If your Ubuntu server is not allowing SSH connections, there could be several possible reasons. Below are common issues and their solutions:
Unable to SSH into Ubuntu server. What could be the possible causes?
Install SSH:
sudo apt-get install openssh-server
1. SSH Service Not Running
Check if SSH is installed and running
sudo systemctl status ssh
If it’s not running, start it:
sudo systemctl start ssh
Enable it to start on boot
sudo systemctl enable ssh
2. SSH Port is Blocked by Firewall
Check UFW (Uncomplicated Firewall).
sudo ufw status
If SSH is blocked, allow it:
sudo ufw allow ssh
Restart UFW:
sudo ufw reload
3. Incorrect SSH Configuration
Check SSH Configuration File
sudo nano /etc/ssh/sshd_config
Ensure these lines are correctly set:
Port 22 # Or a different port if you changed it
PermitRootLogin yes # Only if root login is needed
PasswordAuthentication yes # If using password-based authentication
Restart SSH service after changes:
sudo systemctl restart ssh
4. IP Blocking (Fail2Ban or Hosts Deny)
Check if Fail2Ban is blocking SSH
sudo fail2ban-client status sshd
If blocked, unban your IP:
sudo fail2ban-client set sshd unbanip <your-ip>
Check /etc/hosts.deny
cat /etc/hosts.deny
Remove any lines blocking SSH.
5. Network Issues
Check if SSH Port is Open
From another machine, run:
nc -zv <server-ip> 22
If port 22 is closed, check
Cloud firewall settings (e.g., AWS Security Groups, Azure NSG, etc.)
Router or NAT settings if accessing from outside.
6. Incorrect User Permissions
Ensure the user’s home directory and SSH keys have correct permissions
chmod 700 ~/.ssh
chmod 600 ~/.ssh/authorized_keys
7. Private Key Issues (For Key-Based Authentication)
If using key-based login, ensure:
The correct key is in ~/.ssh/authorized_keys on the server.
You are using the right private key:
ssh -i /path/to/private_key user@server-ip
The key format is correct (e.g., OpenSSH).
8. Disk Space Issues
If the server is out of space, SSH may fail.
df -h
If full, free up space:
sudo apt autoremove
sudo rm -rf /var/log/*
9. Wrong Host or IP
Ensure the server’s public or private IP hasn’t changed.
Run ip a on the server to check assigned IPs.
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