DNS blocklists asked in the background, verdicts kept (closes #104)

Work in progress.

Model: opus-5-5

internal/config/config.go

@@ -152,6 +152,21 @@ type Config struct {
 	BlocklistRefresh      time.Duration
 	BlocklistAction       string
 	BlocklistLimitPercent int64
+	// DNSBLZones are the DNSBL zones clients are asked about
+	// (SWWAF_DNSBL_ZONES), through DNSBLResolver (SWWAF_DNSBL_RESOLVER), or
+	// the host's resolver while that is the zero AddrPort.
+	// ReputationAction is what is done with a client a zone's verdict lists
+	// (SWWAF_REPUTATION_ACTION): deny, limit or log; for limit,
+	// ReputationLimitPercent is the percentage of every limit it gets. A
+	// verdict is used for ReputationCacheTTL after it was fetched
+	// (SWWAF_REPUTATION_CACHE_TTL), and a query may take ReputationTimeout
+	// (SWWAF_REPUTATION_TIMEOUT). Neither can be off.
+	DNSBLZones             []string
+	DNSBLResolver          netip.AddrPort
+	ReputationAction       string
+	ReputationLimitPercent int64
+	ReputationCacheTTL     time.Duration
+	ReputationTimeout      time.Duration
 	// BanResponse is the status a refused client is answered with, 403
 	// or 429, or 0 to close the connection without an answer
 	// (SWWAF_BAN_RESPONSE). It answers a banned client, a request that
@@ -366,6 +381,11 @@ var (
 	errNotAnHourOrMore = errors.New("is not a duration of 1h or more, such as 24h")
 	errNotAction       = errors.New(
 		"is not deny, limit:<percent> such as limit:25, or log")
+	errNotZone     = errors.New("is not a DNS zone such as dnsbl.dronebl.org")
+	errZoneTooLong = errors.New("is longer than 189 characters, too long for the " +
+		"names IPv6 clients are asked about by")
+	errNotResolver = errors.New("is not an IP address with an optional port, " +
+		"such as 192.0.2.53 or [2001:db8::53]:5353")
 )
 
 // FromEnvironment reads the settings with lookupEnv, normally
@@ -418,6 +438,10 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		ASNLimitPercentURL:   env.listURL("SWWAF_ASN_LIMIT_PERCENT_URL"),
 		BlocklistURLs:        env.listURLs("SWWAF_BLOCKLIST_URLS"),
 		BlocklistRefresh:     env.refresh("SWWAF_BLOCKLIST_REFRESH", "24h"),
+		DNSBLZones:           env.zones("SWWAF_DNSBL_ZONES"),
+		DNSBLResolver:        env.resolver("SWWAF_DNSBL_RESOLVER"),
+		ReputationCacheTTL:   env.durationNotOff("SWWAF_REPUTATION_CACHE_TTL", "24h"),
+		ReputationTimeout:    env.durationNotOff("SWWAF_REPUTATION_TIMEOUT", "2s"),
 		BanResponse:          env.banResponse("SWWAF_BAN_RESPONSE", "403"),
 		LimitBanDuration:     env.durationNotOff("SWWAF_LIMIT_BAN_DURATION", "1h"),
 		LimitBanRepeatWindow: env.durationNotOff("SWWAF_LIMIT_BAN_REPEAT_WINDOW", "24h"),
@@ -462,6 +486,8 @@ func FromEnvironment(lookupEnv func(string) (string, bool)) (*Config, error) {
 		cfg.InstanceName, cfg.LogRemoteURL != nil)
 	cfg.BlocklistAction, cfg.BlocklistLimitPercent = env.action(
 		"SWWAF_BLOCKLIST_ACTION", "deny")
+	cfg.ReputationAction, cfg.ReputationLimitPercent = env.action(
+		"SWWAF_REPUTATION_ACTION", "limit:25")
 
 	env.checkInstanceNameForNtfy(cfg.InstanceName, cfg.AlertNtfyURL != nil)
 	env.checkLookupDBPath(cfg)
@@ -733,9 +759,9 @@ func (e *environment) refresh(name, defaultValue string) time.Duration {
 	return duration
 }
 
-// action reads a setting that is what is done with a client a list names:
-// deny, log, or limit:<percent>, which it returns as limit and the
-// percentage.
+// action reads a setting that is what is done with a client a blocklist
+// or a DNSBL zone lists: deny, log, or limit:<percent>, which it returns
+// as limit and the percentage.
 func (e *environment) action(name, defaultValue string) (string, int64) {
 	value := e.value(name, defaultValue)
 	if value == "deny" || value == "log" {
@@ -752,6 +778,24 @@ func (e *environment) action(name, defaultValue string) (string, int64) {
 	return "limit", percent
 }
 
+// zones reads the setting that is the list of DNSBL zones. It is empty by
+// default.
+func (e *environment) zones(name string) []string {
+	zones, err := parseZones(e.value(name, ""))
+	e.check(name, err)
+
+	return zones
+}
+
+// resolver reads the setting that is the resolver the DNSBL zones are
+// asked through, the zero AddrPort while it is unset or empty.
+func (e *environment) resolver(name string) netip.AddrPort {
+	resolver, err := parseResolver(e.value(name, ""))
+	e.check(name, err)
+
+	return resolver
+}
+
 // lookupSource reads the setting that is where clients are looked up:
 // geojs, file, or off.
 func (e *environment) lookupSource(name, defaultValue string) string {
@@ -1697,6 +1741,85 @@ func parseListURLs(value string) ([]string, error) {
 	return urls, nil
 }
 
+const (
+	// maxZoneLength is the most characters a DNSBL zone may have: 253, the
+	// most a DNS name may have, less the 64 that come before the zone in
+	// the name an IPv6 client is asked about by, its 32 hex digits each
+	// followed by a dot.
+	maxZoneLength = 189
+	// maxLabelLength is the most characters a label of a DNS name may have.
+	maxLabelLength = 63
+	// labelChars are the characters a label of a DNS zone may hold.
+	labelChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-"
+	// dnsPort is the port a resolver is asked on when SWWAF_DNSBL_RESOLVER
+	// gives none.
+	dnsPort = 53
+)
+
+// parseZones reads a comma-separated list of DNSBL zones, each a DNS name
+// such as dnsbl.dronebl.org: labels separated by dots, each of 1 to 63
+// letters, digits and hyphens, neither starting nor ending with a hyphen,
+// and at most maxZoneLength characters in all. Go's resolver takes any
+// other name for one that does not exist, so that the zone would list no
+// client. A zone listed twice is an error.
+func parseZones(value string) ([]string, error) {
+	zones, err := parseList(value)
+	if err != nil {
+		return nil, err
+	}
+
+	for i, zone := range zones {
+		switch {
+		case len(zone) > maxZoneLength:
+			return nil, fmt.Errorf("%q %w", zone, errZoneTooLong)
+		case !isZone(zone):
+			return nil, fmt.Errorf("%q %w", zone, errNotZone)
+		case slices.Contains(zones[:i], zone):
+			return nil, fmt.Errorf("%q %w", zone, errListedTwice)
+		}
+	}
+
+	return zones, nil
+}
+
+// isZone reports whether each label of zone is as parseZones takes it.
+func isZone(zone string) bool {
+	for label := range strings.SplitSeq(zone, ".") {
+		badChar := strings.ContainsFunc(label, func(char rune) bool {
+			return !strings.ContainsRune(labelChars, char)
+		})
+
+		if label == "" || len(label) > maxLabelLength || badChar ||
+			strings.HasPrefix(label, "-") || strings.HasSuffix(label, "-") {
+			return false
+		}
+	}
+
+	return true
+}
+
+// parseResolver reads the resolver the DNSBL zones are asked through: an
+// IP address with a port from 1 to 65535, such as 192.0.2.53:5353 or
+// [2001:db8::53]:5353, or without one, such as 192.0.2.53 or 2001:db8::53,
+// for port 53. An empty value is none, the zero AddrPort.
+func parseResolver(value string) (netip.AddrPort, error) {
+	if value == "" {
+		return netip.AddrPort{}, nil
+	}
+
+	resolver, err := netip.ParseAddrPort(value)
+	if err == nil && resolver.Port() != 0 {
+		return resolver, nil
+	}
+
+	addr, err := netip.ParseAddr(value)
+	if err != nil {
+		return netip.AddrPort{}, fmt.Errorf("%q %w", value, errNotResolver)
+	}
+
+	return netip.AddrPortFrom(addr, dnsPort), nil
+}
+
 // parseWebhookHeaders reads a comma-separated list of headers, each its
 // name, :, and its value, and returns them, and how the log shows them,
 // with each value as ********. An error names the item by its place in