How to Use nftables SYN Proxy

SYN proxy in nftables protects servers from SYN flood attacks by completing TCP handshakes before forwarding connections to the real server. This guide walks through the specific troubleshooting steps to diagnose and resolve SYN proxy issues.

Before You Begin

Before you begin, be sure to have the following in place:

• A Linux server with the relevant software installed
• Access to the command line interface
• Appropriate permissions (root or sudo)

Quick Fix

Wrong

No SYN flood protection (server vulnerable to SYN floods)

Wrong: No SYN flood protection

Right

nft add chain ip raw prerouting { type filter hook prerouting priority -300; }\nnft add rule ip raw prerouting tcp dport 80 tcp flags & (syn|rst|ack) == syn notrack\nnft add rule ip filter input tcp dport 80 ct state invalid,untracked synproxy mss 1460 wscale 7 timestamp

Right: SYN proxy to mitigate SYN floods

Output

SYN proxy configured on port 80\n  Notrack: SYN packets bypass connection tracking\n  MSS: 1460, WScale: 7\n  Client must complete TCP handshake with proxy before reaching backend

Prevention

To avoid future issues, follow these best practices:

• Use synproxy in the filter input chain for protected services
• The synproxy statement requires notrack in the raw table for initial SYN packets
• SYN proxy completes the TCP handshake before forwarding to the real server
• Configure MSS, wscale, and timestamp options to match backend
• SYN proxy is transparent to legitimate clients

DodaTech Tools

For further assistance with any of the above issues, consider using DodaTech consulting services or DodaTech tutorials for more in-depth guidance.

Common Mistakes with synproxy

1. Using foldl instead of foldl' causing stack overflow on large lists
2. Forgetting deriving (Show, Eq) on custom data types needed for debugging
3. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable

These mistakes appear frequently in real-world NFTABLES code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ