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// File transfer & sharing

Port 6621: Kerberos V5 FTP Control

Port 6621 is assigned by IANA to kftp (Kerberos V5 FTP Control).

6621

How kftp works

File transfer & sharing
client:66211/32/33/3kftp client:66211/32/33/3kftp

Files are sent over port 6621 in pieces and reassembled on the other side.

Transport
TCP · UDP
Exposure risk
not rated depends on the program listening
Encryption
Unknownnot stated by the registry
IANA status
Assigned1 registered service
Range
Registered (1024-49151)
Category
File transfer & sharing
01 Security

Is port 6621 dangerous?

We have no specific security note for port 6621. 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 6621

ServicePortTransportDescriptionReferenceUpdated
kftp 6621 TCP, UDP Kerberos V5 FTP Control - 2005-08

// source: IANA service name and port number registry

03 Check it

Which program uses port 6621, and is it reachable?

Linux

sudo ss -ltnup 'sport = :6621'      # process listening on 6621
sudo lsof -nP -i :6621

macOS

sudo lsof -nP -i :6621

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 6621 -State Listen | Select LocalAddress, OwningProcess
Get-NetUDPEndpoint -LocalPort 6621
netstat -ano | findstr :6621

From another machine

nmap -p 6621 -sV <host>
sudo nmap -sU -p 6621 <host>
nc -zv <host> 6621

Block it in the firewall

sudo ufw deny 6621/tcp                                         # Linux (ufw)
New-NetFirewallRule -DisplayName "Block 6621" -Direction Inbound -LocalPort 6621 -Protocol TCP -Action Block  # Windows
04 FAQ
What is port 6621 used for?

Port 6621 is assigned by IANA to kftp (Kerberos V5 FTP Control).

Is port 6621 TCP or UDP?

Port 6621 is registered for TCP and UDP.

Is it safe to open port 6621?

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 6621 is open?

On Linux run "ss -ltnup 'sport = :6621'", on macOS "lsof -nP -i :6621", and on Windows "netstat -ano | findstr :6621" to see the local process. From another machine, "nmap -p 6621 <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 :20 File Transfer [Default Data] tcp · udp · sctp :115 Simple File Transfer Protocol tcp · udp

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