AWS SSH Connection Failure: Analysis and Solutions for 'No Supported Authentication Methods Available' Error

Problem Description and Background

When connecting to AWS EC2 instances using PuTTY or FileZilla, users frequently encounter the following error message:

Disconnected: No supported authentication methods available (server sent: publickey)

This error indicates that the SSH client cannot provide an authentication method accepted by the server. While the error message is relatively generic, specific configuration issues are often the root cause in AWS environments.

Core Issue Analysis: File Permission Configuration

According to high-scoring answers from the Stack Overflow community, improper file system permissions are one of the most common causes of this error. The SSH service has strict security requirements for permissions on related files and directories.

Correct permission configuration example:

/home/ec2-user/ - 700
/home/ec2-user/.ssh/ - 700
/home/ec2-user/.ssh/authorized_keys - 600

Let's verify and fix permission issues using a Python script:

import os
import stat

def check_ssh_permissions(home_dir):
    """Check permissions for SSH-related directories and files"""
    ssh_dir = os.path.join(home_dir, '.ssh')
    auth_keys = os.path.join(ssh_dir, 'authorized_keys')
    
    # Check directory permissions
    home_perms = oct(os.stat(home_dir).st_mode)[-3:]
    ssh_perms = oct(os.stat(ssh_dir).st_mode)[-3:]
    
    # Check file permissions
    if os.path.exists(auth_keys):
        key_perms = oct(os.stat(auth_keys).st_mode)[-3:]
    else:
        key_perms = 'File does not exist'
    
    return {
        'home_directory': home_perms,
        'ssh_directory': ssh_perms,
        'authorized_keys': key_perms
    }

def fix_permissions(home_dir):
    """Fix SSH-related permissions"""
    ssh_dir = os.path.join(home_dir, '.ssh')
    auth_keys = os.path.join(ssh_dir, 'authorized_keys')
    
    try:
        # Set home directory permissions to 700
        os.chmod(home_dir, stat.S_IRWXU)
        
        # Set .ssh directory permissions to 700
        if os.path.exists(ssh_dir):
            os.chmod(ssh_dir, stat.S_IRWXU)
        
        # Set authorized_keys permissions to 600
        if os.path.exists(auth_keys):
            os.chmod(auth_keys, stat.S_IRUSR | stat.S_IWUSR)
            
        return "Permissions fixed successfully"
    except Exception as e:
        return f"Permission fix failed: {str(e)}"

Other Common Causes and Solutions

Username Mismatch

Different Amazon Machine Images (AMIs) use different default usernames:

• Amazon Linux AMI: ec2-user
• Ubuntu AMI: ubuntu or root
• CentOS AMI: centos
• RHEL AMI: ec2-user or root

Key Format Issues

PuTTY does not support AWS native PEM format keys and requires conversion using PuTTYgen:

# Correct PuTTYgen usage process:
# 1. Open PuTTYgen
# 2. Select "Conversions" -> "Import key"
# 3. Load PEM file
# 4. Directly click "Save private key" (do not click Generate)
# 5. Save as PPK format

User Home Directory Changes

When using the usermod -d command to change a user's home directory, ensure the new home directory and its .ssh subdirectory have correct permission settings.

System Log Analysis

Detailed error information can be obtained by examining the /var/log/secure log file:

# View SSH authentication logs
tail -f /var/log/secure | grep sshd

Typical permission error logs may display:

Authentication refused: bad ownership or modes for directory /home/ec2-user/.ssh

Troubleshooting Process

1. Verify instance IP address is correct (EC2 restart may change IP)
2. Check if username matches AMI type
3. Confirm PuTTY is using correct PPK format key
4. Validate file system permission configuration
5. Check /var/log/secure for detailed error information
6. Update PuTTY to latest version if necessary

Emergency Recovery Solutions

If completely unable to connect, refer to AWS official key pair recovery tutorial videos to regain access through the EC2 instance console.

Conclusion

AWS SSH connection failures typically stem from configuration issues rather than network problems. Through systematic permission checks, key format verification, and log analysis, most issues can be quickly identified and resolved. Maintaining strict permission management and proper tool configuration is key to ensuring stable SSH connections.