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/config/config.go

@@ -239,6 +239,14 @@ type Config struct {
 	// unless RulesEnabled is false (SWWAF_RULES_ENABLED).
 	RulesDir     string
 	RulesEnabled bool
+	// TrapPaths are the paths a request for which is a clear sign of
+	// attack (SWWAF_TRAP_PATHS), each starting with / and without a ?.
+	TrapPaths []string
+	// ErrorBurstThreshold is the most requests of a client within a minute
+	// that smallwebwaf may refuse after a rule file match or for a missing
+	// or wrong token; one more breaks a limit
+	// (SWWAF_ERROR_BURST_THRESHOLD). 0 is off.
+	ErrorBurstThreshold int64
 	// LogRemoteURL is where every line on stdout is also sent
 	// (SWWAF_LOG_REMOTE_URL), nil while it is unset and nothing is sent.
 	// LogRemoteTLSCAs are the certificates a syslog+tls endpoint's
@@ -378,6 +386,8 @@ var (
 	errNotBytesCount = errors.New("is not response, request or both")
 	errNotPathPrefix = errors.New(
 		"is not a path prefix starting with /, such as /assets/")
+	errNotTrapPath = errors.New(
+		"is not a path starting with / and without a ?, such as /wp-login.php")
 	errNotBoolean      = errors.New("is not true or false")
 	errNotLogRemoteURL = errors.New(
 		"is not syslog+udp, syslog+tcp or syslog+tls with a host and a port, " +
@@ -505,6 +515,8 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		MetricsTopN:          env.numberNotOff("SWWAF_METRICS_TOP_N", "50"),
 		RulesDir:             env.value("SWWAF_RULES_DIR", "/etc/smallwebwaf/rules.d"),
 		RulesEnabled:         env.boolean("SWWAF_RULES_ENABLED", "true"),
+		TrapPaths:            env.trapPaths("SWWAF_TRAP_PATHS"),
+		ErrorBurstThreshold:  env.count("SWWAF_ERROR_BURST_THRESHOLD", "30"),
 		LogRemoteURL:         env.logRemoteURL("SWWAF_LOG_REMOTE_URL"),
 		LogRemoteTLSCAs:      env.certificates("SWWAF_LOG_REMOTE_TLS_CA_FILE"),
 		LogRemoteBuffer:      env.numberNotOff("SWWAF_LOG_REMOTE_BUFFER", "10000"),
@@ -744,6 +756,15 @@ func (e *environment) pathPrefixes(name, defaultValue string) []string {
 	return prefixes
 }
 
+// trapPaths reads the setting that is the list of trap paths. It is empty
+// by default.
+func (e *environment) trapPaths(name string) []string {
+	paths, err := parseTrapPaths(e.value(name, ""))
+	e.check(name, err)
+
+	return paths
+}
+
 // countries reads a setting that is a list of countries.
 func (e *environment) countries(name, defaultValue string) []string {
 	countries, err := parseCountries(e.value(name, defaultValue))
@@ -1536,6 +1557,24 @@ func parsePathPrefixes(value string) ([]string, error) {
 	return prefixes, nil
 }
 
+// parseTrapPaths reads a comma-separated list of trap paths. Each is
+// matched against a request's path as a path rule is, without the query,
+// so a path that does not start with / or holds a ? would never match.
+func parseTrapPaths(value string) ([]string, error) {
+	paths, err := parseList(value)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, path := range paths {
+		if !strings.HasPrefix(path, "/") || strings.Contains(path, "?") {
+			return nil, fmt.Errorf("%q %w", path, errNotTrapPath)
+		}
+	}
+
+	return paths, nil
+}
+
 // countryCodes are the two-letter codes ISO 3166-1 assigns today, and XK,
 // the code in common use for Kosovo. golang.org/x/text/language cannot
 // check them: it also takes withdrawn codes such as su, and reserved ones