Networking & Firewall (Ss/Netstat/Iptables/Nftables/Curl)

Detecting Unauthorized Listeners with NFTables Connection Tracking

🧩 The Challenge

You suspect a process is silently opening a network port that should be blocked, but traditional port scanners aren’t showing the ephemeral connection attempt.

💡 The Fix

Use nftables flow table logging to monitor and log packets that match specific stateful criteria before they hit your drop policies.

nft add rule ip filter input tcp dport 8080 log prefix "UNAUTHORIZED_LISTEN: " counter drop

⚙️ Why It Works

This command forces the kernel to log any packet hitting a specific port to the system message buffer before immediately discarding it, allowing you to trace the source IP even if the service is hidden.

🚀 Pro-Tip: Use dmesg -w to follow these log events in real-time as they are captured by the kernel firewall.

Linux Tips & Tricks | © ngelinux.com | 7/7/2026