Lion Cipher Lab

Write and tune your own Snort rules (without drowning in false positives)

• SOC Analyst / Blue Team
• Intermediate
• ~75 min
• Free · no account required

Why it matters

Anybody can install an IDS. The job — the actual, paid, day-in-day-out job of a detection engineer or a SOC analyst — is writing rules that catch the thing without burying the team, and then keeping them alive as the network changes underneath them. That second half is where most home labs stop and where most real deployments die: a sensor that fires ten thousand alerts a day gets muted, and a muted sensor is worse than no sensor because it lets everyone believe they are covered. This lab gives you the whole loop on traffic you generate yourself: write it, prove it fires, measure how often it…

What you'll learn

• reading a Snort 3 rule field by field: action, protocol, addresses, ports, direction operator
• writing the options block: msg, sid, rev, content, nocase, offset/depth, flow, classtype, priority, reference
• choosing a local SID so your rules never collide with the Talos community set
• capturing your own traffic to a pcap and using it as a repeatable rule test harness
• rewriting a naive signature into a narrow, anchored, cheap one
• understanding the fast pattern matcher and why content is cheap and PCRE is expensive
• measuring false positives instead of guessing at them
• tuning with suppress and event_filter rather than deleting detection
• judgment: when a signature is the right answer and when behavioural or log-based detection is

Steps in this lab

1. Where you are, and what you are about to build
2. Anatomy, part 1 — the rule header (everything before the parenthesis)
3. Anatomy, part 2 — the options block, the 'general' half
4. Anatomy, part 3 — the options block, the half that actually matches
5. SIDs: why your rules start at 1,000,000 and never collide
6. Build a workspace and a rules file Snort will actually load
7. Generate traffic you own, and capture it to a pcap
8. Write your first rule — the naive version — and watch it fire
9. WHY content works the way it does — the fast pattern matcher
10. The naive rule vs the pro rule — same detection, rewritten
11. Change a rule, bump the rev — and see the discipline in practice
12. Measure your false positives instead of arguing about them
13. Tune the volume: event_filter (limit, threshold, both)
14. Tune by source: suppress, the scalpel
15. The third instrument: detection_filter, and choosing between the three
16. Break your own rule with one keystroke — the limit of signatures
17. WHEN a custom rule is the wrong answer
18. Clean up, and the judgment you are taking with you

Open the full interactive lab →