How to fix ssh_exchange_identification: Connection closed by remote host in SSH

Run your SSH command with maximum verbosity to see exactly where the connection is failing:

ssh -vvv user@hostname

Look for output like:
- 'debug1: Local version string: OpenSSH_X.X'
- 'debug1: Remote protocol version X.X, remote software version OpenSSH_X.X'
- 'debug1: match: OpenSSH_X.X'

If you don't see 'Remote protocol version', the server is closing before SSH negotiation completes.

Log into the server (or ask your administrator) and examine the TCP wrapper rules:

cat /etc/hosts.deny
cat /etc/hosts.allow

If you see lines like 'sshd: ALL' in hosts.deny or specific IP restrictions in hosts.allow, your IP may be blocked. Ask your administrator to add your IP to hosts.allow:

# Add to /etc/hosts.allow
sshd: YOUR_IP_ADDRESS
sshd: 192.168.1.0/24

If you have server access, check the Fail2ban status:

sudo fail2ban-client status sshd

To view banned IPs:

sudo fail2ban-client set sshd banip

To unban your IP:

sudo fail2ban-client set sshd unbanip YOUR_IP_ADDRESS

If this is a repeated issue, ask your administrator to adjust Fail2ban sensitivity or whitelist your IP range in /etc/fail2ban/jail.local.

On the remote server, verify the SSH daemon is running:

sudo systemctl status sshd

Restart the SSH service to clear potential stuck connections:

sudo systemctl restart sshd

Examine SSH logs for clues:

# Debian/Ubuntu
sudo tail -f /var/log/auth.log

# RHEL/CentOS/Fedora
sudo tail -f /var/log/secure

# Look for messages like:
# sshd[xxxx]: Connection closed by authenticating user
# sshd[xxxx]: fatal: setgroups: Cannot allocate memory
# sshd[xxxx]: Connection reset by peer

Examine /etc/ssh/sshd_config for overly restrictive settings:

sudo grep -E 'MaxStartups|MaxSessions|LoginGraceTime|UseDNS' /etc/ssh/sshd_config

If MaxStartups is too low (default is 10:30:100), increase it:

# Edit /etc/ssh/sshd_config
MaxStartups 30:30:100

Reload SSH configuration:

sudo systemctl reload sshd

If DNS lookups are slow or failing, they can cause sshd to drop the connection. Add this to /etc/ssh/sshd_config:

UseDNS no

Then reload SSH:

sudo systemctl reload sshd

The server might be out of memory or system resources. Check:

free -m          # Check available memory
df -h            # Check disk space
top -bn1 | head  # Check CPU and memory usage

If the server is low on resources, try to free up space or memory. Then restart the SSH daemon:

sudo systemctl restart sshd

Corrupted host keys can cause connection drops. Check if they exist:

ls -la /etc/ssh/ssh_host_*

If they're missing or corrupted, regenerate them:

# Debian/Ubuntu
sudo dpkg-reconfigure openssh-server

# RHEL/CentOS/Fedora
sudo ssh-keygen -A

Then restart SSH:

sudo systemctl restart sshd