How to set up an SFTP server on Arch Linux

In this guide we are going to set up an sftp server on an Arch Linux system. We will also set up a form of chroot where users can only access sftp with the shared credentials.

The File Transfer Protocol is a standard communication protocol used for the transfer of computer files from a server to a client on a computer network.

FTP isn’t popular today because it Lacks Security. When a file is sent using this protocol, the data, username, and password are all shared in plain text, which means a hacker can access this information with little to no effort. For data to be secure, you need to use an upgraded version of FTP like SFTP.

SFTP Secure File Transfer Protocol is a file transfer protocol that provide secure access to a remote computer to deliver secure communications. It leverages SSH – Secure Socket Shell and is frequently also referred to as ‘Secure Shell File Transfer Protocol’.

Prerequisites

To follow along this guide ensure you have the following:

Arch Linux machine
Root access to the server or a user with root access
Internet access from the server

Table of Content

Ensuring that the server is up to date
Ensuring that the SSH service is installed
Creating users and groups and adding the necessary directories
Configuring the ssh service
Verifying that the set up is working as expected

1. Ensuring that the server is up to date

Before proceeding, ensure your system is up to date. Use this command to refresh the system packages and update them.

sudo pacman -Syyu

2. Ensuring that the SSH service is installed

Verify that the ssh is installed:

$ sudo pacman -Qi openssh
Name            : openssh
Version         : 8.6p1-1
Description     : Premier connectivity tool for remote login with the SSH protocol
Architecture    : x86_64
URL             : https://www.openssh.com/portable.html
Licenses        : custom:BSD
Groups          : None
Provides        : None
Depends On      : glibc  krb5  openssl  libedit  ldns  libxcrypt  libcrypt.so=2-64  zlib  pam
Optional Deps   : xorg-xauth: X11 forwarding
                  x11-ssh-askpass: input passphrase in X
                  libfido2: FIDO/U2F support
Required By     : None
Optional For    : None
Conflicts With  : None
Replaces        : None
Installed Size  : 5.79 MiB
Packager        : Giancarlo Razzolini <grazzolini@archlinux.org>
Build Date      : Mon 19 Apr 2021 11:32:46 AM UTC
Install Date    : Thu 03 Jun 2021 03:23:32 AM UTC
Install Reason  : Explicitly installed
Install Script  : Yes
Validated By    : Signature

If ssh is not installed, install with this command:

sudo pacman -S openssh

Now that it is installed, start the service

sudo systemctl start sshd

Confirm its status

$ sudo systemctl status sshd
● sshd.service - OpenSSH Daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: disabled)
     Active: active (running) since Fri 2021-12-03 10:19:02 UTC; 17min ago
   Main PID: 467 (sshd)
      Tasks: 1 (limit: 4606)
     Memory: 5.0M
     CGroup: /system.slice/sshd.service
             └─467 sshd: /usr/bin/sshd -D [listener] 0 of 10-100 startups

Dec 04 14:53:30 ip-10-2-40-103 sshd[13109]: Unable to negotiate with 141.98.10.246 port 34078: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:53:42 ip-10-2-40-103 sshd[13111]: Unable to negotiate with 141.98.10.246 port 38674: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:53:53 ip-10-2-40-103 sshd[13115]: Unable to negotiate with 141.98.10.246 port 43268: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:54:05 ip-10-2-40-103 sshd[13117]: Unable to negotiate with 141.98.10.246 port 47864: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:54:17 ip-10-2-40-103 sshd[13119]: Unable to negotiate with 141.98.10.246 port 52460: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:54:41 ip-10-2-40-103 sshd[13123]: Unable to negotiate with 141.98.10.246 port 33418: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:54:53 ip-10-2-40-103 sshd[13127]: Unable to negotiate with 141.98.10.246 port 38014: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 14:55:05 ip-10-2-40-103 sshd[13129]: Unable to negotiate with 141.98.10.246 port 42614: no matching key exchange method found. Their offer: diffie-hellman-group14-sha1,diffie-hellman-group-exc>
Dec 04 15:16:10 ip-10-2-40-103 sshd[13191]: Received disconnect from 61.177.173.21 port 60983:11:  [preauth]
Dec 04 15:16:10 ip-10-2-40-103 sshd[13191]: Disconnected from authenticating user root 61.177.173.21 port 60983 [preauth]

3. Creating users and groups and adding the necessary directories

Next we will ensure that the necessary users are present in the system. In my case, I would like to have the sftp users home as /srv/sftp

Let us create the home /srv/sftp with this command:

sudo mkdir /srv/sftp

Then let us create an umbrella group for SFTP only

sudo groupadd sftpusers

Then create an sftp only user called citizix:

sudo useradd -G sftpusers -d /srv/sftp/citizix -s /sbin/nologin citizix

The above options do the following:

-G sftpusers: Create user, append to sftpusers group
-d /srv/sftp/citizix: Set home dir as /srv/sftp/citizix
-s /sbin/nologin: We do not want the user to login, so no ssh login shell
Finally, username as citizix

Then add password to the created user using this command:

$ sudo passwd citizix
Changing password for user citizix.
New password:
Retype new password:
passwd: all authentication tokens updated successfully.

3. Configuring the ssh service

Now that we have installed the necessary software and created the users and groups, let us configure ssh.

Ensure password authentication is enabled for ssh. Edit the config file here /etc/ssh/sshd_config:

sudo vim /etc/ssh/sshd_config

Then ensure this line is not commented:

PasswordAuthentication yes

Next, we need to add rules for the users in the sftpusers group to be considered as sftp. Edit the config file:

sudo vim /etc/ssh/sshd_config

Add this content at the bottom of the file:

Match Group sftpusers  
    X11Forwarding no  
    AllowTcpForwarding no  
    ChrootDirectory /srv/sftp
    ForceCommand internal-sftp

Then restart sshd to reload the config:

sudo systemctl restart sshd

Verify that sshd is running as expected:

$ sudo systemctl status sshd
● sshd.service - OpenSSH Daemon
     Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: disabled)
     Active: active (running) since Sat 2021-12-04 15:48:19 UTC; 18s ago
   Main PID: 14269 (sshd)
      Tasks: 1 (limit: 4606)
     Memory: 892.0K
     CGroup: /system.slice/sshd.service
             └─14269 sshd: /usr/bin/sshd -D [listener] 0 of 10-100 startups

Dec 04 15:48:19 ip-10-2-40-103 systemd[1]: Started OpenSSH Daemon.
Dec 04 15:48:19 ip-10-2-40-103 sshd[14269]: Server listening on 0.0.0.0 port 22.
Dec 04 15:48:19 ip-10-2-40-103 sshd[14269]: Server listening on :: port 22.

4. Verifying that the set up is working as expected

After successfully creating the user and adding sftp configurations, let is test the set up using the command:

❯ sftp citizix@10.2.11.8
citizix@<meta charset="utf-8">10.2.11.8's password:
Connected to <meta charset="utf-8">10.2.11.8.
sftp>

Now we have sftp server up and running with a user configured!

The users will be able to login to the server and access files and directories located in their home directory. If you want to give the user to other directories outside their own directory, just make sure the user has enough rights to access. These directories and files have to be within the sftp directory – /srv/sftp.

Example: if i want user to access the directory /srv/sftp/paymentfiles, do the following:

Create the directory

sudo mkdir /srv/sftp/paymentfiles

Then assign the user(citizix) access by making them own the directory:

sudo chown citizix:sftpusers /srv/sftp/paymentfiles

That is it. Users should now have access.

Conclusion

We managed to set up sftp server in an Archlinux Server in this guide.