FTP port 21 blocked
How to diagnose and fix firewall rules blocking FTP port 21.
Diagnose this automatically
Test whether the network port is reachable before investigating authentication problems.
Shareable output
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Port 21 is the default FTP control port.
If blocked, clients cannot establish an FTP connection.
Common causes
- Firewall rule blocking inbound traffic
- ISP blocking FTP ports
- Server security group restrictions
Fix checklist
- Open port 21 in the firewall.
- Verify cloud security group rules.
- Check server listening ports.
Quick commands
nc -vz <host> 21
nmap -p 21 <host>FTPMonitor Help
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Related
- ECONNREFUSED on port 21 — Why FTP port 21 returns connection refused, and how to isolate service-down vs firewall vs wrong host/port.
- FTP connection refused — Why FTP servers refuse connections and how to diagnose port, service, and firewall issues.
- FTP connection reset by peer — Why FTP connections reset unexpectedly and how to diagnose firewall or server termination issues.
- FTP server not responding — Why FTP servers fail to respond and how to diagnose network, service, or routing problems.
- TCP connection timeout (firewall issue) — Why FTP/SFTP connections time out and how to diagnose firewall, routing, or allowlist blocking.
- TCP connection failed — General TCP failure causes when connecting to FTP, FTPS, or SFTP services.
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