The Core Rule Set, run by Coraza, on each request's method, URL and headers (closes #25)

Coraza v3.8.1 runs the Core Rule Set 4.25.0 (coraza-coreruleset v4.25.0)
after the rule files, with the six changes and the default
SWWAF_WAF_DISABLED_RULES that SPEC.md gives; no body, no response.
SWWAF_WAF_MODE, SWWAF_WAF_PARANOIA_LEVEL, SWWAF_WAF_ANOMALY_THRESHOLD and
SWWAF_WAF_EXEMPT_PATHS as specified; SWWAF_WAF_DISABLED_RULES refuses
900000 to 900999, smallwebwaf's own rules among them. A request with more
query parameters than Coraza reads, 1000, adds 5 (rule 900300). In block
mode a match is refused with 403, an offence counted toward the error
burst; in detect mode it is let through. Both log waf_rule_ids, waf_score
and duration_waf, raise waf_block, and count
smallwebwaf_waf_matches_total.

Judgement call: waf_block is raised in block mode too.
Deviation: no engine-error path; with no body read, Coraza cannot fail.

Model: opus-5-5

internal/proxy/proxy.go

@@ -21,6 +21,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/reputation"
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 	"sneak.berlin/go/smallwebwaf/internal/rules"
+	"sneak.berlin/go/smallwebwaf/internal/waf"
 )
 
 // How smallwebwaf keeps connections to the app open between requests.
@@ -75,9 +76,10 @@ type Params struct {
 	// Alerts receive the alert for each ban the proxy makes or makes
 	// permanent, for each count over an anomaly threshold, for each request
 	// whose client a blocklist, the CrowdSec decision list, a DNSBL zone or
-	// AbuseIPDB lists, and for GeoJS failing, a fetch of a list failing, a
-	// query to a DNSBL zone or a check with AbuseIPDB failing, or the day's
-	// AbuseIPDB checks used up.
+	// AbuseIPDB lists, for each request the Core Rule Set scores at or over
+	// SWWAF_WAF_ANOMALY_THRESHOLD, and for GeoJS failing, a fetch of a list
+	// failing, a query to a DNSBL zone or a check with AbuseIPDB failing,
+	// or the day's AbuseIPDB checks used up.
 	Alerts *alerts.Queue
 }
 
@@ -144,12 +146,13 @@ func New(params Params) *Server {
 			NamedNetblocks: params.Config.WatchNets,
 			Alerts:         params.Alerts,
 		}),
-		lookupFile: params.LookupFile,
-		lists:      lists,
-		dnsbl:      dnsbl,
-		abuseIPDB:  abuseIPDB,
-		rules:      params.Rules,
-		alerts:     params.Alerts,
+		lookupFile:  params.LookupFile,
+		lists:       lists,
+		dnsbl:       dnsbl,
+		abuseIPDB:   abuseIPDB,
+		rules:       params.Rules,
+		coreRuleSet: newCoreRuleSet(params.Config),
+		alerts:      params.Alerts,
 	}
 	h.geojs = lookup.New(lookup.Params{
 		URL:     params.GeoJSURL,
@@ -228,26 +231,48 @@ func newReputation(
 	return lists, dnsbl, abuseIPDB
 }
 
+// newCoreRuleSet returns the Core Rule Set at SWWAF_WAF_PARANOIA_LEVEL,
+// without the rules SWWAF_WAF_DISABLED_RULES switches off, or nil while
+// SWWAF_WAF_MODE is off.
+func newCoreRuleSet(cfg *config.Config) *waf.CoreRuleSet {
+	if cfg.WAFMode == config.WAFModeOff {
+		return nil
+	}
+
+	coreRuleSet, err := waf.New(waf.Params{
+		ParanoiaLevel: cfg.WAFParanoiaLevel, DisabledRules: cfg.WAFDisabledRules,
+	})
+	if err != nil {
+		// The Core Rule Set is built in, and the settings cannot break it:
+		// the paranoia level is from 1 to 4, and the id of no rule switches
+		// nothing off.
+		panic(err)
+	}
+
+	return coreRuleSet
+}
+
 // handler is the proxy. It holds what every request shares; what belongs
 // to one request is in a request.
 type handler struct {
-	config     *config.Config
-	requestLog io.Writer
-	processLog *slog.Logger
-	errorLog   *log.Logger
-	transport  http.RoundTripper
-	now        func() time.Time
-	metrics    *metrics.Metrics
-	limiter    *ratelimit.Limiter
-	ledger     *bans.Ledger
-	geojs      *lookup.GeoJS
-	anomalies  *anomaly.Counters
-	lookupFile *lookup.File
-	lists      *reputation.Lists
-	dnsbl      *reputation.DNSBL
-	abuseIPDB  *reputation.AbuseIPDB
-	rules      *rules.Files
-	alerts     *alerts.Queue
+	config      *config.Config
+	requestLog  io.Writer
+	processLog  *slog.Logger
+	errorLog    *log.Logger
+	transport   http.RoundTripper
+	now         func() time.Time
+	metrics     *metrics.Metrics
+	limiter     *ratelimit.Limiter
+	ledger      *bans.Ledger
+	geojs       *lookup.GeoJS
+	anomalies   *anomaly.Counters
+	lookupFile  *lookup.File
+	lists       *reputation.Lists
+	dnsbl       *reputation.DNSBL
+	abuseIPDB   *reputation.AbuseIPDB
+	rules       *rules.Files
+	coreRuleSet *waf.CoreRuleSet
+	alerts      *alerts.Queue
 }
 
 // newTransport returns what carries requests to the app. It never goes