Leave SWWAF_RATE_LIMIT_EXEMPT_PATHS out of the request rate limits (closes #77)

A request is neither counted nor refused by the request rate limits
when its path as sent, the path the app receives, not percent-decoded,
starts with one of the comma-separated prefixes in
SWWAF_RATE_LIMIT_EXEMPT_PATHS, so /%61ssets/x is not under /assets/. A
request whose decoded path contains .. or a backslash, or whose path as
sent holds an encoded slash, is never exempt, since an app may act on
it as a path outside every prefix, such as /assets/..%2Flogin as
/login. The static lists, bans and the country lists still apply, and
its log line has no counts. The setting is empty by default, and a
prefix that does not start with / stops the start. README.md documents
it.

Model: opus-5-5

internal/config/config.go

@@ -80,6 +80,10 @@ type Config struct {
 	RateLimitPerMinute int64
 	RateLimitPerHour   int64
 	RateLimitPerDay    int64
+	// RateLimitExemptPaths are the path prefixes whose requests the rate
+	// limits neither count nor refuse (SWWAF_RATE_LIMIT_EXEMPT_PATHS).
+	// Each starts with /.
+	RateLimitExemptPaths []string
 	// DeniedCountries are the countries whose clients are refused
 	// (SWWAF_DENIED_COUNTRIES). ExclusivelyAllowedCountries, when not
 	// empty, are the only countries whose clients are let through
@@ -179,8 +183,10 @@ var (
 		"is not the length of an IPv4 netblock, from 0 to 32, such as 24")
 	errNotAbsolutePath = errors.New(
 		"is not an absolute path, such as /var/lib/smallwebwaf")
-	errShortToken = errors.New("is shorter than 32 characters")
-	errNotMode    = errors.New("is not enforce or observe")
+	errShortToken    = errors.New("is shorter than 32 characters")
+	errNotMode       = errors.New("is not enforce or observe")
+	errNotPathPrefix = errors.New(
+		"is not a path prefix starting with /, such as /assets/")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -210,6 +216,7 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		RateLimitPerMinute:      env.count("SWWAF_RATE_LIMIT_PER_MINUTE", "1000"),
 		RateLimitPerHour:        env.count("SWWAF_RATE_LIMIT_PER_HOUR", "10000"),
 		RateLimitPerDay:         env.count("SWWAF_RATE_LIMIT_PER_DAY", "50000"),
+		RateLimitExemptPaths:    env.pathPrefixes("SWWAF_RATE_LIMIT_EXEMPT_PATHS", ""),
 		DeniedCountries:         env.countries("SWWAF_DENIED_COUNTRIES", ""),
 		ExclusivelyAllowedCountries: env.countries(
 			"SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES", ""),
@@ -350,6 +357,14 @@ func (e *environment) count(name, defaultValue string) int64 {
 	return count
 }
 
+// pathPrefixes reads a setting that is a list of path prefixes.
+func (e *environment) pathPrefixes(name, defaultValue string) []string {
+	prefixes, err := parsePathPrefixes(e.value(name, defaultValue))
+	e.check(name, err)
+
+	return prefixes
+}
+
 // 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))
@@ -637,6 +652,23 @@ func parseNetblock(value string) (netip.Prefix, error) {
 	return netip.PrefixFrom(addr, addr.BitLen()), nil
 }
 
+// parsePathPrefixes reads a comma-separated list of path prefixes, each
+// starting with /.
+func parsePathPrefixes(value string) ([]string, error) {
+	prefixes, err := parseList(value)
+	if err != nil {
+		return nil, err
+	}
+
+	for _, prefix := range prefixes {
+		if !strings.HasPrefix(prefix, "/") {
+			return nil, fmt.Errorf("%q %w", prefix, errNotPathPrefix)
+		}
+	}
+
+	return prefixes, 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

internal/config/config_test.go

@@ -35,6 +35,7 @@ const (
 	rateLimitPerMinute      = "SWWAF_RATE_LIMIT_PER_MINUTE"
 	rateLimitPerHour        = "SWWAF_RATE_LIMIT_PER_HOUR"
 	rateLimitPerDay         = "SWWAF_RATE_LIMIT_PER_DAY"
+	rateLimitExemptPaths    = "SWWAF_RATE_LIMIT_EXEMPT_PATHS"
 	deniedCountries         = "SWWAF_DENIED_COUNTRIES"
 	allowedCountries        = "SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES"
 	banResponse             = "SWWAF_BAN_RESPONSE"
@@ -139,6 +140,10 @@ func TestDefaults(t *testing.T) {
 		t.Errorf("%s gave %v, want %v", logRequestHeaders, cfg.LogRequestHeaders,
 			wantHeaders)
 	}
+
+	if len(cfg.RateLimitExemptPaths) != 0 {
+		t.Errorf("%s gave %v, want none", rateLimitExemptPaths, cfg.RateLimitExemptPaths)
+	}
 }
 
 func TestValuesAsSet(t *testing.T) {
@@ -163,6 +168,7 @@ func TestValuesAsSet(t *testing.T) {
 		rateLimitPerMinute:      "60",
 		rateLimitPerHour:        "600",
 		rateLimitPerDay:         "6000",
+		rateLimitExemptPaths:    "/assets/, /favicon.ico",
 		deniedCountries:         "cn, RU,kp,Xk",
 		allowedCountries:        "de",
 		banResponse:             "429",
@@ -215,6 +221,24 @@ func TestValuesAsSet(t *testing.T) {
 	wantNetblocks(t, cfg.DenyNets, "198.51.100.0/24")
 	wantCountries(t, deniedCountries, cfg.DeniedCountries, "CN", "RU", "KP", "XK")
 	wantCountries(t, allowedCountries, cfg.ExclusivelyAllowedCountries, "DE")
+
+	if !slices.Equal(cfg.RateLimitExemptPaths, []string{"/assets/", "/favicon.ico"}) {
+		t.Errorf("%s gave %v, want /assets/ and /favicon.ico",
+			rateLimitExemptPaths, cfg.RateLimitExemptPaths)
+	}
+}
+
+func TestPathPrefixNotStartingWithSlashStopsTheStart(t *testing.T) {
+	t.Parallel()
+
+	_, err := config.FromEnvironment(
+		environment{rateLimitExemptPaths: "/favicon.ico,assets/"}.lookupEnv)
+
+	want := rateLimitExemptPaths + `: "assets/" is not a path prefix ` +
+		`starting with /, such as /assets/`
+	if err == nil || err.Error() != want {
+		t.Errorf("error %v, want %s", err, want)
+	}
 }
 
 func TestInstanceNameAndLoggedHeadersAsSet(t *testing.T) {
@@ -368,8 +392,8 @@ func TestInvalidValueStopsTheStart(t *testing.T) {
 		{rateLimitPerMinute, "1K"},
 		{rateLimitPerHour, "0"},
 		{rateLimitPerHour, "1.5"},
-		{rateLimitPerDay, "-1"},
-		{rateLimitPerDay, "lots"},
+		{rateLimitPerDay, "-1"}, {rateLimitPerDay, "lots"},
+		{rateLimitExemptPaths, "/assets/,,/static/"},
 		{deniedCountries, "nk"},
 		{deniedCountries, "kp,,ir"},
 		{deniedCountries, "prk"},
@@ -505,6 +529,7 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		rateLimitPerMinute:      "1000",
 		rateLimitPerHour:        "10000",
 		rateLimitPerDay:         "50000",
+		rateLimitExemptPaths:    "",
 		deniedCountries:         "",
 		allowedCountries:        "",
 		banResponse:             "403",