Port Checker: From IANA Assignments to Security Hardening#

When troubleshooting server issues, I often need to look up which service runs on which port. Online references are either incomplete or poorly formatted. So I compiled a quick reference for common ports, along with the technical details behind them.

Port Fundamentals#

A port number is a 16-bit unsigned integer, ranging from 0 to 65535. IANA (Internet Assigned Numbers Authority) divides ports into three ranges:

0-1023: Well-Known Ports, require root privileges to bind
1024-49151: Registered Ports, available for regular applications
49152-65535: Dynamic Ports, used for client temporary connections

Common web services are in the well-known range: HTTP on 80, HTTPS on 443, SSH on 22.

TCP vs UDP: Protocol Differences#

The same port number can be used simultaneously by TCP and UDP - they’re independent namespaces. Take DNS port 53:

53/TCP  - DNS zone transfers (master-slave sync)
53/UDP  - DNS queries/responses (daily resolution)

TCP is connection-oriented, establishing a reliable channel via three-way handshake. UDP is connectionless, fire-and-forget, fast but prone to packet loss.

Practical testing:

# TCP connection test
telnet example.com 80

# UDP test (requires nc)
nc -u -v example.com 53

Common Ports Quick Reference#

Web Services#

Port	Protocol	Service	Notes
80	TCP	HTTP	Plain text, insecure
443	TCP	HTTPS	TLS encrypted, recommended
8080	TCP	HTTP Alt	Common for development
8443	TCP	HTTPS Alt	Tomcat default HTTPS
3000	TCP	Dev Server	React/Vue dev server

Databases#

Port	Protocol	Service	Notes
3306	TCP	MySQL	Most popular open-source DB
5432	TCP	PostgreSQL	Feature-rich database
6379	TCP	Redis	In-memory key-value store
27017	TCP	MongoDB	Document database
9200	TCP	Elasticsearch	Search engine

Remote Access#

Port	Protocol	Service	Notes
22	TCP	SSH	Secure remote login
23	TCP	Telnet	Insecure plain-text login (deprecated)
3389	TCP	RDP	Windows Remote Desktop
5900	TCP	VNC	Cross-platform remote desktop

Port Scanning and Security#

Scanning with Nmap#

# Scan common ports
nmap -F target.com

# Scan specific ports
nmap -p 22,80,443 target.com

# Scan port range
nmap -p 1-1000 target.com

# Identify service versions
nmap -sV target.com

Security Hardening Tips#

Close Unnecessary Ports

# Check listening ports
netstat -tulpn

# Or use ss
ss -tulpn

Firewall Rules

# Ubuntu UFW
sudo ufw allow 22/tcp
sudo ufw allow 80/tcp
sudo ufw allow 443/tcp
sudo ufw enable

# CentOS firewalld
firewall-cmd --permanent --add-port=22/tcp
firewall-cmd --permanent --add-port=80/tcp
firewall-cmd --reload

Change Default Ports

Many attack scripts scan default ports. Changing them isn’t a complete solution, but avoids most automated scans:

# Change SSH to 2222
vim /etc/ssh/sshd_config
# Port 22 → Port 2222
systemctl restart sshd

Port Conflict Troubleshooting#

During development, ports often get occupied. Here’s how to troubleshoot:

# Linux/Mac
lsof -i :3000
kill -9 <PID>

# Windows
netstat -ano | findstr :3000
taskkill /PID <PID> /F

# View all listening ports
netstat -tulpn | grep LISTEN

Myth: 65535 Connection Limit#

Theoretically, TCP can handle far more than 65535 connections. The key is the four-tuple:

{source IP, source port, destination IP, destination port}

As long as the four-tuple is unique, a new connection can be established. So a server can support far more than 65535 concurrent connections.

Real limitations come from:

File descriptor limits: ulimit -n
Memory: Each connection uses several KB
CPU: Connection management overhead

Optimization methods:

# Increase file descriptor limit
ulimit -n 100000

# Kernel parameter tuning
vim /etc/sysctl.conf
net.core.somaxconn = 4096
net.ipv4.tcp_max_syn_backlog = 4096

Real Case: Slow SSH Connections#

SSH connections hanging at “Connecting…” for a long time might be caused by DNS reverse lookups.

# Edit sshd_config
UseDNS no  # Disable DNS reverse lookup
GSSAPIAuthentication no  # Disable GSSAPI authentication

Restart and connection speed will improve noticeably.

Tool Recommendation#

Based on this knowledge, I built: Port Checker

Features:

Category-based lookup (Web/Database/Mail/Remote)
Protocol annotation (TCP/UDP)
Service descriptions and security recommendations

The database includes 16 most commonly used ports, covering most development scenarios.