How to setup an SFTP server on CentOS

How to setup an SFTP server on CentOS

This tutorial explains how to setup and use an SFTP server on CentOS. Before I start, let me explain what actually SFTP represents and what it is used for. Currently, most people know that we can use normal FTP for transferring, downloading or uploading data from a server to client or client to server. But this protocol is getting hacked easily (if TLS is not used) by anonymous intruders as it the ports are wide open to anyone. Therefore, SFTP has been introduced to as another alternative to meet the main purpose to strengthen the security level.if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-box-3-0’)};

SFTP stands for SSH File Transfer Protocol or Secure File Transfer Protocol. It uses a separate protocol packaged with SSH to provide a secure connection.if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-medrectangle-3-0’)};

1. Preliminary Note

For this tutorial, I am using CentOS 7 in the 64bit version. The same steps will work on CentOS 6 as well. The tutorial result will show how a client can be provided with access to the SFTP server but unable to login to the server itself by SSH.

2. SFTP Installation

Unlike normal FTP, there’s no need to install additional packages in order to use SFTP. We just require the prebuild SSHd package that got already installed during installation on the server. Therefore, just check to confirm if you already have the required SSH package. Below are the steps:

Run:

rpm -qa|grep ssh

The output should be similar to this:

[[email protected] ~]# rpm -qa|grep ssh
libssh2-1.4.3-10.el7_2.1.x86_64
openssh-7.4p1-13.el7_4.x86_64
openssh-server-7.4p1-13.el7_4.x86_64
openssh-clients-7.4p1-13.el7_4.x86_64

That’s all, now we’ll go on how to make the SFTP configuration.

3. SFTP Configuration

Once all prerequisites of installation are done, we’ll step over to configuration phase. For best practice, we need to create a group and user so that we can manage all user that shall get SFTP access. But first, let’s create an additional folder called data. Below are the steps:

mkdir -p /data/sftp
chmod 701 /data

Basically what I’m trying to do with the above step is to get a separate folder as main directory for the SFTP access. All user directories for the SFTP users will be subdirectories of this data folder.

Let’s create a group for the SFTP user, below are the steps:if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-medrectangle-4-0’)};

groupadd sftpusers

Then create a user ‘howtoforge’ and assign it to the SFTPUSERS group. Below are the steps:

useradd -g sftpusers -d /upload -s /sbin/nologin mysftpuser
passwd mysftpuser
Changing password for user mysftpuser.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

Below is the explanation of the above commands:

  1. I create a user and include the user into sftpusers group using -g command.
  2. I assign the main directory for the user to be in the /upload directory by setting the -d /upload command. This means that later the /upload folder will be under /data/mysftpuser/upload.
  3. I limit the access to the /sbin/nologin shell to ensure the user is only able to use the SFTP protocol, not SSH.
  4. I name the user “mysftpuser”.
  5. Set password for user “mysftpuser”.


Now let’s create the /upload folder under /data/mysftpuser, then assign appropriate ownership to the folder.

mkdir -p /data/mysftpuser/upload
chown -R root:sftpusers /data/mysftpuser
chown -R mysftpuser:sftpusers /data/mysftpuser/upload

Once done, verify that the new folder under the directory /data exists and that we made the configuration correct.


[[email protected] ~]# ls -ld /data/
drwx-----x. 5 root root 54 Mar 22 14:29 /data/
[[email protected] ~]# ls -ld /data/mysftpuser
drwxr-xr-x. 3 root sftpusers 20 Mar 22 14:29 /data/mysftpuser
[[email protected] ~]# ls -ld /data/mysftpuser/upload
drwxr-xr-x. 2 mysftpuser sftpusers 6 Mar 22 14:29 /data/mysftpuser/upload
[[email protected] ~]# cat /etc/passwd|grep mysftpuser
mysftpuser:x:1001:1001::/upload:/sbin/nologin

Now configure the SSH protocol to create an SFTP process. This can be done by editing the configuration file under /etc/ssh/sshd_config. Below are the steps:

nano /etc/ssh/sshd_config

Add the following lines at the end of the file.

Match Group sftpusers
ChrootDirectory /data/%u
ForceCommand internal-sftp

Once done restart the SSH services, below are the steps:

service sshd status
Redirecting to /bin/systemctl status sshd.service
? sshd.service - OpenSSH server daemon
Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
Active: active (running) since Thu 2018-03-22 14:21:38 CET; 16min ago
Docs: man:sshd(8)
man:sshd_config(5)
Main PID: 942 (sshd)
CGroup: /system.slice/sshd.service
??942 /usr/sbin/sshd -D
Mar 22 14:21:37 localhost.localdomain systemd[1]: Starting OpenSSH server daemon...
Mar 22 14:21:38 localhost.localdomain sshd[942]: Server listening on 0.0.0.0 port 22.
Mar 22 14:21:38 localhost.localdomain sshd[942]: Server listening on :: port 22.
Mar 22 14:21:38 localhost.localdomain systemd[1]: Started OpenSSH server daemon.
Mar 22 14:21:49 localhost.localdomain sshd[1375]: Accepted password for root from 192.168.1.9 port 59465 ssh2
service sshd restart
[[email protected] ~]# service sshd restart
Redirecting to /bin/systemctl restart sshd.service

4. Testing SFTP

Now everything has been configured, so let’s make a test to ensure the setup meets our purpose.
I’ll access SFTP by using another server called TEST01. First, I’ll verify the Port of the SFTP server . To do that I’ll use the nmap function. If your client server didn’t have it you may download and install it with yum as shown below:

yum list nmap
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: centos.copahost.com
* epel: mirror.imt-systems.com
* extras: mirror.imt-systems.com
* remi-php72: remi.mirror.wearetriple.com
* remi-safe: remi.mirror.wearetriple.com
* updates: ftp.rz.uni-frankfurt.de
Available Packages
nmap.x86_64 2:6.40-7.el7
yum install nmap -y
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: centos.copahost.com
* epel: mirror.imt-systems.com
* extras: mirror.imt-systems.com
* remi-php72: remi.mirror.wearetriple.com
* remi-safe: remi.mirror.wearetriple.com
* updates: ftp.rz.uni-frankfurt.de
Resolving Dependencies
--> Running transaction check
---> Package nmap.x86_64 2:6.40-7.el7 will be installed
--> Processing Dependency: nmap-ncat = 2:6.40-7.el7 for package: 2:nmap-6.40-7.el7.x86_64
--> Running transaction check
---> Package nmap-ncat.x86_64 2:6.40-7.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
=============================================================================================================================================================================================
Package Arch Version Repository Size
=============================================================================================================================================================================================
Installing:
nmap x86_64 2:6.40-7.el7 base 4.0 M
Installing for dependencies:
nmap-ncat x86_64 2:6.40-7.el7 base 201 k
Transaction Summary
=============================================================================================================================================================================================
Install 1 Package (+1 Dependent package)
Total download size: 4.2 M
Installed size: 17 M
Downloading packages:
(1/2): nmap-ncat-6.40-7.el7.x86_64.rpm | 201 kB 00:00:01
(2/2): nmap-6.40-7.el7.x86_64.rpm | 4.0 MB 00:00:14
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Total 300 kB/s | 4.2 MB 00:00:14
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Installing : 2:nmap-ncat-6.40-7.el7.x86_64 1/2
Installing : 2:nmap-6.40-7.el7.x86_64 2/2
Verifying : 2:nmap-ncat-6.40-7.el7.x86_64 1/2
Verifying : 2:nmap-6.40-7.el7.x86_64 2/2
Installed:
nmap.x86_64 2:6.40-7.el7
Dependency Installed:
nmap-ncat.x86_64 2:6.40-7.el7
Complete!
[[email protected] ~]# nmap -n 192.168.1.100
Starting Nmap 6.40 ( http://nmap.org ) at 2018-03-22 14:51 CET
Nmap scan report for 192.168.1.100
Host is up (0.000016s latency).
Not shown: 997 closed ports
PORT STATE SERVICE
22/tcp open ssh
Nmap done: 1 IP address (1 host up) scanned in 0.12 seconds


You’ll notice that currently on our SFTP server, the only open port is SSH 22. Now, let’s try to access the SFTP server (IP: 192.168.1.100 in my case) from TEST01 client. Below are the steps:

[[email protected] /]# sftp [email protected]
[email protected]'s password:
Connected to 192.168.1.100.
sftp> pwd
Remote working directory: /upload
sftp>

Great! Now our SFTP server is accessible from outside. Notice that the default directory is /upload . This means that SFTP will only show the default path as /upload even though our previous configuration made in the SFTP server directory is /data/mysftpuser/upload.
Now let’s try to get a file from the SFTP server directory into our testing client. First, let’s create a test file under /data/mysftpuser/upload. Below are the steps:

cd /data/mysftpuser/upload
touch testing_file.txt

Then go back to our testing site TEST01 and see if we able to get and download the created file.

[[email protected] /]# sftp [email protected]
[email protected]'s password:
Connected to 192.168.1.100.
sftp> pwd
Remote working directory: /upload
sftp> ls
testing_file.txt
sftp> get testing_file.txt
Fetching /upload/testing_file.txt to testing_file.txt
sftp> quit

Excellent! Our SFTP test has been successful, let’s try to access SSH using the user mysftpuser. As previously, we’ve set configuration as /sbin/nologin, therefore the user won’t be able to use SSH services:

[[email protected] ~]# ssh [email protected]
[email protected]'s password:
This service allows sftp connections only.
Connection to 192.168.1.100 closed.

Nice! Now we have a secured SFTP server up and running.

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