UDP 21

Looking for information on Protocol UDP 21? This page will attempt to provide you with as much port information as possible on UDP Port 21.

UDP Port 21 may use a defined protocol to communicate depending on the application. A protocol is a set of formalized rules that explains how data is communicated over a network. Think of it as the language spoken between computers to help them communicate more efficiently.

Protocol HTTP for example defines the format for communication between internet browsers and web sites. Another example is the IMAP protocol that defines the communication between IMAP email servers and clients or finally, the SSL protocol which states the format to use for encrypted communications.

UDP Port 21

Here is what we know about protocol UDP Port 21. If you have information on UDP port 21 that is not reflected on this page, simply leave a comment and we’ll update our information.

PORT 21 – Information

• Port Number: 21
• TCP / UDP: UDP
• Delivery: No
• Protocol / Name: ftp-control
• Port Description: This is the FTP service control port. Firewall rules focus on this port, then open port 20 only when required for a data transfer. Security Concerns with FTP: Cleartext, re-usable passwords. – Portal for user account grinding. – FTP Bounce, where attacker uses ftp’s "port" command to redirect the FTP transfer to a port & IP other than default port 20 on the FTP server. Attacks can include "bouncing" internal network scans, email forging/flooding, etc. CERT Advisories: CA-97.16, CA-99.13. Disable this service on non-FTP servers. Open at perimeter only with static route to internal FTP server(s).
• Virus / Trojan: No

  Tip! Use our free Digital Footprint and Firewall Test to help verify you are not infected.

Side note: UDP port 21 uses the Datagram Protocol, a communications protocol for the Internet network layer, transport layer, and session layer. This protocol when used over PORT 21 makes possible the transmission of a datagram message from one computer to an application running in another computer. Like TCP (Transmission Control Protocol), UDP is used with IP (the Internet Protocol) but unlike TCP on Port 21, UDP Port 21 is connectionless and does not guarantee reliable communication; it’s up to the application that received the message on Port 21 to process any errors and verify correct delivery.

Because protocol UDP port 21 was flagged as a virus (colored red) does not mean that a virus is using port 21, but that a Trojan or Virus has used this port in the past to communicate.

UDP 21 – Disclaimer

We do our best to provide you with accurate information on PORT 21 and work hard to keep our database up to date. This is a free service and accuracy is not guaranteed. We do our best to correct any errors and welcome feedback!