When setting up Monit's web interface, many administrators encounter the frustrating ERR_EMPTY_RESPONSE error. This typically indicates that while the port might be open, Monit isn't properly listening or serving content on that port due to configuration or firewall issues.

The provided configuration shows several potential points of failure:

set httpd port 2812 and
  use address mywebsite.com
  allow localhost
  allow admin:password

The iptables rules appear correct at first glance:

#monit interface
-A OUTPUT -p tcp --dport 2812 -j ACCEPT
-A INPUT -p tcp --dport 2812 -j ACCEPT

1. Binding Issues: The 'use address' directive must match the server's actual IP or hostname that's resolvable from your network

2. Access Control: The 'allow' statements should include both localhost AND any external IPs that need access

3. Firewall Chains: Iptables rules might need to account for specific network interfaces

First, verify Monit is actually running:

sudo monit status
ps aux | grep monit

An improved configuration would be:

set httpd port 2812
    use address 0.0.0.0  # bind to all interfaces
    allow 0.0.0.0/0.0.0.0 # temporary wide open
    allow admin:password
    with ssl {           # enable SSL for security
        pemfile: /etc/ssl/certs/monit.pem
    }

More comprehensive firewall rules should include:

# Allow established connections
-A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT

# Monit specific rules
-A INPUT -p tcp -m tcp --dport 2812 -m conntrack --ctstate NEW -j ACCEPT
-A OUTPUT -p tcp -m tcp --sport 2812 -m conntrack --ctstate ESTABLISHED -j ACCEPT

1. Check if Monit is listening:

sudo netstat -tulnp | grep 2812
sudo ss -tulnp | grep monit

2. Test local connection first:

curl -v http://localhost:2812

3. Verify firewall rules are properly loaded:

sudo iptables -L -n -v

If issues persist, check:

# Monit logs
tail -f /var/log/monit.log

# System logs for errors
journalctl -u monit -f

# SELinux context (if applicable)
semanage port -l | grep 2812

---

When configuring Monit's web interface, many administrators encounter the ERR_EMPTY_RESPONSE error despite seemingly correct firewall rules. This typically stems from multiple configuration layers that need alignment.

First, confirm Monit is actually running and bound to the correct interface:

sudo netstat -tulnp | grep 2812
tcp6       0      0 :::2812                 :::*                    LISTEN      1234/monit

If you don't see Monit listening, the service might not be running properly. Check its status:

sudo systemctl status monit

The iptables rules shown in the original post only handle TCP traffic. Modern systems often use IPv6 by default. Try these enhanced rules:

# IPv4 rules
-A INPUT -p tcp --dport 2812 -j ACCEPT
# IPv6 equivalent
-A INPUT -p tcp6 --dport 2812 -j ACCEPT

The original configuration has several potential issues. Here's an improved version:

set httpd port 2812
    with SSL {
        pemfile: /etc/ssl/certs/monit.pem
    }
    use address 0.0.0.0
    allow @monit
    allow @users readonly
    allow admin:CHANGE_THIS_PASSWORD

Key improvements:

• Added SSL support (highly recommended)
• Bound to all interfaces (0.0.0.0)
• Implemented proper ACL groups

Use these commands to test connectivity:

# From localhost
curl -v http://localhost:2812

# From remote (replace with actual IP)
telnet mywebsite.com 2812
nc -zv mywebsite.com 2812

On RedHat-based systems, SELinux might block access. Check and modify policies:

sudo ausearch -m avc -ts recent | grep monit
sudo setsebool -P httpd_can_network_connect 1

For AWS/GCP environments where you might want to restrict access:

set httpd port 2812
    use address 127.0.0.1
    allow 127.0.0.1
    allow 10.0.0.0/8  # Internal network
    allow admin:securePassword

Then use SSH tunneling for secure access:

ssh -L 2812:localhost:2812 user@mywebsite.com

Enable verbose logging in Monit:

set logfile syslog facility log_daemon
set alert admin@mywebsite.com
set mail-format {
    subject: Monit Alert: $SERVICE $EVENT
    message: Monit $ACTION $SERVICE at $DATE: $DESCRIPTION
}

Check Monit logs after configuration changes:

sudo tail -f /var/log/monit.log
journalctl -u monit -f

1. Restart Monit: sudo systemctl restart monit
2. Reload firewall rules: sudo iptables-restore < /etc/iptables.rules
3. Test locally first, then remotely

Remember that changes might take a few moments to propagate. If all else fails, consider temporarily disabling the firewall (for testing only) to isolate the issue.