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// File transfer & sharing

Port 20: File Transfer [Default Data]

Port 20 is assigned by IANA to ftp-data (File Transfer [Default Data]).

20

How ftp-data works

File transfer & sharing
client:201/32/33/3ftp-data client:201/32/33/3ftp-data

Files are sent over port 20 in pieces and reassembled on the other side.

Transport
TCP · UDP · SCTP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Well-known (0-1023)
Category
File transfer & sharing
01 Security

Is port 20 dangerous?

We have no specific security note for port 20. Its risk depends on the program actually listening on it. General advice for file transfer & sharing services:

  • Prefer encrypted protocols (SFTP, FTPS, SMB 3 with encryption).
  • Disable anonymous access and restrict shares or exports to known hosts.
  • Never publish file sharing protocols to the Internet.
02 Registry

IANA registry entries for port 20

ServicePortTransportDescriptionReferenceUpdated
ftp-data 20 TCP, UDP, SCTP File Transfer [Default Data] - -

// source: IANA service name and port number registry

03 Check it

Which program uses port 20, and is it reachable?

Linux

sudo ss -ltnup 'sport = :20'      # process listening on 20
sudo lsof -nP -i :20

macOS

sudo lsof -nP -i :20

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 20 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 20
netstat -ano | findstr :20

From another machine

nmap -p 20 -sV <host>
sudo nmap -sU -p 20 <host>
nc -zv <host> 20

Block it in the firewall

sudo ufw deny 20/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 20" -Direction Inbound -LocalPort 20 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 20 used for?

Port 20 is assigned by IANA to ftp-data (File Transfer [Default Data]).

Is port 20 TCP or UDP?

Port 20 is registered for TCP and UDP and SCTP.

Is it safe to open port 20?

It depends on the program that listens on it. Open it only for a service you know and only to the networks that need it. Prefer encrypted protocols (SFTP, FTPS, SMB 3 with encryption).

How do I check if port 20 is open?

On Linux run "ss -ltnup 'sport = :20'", on macOS "lsof -nP -i :20", and on Windows "netstat -ano | findstr :20" to see the local process. From another machine, "nmap -p 20 <host>" shows whether it is reachable.

05 Related
:445 SMB tcp · udp · critical risk :21 FTP tcp · udp · sctp · high risk :139 NetBIOS Session tcp · udp · high risk :69 TFTP udp · tcp · high risk :2049 NFS tcp · udp · sctp · high risk :873 rsync tcp · udp · high risk :115 Simple File Transfer Protocol tcp · udp :152 Background File Transfer Program tcp · udp

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