Trap paths, and the error burst banning a client refused too often (closes #115)

SWWAF_TRAP_PATHS: a request whose path, as a path rule sees it, is one
of them is a clear sign of attack, banned as a ban rule's match is; the
ban's notes give its trap_path. Checked after the rate limits, before the
rule files.

SWWAF_ERROR_BURST_THRESHOLD (default 30, or off): more refusals in a
minute after a block or ban rule or a trap path, or for a missing or
wrong token, ban the client as a broken limit does. Counted in
clients.json's minute_refusals; limit_hit error_burst, notes kind
refusals.

A token refusal is now the offence token_refused, and
smallwebwaf_offences_total counts every kind the history does.

Judgement call: the threshold is not lowered by a client's limit percentage.

Model: opus-5-5

internal/proxy/bans.go

@@ -1,6 +1,7 @@
 package proxy
 
 import (
+	"net/http"
 	"net/netip"
 	"time"
 
@@ -9,7 +10,6 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/ratelimit"
 	"sneak.berlin/go/smallwebwaf/internal/reputation"
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
-	"sneak.berlin/go/smallwebwaf/internal/rules"
 )
 
 // banResponse is a refusal answered with SWWAF_BAN_RESPONSE, and logged
@@ -83,6 +83,48 @@ func (rq *request) countBytes() {
 	}
 }
 
+// countRefusal counts the request for the error burst once it has been
+// answered, if smallwebwaf refused it after a rule file match or a trap
+// path, or for a missing or wrong token, and in observe mode if enforce
+// mode would have: more than SWWAF_ERROR_BURST_THRESHOLD such refusals of
+// the client within a minute break a limit. A client in SWWAF_ALLOW_NETS,
+// which the checks skip, is not counted, and nothing is while the
+// threshold is off.
+func (rq *request) countRefusal() {
+	cfg := rq.h.config
+	if cfg.ErrorBurstThreshold == 0 {
+		return
+	}
+
+	// In observe mode, a request that enforce mode would have refused
+	// before it reached the endpoint has had no token refused there.
+	tokenRefused := rq.tokenRefused && rq.line.WouldAction == "" &&
+		!isInside(rq.client, cfg.AllowNets)
+	if !rq.attack && !rq.ruleBlocked && !tokenRefused {
+		return
+	}
+
+	now := rq.h.now()
+
+	hit, over := rq.h.limiter.CountRefusal(rq.h.clientGroup(rq.client), now,
+		cfg.ErrorBurstThreshold)
+	if !over {
+		return
+	}
+
+	// What the client was sent, or in observe mode would have been.
+	status := rq.out.status
+
+	switch rq.line.WouldAction {
+	case requestlog.ActionRuleBlocked:
+		status = http.StatusForbidden
+	case requestlog.ActionBanned:
+		status = cfg.BanResponse
+	}
+
+	rq.banForLimit(now, hit, status)
+}
+
 // countedBytes returns the request's bytes, once it has ended, as the
 // byte limits and the anomaly thresholds count them: the response's body
 // bytes, the request's, or both, as SWWAF_BYTES_COUNT says. For an
@@ -107,20 +149,27 @@ func (rq *request) countedBytes() int64 {
 }
 
 // banForLimit bans the client's netblock at now for a broken limit, the
-// one hit names, and notes the offence for the log line. status is what
-// the client was sent, or is sent: SWWAF_BAN_RESPONSE for a request over
-// a rate limit, the app's answer for one whose bytes broke a byte limit.
-// The ban's notes give the client's limit percentage for that kind of
-// limit. The ban sets the client's counters back to zero. In observe mode
-// it makes no ban and sets nothing back, and raises the alert for the ban
-// it would have made, if that alert would be sent.
+// one hit names, notes the offence for the log line and counts the hit in
+// the metrics. status is what the client was sent, or is sent:
+// SWWAF_BAN_RESPONSE for a request over a rate limit, the app's answer for
+// one whose bytes broke a byte limit, the refusal for one that broke the
+// error burst. The ban's notes give the client's limit percentage for a
+// rate limit or a byte limit; the error burst is not lowered. The ban sets
+// the client's counters back to zero. In observe mode it makes no ban and
+// sets nothing back, and raises the alert for the ban it would have made,
+// if that alert would be sent.
 func (rq *request) banForLimit(now time.Time, hit ratelimit.Hit, status int) {
-	rq.line.LimitHit = hit.Window
-	if hit.Kind == ratelimit.KindBytes {
-		rq.line.LimitHit += "_bytes" // as counts names the byte totals
+	switch hit.Kind {
+	case ratelimit.KindBytes:
+		rq.line.LimitHit = hit.Window + "_bytes" // as counts names the byte totals
+	case ratelimit.KindRefusals:
+		rq.line.LimitHit = requestlog.LimitHitErrorBurst
+	default:
+		rq.line.LimitHit = hit.Window
 	}
 
 	rq.line.Offence = requestlog.OffenceLimit
+	rq.h.metrics.LimitHit(hit)
 
 	netblock := rq.h.netblock(rq.client)
 	if rq.h.config.Observe && !rq.wouldAlertBan(netblock, now, bans.CauseLimit) {
@@ -140,13 +189,13 @@ func (rq *request) banForLimit(now time.Time, hit ratelimit.Hit, status int) {
 		Requests:   rq.netblockRequests(netblock),
 	}
 
-	percent := rq.limitPercent
-	if hit.Kind == ratelimit.KindBytes {
-		percent = rq.bytesPercent
+	switch hit.Kind {
+	case ratelimit.KindRequests:
+		notes.LimitPercent, notes.LimitPercentSetting = rq.limitPercent.logged()
+	case ratelimit.KindBytes:
+		notes.LimitPercent, notes.LimitPercentSetting = rq.bytesPercent.logged()
 	}
 
-	notes.LimitPercent, notes.LimitPercentSetting = percent.logged()
-
 	if rq.h.config.Observe {
 		ban, wouldBan := rq.h.ledger.WouldBanForLimit(netblock, now, notes)
 		if wouldBan {
@@ -166,25 +215,22 @@ func (rq *request) banForLimit(now time.Time, hit ratelimit.Hit, status int) {
 }
 
 // banForAttack bans the client's netblock at now for a clear sign of
-// attack, the match of rule, a ban rule. In observe mode it makes no ban,
-// and raises the alert for the ban it would have made, if that alert
-// would be sent.
-func (rq *request) banForAttack(now time.Time, rule rules.Rule) {
+// attack, which notes name: the ban rule that matched, or the trap path
+// asked for. It fills in the rest of the notes. In observe mode it makes
+// no ban, and raises the alert for the ban it would have made, if that
+// alert would be sent.
+func (rq *request) banForAttack(now time.Time, notes bans.Notes) {
 	netblock := rq.h.netblock(rq.client)
 	if rq.h.config.Observe && !rq.wouldAlertBan(netblock, now, bans.CauseAttack) {
 		return
 	}
 
-	notes := bans.Notes{
-		ASN:        rq.line.ASN,
-		ASName:     rq.line.ASName,
-		Country:    rq.line.Country,
-		RuleID:     rule.ID,
-		Target:     rule.Target,
-		Reputation: rq.reputation,
-		Request:    rq.noted(now, rq.h.config.BanResponse),
-		Requests:   rq.netblockRequests(netblock),
-	}
+	notes.ASN = rq.line.ASN
+	notes.ASName = rq.line.ASName
+	notes.Country = rq.line.Country
+	notes.Reputation = rq.reputation
+	notes.Request = rq.noted(now, rq.h.config.BanResponse)
+	notes.Requests = rq.netblockRequests(netblock)
 
 	if rq.h.config.Observe {
 		ban, wouldBan := rq.h.ledger.WouldBanForAttack(netblock, now, notes)