Possible SYN flooding on port 80. Sending cookies. Networking Origin: Network OSI layer 4 · Transport Last updated 5 Nov 2026

Networking: TCP: request_sock_TCP: Possible SYN flooding on port 80. Sending cookies. Check SNMP counters.

The server's pending-connection queue filled, so the kernel switched to SYN cookies to survive.

Exact message you may see Possible SYN flooding on port 80TCP: request_sock_TCP: Possible SYN floodingSending cookies. Check SNMP counters.
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Seen on: Linux Networking

Meaning

It can be a real SYN flood or just a traffic spike with a small backlog. Raise net.core.somaxconn and net.ipv4.tcp_max_syn_backlog, keep tcp_syncookies=1, rate limit with the firewall or a CDN/WAF, and check the application accepts connections quickly.

Common causes

  • Actual SYN flood attack
  • Traffic spike with small listen backlog
  • Slow accept() loop in the application
  • No CDN/rate limiting in front
  • Low tcp_max_syn_backlog

⚡ Quick fix

  1. Keep net.ipv4.tcp_syncookies=1
  2. Raise somaxconn and tcp_max_syn_backlog
  3. Rate limit or move behind a CDN/WAF

Detailed fix by platform

Linux

  1. bash
    sysctl net.ipv4.tcp_syncookies net.ipv4.tcp_max_syn_backlog net.core.somaxconn
netstat -s | grep -i "SYNs to LISTEN"

How to diagnose

  1. Source — Attack or spike?
  2. Backlog — Large enough?
  3. SYN cookies — On?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

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