{
  "id": "networking/tcp-syn-cookies-enabled",
  "signature": "TCP: SYN cookies enabled due to high connection rate on port 443, dropping connections",
  "signature_zh": "TCP：由于端口443上的高连接速率，已启用SYN Cookie，正在丢弃连接",
  "regex": "SYN cookies enabled|possible SYN flooding|tcp_syncookies",
  "domain": "networking",
  "category": "network_error",
  "subcategory": null,
  "root_cause": "The kernel detected a high rate of incoming SYN packets on a port, triggering SYN cookies as a defense against SYN flood attacks, which can cause legitimate connections to be dropped if the cookie validation fails.",
  "root_cause_type": "generic",
  "root_cause_zh": "内核检测到端口上高传入SYN数据包速率，触发SYN Cookie作为防御SYN洪水攻击的措施，如果Cookie验证失败，可能导致合法连接被丢弃。",
  "versions": [
    {
      "version": "Linux 5.15+",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Linux 6.1+",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Ubuntu 22.04",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    },
    {
      "version": "Debian 12",
      "introduced": null,
      "deprecated": null,
      "removed": null,
      "behavior_change": null,
      "status": "active"
    }
  ],
  "os_specific": {},
  "dead_ends": [
    {
      "action": "",
      "why_fails": "This leaves the server vulnerable to SYN flood attacks and may cause resource exhaustion under high traffic.",
      "fail_rate": 0.7,
      "condition": "",
      "sources": []
    },
    {
      "action": "",
      "why_fails": "A high backlog does not prevent SYN cookies from being enabled when the SYN rate exceeds the threshold.",
      "fail_rate": 0.85,
      "condition": "",
      "sources": []
    }
  ],
  "workarounds": [
    {
      "action": "Increase the SYN backlog and reduce the threshold for SYN cookies: sysctl -w net.ipv4.tcp_max_syn_backlog=8192 && sysctl -w net.ipv4.tcp_synack_retries=2",
      "success_rate": 0.85,
      "how": "Increase the SYN backlog and reduce the threshold for SYN cookies: sysctl -w net.ipv4.tcp_max_syn_backlog=8192 && sysctl -w net.ipv4.tcp_synack_retries=2",
      "condition": "",
      "sources": []
    },
    {
      "action": "Use a load balancer or reverse proxy (e.g., Nginx) to distribute incoming connections and reduce the SYN rate on the backend server",
      "success_rate": 0.9,
      "how": "Use a load balancer or reverse proxy (e.g., Nginx) to distribute incoming connections and reduce the SYN rate on the backend server",
      "condition": "",
      "sources": []
    }
  ],
  "workarounds_zh": [
    "Increase the SYN backlog and reduce the threshold for SYN cookies: sysctl -w net.ipv4.tcp_max_syn_backlog=8192 && sysctl -w net.ipv4.tcp_synack_retries=2",
    "Use a load balancer or reverse proxy (e.g., Nginx) to distribute incoming connections and reduce the SYN rate on the backend server"
  ],
  "transition_graph": {
    "leads_to": [],
    "preceded_by": [],
    "frequently_confused_with": []
  },
  "official_doc_url": "https://www.kernel.org/doc/html/latest/networking/ip-sysctl.html",
  "official_doc_section": null,
  "error_code": null,
  "verification_tier": "ai_generated",
  "confidence": 0.86,
  "fix_success_rate": 0.85,
  "resolvable": "true",
  "first_seen": "2024-01-20",
  "last_confirmed": "2024-06-01",
  "last_updated": "2024-06-01",
  "evidence_count": 1,
  "tags": [],
  "locale": "en",
  "aliases": []
}