Port 21 FTP
Port 21 is the IANA-assigned ftp port for File Transfer Protocol [Control] (TCP / UDP / SCTP).
What is port 21 used for?
Port 21 is assigned to ftp and uses TCP / UDP / SCTP. The IANA registry describes the service as “File Transfer Protocol [Control]”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach ftp connects to port 21. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 21 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Browse all File transfer and sharing ports
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [Jon_Postel]. Reference: [RFC 959].
Check and block port 21
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :21'
sudo ss -ulpn 'sport = :21'
sudo ss -Slpn 'sport = :21'Check on Windows
Get-NetTCPConnection -LocalPort 21 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 21 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 21/tcp
sudo ufw deny 21/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 21 -j DROP
sudo iptables -A INPUT -p udp --dport 21 -j DROP
sudo iptables -A INPUT -p sctp --dport 21 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled. UFW and the built-in Windows cmdlets cover TCP and UDP only; this port also lists SCTP, so use nftables, iptables or the vendor's diagnostic tool for those protocols. SCTP and DCCP handling also depends on kernel support, including their diagnostic modules, so a missing result may mean the module is not loaded rather than that nothing is listening.
Is port 21 safe to expose?
Classic FTP sends usernames, passwords and file contents in clear text; prefer SFTP or FTPS and restrict access by IP. Expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Where port 21 is usually used
Most traffic on port 21 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Transferring build artifacts, backups or exports between trusted hosts.
- Mounting or browsing shared directories inside a private network.
- Scheduled sync jobs that connect on a fixed port.
List an FTP root
FTP sends credentials in clear text. Use this only on a trusted network and prefer SFTP for real transfers.
curl -v ftp://127.0.0.1:21/Common problems with port 21
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 21. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :21' or sudo lsof -i :21. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 21 at a glance
| Port number | 21 |
|---|---|
| Service name | ftp |
| Transport protocol | TCP / UDP / SCTP |
| Category | File transfer and sharing |
| Range | Well-known port (0–1023) |
| Risk level | Medium |
| Registered assignee | [Jon_Postel] |
| Reference | [RFC 959] |
| Registry description | File Transfer Protocol [Control] |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 21 used for?
Port 21 (FTP) uses TCP / UDP / SCTP. File Transfer Protocol [Control].
How do I check if port 21 is open?
On Linux run `sudo ss -tlpn 'sport = :21'` and `sudo ss -ulpn 'sport = :21'` and `sudo ss -Slpn 'sport = :21'`. On Windows run `Get-NetTCPConnection -LocalPort 21 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 21 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 21; empty output usually means nothing is bound to it. UFW and the built-in Windows cmdlets cover TCP and UDP only; SCTP needs nftables, iptables or the vendor's diagnostic tool.
How do I block port 21?
With UFW run `sudo ufw deny 21/tcp` and `sudo ufw deny 21/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 21 -j DROP` and `sudo iptables -A INPUT -p udp --dport 21 -j DROP` and `sudo iptables -A INPUT -p sctp --dport 21 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot. UFW covers TCP and UDP only; use iptables or nftables for SCTP.
Is port 21 safe to expose?
Classic FTP sends usernames, passwords and file contents in clear text; prefer SFTP or FTPS and restrict access by IP. Nevertheless, expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy