File Transfer Protocol

Old and popular protocol to share files. It uses TCP.

Client-server architecture.

ports: 20, 21

20: data port 21: ftp server command port

The ports of client are usually unprivileged, i.e. a number larger than 1023


FTP runs in 2 modes.

Active Mode

  1. Client prepares a random port P1 to receive data
  2. Client sends FTP command PORT P1 to the server port 21 from a random port P2
  3. Server initializes one data channel from its port 20 to port P1 on client

Problem: port P1 is usually behind firewall.

Passive Mode

  1. Client sends PASV command to server port 21 from an arbitrary port P2
  2. Server sends an IP address and a port P3 to client port P2
  3. Client initializes one data channel from another port P1 to port P3 of server

There is an additional mode, extended passive mode.

In FTP, what are the differences between passive and extended passive modes?

Passive mode is more common than active mode.


very secure FTP daemon

Most popular FTP.


There are generally 3 main config files.

On RedHat/CentOS:

/etc/vsftpd/vsftpd.conf # main
/etc/vsftpd/ftpusers    # list of users DISALLOWED to log in ftp, such as root
/etc/vsftpd/user_list   # another user list
                        # either blacklist or whitelist, controlled by main config file

On Ubuntu:

# Not sure where the user_list file is

Shared directory:

  • different distribution has different locations


  • some possible locations
  • can be changed in config file

Some config snippet:



By default, everyone can access the default shared directory.

Read the man page of specific distribution to get more tuned config.

Debian vsftpd.conf Man Page

Disable anonymous user

In /etc/vsftpd/conf:


Allow anonymous upload

Allow anonymous upload for Vsftpd?

Change config

  • In /etc/vsftpd/conf:
  • And restart ftp service.

Change permission of directory used to upload

chown -R ftp /var/ftp

Not sure if steps above work, but consider SELinux and search around when there is problem.

FTP User

FTP uses user to manage. Clients must log in as a user.

There are 3 type of users for FTP

  1. system user
    • all the normal users used to login the system
    • same permission as that user
    • default directory is the home directory
    • all users of the system can be used to log in FTP, so we need “blacklist” (as mentioned above)
  2. anonymous user
    • no authentication required
    • actually a user called “ftp” whose home is “/var/ftp/”
    • will be created when installing vsftpd
    • can be disabled
  3. FTP virtual user
    • FTP-only users

No need to change any config if anonymous user is enough.

How to Use FTP

GUI is very simple to use.

For CLI use ftp command.

Sample Usage

An FTP directory where people can upload but cannot see the content:

cd /var/ftp/
mkdir dropbox-upload
chmod 2733 dropbox-upload