How to fix Connection to hostname closed by remote host. in SSH

Use SSH verbose flags to see detailed connection logs:

ssh -vvv user@hostname

The three -v flags produce maximum verbosity. Look for where the output stops—this shows exactly when the server closes the connection. Common indicators:
- "Server version" = TCP accepted, negotiation started
- "Offering public key" = SSH handshake started but server rejected auth
- Nothing after key offer = server closed after seeing key offer

SSH server logs reveal the exact reason for closure. If you can access the server another way (physical console, different SSH account, cloud console), check:

# Debian/Ubuntu
sudo tail -50 /var/log/auth.log | grep sshd

# RHEL/CentOS/Fedora
sudo tail -50 /var/log/secure | grep sshd

# Modern systemd systems
sudo journalctl -u ssh -n 50
sudo journalctl -u sshd -n 50

Look for messages like:
- "Too many authentication failures"
- "MaxSessions exceeded"
- "Invalid user"
- "Received disconnect"
- "Fatal: no more authentication methods available"

If the server banned you due to too many failed attempts, it may have a temporary ban. Wait 5-10 minutes (default fail2ban unban time) or restart the SSH daemon:

If you have access via another method:

# Restart SSH daemon (choose one based on your system)
sudo systemctl restart ssh      # Ubuntu/Debian
sudo systemctl restart sshd     # RHEL/CentOS/Fedora
sudo service ssh restart        # Older Debian/Ubuntu
sudo service sshd restart       # Older RHEL/CentOS

Then try connecting again from a different IP if possible.

Ensure you're using the correct username for the system and supported authentication method:

# Try with explicit username
ssh -u ubuntu@hostname              # Ubuntu AMIs use 'ubuntu'
ssh -u ec2-user@hostname            # Amazon Linux uses 'ec2-user'
ssh -u root@hostname                # Some images use 'root'
ssh -u centos@hostname              # CentOS uses 'centos'

# Try with explicit key
ssh -i ~/.ssh/key.pem user@hostname

# Try with password (if key auth fails)
ssh -o PubkeyAuthentication=no user@hostname

If none work, the account may be disabled or the authentication method is rejected by server configuration.

Verify the SSH daemon is running and listening on the expected port (default 22):

If you have access to the server:

# Check if sshd is running
sudo systemctl status ssh     # or sshd on RHEL

# Check what ports SSH is listening on
sudo ss -tlnp | grep ssh
sudo netstat -tlnp | grep ssh

Expected output should show sshd listening on 0.0.0.0:22 or [::]:22

If not running, start it:

sudo systemctl start ssh      # Ubuntu/Debian
sudo systemctl start sshd     # RHEL/CentOS

Check if the server configuration restricts your access:

# Review key SSH settings that might block you
sudo grep -E "^(AllowUsers|DenyUsers|PermitRootLogin|MaxAuthTries|MaxSessions)" /etc/ssh/sshd_config

Common issues:
- AllowUsers user1 user2 — Your username not in list
- DenyUsers user — Your username explicitly blocked
- PermitRootLogin no — Root login disabled but you're trying as root
- MaxAuthTries 2 — Very low limit, you've exceeded it

Fix by editing /etc/ssh/sshd_config with proper permissions, then restart SSH:

sudo nano /etc/ssh/sshd_config
sudo systemctl restart ssh

If the server is out of resources, SSH may close the connection:

Check from server console or if you can access another way:

# Disk space
df -h

# Memory
free -h

# Check for OOM (Out of Memory) errors
sudo dmesg | tail -20

If disk is full (100%), you cannot log in via SSH. Free space:

# Find large files
du -sh /*
rm -rf /tmp/*  # Remove temp files

If memory is exhausted, restart the system or kill memory-hungry processes.

If you suspect Fail2Ban or firewall blocking, try from a different network:

# From a different computer or through a VPN
ssh -v user@hostname

# Or use a web-based console if available (AWS/Azure/GCP)
# This bypasses network restrictions

If the connection works from another IP, you're likely banned by Fail2Ban or firewall rules. Access the server via console and check:

# Check Fail2Ban status
sudo fail2ban-client status sshd
sudo fail2ban-client set sshd unbanip YOUR_IP