SSH service Linux

ssh.txt

* start the SSH service with systemctl

sudo systemctl enable ssh
sudo systemctl start ssh
sudo systemctl restart sshd

* Allow Or Deny SSH Access To A Particular User Or Group In Linux
sudo nano /etc/ssh/sshd_config

* Add or edit the following line:
AllowUsers myUserName

* You can also specify more than one user as shown below.
AllowUsers myUserName otherUser

* To allow an entire group, say for example root, add/edit the following line:
AllowGroups root

* Deny SSH Access to a user or group
DenyUsers myUserName
DenyGroups root

* Root ssh access is considered a bad practice in terms of security.
* Find the following line, Uncomment it, and set the value to no.
PermitRootLogin no


* Regenerate SSH keys
sudo rm /etc/ssh/ssh_host*
sudo ssh-keygen -A

* Regenerate with custom SSH key (ed25519 is 256bits)
sudo ssh-keygen -b 4096 -h -f /etc/ssh/ssh_host_rsa_key
sudo ssh-keygen -t ed25519 -h -f /etc/ssh/ssh_host_ed25519_key

+ Note
  Host keys needs to be copied to /etc/ssh, 
  permissions changed to 644 for public key, 
  600 for the private key, 
  and owner changed to root:root.