kex_exchange_identification: Connection reset by peer

A comprehensive guide to troubleshooting the SSH connection error "kex_exchange_identification: read: Connection reset by peer"

Problem Overview

The kex_exchange_identification: read: Connection reset by peer error occurs when an SSH client attempts to establish a connection with an SSH server, but the connection is abruptly terminated during the initial key exchange process. This error typically indicates that:

1. The SSH server process malfunctioned or crashed
2. A firewall or security mechanism is interfering with the connection
3. Network configuration issues are preventing proper communication

Common Causes and Solutions

1. Server-Side Issues

SSH Service Problems

The SSH daemon might be running but in an unstable state:

# Check SSH service status
sudo systemctl status sshd

# Restart SSH service (common fix)
sudo systemctl restart sshd

# For comprehensive cleanup, find and kill existing SSH processes
sudo netstat -ap | grep :22
sudo kill <PID_of_sshd_process>
sudo systemctl start sshd

WARNING

After OpenSSH upgrades, always restart the sshd service to ensure compatibility between client and server versions.

Configuration File Issues

Check server configuration files that might be blocking connections:

# Check hosts.allow and hosts.deny configurations
cat /etc/hosts.allow
cat /etc/hosts.deny

If /etc/hosts.deny contains sshd: ALL, it will block all SSH connections. Modify this file to allow your specific IP range or comment out the restrictive line:

# Comment out the restrictive line
sudo sed -i 's/ALL: ALL/#ALL: ALL/g' /etc/hosts.deny

File Permission Problems

SSH is strict about file permissions. Incorrect permissions can cause connection failures:

# Check and fix directory permissions (example for /var/empty/sshd)
sudo mkdir /var/empty/sshd
sudo chmod 0711 /var/empty/sshd

# Ensure SSH key permissions are correct
chmod 600 ~/.ssh/id_rsa
chmod 644 ~/.ssh/known_hosts

2. Network and Firewall Issues

Routing Table Problems

Incorrect routing can cause SSH connection issues:

# Check routing table
route -n

# Remove problematic routes (example)
sudo route del 192.168.50.0/24 via 192.168.50.1

Firewall and Security Groups

Check if firewalls or security groups are blocking the connection:

# Check iptables rules
sudo iptables -L

# Monitor connections with tcpdump
sudo tcpdump -i any dst destination_IP and src Source_IP and dst port 22

Ensure TCP forwarding is allowed in SSH configuration if using port forwarding:

# Edit sshd_config
sudo nano /etc/ssh/sshd_config

# Set AllowTcpForwarding to yes
AllowTcpForwarding yes

# Test configuration and restart service
sudo sshd -t
sudo systemctl restart sshd

3. Client-Side Issues

Multiple Network Interfaces

Having multiple active network interfaces with similar IP addresses can cause conflicts:

# Disable unnecessary network interfaces
nmcli conn down <interface-name>

DNS Resolution Problems

DNS issues can interfere with SSH connections:

# Change DNS servers to reliable alternatives (Google DNS)
# Edit /etc/resolv.conf or network manager settings
nameserver 8.8.8.8
nameserver 8.8.4.4

Proxy and Application Settings

Some applications (like VSCode) may use proxy settings that interfere with SSH:

• Check your system's proxy settings
• Try executing SSH commands in a standard terminal instead of IDE-integrated terminals
• Verify that forwarded ports are properly configured

4. Version Compatibility Issues

Mismatched SSH client and server versions can cause connection problems:

# Use verbose output to debug connection issues
ssh -vvv user@server

# Check versions on both client and server
ssh -V
ssh user@server "ssh -V"

Special Cases

GitLab CI/CD Pipelines

When encountering this error in GitLab pipelines, ensure proper SSH key handling:

before_script:
  - mkdir -p ~/.ssh
  - echo "$SSH_PRIVATE_KEY" | tr -d '\r' > ~/.ssh/id_rsa
  - chmod 600 ~/.ssh/id_rsa
  - eval "$(ssh-agent -s)"
  - ssh-add ~/.ssh/id_rsa
  - ssh-keyscan -H $SERVER_IP >> ~/.ssh/known_hosts

Regional Restrictions

In some regions, internet restrictions may block SSH connections:

• Check if you're in a region with internet restrictions
• Consider using VPN services if legitimate access is being blocked
• Verify with your network administrator about any filtering policies

Debugging Workflow

Follow this systematic approach to diagnose and resolve the issue:

Step-by-Step Debugging Process

1. Check server logs: /var/log/auth.log or /var/log/secure
2. Verify SSH service status on the server
3. Test basic connectivity: ping server_ip
4. Check firewall rules on both client and server
5. Examine SSH configuration files
6. Test with verbose output: ssh -vvv user@server
7. Verify file permissions for SSH directories and keys
8. Check for multiple active connections to the same server

Prevention Best Practices

• Regularly update SSH client and server software
• Maintain proper file permissions for SSH keys and directories
• Monitor server logs for authentication attempts
• Use consistent network configurations
• Implement proper firewall rules instead of relying on hosts.deny for security

When to Seek Further Help

If none of these solutions resolve the issue, consider:

• Consulting with your network administrator
• Checking for hardware or infrastructure problems
• Examining system resource constraints on the server
• Looking for unusual patterns in authentication logs that might indicate targeted blocking

This error, while frustrating, is typically resolvable through systematic investigation of server configurations, network settings, and security policies affecting SSH communications.