That dreaded moment when your SSH client establishes a TCP connection but fails during the identification exchange phase. The error message "ssh_exchange_identification: read: Connection reset by peer" typically occurs after the TCP three-way handshake completes but before SSH protocol negotiation begins.

From my sysadmin experience, these are the most frequent causes:

# 1. SSH server's MaxStartups limit reached
grep MaxStartups /etc/ssh/sshd_config

# 2. TCP wrappers (/etc/hosts.deny) blocking
grep sshd: /etc/hosts.deny

# 3. Resource exhaustion on server
ss -ant | grep 443 | wc -l

# 4. Network-level interference (AWS Security Groups example)
aws ec2 describe-security-groups --group-ids sg-xyz123

Before randomly tweaking configurations, try these targeted diagnostics:

# Check if SSH daemon is actually listening
netstat -tulnp | grep sshd
ss -ltn | grep 443

# Verify connection at raw TCP level
nc -zv xxx.xxx.xxx 443
telnet xxx.xxx.xxx 443

# Inspect firewall logs (RHEL example)
journalctl -u firewalld --since "1 hour ago" | grep ssh

When I encounter this in production environments, these adjustments typically resolve it:

# Increase MaxStartups with rate limiting
echo "MaxStartups 30:10:60" >> /etc/ssh/sshd_config

# Adjust TCP keepalive settings
echo "ClientAliveInterval 30" >> /etc/ssh/sshd_config
echo "TCPKeepAlive yes" >> /etc/ssh/sshd_config

# For AWS environments, ensure security group allows both inbound/outbound
aws ec2 authorize-security-group-ingress \
    --group-id sg-xyz123 \
    --protocol tcp \
    --port 443 \
    --cidr 0.0.0.0/0

If standard fixes don't work, try these advanced troubleshooting steps:

# Packet capture during connection attempt
tcpdump -i eth0 -nn port 443 -w ssh_debug.pcap

# Run SSH in debug mode on server
/usr/sbin/sshd -d -p 443

# Check for SELinux interference
ausearch -m avc -ts recent | grep ssh
setenforce 0 # temporary disable for testing

Implement these monitoring solutions to catch issues proactively:

# Zabbix item prototype for SSH connection tracking
UserParameter=sshd.connections[*],netstat -ant | grep ':443 ' | grep -v LISTEN | wc -l

# Nagios check for MaxStartups threshold
define command {
    command_name check_sshd_startups
    command_line /usr/lib/nagios/plugins/check_ssh -p 443 -H $HOSTADDRESS$ -t 30 -w 20 -c 25
}

# Common SSH troubleshooting commands (run on client side)
ssh -vvv user@hostname  # Verbose output for diagnostics
telnet hostname 22      # Basic port connectivity test
nc -zv hostname 22      # Alternative network check

This specific error occurs during the initial SSH protocol exchange, before authentication begins. The client establishes a TCP connection but gets abruptly terminated during version string exchange.

# Server-side configuration to check
cat /etc/ssh/sshd_config | grep -i "MaxStartups"
cat /etc/hosts.allow
cat /etc/hosts.deny

When using non-standard ports (like 443 in your case), additional factors come into play:

# Check if the port is actually listening
netstat -tulnp | grep 443
ss -tulnp | grep 443  # Modern alternative

# Firewall rules verification
iptables -L -n -v | grep 443
firewall-cmd --list-all  # For firewalld systems

Many servers implement connection rate limiting. Check these settings:

# Example of adjusting connection limits
sudo sed -i 's/#MaxStartups 10:30:100/MaxStartups 20:50:200/' /etc/ssh/sshd_config
sudo systemctl restart sshd

# Temporary workaround for testing
sudo iptables -A INPUT -p tcp --dport 443 -m connlimit --connlimit-above 5 -j REJECT

For persistent cases, packet capture reveals the truth:

# Server-side capture
tcpdump -i eth0 'port 443' -w ssh_debug.pcap

# Client-side capture
tcpdump -i any 'host xxx.xxx.xxx and port 443' -vvv

Sometimes the issue stems from specific auth methods:

# Force password auth for testing
ssh -o PreferredAuthentications=password -o PubkeyAuthentication=no user@host

# Or force public key auth
ssh -o PreferredAuthentications=publickey -o PasswordAuthentication=no user@host

Complete SSH subsystem reinstallation often resolves obscure issues:

# For Debian/Ubuntu
sudo apt purge openssh-server && sudo apt install openssh-server

# For RHEL/CentOS
sudo yum remove openssh-server && sudo yum install openssh-server