to public DNS
# Google DNS: 8.8.8.8, 8.8.4.4
# Cloudflare DNS: 1.1.1.1, 1.0.0.1

# Test with specific DNS server
nslookup google.com 8.8.8.8

# Check if port 53 is reachable
telnet 8.8.8.8 53

4. Local Domain Not Working (.local)

Error: myapp.local doesn't resolve after editing hosts file

Solutions:

1. Verify hosts file syntax:

   # Correct format (space or tab between IP and domain)
   127.0.0.1       myapp.local
   
   # Incorrect (no space)
   127.0.0.1myapp.local
   
   # Incorrect (wrong IP)
   localhost       myapp.local

2. Flush DNS cache (see commands above)
3. Restart browser - some browsers aggressively cache
4. Check for typos - domain names are case-insensitive but must match exactly
5. Test with ping:

   ping myapp.local
   # Should reply from 127.0.0.1

6. macOS .local issue: On macOS, .local is reserved for Bonjour/mDNS. Use .localhost or .test instead.

Q: Can I use wildcards in the hosts file?

A: No. The hosts file does not support wildcards. You cannot use *.myapp.local to match all subdomains. You must add each subdomain explicitly:

127.0.0.1       myapp.local
127.0.0.1       api.myapp.local
127.0.0.1       admin.myapp.local

For wildcard DNS, you need a local DNS server like dnsmasq.

Q: How do I set up wildcard local domains (*.local)?

A: Use dnsmasq (Linux/Mac) or Acrylic DNS Proxy (Windows). Example with dnsmasq:

# Install dnsmasq (Mac)
brew install dnsmasq

# Configure wildcard
echo "address=/.local/127.0.0.1" >> /usr/local/etc/dnsmasq.conf

# Start dnsmasq
sudo brew services start dnsmasq

# Configure system to use dnsmasq
sudo mkdir -p /etc/resolver
echo "nameserver 127.0.0.1" | sudo tee /etc/resolver/local

Now all *.local domains resolve to 127.0.0.1.

Q: Why does my hosts file not work on some websites?

A: Some browsers and apps use DNS over HTTPS (DoH) which bypasses the hosts file. To fix:

• Chrome: Settings → Privacy → Turn off "Use secure DNS"
• Firefox: Settings → Network Settings → Disable "DNS over HTTPS"

Alternatively, some apps hard-code DNS servers (like 8.8.8.8) and ignore system DNS settings entirely.

Q: What's the difference between localhost and 127.0.0.1?

A: localhost is a hostname that resolves to a loopback IP address. 127.0.0.1 is the actual IPv4 loopback address.

• localhost can resolve to 127.0.0.1 (IPv4) or ::1 (IPv6) depending on system configuration
• Most systems prefer IPv6, so localhost might try ::1 first
• Using 127.0.0.1 explicitly forces IPv4, avoiding any IPv6 issues
• Both are defined in hosts file by default

Q: How long does DNS propagation take?

A: DNS propagation depends on TTL (Time To Live) values:

• Typical TTL: 300 seconds (5 minutes) to 86400 seconds (24 hours)
• Best case: 5-15 minutes if TTL is low
• Worst case: 24-48 hours if TTL is high and caches are stale
• Pro tip: Lower TTL to 300 seconds 24-48 hours BEFORE making DNS changes, then change DNS, then wait for propagation, then raise TTL back to normal (3600-86400)

Q: Can I block ads using the hosts file?

A: Yes! Redirect ad domains to 0.0.0.0 (or 127.0.0.1):

0.0.0.0       ads.example.com
0.0.0.0       tracker.example.com
0.0.0.0       malware-site.com

This prevents your computer from connecting to those domains. However:

• Only blocks on this specific device
• Doesn't block inline ads (same domain as content)
• Can break some websites if you block too aggressively
• Better alternatives: uBlock Origin browser extension, Pi-hole for network-wide blocking

There are community-maintained hosts files with thousands of ad/tracker domains: Steven Black's hosts