Common Connectivity Problems and Solutions

Website accessibility issues often stem from overly restrictive firewall rules or misconfigured allow lists. Understanding typical scenarios helps identify problems quickly.

Legitimate Users Blocked: This common issue occurs when firewall rules are too aggressive or when legitimate traffic triggers security filters. Users might see connection timeouts, "Access Denied" messages, or experience intermittent connectivity:

# Diagnostic commands for blocked users
# Check if specific IP is blocked
iptables -L -n -v | grep "192.168.1.100"
fail2ban-client status | grep "Banned IP list"

# Check recent blocks in logs
tail -f /var/log/syslog | grep "DROPPED\|REJECTED"
tail -f /var/log/ufw.log | grep "BLOCK"

# Test connectivity from user's perspective
# Using curl with verbose output
curl -v https://example.com
curl -I https://example.com  # Headers only

# Check specific ports
nmap -p 80,443 example.com
nc -zv example.com 443

When legitimate users are blocked, implement temporary fixes while investigating:

#!/usr/bin/env python3
# Emergency unblock script
import sys
import subprocess
import logging

logging.basicConfig(level=logging.INFO, 
                   format='%(asctime)s - %(message)s')

def unblock_ip(ip_address, reason="Emergency unblock"):
    """Safely unblock an IP address across multiple systems"""
    
    unblock_commands = [
        # iptables
        f"iptables -D INPUT -s {ip_address} -j DROP",
        f"iptables -D INPUT -s {ip_address} -j REJECT",
        
        # fail2ban
        f"fail2ban-client set sshd unbanip {ip_address}",
        f"fail2ban-client set apache-auth unbanip {ip_address}",
        f"fail2ban-client set nginx-http-auth unbanip {ip_address}",
        
        # UFW
        f"ufw delete deny from {ip_address}",
        
        # ipset
        f"ipset del blacklist {ip_address}"
    ]
    
    results = []
    for cmd in unblock_commands:
        try:
            result = subprocess.run(cmd.split(), 
                                  capture_output=True, 
                                  text=True)
            if result.returncode == 0:
                results.append(f"Success: {cmd}")
                logging.info(f"Unblocked {ip_address} via: {cmd}")
        except Exception as e:
            results.append(f"Failed: {cmd} - {str(e)}")
    
    # Log the action
    with open("/var/log/emergency_unblock.log", "a") as f:
        f.write(f"{datetime.now()} - Unblocked {ip_address} - Reason: {reason}\n")
    
    return results

if __name__ == "__main__":
    if len(sys.argv) < 2:
        print("Usage: emergency_unblock.py <ip_address> [reason]")
        sys.exit(1)
    
    ip = sys.argv[1]
    reason = sys.argv[2] if len(sys.argv) > 2 else "Emergency unblock"
    
    results = unblock_ip(ip, reason)
    for result in results:
        print(result)

Geographic Restrictions Gone Wrong: Geographic blocking can inadvertently affect legitimate users using VPNs, traveling, or accessing from unexpected locations:

# Debug geographic restrictions
# Check GeoIP database accuracy
geoiplookup 8.8.8.8  # Should show US

# Test specific country blocks
# Create test script
cat > test_geo_block.sh << 'EOF'
#!/bin/bash
# Test access from different geographic locations

LOCATIONS=(
    "US:8.8.8.8"
    "UK:81.92.204.172"
    "JP:202.12.29.210"
    "AU:139.130.4.5"
)

for loc in "${LOCATIONS[@]}"; do
    IFS=':' read -r country ip <<< "$loc"
    echo "Testing from $country ($ip):"
    curl -H "X-Forwarded-For: $ip" -I https://example.com
    echo "---"
done
EOF

chmod +x test_geo_block.sh
./test_geo_block.sh