Ravindra BagaleCourses & study guides मराठी Track your progress

40. DoS and DDoS – Availability Attacks

40.5 Application-Layer DoS – Slow and Heavy Requests

By Last updated:

Application-layer DoS does not always need Tbps. Ideas defenders must know:

  • Many requests to expensive URLs (search, report export, login without cache)
  • Slowloris-class concepts: hold many connections open slowly so worker slots exhaust (Apache/nginx worker limits matter)
  • Large slow POST bodies / incomplete headers (timeouts help)

Blue fixes that actually help SMEs:

  • limit_req / limit_conn in nginx; Apache mod_evasive / reqtimeout modules (distro docs)
  • Short timeouts; sensible worker_connections / MaxRequestWorkers
  • WAF / bot management at CDN
  • Cache static + safe GETs at edge
# nginx snippet for YOUR lab origin – rate limit zone (example values; tune!)
limit_req_zone $binary_remote_addr zone=one:10m rate=5r/s;
server {
    listen 80;
    server_name orders.lab;
    location / {
        limit_req zone=one burst=10 nodelay;
        proxy_read_timeout 10s;
        # root /var/www/orders;  ...
    }
}

# Amazon Linux style – after editing nginx config on YOUR VM
sudo nginx -t
sudo service nginx reload
Red team (attacker) does Blue team (defender) detects / stops
Many concurrent slow connections to exhaust workers limit_conn; shorter timeouts; more workers behind LB carefully
HTTP flood on uncached dynamic page CDN/WAF; cache; rate limit; CAPTCHA on abuse
Targets login without lockout / rate limit Fail2ban-ish / WAF rules; MFA; progressive delays

Ravindra Bagale's Tip

Students set worker_processes auto but forget the rate limit. You need both size and limits. Interview: "application DoS is often cheap RPS that hurts origin CPU – I rate-limit and cache."

Lab

Disposable Ubuntu/Amazon Linux VM host-only: install nginx, serve a tiny PHP/index.html for Raja-Rani Traders order page. From Kali use ab or siege with low concurrency and short run against http://192.168.56.XX/ only. Note latency. Add limit_req. Re-run same load. Document before/after. Stop if host CPU pegs – this is a classroom, not a burn-in.

Extended teaching notes

Why this lesson matters

Availability attacks try to exhaust bandwidth, connections or app workers. Learning them is about detection and defence — never about stressing networks you do not own (that can be illegal and harmful).

  • Rate limits and WAF/CDN at the edge.
  • SYN cookies / tuning on servers.
  • Autoscaling with care (cost controls).
  • Keep IoT devices off default passwords (Mirai lesson).
  • Lab simulations only on isolated networks you own.

Mini case study

Suppose we are on a purple-team exercise for an Indian mid-size company (sample data). Red notes a technique from this lesson against a lab clone. Blue must show the control that stops it, the log that would have fired, and the ticket that hardens production. That is the standard this course trains you for.

Defender's worksheet (do this today)

  1. Write the lesson title, date and the exact lab target you are allowed to use.
  2. List three owned systems (VMs, apps, or cloud accounts) where this lesson applies.
  3. Run one safe check from the lesson on a lab system only. Paste the command and output.
  4. Write a two-sentence fix you would give a teammate.
  5. Name one related public advisory (CERT-In, CISA, NVD, or a vendor bulletin) with its date.

Indian workplace (sample data)

Suppose we are a junior analyst at Infosys, TCS, Wipro or a fintech SOC in Bengaluru/Pune. Turn this lesson into one checklist item with an owner, a due date and a place to store evidence (ticket + screenshot).

Global practice

Compare your notes with OWASP, CIS Benchmarks, NIST CSF/IR guidance or a cloud Well-Architected security question. Write five lines on what overlaps.

Everyday analogy

Explain the risk and the fix to a family member without jargon. If they can repeat the warning, you understand it well enough to teach juniors.

Key terms

  • Control: a safeguard (patch, config, process, detection).
  • Finding: a weakness backed by evidence.
  • Remediation: the fix plus how you verified it.
  • Scope: systems you have written permission to touch.
  • Least privilege: only the access the task needs.
  • Defence in depth: several layers so one failure does not end the story.

Common mistakes

  • Skipping written permission "just for a quick scan".
  • Practising on hostel, office or public targets without authorisation.
  • Collecting tool output without writing the fix.
  • Copying attack steps onto production to "verify". Defenders verify with versions, configs and patches.
  • Publishing screenshots that contain secrets, cookies or internal hostnames.

Recap

  • Evidence, fix, owner, date.
  • Own lab only.
  • Fix > spectacle.