Set up an FTP server
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File Transfer Protocol (FTP) is a TCP protocol for downloading files between computers. In the past, it has also been used for uploading but, as that method does not use encryption, user credentials as well as data transferred in the clear and are easily intercepted. So if you are here looking for a way to upload and download files securely, see the OpenSSH documentation instead.
FTP works on a client/server model. The server component is called an FTP daemon. It continuously listens for FTP requests from remote clients. When a request is received, it manages the login and sets up the connection. For the duration of the session it executes any of commands sent by the FTP client.
Access to an FTP server can be managed in two ways:
-
Anonymous
-
Authenticated
In the Anonymous mode, remote clients can access the FTP server by using the default user account called “anonymous” or “ftp” and sending an email address as the password. In the Authenticated mode a user must have an account and a password. This latter choice is very insecure and should not be used except in special circumstances. If you are looking to transfer files securely see SFTP in the section on OpenSSH-Server. User access to the FTP server directories and files is dependent on the permissions defined for the account used at login. As a general rule, the FTP daemon will hide the root directory of the FTP server and change it to the FTP Home directory. This hides the rest of the file system from remote sessions.
vsftpd - FTP Server Installation
vsftpd is an FTP daemon available in Ubuntu. It is easy to install, set up, and maintain. To install vsftpd you can run the following command:
sudo apt install vsftpd
Anonymous FTP Configuration
By default vsftpd is not configured to allow anonymous download. If you wish to enable anonymous download edit /etc/vsftpd.conf by changing:
anonymous_enable=YES
During installation a ftp user is created with a home directory of /srv/ftp. This is the default FTP directory.
If you wish to change this location, to /srv/files/ftp for example, simply create a directory in another location and change the ftp user’s home directory:
sudo mkdir -p /srv/files/ftp
sudo usermod -d /srv/files/ftp ftp
After making the change restart vsftpd:
sudo systemctl restart vsftpd.service
Finally, copy any files and directories you would like to make available through anonymous FTP to /srv/files/ftp, or /srv/ftp if you wish to use the default.
User Authenticated FTP Configuration
By default vsftpd is configured to authenticate system users and allow them to download files. If you want users to be able to upload files, edit /etc/vsftpd.conf:
write_enable=YES
Now restart vsftpd:
sudo systemctl restart vsftpd.service
Now when system users login to FTP they will start in their home directories where they can download, upload, create directories, etc.
Similarly, by default, anonymous users are not allowed to upload files to FTP server. To change this setting, you should uncomment the following line, and restart vsftpd:
anon_upload_enable=YES
Warning
Enabling anonymous FTP upload can be an extreme security risk. It is best to not enable anonymous upload on servers accessed directly from the Internet.
The configuration file consists of many configuration parameters. The information about each parameter is available in the configuration file. Alternatively, you can refer to the man page, man 5 vsftpd.conf for details of each parameter.
Securing FTP
There are options in /etc/vsftpd.conf to help make vsftpd more secure. For example users can be limited to their home directories by uncommenting:
chroot_local_user=YES
You can also limit a specific list of users to just their home directories:
chroot_list_enable=YES
chroot_list_file=/etc/vsftpd.chroot_list
After uncommenting the above options, create a /etc/vsftpd.chroot_list containing a list of users one per line. Then restart vsftpd:
sudo systemctl restart vsftpd.service
Also, the /etc/ftpusers file is a list of users that are disallowed FTP access. The default list includes root, daemon, nobody, etc. To disable FTP access for additional users simply add them to the list.
FTP can also be encrypted using FTPS. Different from SFTP, FTPS is FTP over Secure Socket Layer (SSL). SFTP is a FTP like session over an encrypted SSH connection. A major difference is that users of SFTP need to have a shell account on the system, instead of a nologin shell. Providing all users with a shell may not be ideal for some environments, such as a shared web host. However, it is possible to restrict such accounts to only SFTP and disable shell interaction.
To configure FTPS, edit /etc/vsftpd.conf and at the bottom add:
ssl_enable=YES
Also, notice the certificate and key related options:
rsa_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
rsa_private_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
By default these options are set to the certificate and key provided by the ssl-cert package. In a production environment these should be replaced with a certificate and key generated for the specific host. For more information on certificates see Security - Certificates.
Now restart vsftpd, and non-anonymous users will be forced to use FTPS:
sudo systemctl restart vsftpd.service
To allow users with a shell of /usr/sbin/nologin access to FTP, but have no shell access, edit /etc/shells adding the nologin shell:
# /etc/shells: valid login shells
/bin/csh
/bin/sh
/usr/bin/es
/usr/bin/ksh
/bin/ksh
/usr/bin/rc
/usr/bin/tcsh
/bin/tcsh
/usr/bin/esh
/bin/dash
/bin/bash
/bin/rbash
/usr/bin/screen
/usr/sbin/nologin
This is necessary because, by default vsftpd uses PAM for authentication, and the /etc/pam.d/vsftpd configuration file contains:
auth required pam_shells.so
The shells PAM module restricts access to shells listed in the /etc/shells file.
Most popular FTP clients can be configured to connect using FTPS. The lftp command line FTP client has the ability to use FTPS as well.
References
- See the vsftpd website for more information.