Red Hat Linux 9: Red Hat Linux Security Guide
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5.6. Securing FTP

The File Transport Protocol (FTP) is an older TCP protocol designed to transfer files over a network. Because all transactions with the server, including user authentication, are unencrypted, it is considered an insecure protocol and should be carefully configured.

	Note
	Red Hat Linux 9 does not ship with the `xinetd`-based `wu-ftpd` service. However, instructions for securing it remain in this section for legacy systems.

Red Hat Linux provides three FTP servers.

gssftpd — A kerberized xinetd-based FTP daemon which does not pass authentication information over the network.
Red Hat Content Accelerator (tux) — A kernel-space Web server with FTP capabilities.
vsftpd — A standalone, security oriented implementation of the FTP service.

The following security guidelines are for setting up the wu-ftpd and vsftpd services.

	Warning
	If you activate both the `wu-ftpd` and `vsftpd` services, the `xinetd`-based `wu-ftpd` service will handle FTP connections.

5.6.1. FTP Greeting Banner

Before submitting a user name and password, all users are presented with a greeting banner. By default, this banner includes version information useful to crackers trying to identify weaknesses in a system.

To change the greeting banner for vsftpd, add the following directive to /etc/vsftpd/vsftpd.conf:

ftpd_banner=<insert_greeting_here>

Replace <insert_greeting_here> in the above directive with the text of your greeting message.

To change the greeting banner for wu-ftpd, add the following directives to /etc/ftpusers:

greeting text <insert_greeting_here>

Replace <insert_greeting_here> in the above directive with the text of your greeting message.

For mutli-line banners, it is best to use a banner file. To simplify management of multiple banners, we will place all banners in a new directory called /etc/banners/. The banner file for FTP connections in this example will be /etc/banners/ftp.msg. Below is an example of what such a file may look like:

####################################################
# Hello, all activity on ftp.example.com is logged.#
####################################################

	Note
	It is not necessary to begin each line of the file with `220` as specified in Section 5.1.1.1 TCP Wrappers and Connection Banners.

To reference this greeting banner file for vsftpd, add the following directive to /etc/vsftpd/vsftpd.conf:

banner_file=/etc/banners/ftp.msg

To reference this greeting banner file for wu-ftpd, add the following directives to /etc/ftpusers:

greeting terse
banner /etc/banners/ftp.msg

It also is possible to send additional banners to incoming connections using TCP wrappers as described in Section 5.1.1.1 TCP Wrappers and Connection Banners.

5.6.2. Anonymous Access

For both wu-ftpd and vsftpd, the presence of the /var/ftp/ directory activates the anonymous account.

The easiest way to create this directory is to install the vsftpd package. This package sets a directory tree up for anonymous users and configures the permissions on directories to read-only for anonymous users.

	Note
	For releases before Red Hat Linux 9, you must install the `anonftp` package to create the `/var/ftp/` directory.

By default the anonymous user cannot write to any directories.

	Caution
	If enabling anonymous access to an FTP server, be careful where you store sensitive data.

5.6.2.1. Anonymous Upload

If you want to allow anonymous users to upload, it is recommended you create a write-only directory within /var/ftp/pub/.

To do this type:

mkdir /var/ftp/pub/upload

Next change the permissions so that anonymous users cannot see what is within the directory by typing:

chmod 730 /var/ftp/pub/upload

A long format listing of the directory should look like this:

drwx-wx---    2 root     ftp          4096 Feb 13 20:05 upload

	Warning
	Administrators who allow anonymous users to read and write in directories often find that their server become a repository of stolen software.

Additionally, under vsftpd, add the following line to /etc/vsftpd/vsftpd.conf:

anon_upload_enable=YES

5.6.3. User Accounts

Because FTP passes unencrypted usernames and passwords over insecure networks for authentication, it is a good idea to deny system users access to the server from their user accounts.

To disable user accounts in wu-ftpd, add the following directive to /etc/ftpusers:

deny-uid *

To disable user accounts in vsftpd, add the following directive to /etc/vsftpd/vsftpd.conf:

local_enable=NO

5.6.3.1. Restricting User Accounts

The easiest way to disable a specific group of accounts, such as the root user and those with sudo privileges from accessing an FTP server is to use a PAM list file as described in Section 4.4.2.4 Disabling Root Using PAM. The PAM configuration file for wu-ftpd is /etc/pam.d/ftp. The PAM configuration file for vsftpd is /etc/pam.d/vsftpd.

It is also possible to perform this test within each service directly.

To disable specific user accounts in wu-ftpd, add the username to /etc/ftpusers:

To disable specific user accounts in vsftpd, add the username to /etc/vsftpd.ftpusers:

5.6.4. Use TCP Wrappers To Control Access

You can use TCP wrappers to control access to either FTP daemon as outlined in Section 5.1.1 Enhancing Security With TCP Wrappers.

5.6.5. Use `xinetd` To Control the Load

If using wu-ftpd, you can use xinetd to control the amount of resources the FTP server consumes and to limit the effects of denial of service attacks. See Section 5.1.2 Enhancing Security With xinetd for more on how to do this.