🧪 DNS Lab

📌 Objective

Understand DNS from a systems perpective

⚙️ Environment

  • Virtualization: VirtualBox
  • OS: ( 1 DNS Server VM + 1 Client VM)

🛠️ Lab Network Topology

🧩 Phase 1 — Direct DNS (No Cache)

Client VM -> dnsmasq (192.168.10.10)

Used for:

  • Break #1 (Wrong DNS Server)
  • Break #2 (Wrong DNS Record)

🧩 Phase 2 — Direct DNS (No Cache)

Clint VM -> systemd-resolved (127.0.0.53) [CACHE] -> dnsmasq (192.168.10.10) [DNS SERVER]

Used for:

  • Break #3 (DNS Caching Behavior)

⚙️ Part 1 — DNS Server Setup (dnsmasq)

Step 1 - Install

sudo apt install dnsmasq

Step 2 - Configure domain mapping

sudo nano /etc/dnsmasq.conf

dnsmasq

Step 3 - Start service

sudo systemctl restart dnsmasq
sudo systemctl status dnsmasq

Status

Step 4 - Test locally

dig @127.0.0.1 myapp.local

Expected result: myapp.local -> 192.168.10.10

Local test

⚙️ Part 2 — Client Setup (Phase 1: No cache)

Client sends queries directly to dnsmasq.

Client -> dnsmasq

Step 1 - Configure resolver manually

sudo nano /etc/resolv.conf

Config

Step 2 - Test

dig myapp.local
ping myapp.local

dig-client-side

ping-client-side

Break 1 - Wrong DNS Server

Change resolver

sudo nano /etc/resolv.conf

Break1

Observed behavior

  • dig myapp.local -> communications error to 8.8.8.8#53, timed out
  • ping -> temporary failure in name resolution
  • ping 192.168.10.10 -> ✅ works

Break1

Interpetation

DNS query sent to wrong server

🧠 Root Cause

Client resolver misconfigured

🔧 Fix

Change the resolver config:

nameserver 192.168.10.10

Break 2 - Wrong DNS Record

Misconfigure dnsmasq.conf

sudo nano /etc/dnsmasq.conf
sudo systemctl restart dnsmasq

Dns config

Observed behavior

Run from client VM:

  • dig myapp.local -> 192.168.10.99
  • ping myapp.local -> fails (Destination Host Unreachable)
  • ping 192.168.10.10 -> ✅ works

Break2

Break2

Interpretation

DNS works but returns incorrect IP

🧠 Root Cause

Conclusion: dnsmasq is misconfigured

🔧 Fix

Change the dnsmasq.conf:

address=/myapp.local/192.168.10.10

Break 3 - Cached DNS

⚙️ Setup

Step 1: Enable caching on client vm:

sudo systemctl enable --now systemd-resolved
sudo ln -sf /run/system/resolve/stub-resolv.conf /etc/resolv.conf
sudo resolvectl dns enp0s8 192.168.10.10

Step 2: Configure server dnsmasq.conf:

address=/myapp.local/192.168.10.99
local-ttl=60
cache-size=1000

sudo systemctl restart dnsmasq

DNS config

Observed behavior

On client VM:

sudo resolvectl query myapp.local

Old IP

Interpretation

Client cache used instead of querying DNS

🧠 Root Cause

TTL not expired -> cached response

🔧 Fix

sudo resolvectl flush-caches