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NOTE: CentOS Enterprise Linux is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux.

5.6. Securing FTP

The File Transport Protocol, or 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.

Red Hat Enterprise 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 vsftpd FTP service.

5.6.1. FTP Greeting Banner

Before submitting a username 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 the /etc/vsftpd/vsftpd.conf file:

ftpd_banner=<insert_greeting_here>

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

For mutli-line banners, it is best to use a banner file. To simplify management of multiple banners, place all banners in a new directory called /etc/banners/. The banner file for FTP connections in this example is /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 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 the /etc/vsftpd/vsftpd.conf file:

banner_file=/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

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.

By default the anonymous user cannot write to any directories.

Caution Caution
 

If enabling anonymous access to an FTP server, be aware of where sensitive data is stored.

5.6.2.1. Anonymous Upload

To allow anonymous users to upload, it is recommended that a write-only directory be created 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 Warning
 

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

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

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 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 vsftpd is /etc/pam.d/vsftpd.

It is also possible to disable user accounts within each service directly.

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

5.6.4. Use TCP Wrappers To Control Access

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

 
 
  Published under the terms of the GNU General Public License Design by Interspire