How to Set Up SSH on Linux for Remote Access

1) Install OpenSSH Server

# Ubuntu / Debian
sudo apt update
sudo apt install -y openssh-server

# CentOS / AlmaLinux
sudo dnf install -y openssh-server
sudo systemctl enable --now sshd

Check status:

# Ubuntu: service name is "ssh"
sudo systemctl status ssh

# RHEL family: service name is "sshd"
sudo systemctl status sshd

2) Open the Firewall (Port 22/TCP by default)

# Ubuntu with UFW
sudo ufw allow OpenSSH
sudo ufw enable
sudo ufw status

# CentOS/AlmaLinux with firewalld
sudo firewall-cmd --permanent --add-service=ssh
sudo firewall-cmd --reload
sudo firewall-cmd --list-all

Tip: If you change the SSH port later (e.g., 2222), update the firewall rule accordingly.

3) Create an SSH Key (Client)

# on your local machine
ssh-keygen -t ed25519 -C "[email protected]"
# press Enter to accept the default path (~/.ssh/id_ed25519)
# set a passphrase for extra security

Copy your public key to the server:

ssh-copy-id user@server-ip

If ssh-copy-id is unavailable, paste the content of ~/.ssh/id_ed25519.pub into ~/.ssh/authorized_keys on the server (permissions: ~/.ssh = 700, authorized_keys = 600).

4) Connect Over SSH

# default port
ssh user@server-ip

# custom port
ssh -p 2222 user@server-ip

5) Hardening the SSH Server

Edit the SSH daemon config and apply best practices.

# Ubuntu
sudo nano /etc/ssh/sshd_config

# CentOS/AlmaLinux
sudo nano /etc/ssh/sshd_config

Recommended settings (uncomment or add):

Port 22                  # or change to a non-standard port like 2222
PermitRootLogin no       # disable direct root login
PasswordAuthentication no   # require keys (set after key login works)
PubkeyAuthentication yes
PermitEmptyPasswords no
ClientAliveInterval 300  # keep-alive
ClientAliveCountMax 2
MaxAuthTries 3

Reload the service:

# Ubuntu
sudo systemctl reload ssh

# CentOS/AlmaLinux
sudo systemctl reload sshd

6) Troubleshooting

• Service not listening? ss -tlnp | grep -E ':(22|2222) '
• Firewall blocked? Confirm rules using ufw status or firewall-cmd --list-all.
• Wrong permissions? SSH requires strict perms:

  chmod 700 ~/.ssh
  chmod 600 ~/.ssh/authorized_keys

• Check logs: sudo journalctl -u ssh -f (Ubuntu) or sudo journalctl -u sshd -f (RHEL family).

Bonus: SSH Config Shortcuts (Client)

Create ~/.ssh/config to save hosts and options:

Host myserver
  HostName 203.0.113.10
  User alex
  Port 2222
  IdentityFile ~/.ssh/id_ed25519
  ServerAliveInterval 60

Now connect with simply:

ssh myserver