Well-Known Ports (0-1023): Complete Overview and IANA Assignments

Q: Well-Known Ports (0-1023): Complete Overview and IANA Assignments

## Well-Known Ports (0-1023) Well-known ports, also called system ports, form the foundation of internet communication. They are assigned by the Internet Assigned Numbers Authority (IANA). ### Characteristics - Range: 0 to 1023 - Assignment: Centrally managed by IANA - Privileges: On Unix/Linux, binding to ports below 1024 requires root (superuser) privileges or the CAP_NET_BIND_SERVICE capability - Convention: Most critical internet services live in this range ### Top 20 Well-Known Ports |

Overview of the well-known port range 0-1023 assigned by IANA. Covers system port requirements, root privileges, and the most important services.

Port Ranges

Detailed Explanation

Well-Known Ports (0-1023)

Well-known ports, also called system ports, form the foundation of internet communication. They are assigned by the Internet Assigned Numbers Authority (IANA).

Characteristics

Range: 0 to 1023
Assignment: Centrally managed by IANA
Privileges: On Unix/Linux, binding to ports below 1024 requires root (superuser) privileges or the CAP_NET_BIND_SERVICE capability
Convention: Most critical internet services live in this range

Top 20 Well-Known Ports

Port	Service	Protocol	Category
20/21	FTP	TCP	File Transfer
22	SSH	TCP	Remote Access
23	Telnet	TCP	Remote Access (deprecated)
25	SMTP	TCP	Email
53	DNS	TCP/UDP	Name Resolution
67/68	DHCP	UDP	Network Config
69	TFTP	UDP	File Transfer
80	HTTP	TCP	Web
88	Kerberos	TCP/UDP	Authentication
110	POP3	TCP	Email
119	NNTP	TCP	Newsgroups
123	NTP	UDP	Time Sync
135	MS RPC	TCP/UDP	Windows RPC
143	IMAP	TCP	Email
161/162	SNMP	UDP	Monitoring
179	BGP	TCP	Routing
389	LDAP	TCP/UDP	Directory
443	HTTPS	TCP	Web (Encrypted)
445	SMB	TCP	File Sharing
636	LDAPS	TCP	Directory (Encrypted)

Running Services Without Root

Modern Linux provides alternatives to running services as root:

# Grant capability to bind to privileged ports
sudo setcap 'cap_net_bind_service=+ep' /usr/bin/your-app

# Use systemd socket activation
[Socket]
ListenStream=80

# Use a reverse proxy (nginx/caddy on 80 → app on 8080)

Security Implications

Well-known ports are heavily scanned by attackers. Best practices:

Close all unused well-known ports
Use fail2ban or similar for SSH (22)
Redirect HTTP (80) to HTTPS (443)
Disable legacy services (Telnet 23, FTP 21) when not needed

Use Case

Performing a security audit of a server to identify which well-known ports are open, closing unnecessary services, and configuring a reverse proxy to avoid running application code as root.

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