Port 53 — DNS

TCP/UDP well-known

What runs on port 53

Domain name resolution. UDP for normal queries, TCP for responses over 512 bytes and for zone transfers.

Security considerations

An open recursive resolver will be abused for DNS amplification attacks. Restrict recursion to your own networks.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :53
sudo ss -lntp | grep :53

# Windows
netstat -ano | findstr :53
Get-NetTCPConnection -LocalPort 53

# is it reachable from outside?
nc -zv example.com 53
curl -v telnet://example.com:53

Freeing the port

# find the process, then stop it
sudo lsof -ti :53 | xargs kill        # Linux / macOS
netstat -ano | findstr :53            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Other common ports

• Port 20 — FTP data
• Port 21 — FTP control
• Port 22 — SSH / SFTP
• Port 23 — Telnet
• Port 25 — SMTP
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 110 — POP3
• Port 123 — NTP
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB
• Port 465 — SMTPS
• Port 514 — Syslog

All port numbers