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Thursday, 30 January 2020

How to Install Secure FTP Server on CentOS 8

How to Install Secure FTP Server on CentOS 8

FTP (File Transfer Protocol) is a standard network protocol used to transfer computer files between clients and server. This protocol uses plain text to transfer data and credentials. However, Sysadmins can overcome this limitation by configuring Secure FTP Server on CentOS 8 that uses FTPS protocol instead of FTP.

In this lab, you will learn how to install Secure FTP server on CentOS 8 using vsftpd.

Instructions in this article are of advance level, if you are new in Linux world then we strongly recommend you to read Red Hat RHCSA 8 Cert Guide: EX200 (Certification Guide) by Pearson IT Certification. It will provides basic to intermediate knowledge about RHEL (Red Hat Enterprise Linux) 8 or CentOS 8.

 

Table of Contents:

How to Install Secure FTP Server on CentOS 8

Environment Specification:

We are using a minimal CentOS 8 virtual machine with following specification.

FTP Server

  • CPU - 3.4 Ghz (2 cores)
  • Memory - 2 GB
  • Storage - 20 GB
  • Operating System - CentOS 8.0
  • Hostname - ftp-server.sysadminlabs.com
  • IP Address - 192.168.116.217 /24

FTP Client

  • CPU - 3.4 Ghz (2 cores)
  • Memory - 2 GB
  • Storage - 20 GB
  • Operating System - CentOS 8.0
  • Hostname - ftp-client.sysadminlabs.com
  • IP Address - 192.168.116.218 /24

 

Installing vsftpd on CentOS 8:

Connect with ftp-server.sysadminlabs.com as root user by using a ssh tool like PuTTY.

vsftpd (Very Secure FTP Daemon) is the software package that has been used since long for configuring FTP services on Linux.

vsftpd is available in default yum repositories and therefore it can be installed using a dnf command.

# dnf install -y vsftpd

Take backup of existing vsftpd.conf file.

# cp /etc/vsftpd/vsftpd.conf /etc/vsftpd/vsftpd.conf.org

Now edit vsftpd configuration file.

# vi /etc/vsftpd/vsftpd.conf

Find and set following directives therein.

anonymous_enable=NO   # disable anonymous users
local_enable=YES   # allow local users
write_enable=YES   # allow ftp write commands
local_umask=022    # set default umask
dirmessage_enable=YES   # enable messages on change directory
xferlog_enable=YES   # enable logging of uploads and downloads
connect_from_port_20=YES  # ensure PORT transfer connections from port 20 (ftp-data)
xferlog_std_format=YES   # keep standard log format
listen=NO    # prevent vsftpd run in stand alone mode
listen_ipv6=YES    # allow vsftpd to listen on IPv6 socket
pam_service_name=vsftpd   # set PAM Service name to vsftpd

 

Configure User List in vsftpd:

Users that are allowed/deny to use FTP service are listed in a user_list file.

Default user_list file is located at /etc/vsftpd/user_list, we can add or remove FTP users in this file.

By default, all the users in the user_list are denied to access FTP service.

We have to explicitly allow users in user_list by setting following directives in vsftpd.conf file.

userlist_enable=YES   # enable vsftpd to load usernames
userlist_deny=NO   # allow access to users in userlist

 

Enable Chroot Jail for vsftpd:

To restrict FTP users in a chrooted environment, add following two directives in vsftpd.conf file.

chroot_local_user=YES   # Create chrooted environment for users
allow_writeable_chroot=YES  # Allow write permission to user on chroot jail directory

 

Generate a TLS certificate for Secure FTP service:

The FTP service does not use encryption. Therefore, it transfers data and login credentials in plain text. This makes FTP service highly vulnerable to Sniffing and Men in the Middle attacks.

However, we can configure Secure FTP (FTPS), to encrypt the communication between server and clients.

Now, generate a TLS (Transport Layer Security) certificate by using following command.

# openssl req -x509 -nodes -keyout /etc/vsftpd/vsftpd.key -out /etc/vsftpd/vsftpd.pem -days 365 -newkey rsa:2048
Generating a RSA private key
.............................+++++
....................+++++
writing new private key to '/etc/vsftpd/vsftpd.key'
-----
You are about to be asked to enter information that will be incorporated
into your certificate request.
What you are about to enter is what is called a Distinguished Name or a DN.
There are quite a few fields but you can leave some blank
For some fields there will be a default value,
If you enter '.', the field will be left blank.
-----
Country Name (2 letter code) [XX]:PK
State or Province Name (full name) []:Sindh
Locality Name (eg, city) [Default City]:Karachi
Organization Name (eg, company) [Default Company Ltd]:Ahmer's SysAdmin Recipes
Organizational Unit Name (eg, section) []:IT LAB
Common Name (eg, your name or your server's hostname) []:ftp-server.sysadminlabs.com
Email Address []:root@ftp-server.sysadminlabs.com

Edit vsftpd.conf file to configure FTP service to use the TLS certificate.

# vi /etc/vsftpd/vsftpd.conf

Add following directives in this file.

ssl_enable=YES       # Enable vsftpd Secure connections
ssl_sslv2=NO      # Disallow SSL v2 protocol connections
ssl_sslv3=NO      # Disallow SSL v3 protocol connections
ssl_tlsv1_2=YES      # Allow TLS v1.2 protocol connections
rsa_cert_file=/etc/vsftpd/vsftpd.pem   # Location of TLS certificate
rsa_private_key_file=/etc/vsftpd/vsftpd.key  # Location of Private Key
allow_anon_ssl=NO     # Disallow Anonymous Access
force_local_data_ssl=YES    # Force users to use SSL connection for data transfer
force_local_logins_ssl=YES    # Force users to use SSL connection for credentials
require_ssl_reuse=NO     # Disable SSL session reuse
ssl_ciphers=HIGH       
pasv_min_port=30000     # Min port number to define a range for PASV connections
pasv_max_port=31000     # Max port number to define a range for PASV connections
debug_ssl=YES      # Dump OpenSSL diagnostics in vsftpd log file

Allow Secure FTP (FTPS) and Passive FTP ports in Linux firewall.

# firewall-cmd --permanent --add-port=30000-31000/tcp
success
# firewall-cmd --permanent --add-port=990/tcp
success
# firewall-cmd --reload
success

Start and enable vsftpd service.

# systemctl enable --now vsftpd.service
Created symlink /etc/systemd/system/multi-user.target.wants/vsftpd.service â /usr/lib/systemd/system/vsftpd.service.

 

Create a CentOS 8 User to access Secure FTP Service:

Create a user for using Secure FTP service.

# useradd ahmer
# passwd ahmer
Changing password for user ahmer.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

Add this user to allowed user_list.

# echo ahmer >> /etc/vsftpd/user_list

Our Secure FTP server has been configured successfully.

 

Testing Secure FTP Server from CentOS 8 client:

To connect to our Secure FTP server, we need a FTP client software.

There are many FTP clients are available in default yum repository of CentOS 8.

We are installing lftp: a sophisticated ftp/http file transfer program. By using lftp client we will test our Secure FTP server.

# dnf install -y lftp

Because our Secure FTP server uses a self-signed TLS certificate, therefore, the lftp client displays follow warning.

"Fatal error: Certificate verification: Not trusted"

To suppress this warning, we can add following directive in lftp configuration file.

# echo "set ssl:verify-certificate no" >> /etc/lftp.conf

Connect to Secure FTP service using lftp command.

# lftp ahmer@ftp-server
Password:
lftp ahmer@ftp-server:~>

Now test our Secure FTP server by executing some FTP commands.

lftp ahmer@ftp-server:~> ls
-rw-r--r--    1 1000     1000           91 Jan 29 16:45 resolv.conf
lftp ahmer@ftp-server:/> put /etc/hosts
214 bytes transferred
lftp ahmer@ftp-server:/> ls
-rw-r--r--    1 1000     1000          214 Jan 30 14:20 hosts
-rw-r--r--    1 1000     1000           91 Jan 29 16:45 resolv.conf
lftp ahmer@ftp-server:/> exit

Our Secure FTP server on CentOS 8 is working fine.

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