[root tip] [How To] Set up your own SSH service

Difficulty: ★★★☆☆

This is a crosspost from my notepad at root.nix.dk which in turn is put together from my old topics from the archived forum [1][2]

SSH server security

When you setup a SSH server on a public IP, your service will be spammed within minutes attempting to brute force your login.

Your first task as admin of a SSH server is to secure it.

Topic assumptions

  1. A Manjaro instance with at least two users, a root user and a user belonging to the %wheel group.
  2. A drop-in config in /etc/sudoers.d/ assigning permissions to the group.
  3. Such file is created with default Manjaro install
    $ cat /etc/sudoers.d/10-installer
    %wheel ALL=(ALL) ALL
    

If you have installed Manjaro by hand you will need to establish such file by hand. Always use visudo as the filepermissions are important.

SSH security

Your SSH server is not the only server you may come across using a public key infrastucture. If you are a using services like Github, GitLab, SourceForge, OSDN and many more - you have come across this before.

1. Public key login

Create key pair

SSH public key infrastructure is actually a key pair consisting of both a public and a private key. The public part has the extension .pub and this is the key you place on the server or service.

You create key pair using the ssh-keygen utility. This utility takes various arguments which affects how the key-pair is generated and where the files are placed.

The defaults are sane but in case you want a stronger key, you can use the man-pages to expand your knowledge.

$ man ssh-keygen

It is recommended to use a separate key for each server and each device you are using to connect with and to tell the keys apart you use a different filename e.g. the name of the service. If you have a Linode cloud instance you could name the file linode or if you are using OSDN you may name it osdn.

To create a key-pair for a service using the service name for the key-pair, you supply the path to the file using the -f argument - in this example I will use the name ssh-service with the cipher appened.

When you create the key you have the option of using a password - or you can leave password blank. This of course lowers the security on client side - but you don’t have to unlock the key-pair on use - and it is a matter of choice - just remember to keep you private keys safe - no matter using key password or not.

EDIT: 2021-10-16T07:45:00Z
Due to recent changes and deprecation of ciphers considered unsafe you should add the -t option to specify the cipher to be used preferably ed25519

$ ssh-keygen -t ed25519  -f ~/.ssh/ssh-service-ed25519.ppk

You don’t need to use the .ppk part, but doing so makes it easy to configure FileZilla as SFTP client using a keyfile.

Transfer the public key to the server

Many VPS providers only provide a root account on the initial device in which case you will have to target root for the keyfile.

Manjaro installations defaults to keyfile only for root login. Therefore you will need to to the other user account created on the device to be able to upoload the keyfile.

To upload the public part to the server - you can use the scp command which is an abbreviation of secure copy. The command consist of three parts:

  • The command itself
  • The path to the file to be copied
  • Server and where to put the file

In case of a VPS with only a root user

$ scp ./ssh/ssh-service-ed25519.ppk.pub  root@host:/root/.ssh

Or in case of a rapsberry pi in your local network

$ scp ./ssh/ssh-service-ed25519.ppk.pub  $user@host:/home/$USER/.ssh/

Another option is to use ssh-copy to send the file to your ssh service

ssh-copy-id -i ~/.ssh/ssh-service-ed25519.ppk.pub $USER@server.domain.tld

Log off the server and test your connection using your identity file

ssh $USER@server.domain.tld -i ~/.ssh/ssh-service-ed25519.ppk.pub

If you - in the above key-generation supplied a password to protect your key-pair - you will be prompted to unlock the key-pair.

If you did it right - you are now in a remote shell without being prompted for credentials.

2. Disable password login

When you are logged in - using the keyfile - edit the file /etc/ssh/sshd_config

sudo nano /etc/ssh/sshd_config

Scroll down to the lines reading:

# To disable tunneled clear text passwords, change to no here!
#PasswordAuthentication yes
#PermitEmptyPasswords no

And change them to:

PasswordAuthentication no

3. Change port

While you are logged in - seriously consider changing the port from the default 22 to something else! Consult /etc/services to avoid a collision with known services - but in reality any port over 10000 can be used.

The port is located at the top of the configuration

# Port 22
#AddressFamily any
#ListenAddress 0.0.0.0
#ListenAddress ::

Uncomment the line and change the port number

Port 33000

Save the changes and restart the sshd daemon

# systemctl reload sshd

4. Connecting to the server

Connecting to SSH server can be automated by means of the user’s local configuration. Some points to remember:

  • the file is parsed from top to bottom
  • the first match is used

so don’t keep duplicated entries!

If you plan to connect to the same server using different users, this can easily be done using different names for the Host entry.

Every server or service is designated by the Host - all lines between Host entries belong to the preceding Host.

$ nano ~/.ssh/config

Add your server details - examples

Host nickname
  Hostname server.domain.tld
  IdentityFile ~/.ssh/ssh-service-ed25519.ppk
  user root
  Port 33000
Host fido.domain.tld
  IdentityFile ~/.ssh/fido.ppk
  user fido
ssh nickname

:information_source: Note on host key fingerprint

Upon first connection to a - previously unknown - ssh service you will be challenged before connection - something like

$ ssh user@hostname
The authenticity of host 'hostname (v.x.y.z)' can't be established.
ED25519 key fingerprint is SHA256:P4QBIqLt6g6JU5P3po0WRLF+mr0ypYhhG3iGgCprM20.
This host key is known by the following other names/addresses:
    ~/.ssh/known_hosts:15: v.x.y.z
Are you sure you want to continue connecting (yes/no/[fingerprint])?

The ssh client will always try to match the remote system to a list of known hosts found in ~/.ssh/known_hosts and if no match is found the client challenge you to trust the host. If you accept the challenge - the host key fingerprint will be added to the list.

If you - for any reason want to skip this check you can do so by adding an option to your connection command

ssh -o StrictHostKeyChecking=no

If you are wearing a blackhat and have a host which regularly changes IP or if you only connect to your own systems you could opt to completely ignore the fingerprints by adding it to your local ~/.ssh/config

Host *
  StrictHostKeyChecking no

:information_source: A note on passwordless keyfiles

When you have a lot of key-pairs (using no password) - you may run into the message - too many failed logins and subsequently - on a Manjaro system - the user may be locked.

You can create a workaround for this by disabling identity agent for the host or you can specify the the identity file on the command line when you connect to the host.

This can be done for all or for a single host

Host nickname
  IdentityAgent none
...

Or using a wildcard

Host *
  IdentityAgent none
...

4. Secure your key-pairs

You don’t want to loose your key collection.

IMPORTANT: Keep a copy of your ~/.ssh folder in a secure location and if you are using password less keys - keep them safe by:

  • using a strong password on your system
  • root account can always bypass permissions so
    • either disabe the root account
      or use a strong password that is not your user password
  • it is mandatory to ensure the files is only readable by your user and they need not being executable
    chown $USER:$USER .ssh -R
    chmod u-x,go-rwx ~/.ssh/*
    

5. Firewall

Next step is to ensure only allowed services are accessible by implementing a firewall.

Firewalld

Firewalld a very good firewall service - well suited on systemd based systems like Manjaro. Firewalld can be configured using the term application since an application is merely a definition of which ports should be allowed - e.g. a http application or ssh or smtp.

When you configure the firewall you use zones to define where you are and services to define what you allow. Install firewalld:

# pacman -Syu firewalld

Then enable and start the service

sudo systemctl enable --now firewalld

When firewalld is enabled and started the default zone is public which allows the computer to be visible but all ports closed.

Adding a specific service (application) is done using the command line but you can also use the applet found in your system configuration area. A systray application is available if you install the dependencies for it (python-pyqt5). Adding services has immediate effect - no need to reload the service. Simply add the service to the allowed service to the desired zone

Example - adding http to public zone

# firewall-cmd --zone=public --add-service=http
success

It is important to realize that changes you make on the fly are not permanent. To make a certain service available permanently, add the --permanent argument

# firewall-cmd --permanent --zone=public --add-service=http
success

What if you want to add your own service definition? Continuing with ssh - you want to run your ssh server on a non default port.

Browse the folder /usr/lib/firewalld/services and make a copy of an appropriate service definition.

Copy the ssh.xml service definition to /etc/firewalld/services

# cp /usr/lib/firewalld/services/ssh.xml /etc/firewalld/services/my-ssh.xml

Edit the service definition

# nano /etc/firewalld/services/my-ssh.xml

Change the port to match your service and the short name to distinguish from the original service.

<?xml version="1.0" encoding="utf-8"?>
<service>
  <short>My SSH service</short>
  <description>Secure Shell (SSH) is a protocol for logging into and executing commands on remote machines. It provides secure encrypted communications. If you plan on accessing your machine remotely via SSH over a firewalled interface, enable this option. You need the openssh-server package installed for this option to be useful.</description>
  <port protocol="tcp" port="30000"/>
</service>

Wait 5-10 seconds for the service file to be recognized and activate it (Same rule on permanent applies)

# firewall-cmd --zone=public --add-service=my-ssh --permanent
success

That’s it - more info on firewalld can be found using the dedicated webpage

17 Likes

No I didn’t - all configs - at least mine - goes into ~/.ssh/config

I got the error:

❯ chown $USER:$USER .ssh -R && chmod 600 ~/.ssh -R
chmod: cannot access '/home/zesko/.ssh/ed25519': Permission denied
chmod: cannot access '/home/zesko/.ssh/ed25519.pub': Permission denied
chmod: cannot access '/home/zesko/.ssh/config': Permission denied

Fix:

That should be chown $USER:$USER ~/.ssh -R && chmod 600 ~/.ssh/* -R

3 Likes

Thank you for making me aware of this typo :slight_smile:

1 Like