PORT 53 / TCP/UDPInfrastructure & CloudExposure Risk: Medium

Port 53: DNS — Domain Name System Resolver & Server

Translates domain names to IP addresses. Uses UDP for standard queries and TCP for zone transfers (AXFR) or responses exceeding 512 bytes.

Default Service

DNS

Process: named / systemd-resolved / dnsmasq

Transport Protocol

TCP/UDP

IANA Registered Port 53

Docker Port Flag

-p 53:53/udp -p 53:53/tcp

Container to Host Mapping

How to Check & Kill Process on Port 53 (EADDRINUSE Fix)

macOS & Linux (lsof / kill)

# Find process using port 53
sudo lsof -i :53

# Instantly kill process on port 53
lsof -ti:53 | xargs kill -9

Windows (PowerShell & CMD)

# Find PID listening on port 53
netstat -ano | findstr :53

# Force kill by PID
taskkill /PID <PID_NUMBER> /F

Cross-Platform Node.js One-Liner

# Kill port 53 via npx
npx kill-port 53

# Or diagnose via Castov CLI
npx castov fix "EADDRINUSE :::53"

Linux Firewall (UFW) & Connectivity Test

# Test if port 53 is reachable
nc -zv localhost 53

# Allow or deny port 53 in UFW
sudo ufw allow 53/tcp

Frequently Asked Questions about Port 53

What is Port 53 used for?

Port 53 is the standard network port for DNS (Domain Name System Resolver & Server) operating over TCP/UDP. The typical system daemon or process is named / systemd-resolved / dnsmasq.

How do I kill a process running on Port 53 (EADDRINUSE)?

On macOS and Linux, run: lsof -ti:53 | xargs kill -9 (or npx kill-port 53). On Windows PowerShell/CMD, run: netstat -ano | findstr :53 and then taskkill /PID <PID> /F.

Is it safe to expose Port 53 to the internet?

Port 53 has a Medium security risk profile. Translates domain names to IP addresses. Uses UDP for standard queries and TCP for zone transfers (AXFR) or responses exceeding 512 bytes.