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Secure FTP Server - vsftpd

Prerequisites

  • Proficiency with a command-line editor (we are using vi in this example)
  • A heavy comfort level with issuing commands from the command-line, viewing logs, and other general systems administrator duties
  • An understanding of PAM, as well as openssl commands is helpful.
  • All commands are run as the root user or sudo

Introduction

vsftpd is the Very Secure FTP Daemon (FTP being the file transfer protocol). It has been available for many years now, and is actually the default FTP daemon in Rocky Linux, as well as many other Linux distributions.

vsftpd allows for the use of virtual users with pluggable authentication modules (PAM). These virtual users don't exist in the system, and have no other permissions except to use FTP. This means that if a virtual user gets compromised, the person with those credentials would have no other permissions once they gained access. Using this setup is very secure indeed, but does require a bit of extra work.

Installing vsftpd

We also need to make sure openssl is installed. If you are running a web server, this probably is already installed, but just to make sure, you can run:

dnf install vsftpd openssl

You will also want to enable the vsftpd service:

systemctl enable vsftpd

But don't start the service just yet.

Configuring vsftpd

We want to make sure that some settings are disabled and that others are enabled. Generally, when you install vsftpd, it comes with the most sane options already set, but it is a good idea to make sure.

To check the configuration file and make changes as necessary, run:

vi /etc/vsftpd/vsftpd.conf

Look for the line "anonymous_enable=" and make sure that it is set to "NO" and that it is NOT remarked/commented out. (Remarking out this line will enable anonymous logins). The line should look like this when it is correct:

anonymous_enable=NO

Make sure that local_enable is set to yes:

local_enable=YES

Add a line for the local root user. If the server that you are installing this on is a web server, we assume that you will be using the Apache Web Server Multi-Site Setup, and that your local root will reflect that. If your setup is different, or if this is not a web server, adjust the local_root setting:

local_root=/var/www/sub-domains

Make sure that write_enable is set to yes as well:

write_enable=YES

Find the line to "chroot_local_users", and remove the remark. Then add two lines below so that it looks like this:

chroot_local_user=YES
allow_writeable_chroot=YES
hide_ids=YES

Beneath this, we want to add an entirely new section that will deal with virtual users:

# Virtual User Settings
user_config_dir=/etc/vsftpd/vsftpd_user_conf
guest_enable=YES
virtual_use_local_privs=YES
pam_service_name=vsftpd
nopriv_user=vsftpd
guest_username=vsftpd

We need to add a section near the bottom of the file to force passwords sent over the internet to be encrypted. We need openssl installed and we will need to create the certificate file for this as well.

Start by adding these lines at the bottom of the file:

rsa_cert_file=/etc/vsftpd/vsftpd.pem
rsa_private_key_file=/etc/vsftpd/vsftpd.key
ssl_enable=YES
allow_anon_ssl=NO
force_local_data_ssl=YES
force_local_logins_ssl=YES
ssl_tlsv1=YES
ssl_sslv2=NO
ssl_sslv3=NO

pasv_min_port=7000
pasv_max_port=7500

Now save your configuration. (That's SHIFT:wq if using vi.)

Setting Up The RSA Certificate

We need to create the vsftpd RSA certificate file. The author generally figures that a server is good for 4 or 5 years, so set the number of days for this certificate based on the number of years you believe you'll have the server up and running on this hardware.

Edit the number of days as you see fit, and then use the below format of the command to create the certificate and private key files:

openssl req -x509 -nodes -days 1825 -newkey rsa:2048 -keyout /etc/vsftpd/vsftpd.key -out /etc/vsftpd/vsftpd.pem

Like all certificate creation processes, this will start a script that will ask you for some information. This is not a difficult process. Many fields can be left blank.

The first field is the country code field, fill this one in with your country two letter code:

Country Name (2 letter code) [XX]:

Next comes the state or province, fill this in by typing the whole name, not the abbreviation:

State or Province Name (full name) []:

Next is the locality name. This is your city:

Locality Name (eg, city) [Default City]:

Next is the company or organizational name. You can leave this blank or fill it in as you see fit:

Organization Name (eg, company) [Default Company Ltd]:

Next is the organizational unit name. You can fill this in if the server is for a specific division, or leave it blank:

Organizational Unit Name (eg, section) []:

The next field should be filled in, however you can decide how you want to fill it in. This is the common name of your server. Example: webftp.domainname.ext:

Common Name (eg, your name or your server's hostname) []:

Finally, there is the email field, which you can certainly leave blank without issue:

Email Address []:

Once you have completed the form, the certificate will be created.

Setting Up Virtual Users

As stated earlier, using virtual users for vsftpd is much more secure, because they have no system privileges at all. That said, we need to add a user for the virtual users to use. We also need to add a group:

groupadd nogroup

And then:

useradd --home-dir /home/vsftpd --gid nogroup -m --shell /bin/false vsftpd

The user must match the guest_username= line in the vsftpd.conf file.

Now navigate to the configuration directory for vsftpd:

cd /etc/vsftpd

We need to create a new password database that will be used to authenticate our virtual users. We need to create a file to read the virtual users and passwords from that will create the database.

In the future, when adding new users, we will want to duplicate this process as well:

vi vusers.txt

The user and password are line separated, so simply type the user, hit enter, and then type the password. Continue until you have added all of the users you currently want to have access to the system. Example:

user_name_a
user_password_a
user_name_b
user_password_b

Once the text file is created, we now want to generate the password database that vsftpd will use for the virtual users. This is done with db_load. db_load is provided by libdb-utils which should be loaded on your system, but if it is not, you can simply install it with:

dnf install libdb-utils

Create the database from the text file with:

db_load -T -t hash -f vusers.txt vsftpd-virtual-user.db

We need to take just a moment here to reference what db_load is doing here:

  • The -T is used to easily allow the import of a text file into the database.
  • The -t says to specify the underlying access method.
  • The hash is the underlying access method we are specifying.
  • The -f says to read from a specified file.
  • The specified file is vusers.txt .
  • And the database we are creating or adding to is vsftpd-virtual-user.db.

Change the default permissions of the database file:

chmod 600 vsftpd-virtual-user.db

And remove the vusers.txt file:

rm vusers.txt

When adding new users, simply use vi to create a new vusers.txt file, and re-run the db_load command, which will add the new user/s to the database.

Setting Up PAM

vsftpd installs a default pam file when you install the package. We are going to replace this with our own content, so always make a backup copy of the old file first.

Make a directory for your backup file in /root:

mkdir /root/backup_vsftpd_pam

Then copy the pam file to this directory:

cp /etc/pam.d/vsftpd /root/backup_vsftpd_pam/

Now edit the original file:

vi /etc/pam.d/vsftpd

Remove everything in this file except the "#%PAM-1.0" and then add in the following lines:

auth       required     pam_userdb.so db=/etc/vsftpd/vsftpd-virtual-user
account    required     pam_userdb.so db=/etc/vsftpd/vsftpd-virtual-user
session    required     pam_loginuid.so

Save your changes and exit (SHIFT:wq in vi).

This will enable login for your virtual users defined in vsftpd-virtual-user.db, and will disable local logins.

Setting Up The Virtual User's Configuration

Each virtual user has their own configuration file, which specifies their own local_root directory. This local root must be owned by the user "vsftpd" and the group "nogroup".

Remember that this was set up in the Setting Up Virtual Users section above. To change the ownership for the directory, simply type this at the command line:

chown vsftpd.nogroup /var/www/sub-domains/whatever_the_domain_name_is/html

We need to create the file that contains the virtual user's configuration:

vi /etc/vsftpd/vsftpd_user_conf/username

This will have a single line in it that specifies the virtual user's local_root:

local_root=/var/www/sub-domains/com.testdomain/html

This file path is specified in the "Virtual User" section of the vsftpd.conf file.

Starting vsftpd

Once all of this is completed, start the vsftpd service and then test your users, assuming that the service starts correctly:

systemctl restart vsftpd

Testing vsftpd

You can test your setup using the command line on a machine and test access to the machine using FTP. That said, the easiest way to test is to test with an FTP client, such as FileZilla.

When you test with a virtual user to the server running vsftpd, you should get an SSL certificate trust message. This trust message is saying to the person using the FTP client that the server uses a certificate and asks them to approve the certificate before continuing. Once connected as a virtual user, you should be able to place files in the "local_root" folder that we setup for that user.

If you are unable to upload a file, then you may need to go back and make sure that each of the above steps is completed. For instance, it could be that the ownership permissions for the "local_root" have not been set to the "vsftpd" user and the "nogroup" group.

Conclusion

vsftpd is a popular and common ftp server and can be set up as a stand alone server, or as part of an Apache Hardened Web Server. If set up to use virtual users and a certificate, it is quite secure.

While there are quite a number of steps to setting up vsftpd as outlined in this document, taking the extra time to set it up correctly will ensure that your server is as secure as it can be.

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