15.2. FTP

The File Transfer Protocol (FTP) is one of the oldest and most commonly used protocols found on the Internet today. Its purpose is to reliably transfer files between computer hosts on a network without requiring the user to log directly in to the remote host or to have knowledge of how to use the remote system. It allows users to access files on remote systems using a standard set of simple commands.
This section outlines the basics of the FTP protocol and introduces vsftpd, which is the preferred FTP server in Red Hat Enterprise Linux.

15.2.1. The File Transfer Protocol

FTP uses a client-server architecture to transfer files using the TCP network protocol. Because FTP is a rather old protocol, it uses unencrypted user name and password authentication. For this reason, it is considered an insecure protocol and should not be used unless absolutely necessary. However, because FTP is so prevalent on the Internet, it is often required for sharing files to the public. System administrators, therefore, should be aware of FTP's unique characteristics.
This section describes how to configure vsftpd to establish connections secured by TLS and how to secure an FTP server with the help of SELinux. A good substitute for FTP is sftp from the OpenSSH suite of tools. For information about configuring OpenSSH and about the SSH protocol in general, refer to Chapter 11, OpenSSH.
Unlike most protocols used on the Internet, FTP requires multiple network ports to work properly. When an FTP client application initiates a connection to an FTP server, it opens port 21 on the server — known as the command port. This port is used to issue all commands to the server. Any data requested from the server is returned to the client via a data port. The port number for data connections, and the way in which data connections are initialized, vary depending upon whether the client requests the data in active or passive mode.
The following defines these modes:
active mode
Active mode is the original method used by the FTP protocol for transferring data to the client application. When an active-mode data transfer is initiated by the FTP client, the server opens a connection from port 20 on the server to the IP address and a random, unprivileged port (greater than 1024) specified by the client. This arrangement means that the client machine must be allowed to accept connections over any port above 1024. With the growth of insecure networks, such as the Internet, the use of firewalls for protecting client machines is now prevalent. Because these client-side firewalls often deny incoming connections from active-mode FTP servers, passive mode was devised.
passive mode
Passive mode, like active mode, is initiated by the FTP client application. When requesting data from the server, the FTP client indicates it wants to access the data in passive mode and the server provides the IP address and a random, unprivileged port (greater than 1024) on the server. The client then connects to that port on the server to download the requested information.
While passive mode does resolve issues for client-side firewall interference with data connections, it can complicate administration of the server-side firewall. You can reduce the number of open ports on a server by limiting the range of unprivileged ports on the FTP server. This also simplifies the process of configuring firewall rules for the server.

15.2.2. The vsftpd Server

The Very Secure FTP Daemon (vsftpd) is designed from the ground up to be fast, stable, and, most importantly, secure. vsftpd is the only stand-alone FTP server distributed with Red Hat Enterprise Linux, due to its ability to handle large numbers of connections efficiently and securely.
The security model used by vsftpd has three primary aspects:
  • Strong separation of privileged and non-privileged processes — Separate processes handle different tasks, and each of these processes runs with the minimal privileges required for the task.
  • Tasks requiring elevated privileges are handled by processes with the minimal privilege necessary — By taking advantage of compatibilities found in the libcap library, tasks that usually require full root privileges can be executed more safely from a less privileged process.
  • Most processes run in a chroot jail — Whenever possible, processes are change-rooted to the directory being shared; this directory is then considered a chroot jail. For example, if the /var/ftp/ directory is the primary shared directory, vsftpd reassigns /var/ftp/ to the new root directory, known as /. This disallows any potential malicious hacker activities for any directories not contained in the new root directory.
Use of these security practices has the following effect on how vsftpd deals with requests:
  • The parent process runs with the least privileges required — The parent process dynamically calculates the level of privileges it requires to minimize the level of risk. Child processes handle direct interaction with the FTP clients and run with as close to no privileges as possible.
  • All operations requiring elevated privileges are handled by a small parent process — Much like the Apache HTTP Server, vsftpd launches unprivileged child processes to handle incoming connections. This allows the privileged, parent process to be as small as possible and handle relatively few tasks.
  • All requests from unprivileged child processes are distrusted by the parent process — Communication with child processes is received over a socket, and the validity of any information from child processes is checked before being acted on.
  • Most interactions with FTP clients are handled by unprivileged child processes in a chroot jail — Because these child processes are unprivileged and only have access to the directory being shared, any crashed processes only allow the attacker access to the shared files.

15.2.2.1. Starting and Stopping vsftpd

To start the vsftpd service in the current session, type the following at a shell prompt as root:
~]# systemctl start vsftpd.service
To stop the service in the current session, type as root:
~]# systemctl stop vsftpd.service
To restart the vsftpd service, run the following command as root:
~]# systemctl restart vsftpd.service
This command stops and immediately starts the vsftpd service, which is the most efficient way to make configuration changes take effect after editing the configuration file for this FTP server. Alternatively, you can use the following command to restart the vsftpd service only if it is already running:
~]# systemctl try-restart vsftpd.service
By default, the vsftpd service does not start automatically at boot time. To configure the vsftpd service to start at boot time, type the following at a shell prompt as root:
~]# systemctl enable vsftpd.service
Created symlink from /etc/systemd/system/multi-user.target.wants/vsftpd.service to /usr/lib/systemd/system/vsftpd.service.
For more information on how to manage system services in Red Hat Enterprise Linux 7, see Chapter 9, Managing Services with systemd.

15.2.2.2. Starting Multiple Copies of vsftpd

Sometimes, one computer is used to serve multiple FTP domains. This is a technique called multihoming. One way to multihome using vsftpd is by running multiple copies of the daemon, each with its own configuration file.
To do this, first assign all relevant IP addresses to network devices or alias network devices on the system. For more information about configuring network devices, device aliases, and additional information about network configuration scripts, see the Red Hat Enterprise Linux 7 Networking Guide.
Next, the DNS server for the FTP domains must be configured to reference the correct machine. For information about BIND, the DNS protocol implementation used in Red Hat Enterprise Linux, and its configuration files, see the Red Hat Enterprise Linux 7 Networking Guide.
For vsftpd to answer requests on different IP addresses, multiple copies of the daemon must be running. To facilitate launching multiple instances of the vsftpd daemon, a special systemd service unit (vsftpd@.service) for launching vsftpd as an instantiated service is supplied in the vsftpd package.
In order to make use of this service unit, a separate vsftpd configuration file for each required instance of the FTP server must be created and placed in the /etc/vsftpd/ directory. Note that each of these configuration files must have a unique name (such as /etc/vsftpd/vsftpd-site-2.conf) and must be readable and writable only by the root user.
Within each configuration file for each FTP server listening on an IPv4 network, the following directive must be unique:
listen_address=N.N.N.N
Replace N.N.N.N with a unique IP address for the FTP site being served. If the site is using IPv6, use the listen_address6 directive instead.
Once there are multiple configuration files present in the /etc/vsftpd/ directory, individual instances of the vsftpd daemon can be started by executing the following command as root:
~]# systemctl start vsftpd@configuration-file-name.service
In the above command, replace configuration-file-name with the unique name of the requested server's configuration file, such as vsftpd-site-2. Note that the configuration file's .conf extension should not be included in the command.
If you want to start several instances of the vsftpd daemon at once, you can make use of a systemd target unit file (vsftpd.target), which is supplied in the vsftpd package. This systemd target causes an independent vsftpd daemon to be launched for each available vsftpd configuration file in the /etc/vsftpd/ directory. Execute the following command as root to enable the target:
~]# systemctl enable vsftpd.target
Created symlink from /etc/systemd/system/multi-user.target.wants/vsftpd.target to /usr/lib/systemd/system/vsftpd.target.
The above command configures the systemd service manager to launch the vsftpd service (along with the configured vsftpd server instances) at boot time. To start the service immediately, without rebooting the system, execute the following command as root:
~]# systemctl start vsftpd.target
See Section 9.3, “Working with systemd Targets” for more information on how to use systemd targets to manage services.
Other directives to consider altering on a per-server basis are:
  • anon_root
  • local_root
  • vsftpd_log_file
  • xferlog_file

15.2.2.3. Encrypting vsftpd Connections Using TLS

In order to counter the inherently insecure nature of FTP, which transmits user names, passwords, and data without encryption by default, the vsftpd daemon can be configured to utilize the TLS protocol to authenticate connections and encrypt all transfers. Note that an FTP client that supports TLS is needed to communicate with vsftpd with TLS enabled.

Note

SSL (Secure Sockets Layer) is the name of an older implementation of the security protocol. The new versions are called TLS (Transport Layer Security). Only the newer versions (TLS) should be used as SSL suffers from serious security vulnerabilities. The documentation included with the vsftpd server, as well as the configuration directives used in the vsftpd.conf file, use the SSL name when referring to security-related matters, but TLS is supported and used by default when the ssl_enable directive is set to YES.
Set the ssl_enable configuration directive in the vsftpd.conf file to YES to turn on TLS support. The default settings of other TLS-related directives that become automatically active when the ssl_enable option is enabled provide for a reasonably well-configured TLS set up. This includes, among other things, the requirement to only use the TLS v1 protocol for all connections (the use of the insecure SSL protocol versions is disabled by default) or forcing all non-anonymous logins to use TLS for sending passwords and data transfers.

Example 15.3. Configuring vsftpd to Use TLS

In this example, the configuration directives explicitly disable the older SSL versions of the security protocol in the vsftpd.conf file:
ssl_enable=YES
ssl_tlsv1=YES
ssl_sslv2=NO
ssl_sslv3=NO
Restart the vsftpd service after you modify its configuration:
~]# systemctl restart vsftpd.service
See the vsftpd.conf(5) manual page for other TLS-related configuration directives for fine-tuning the use of TLS by vsftpd.

15.2.2.4. SELinux Policy for vsftpd

The SELinux policy governing the vsftpd daemon (as well as other ftpd processes), defines a mandatory access control, which, by default, is based on least access required. In order to allow the FTP daemon to access specific files or directories, appropriate labels need to be assigned to them.
For example, in order to be able to share files anonymously, the public_content_t label must be assigned to the files and directories to be shared. You can do this using the chcon command as root:
~]# chcon -R -t public_content_t /path/to/directory
In the above command, replace /path/to/directory with the path to the directory to which you want to assign the label. Similarly, if you want to set up a directory for uploading files, you need to assign that particular directory the public_content_rw_t label. In addition to that, the allow_ftpd_anon_write SELinux Boolean option must be set to 1. Use the setsebool command as root to do that:
~]# setsebool -P allow_ftpd_anon_write=1
If you want local users to be able to access their home directories through FTP, which is the default setting on Red Hat Enterprise Linux 7, the ftp_home_dir Boolean option needs to be set to 1. If vsftpd is to be allowed to run in standalone mode, which is also enabled by default on Red Hat Enterprise Linux 7, the ftpd_is_daemon option needs to be set to 1 as well.
See the ftpd_selinux(8) manual page for more information, including examples of other useful labels and Boolean options, on how to configure the SELinux policy pertaining to FTP. Also, see the Red Hat Enterprise Linux 7 SELinux User's and Administrator's Guide for more detailed information about SELinux in general.

15.2.3. Additional Resources

For more information about vsftpd, see the following resources.

15.2.3.1. Installed Documentation

  • The /usr/share/doc/vsftpd-version-number/ directory — Replace version-number with the installed version of the vsftpd package. This directory contains a README file with basic information about the software. The TUNING file contains basic performance-tuning tips and the SECURITY/ directory contains information about the security model employed by vsftpd.
  • vsftpd-related manual pages — There are a number of manual pages for the daemon and the configuration files. The following lists some of the more important manual pages.
    Server Applications
    • vsftpd(8) — Describes available command-line options for vsftpd.
    Configuration Files
    • vsftpd.conf(5) — Contains a detailed list of options available within the configuration file for vsftpd.
    • hosts_access(5) — Describes the format and options available within the TCP wrappers configuration files: hosts.allow and hosts.deny.
    Interaction with SELinux
    • ftpd_selinux(8) — Contains a description of the SELinux policy governing ftpd processes as well as an explanation of the way SELinux labels need to be assigned and Booleans set.

15.2.3.2. Online Documentation

About vsftpd and FTP in General
Red Hat Enterprise Linux Documentation
  • Red Hat Enterprise Linux 7 Networking Guide — The Networking Guide for Red Hat Enterprise Linux 7 documents relevant information regarding the configuration and administration of network interfaces, networks, and network services in this system. It provides an introduction to the hostnamectl utility and explains how to use it to view and set host names on the command line, both locally and remotely.
  • Red Hat Enterprise Linux 7 SELinux User's and Administrator's Guide — The SELinux User's and Administrator's Guide for Red Hat Enterprise Linux 7 describes the basic principles of SELinux and documents in detail how to configure and use SELinux with various services such as the Apache HTTP Server, Postfix, PostgreSQL, or OpenShift. It explains how to configure SELinux access permissions for system services managed by systemd.
  • Red Hat Enterprise Linux 7 Security Guide — The Security Guide for Red Hat Enterprise Linux 7 assists users and administrators in learning the processes and practices of securing their workstations and servers against local and remote intrusion, exploitation, and malicious activity. It also explains how to secure critical system services.
Relevant RFC Documents
  • RFC 0959 — The original Request for Comments (RFC) of the FTP protocol from the IETF.
  • RFC 1123 — The small FTP-related section extends and clarifies RFC 0959.
  • RFC 2228FTP security extensions. vsftpd implements the small subset needed to support TLS and SSL connections.
  • RFC 2389 — Proposes FEAT and OPTS commands.
  • RFC 2428IPv6 support.