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Under the Hood: How Gcore Battled a 6 Tbps DDoS Monster

When a multi-vector botnet spiked past 6 terabits per second, Gcore’s global edge had minutes—not hours—to keep enterprise customers online across the US, EU, UK, AU, and India. Here’s the engineering playbook: Anycast to absorb, BGP to reroute, L7 behavioral filters to separate human from bot, and real-time threat intel to cut off the botnet’s oxygen.

Why trust CyberDudeBivash?

• Executive-first reporting that translates packet floods into business risk, SLA exposure, and revenue impact.
• Defense guidance aligned to CISA (US), ENISA/NIS2 (EU), NCSC (UK), ACSC (AU), and CERT-In (IN).
• Hands-on mitigation checklists for CTOs, CISOs, SRE/DevOps, and Cloud Architects.

Executive Summary 

• What happened: A coordinated, multi-vector DDoS peaked above 6 Tbps, mixing L3/L4 floods (UDP amplification, TCP SYN/ACK) with L7 HTTP/HTTP2 bursts aimed at API and CDN edges.
• Why it worked (temporarily): Carpet-bombing across many prefixes, HTTP/2 rapid reset/stream abuse, and geo-distributed bot nodes to evade simple geo-blocks.
• How it was contained: Anycast load-spread, automated BGP blackholing/specifics, ACL offload at NIC/ASIC, scrubbing centers, real-time behavioral and challenge filters, and cache-warming for static content.
• Outcome for customers: Core sites and APIs stayed reachable; some regional rate-limits/throttles applied during peak windows.

Attack Anatomy

• L3/L4 Volumetric: DNS/CLDAP/NTP amplifications; UDP reflection with spoofed sources; TCP SYN + ACK floods to exhaust state tables.
• L7 App-Layer: HTTP(S)/HTTP2 bursts with low-bytes, high-connection churn, randomized headers, and JA3/TLS fingerprint rotation.
• Tactics: Carpet-bombing (spreading attacks over many /24s), pulse waves to destabilize autoscaling, and mixed-vector switching every 3–5 minutes.

Minute-by-Minute Playbook

1. 00:00–00:02 – Spike Detection: Anycast telemetry flags asymmetrical edges; NetFlow/sFlow anomalies; SYN backlog alerts.
2. 00:02–00:05 – Volumetric Controls: Upstream RTBH for dirty sources, BGP more specifics to steer hot prefixes into scrubbing POPs, NIC/ASIC ACL for stateless drops (bogons, known amplifiers).
3. 00:05–00:10 – L7 Filtering: Progressive rate-limit + token-bucket, HTTP/2 stream cap, JA3/TLS fingerprint deny/score, JavaScript challenges and managed bot rules for high-risk paths.
4. 00:10–00:20 – Stabilization: Cache-warm static assets; reroute APIs to healthiest regions; enable read-only modes for noisy endpoints.
5. 00:20+ – Containment: Threat intel pushes /24 and ASN blocks; ISP partners rate-limit known reflectors; watch for botnet re-formation.

How the Edge Actually Absorbed 6 Tbps

• Anycast Everywhere: Same service IP advertised from many POPs. Floods are deflected into the whole mesh, not a single DC.
• Tiered Scrubbing: Hardware ACLs drop junk first, then FPGA/ASIC mitigation, then software L7 engines for the tricky stuff.
• Programmable BGP: Automation injects more-specific routes to drag hotspots into overprovisioned scrubbing capacity; RTBH for sacrificial hosts.
• Behavioral L7: Per-signal scoring (header entropy, cookie behavior, TLS/JA3, solver success, path entropy) → graduated challenges and blocklists.

What This Means for Your Team

US/EU/UK/AU/IN enterprises must assume terabit-scale DDoS is a cost of doing business. Your resilience comes from architecture, not just one vendor.

Customer-Side Hardening

• Put APIs behind Anycast + WAF/CDN; pin to providers with proven multi-Tbps scrubbing.
• Adopt “aggressive caching” for static; separate API subdomains from marketing sites.
• Define L7 surge playbooks: temporary captchas, token buckets, per-IP/ASN ceilings, and queue pages for non-critical paths.
• Autoscaling guardrails: cap max replicas and protect databases with circuit breakers to avoid self-DDoS.
• ISP coordination: Pre-agree RTBH and BGP communities; test with your transit partners quarterly.

KPIs the C-Suite Should Watch

• Time-to-Mitigate (TTM): detection → stable filtering < 10 minutes.
• Residual Error Rate: % of good requests challenged/blocked during peak (aim < 2%).
• Regional Availability: SLO by market (US/EU/UK/AU/IN) and by product (web vs. API).
• Cost Containment: egress, autoscale, and database CPU spend during attack windows.

Compliance & Framework Mapping

• US (CISA/NIST CSF): PR.PT-5 (DDOS protection), DE.AE-2 (anomalies), RS.MI-1 (mitigation).
• EU (NIS2/ENISA): Essential entities: network resilience, incident handling, provider oversight.
• UK (NCSC CAF): D2 protective technology, M2 anomalies & events.
• AU (ACSC): Essential Eight hardening of internet-facing services; ISP collaboration.
• India (CERT-In): 180-day log retention; material incident reporting; ISP coordination notes.

FAQ

Is 6 Tbps the new normal?

Terabit-scale is increasingly common due to cheap bandwidth and weaponized reflection. Design for multi-Tbps, not just Gbps.Can pure L7 floods take us down without big bandwidth?

Yes—connection churn and CPU-heavy requests can starve origin resources. Rate-limit and offload to edge compute/WAF.What single change pays off most?

Anycast fronting + strict L7 policies for API and app endpoints, with pre-baked surge modes you can toggle in minutes.

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Editor’s Picks — DDoS Resilience Stack

Anycast CDN + WAF
Multi-Tbps edge scrubbing for US/EU/UK/AU/INServer EDR with eBPF
Protect origin boxes during L7 floodsNetFlow/PCAP Observability
Real-time anomaly & ASN attributionSASE / ZTNA
Lock down admin surfaces and origins

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