CrowdSec: Let the Bad IPs Work Against Themselves
CrowdSec isn’t a firewall. It doesn’t block traffic by default, and it won’t replace iptables or fail2ban. What it does — and does well — is watch logs, recognize bad behavior, and tell you what should be blocked. Then, if you want, it pulls in a global blocklist based on what others have seen too.
It’s collaborative defense, not threat intel as a service. No licensing headaches. No telemetry you can’t inspect. And it doesn’t care whether you’re running a web server, mail server, or just SSH — if something’s poking it the wrong way, CrowdSec sees it.
Where It Actually Helps
| Capability | How It’s Used in the Wild |
| Log parsing engine | Reads local logs from nginx, sshd, dovecot, postfix, you name it |
| Behavior detection | Applies scenarios (like brute force, scraping, port scanning) via YAML |
| Bouncer system | Lets you decide how to block (iptables, nftables, HTTP 403, etc.) |
| Community blocklist | Pulls threat data from thousands of other users — optional, not mandatory |
| Custom scenarios | Define what you consider hostile — very flexible |
| Metrics + alerting | Includes Prometheus-style exporter and alert APIs |
| Privacy-aware | All logs stay local; only anonymized decisions are shared |
| Multi-service setups | Supports dozens of log sources out of the box |
What You Need to Run It
CrowdSec works on most modern Linux distributions. You can treat it like a lightweight daemon that reads your logs and makes suggestions (or enforces them, if you let it).
– OS: Debian, Ubuntu, CentOS, Alpine, FreeBSD (limited)
– Requirements: Go runtime (precompiled binaries), access to syslog or local logs
– No kernel modules: It sits in userspace
– Bouncers: Optional add-ons (written in Go, Python, C, etc.) to block traffic based on CrowdSec signals
– Control: Everything is configured via .yaml and command-line
Setup: Getting It Working (Debian/Ubuntu Example)
- Install CrowdSec:
curl -s https://install.crowdsec.net | bash
- Enable and start the service:
systemctl enable crowdsec
systemctl start crowdsec
- Install a bouncer (e.g. firewall):
sudo apt install crowdsec-firewall-bouncer-iptables
- Check what’s being analyzed:
cscli metrics
cscli decisions list
- Join the community blocklist (optional):
cscli console enroll
Where People Actually Use It
– SSH servers open to the internet (brute force detection in seconds)
– Web servers under scraping, scanning, or bot load
– VPS providers needing lightweight, automated abuse handling
– Teams replacing fail2ban with something smarter
– Networks where signals from outside matter more than just local thresholds
It’s also a good fit for hybrid infra — where some stuff is on-prem and some in the cloud, but all of it faces random garbage traffic.
What It’s Good At (And What’s Not Perfect)
Why sysadmins keep it:
– Very little tuning needed out of the box
– Playbooks/scenarios are human-readable
– Doesn’t try to do too much — just blocks what’s misbehaving
– Community threat feed can be surprisingly effective
– CLI tools are actually pleasant to use
But heads up:
– Doesn’t replace a proper WAF or DDoS filter
– Needs decent log coverage — no logs, no detections
– IPv6 support is improving, but not flawless
– You must set up a bouncer — detection doesn’t equal blocking by default
– Advanced tuning can get YAML-heavy
Final Thought
CrowdSec feels like it was written by people who’ve been hit by real bots and just wanted a better way to deal with them. It doesn’t scream for attention, and it doesn’t pretend to be a silver bullet. But once it’s running, it keeps your logs quieter — and your firewall a bit smarter.
