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42. Evading IDS, Firewalls and Honeypots – Detection Games

42.5 Firewall Types – What "Evasion" Means to Blue

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Types you must recite:

Type Idea Typical miss
Packet filter Per-packet IP/port (classic iptables filter, simple SG) No state; no app; fragmented/ack tricks on old gear
Stateful Tracks TCP/UDP "this is a reply" (firewalld/nftables conntrack) UDP "state" weak; policy too wide (any/any)
WAF HTTP/S app rules (OWASP CRS / ModSecurity class) Bypass via alternate encoding if parser off; still need code fix
NGFW App-ID, user-ID, SSL decrypt (policy), IPS blade Decrypt gaps; "allow the vendor" too broad – SUNBURST-class lesson

Red "firewall evasion" in exam-language often means: policy hole, wrong direction (egress open), approved app abused, not a magic packet. Blue translation: review allow-lists, log denies, default-deny inbound and think about outbound.

# Amazon Linux lab – see what is actually allowed
sudo firewall-cmd --list-all
sudo firewall-cmd --get-log-denied
sudo firewall-cmd --set-log-denied=all
# Example rich rule: log + accept SSH only from Kali host-only (lab)
sudo firewall-cmd --permanent --add-rich-rule='rule family="ipv4" source address="192.168.56.10" port port="22" protocol="tcp" log prefix="SSH-KALI " level="info" accept'
sudo firewall-cmd --reload
sudo firewall-cmd --list-all
# Watch kernel log (Amazon Linux /var/log/messages; Ubuntu often /var/log/kern.log)
sudo tail -n 50 /var/log/messages

Ubuntu ufw note: sudo ufw status verbose then sudo service ufw status. Don't mix ufw + firewalld on one brain-dead VM without a plan.

Red team (attacker) does Blue team (defender) detects / stops
Uses an allowed outbound 443 path for C2 Egress proxy / allow-list destinations; DNS + TLS logs
Finds forgotten public zone port (3306 on internet) Regular --list-all; default-deny; SG = 22 from admin IP only
Walks WAF with odd encodings Enable WAF parsers; still patch the app (Chapter 41 XSS lesson)

Ravindra Bagale's Tip

Students leave port 22 open to 0.0.0.0/0 in the security group and call themselves "firewalld experts". The cloud SG is the first firewall. Interview: show --list-all and an AWS SG screenshot for the same lab. If this isn't clear, read it again.

Lab

On OWN Amazon Linux 192.168.56.40: sudo firewall-cmd --list-all before/after adding the SSH-from-Kali rich rule. From Kali, ssh (or nmap -p 22) – confirm log prefix SSH-KALI. From a different lab IP if you have one, confirm deny log. Snapshot.

Extended teaching notes

Why this lesson matters

Hardening is deleting unnecessary risk: fewer packages, fewer open ports, fewer privileged accounts, faster patches, better logs.

Hardening related services

[('SSH', 'Key auth, no root login, Fail2ban or equivalent, SG limited to your IP/VPN.', '', ''), ('SMB / NFS / LDAP / SNMP', 'Do not expose to the internet; patch; strong auth; disable unused versions (e.g. SMBv1).', '', ''), ('SMTP', 'Authenticated relay only; SPF/DKIM/DMARC for your domains.', '', ''), ('DNS', 'Restrict zone transfers; monitor for unexpected records (crt.sh for certs).', '', ''), ('Firewalls', 'Host firewall + cloud SG/NSG; default deny; document every allow.', '', '')]

Real examples

Global: CIS Benchmarks exist for Linux, Windows and clouds. Indian company (sample): a Pune team requires SSH keys only and weekly patch windows for internet-facing hosts.

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.