Bouncers and the firewall bouncer

1. The role of a bouncer #

Enforcing decisions.

The principle #

A bouncer (remediation component) enforces LAPI decisions. It authenticates with a key generated by cscli. The engine and bouncers can be on different machines.

sudo cscli bouncers add mon-bouncer
sudo cscli bouncers list
sudo cscli bouncers delete mon-bouncer

The key is shown only once. Keep it in a secrets manager. Use one key per bouncer, so you can revoke just one.

Where to block? #

The full list is on the Hub. The Ingress Nginx bouncer is also deprecated.

Live or stream? #

In a real test with the Traefik plugin, a freshly banned IP kept passing in live mode while its “allowed” status stayed cached. In stream mode, it was blocked at the next sync.

The decisions API #

All bouncers use this API. An IP with no decision returns null; without a valid key, the response is 403.

curl -s -H "X-Api-Key: $KEY" "http://127.0.0.1:8080/v1/decisions?ip=203.0.113.50"
# [{"duration":"3h59m44s","origin":"crowdsec","scenario":"crowdsecurity/ssh-bf","scope":"Ip","type":"ban","value":"203.0.113.50"}]

curl -s -H "X-Api-Key: $KEY" "http://127.0.0.1:8080/v1/decisions/stream?startup=true"
# {"new": [...], "deleted": [...]}

2. The firewall bouncer #

Installation, configuration and checks.

Install #

sudo apt install crowdsec-firewall-bouncer-nftables
# ou : crowdsec-firewall-bouncer-iptables
sudo systemctl status crowdsec-firewall-bouncer
sudo cscli bouncers list

On the same machine as the LAPI, the package creates its key by itself. Otherwise, generate it and copy it into the configuration.

Configure #

The file is /etc/crowdsec/bouncers/crowdsec-firewall-bouncer.yaml. Put your settings in a .yaml.local file so they survive upgrades.

mode: nftables
update_frequency: 10s
log_mode: file
log_dir: /var/log/
log_level: info
api_url: http://127.0.0.1:8080/
api_key: YOUR_KEY
disable_ipv6: false
deny_action: DROP
deny_log: false
nftables:
  ipv4:
    enabled: true
    set-only: false
    table: crowdsec
    chain: crowdsec-chain
    priority: -10
  ipv6:
    enabled: true
    set-only: false
    table: crowdsec6
    chain: crowdsec6-chain
    priority: -10

Check #

sudo cscli decisions add --ip 203.0.113.50 --duration 10m
sudo nft list table ip crowdsec
sudo cscli metrics show bouncers
sudo cscli decisions delete --ip 203.0.113.50

table ip crowdsec {
	counter crowdsec-blacklists-cscli {
		packets 3 bytes 180
	}
	set crowdsec-blacklists-cscli {
		type ipv4_addr
		flags timeout
		elements = { 203.0.113.50 timeout 9m58s expires 9m50s }
	}
}

Do not ban yourself. Before testing, add your admin address to an allowlist and keep console access at your hosting provider.

Sources #

• CrowdSec : Remediation Components
• CrowdSec : Firewall

Commands and blocking verified on CrowdSec 1.8.2 and firewall bouncer 0.0.38 in nftables mode.