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

A request whose path, as the client sent it and before the query
string, starts with one of the comma-separated prefixes in
SWWAF_RATE_LIMIT_EXEMPT_PATHS is neither counted nor refused by the
request rate limits; the static lists, bans and the country lists still
apply. The setting is empty by default, and a prefix that does not start
with / stops the start. README.md documents it.

Judgement call: plain prefix on the path as sent, as the issue rules, so
/assets/../login matches /assets/; traefik removes such dot segments by
default, but a request that reaches smallwebwaf uncleaned is matched as
sent.

Model: opus-5-5

internal/config/config.go

@@ -70,6 +70,10 @@ 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
@@ -161,7 +165,9 @@ 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")
+	errShortToken    = errors.New("is shorter than 32 characters")
+	errNotPathPrefix = errors.New(
+		"is not a path prefix starting with /, such as /assets/")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -188,6 +194,7 @@ 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", ""),
@@ -315,6 +322,14 @@ 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))
@@ -593,6 +608,23 @@ 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

@@ -33,6 +33,7 @@ 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"
@@ -118,6 +119,10 @@ func TestDefaults(t *testing.T) {
 	wantNetblocks(t, cfg.DenyNets)
 	wantCountries(t, deniedCountries, cfg.DeniedCountries)
 	wantCountries(t, allowedCountries, cfg.ExclusivelyAllowedCountries)
+
+	if len(cfg.RateLimitExemptPaths) != 0 {
+		t.Errorf("%s gave %v, want none", rateLimitExemptPaths, cfg.RateLimitExemptPaths)
+	}
 }
 
 func TestValuesAsSet(t *testing.T) {
@@ -141,6 +146,7 @@ func TestValuesAsSet(t *testing.T) {
 		rateLimitPerMinute:      "60",
 		rateLimitPerHour:        "600",
 		rateLimitPerDay:         "6000",
+		rateLimitExemptPaths:    "/assets/, /favicon.ico",
 		deniedCountries:         "cn, RU,kp,Xk",
 		allowedCountries:        "de",
 		banResponse:             "429",
@@ -192,6 +198,24 @@ 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 TestCodeOnBothCountryListsStopsTheStart(t *testing.T) {
@@ -334,6 +358,7 @@ func TestInvalidValueStopsTheStart(t *testing.T) {
 		{rateLimitPerHour, "1.5"},
 		{rateLimitPerDay, "-1"},
 		{rateLimitPerDay, "lots"},
+		{rateLimitExemptPaths, "/assets/,,/static/"},
 		{deniedCountries, "nk"},
 		{deniedCountries, "kp,,ir"},
 		{deniedCountries, "prk"},
@@ -441,6 +466,7 @@ func TestLogsEachSettingWithItsValue(t *testing.T) {
 		rateLimitPerMinute:      "1000",
 		rateLimitPerHour:        "10000",
 		rateLimitPerDay:         "50000",
+		rateLimitExemptPaths:    "",
 		deniedCountries:         "",
 		allowedCountries:        "",
 		banResponse:             "403",

internal/proxy/proxy_test.go

@@ -58,6 +58,7 @@ const (
 	denyNets                = "SWWAF_DENY_NETS"
 	rateLimitPerMinute      = "SWWAF_RATE_LIMIT_PER_MINUTE"
 	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"

internal/proxy/ratelimits_test.go

@@ -69,3 +69,35 @@ func TestRateLimitRefusesBeforeTheApp(t *testing.T) {
 		t.Errorf("the app was called %d times, want 4", calls.Load())
 	}
 }
+
+func TestRateLimitExemptPathsAreNeitherCountedNorRefused(t *testing.T) {
+	t.Parallel()
+
+	const denied = "192.0.2.50" // in SWWAF_DENY_NETS
+
+	s, _, _ := startWithClock(t, "", map[string]string{
+		rateLimitPerMinute:   "1",
+		rateLimitExemptPaths: "/assets/,/favicon.ico",
+		denyNets:             denied,
+	})
+
+	// With a limit of one request a minute, the requests for paths under a
+	// prefix are not counted, so client's first request for / is within
+	// the limit; and once client has reached it, they are not refused.
+	s.request(client, "/assets/app.js", http.StatusOK, requestlog.ActionForward)
+	s.request(client, "/favicon.ico?v=2", http.StatusOK, requestlog.ActionForward)
+	s.get(client, http.StatusOK, requestlog.ActionForward)
+
+	line := s.request(client, "/assets/app.js", http.StatusOK, requestlog.ActionForward)
+	if line.LimitHit != "" {
+		t.Errorf("log line has limit_hit %q, want none", line.LimitHit)
+	}
+
+	// A path outside every prefix is counted: /assets is not under
+	// /assets/, and breaks the limit.
+	s.request(client, "/assets", http.StatusForbidden, requestlog.ActionRateLimited)
+
+	// A ban and SWWAF_DENY_NETS still refuse a path under a prefix.
+	s.request(client, "/assets/app.js", http.StatusForbidden, requestlog.ActionBanned)
+	s.request(denied, "/assets/app.js", http.StatusForbidden, requestlog.ActionDenied)
+}

internal/proxy/request.go

@@ -8,6 +8,8 @@ import (
 	"net/http/httputil"
 	"net/netip"
 	"os"
+	"slices"
+	"strings"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -115,13 +117,16 @@ func (h *handler) newRequest(w http.ResponseWriter, r *http.Request) *request {
 // client either refuses is not looked up, and then the country lists; a
 // request any of them refuses is not counted for the rate limits. Then
 // come the rate limits, unless the client is in
-// SWWAF_RATE_LIMIT_EXEMPT_NETS, so that every other request is counted,
+// SWWAF_RATE_LIMIT_EXEMPT_NETS or the request's path, as the client sent
+// it and the log line shows it, starts with one of
+// SWWAF_RATE_LIMIT_EXEMPT_PATHS, so that every other request is counted,
 // one refused for its size too. Every refusal but the size limit's is
 // answered with SWWAF_BAN_RESPONSE. ctx is the request's own context.
 func (rq *request) check(ctx context.Context) *refusal {
 	cfg := rq.h.config
 	allowed := isInside(rq.client, cfg.AllowNets)
-	exempt := isInside(rq.client, cfg.RateLimitExemptNets)
+	exempt := isInside(rq.client, cfg.RateLimitExemptNets) ||
+		startsWithAny(rq.in.URL.EscapedPath(), cfg.RateLimitExemptPaths)
 	now := rq.h.now()
 
 	if !allowed && isInside(rq.client, cfg.DenyNets) {
@@ -152,6 +157,13 @@ func (rq *request) check(ctx context.Context) *refusal {
 	return nil
 }
 
+// startsWithAny reports whether path starts with one of prefixes.
+func startsWithAny(path string, prefixes []string) bool {
+	return slices.ContainsFunc(prefixes, func(prefix string) bool {
+		return strings.HasPrefix(path, prefix)
+	})
+}
+
 // forward passes the request to the app and the app's answer back. ctx
 // is the request's own context.
 func (rq *request) forward(ctx context.Context) {

internal/smallwebwaf/smallwebwaf_test.go

@@ -400,6 +400,7 @@ func wantStartingLine(t *testing.T, line map[string]any, appURL, dir string) {
 		"SWWAF_RATE_LIMIT_PER_MINUTE":           "1000",
 		"SWWAF_RATE_LIMIT_PER_HOUR":             "10000",
 		rateLimitPerDay:                         "50000",
+		"SWWAF_RATE_LIMIT_EXEMPT_PATHS":         "",
 		"SWWAF_DENIED_COUNTRIES":                "",
 		"SWWAF_EXCLUSIVELY_ALLOWED_COUNTRIES":   "",
 		"SWWAF_BAN_RESPONSE":                    "403",