dns: Add preferred_by rule item

route/rule/rule_dns.go

@@ -329,6 +329,11 @@ func NewDefaultDNSRule(ctx context.Context, logger log.ContextLogger, options op
 		rule.items = append(rule.items, item)
 		rule.allItems = append(rule.allItems, item)
 	}
+	if len(options.PreferredBy) > 0 {
+		item := NewPreferredByDNSItem(ctx, options.PreferredBy)
+		rule.items = append(rule.items, item)
+		rule.allItems = append(rule.allItems, item)
+	}
 	if options.RuleSetIPCIDRAcceptEmpty { //nolint:staticcheck
 		if legacyDNSMode {
 			deprecated.Report(ctx, deprecated.OptionRuleSetIPCIDRAcceptEmpty)

route/rule/rule_item_preferred_by_dns.go

@@ -0,0 +1,74 @@
+package rule
+
+import (
+	"context"
+	"strings"
+
+	"github.com/sagernet/sing-box/adapter"
+	E "github.com/sagernet/sing/common/exceptions"
+	F "github.com/sagernet/sing/common/format"
+	"github.com/sagernet/sing/service"
+
+	mDNS "github.com/miekg/dns"
+)
+
+var _ RuleItem = (*PreferredByDNSItem)(nil)
+
+type PreferredByDNSItem struct {
+	ctx           context.Context
+	transportTags []string
+	transports    []adapter.DNSTransportWithPreferredDomain
+}
+
+func NewPreferredByDNSItem(ctx context.Context, transportTags []string) *PreferredByDNSItem {
+	return &PreferredByDNSItem{
+		ctx:           ctx,
+		transportTags: transportTags,
+	}
+}
+
+func (r *PreferredByDNSItem) Start() error {
+	transportManager := service.FromContext[adapter.DNSTransportManager](r.ctx)
+	for _, transportTag := range r.transportTags {
+		rawTransport, loaded := transportManager.Transport(transportTag)
+		if !loaded {
+			return E.New("DNS server not found: ", transportTag)
+		}
+		transportWithPreferredDomain, withPreferredDomain := rawTransport.(adapter.DNSTransportWithPreferredDomain)
+		if !withPreferredDomain {
+			return E.New("DNS server type does not support preferred_by: ", rawTransport.Type())
+		}
+		r.transports = append(r.transports, transportWithPreferredDomain)
+	}
+	return nil
+}
+
+func (r *PreferredByDNSItem) Match(metadata *adapter.InboundContext) bool {
+	var domainHost string
+	if metadata.Domain != "" {
+		domainHost = metadata.Domain
+	} else {
+		domainHost = metadata.Destination.Fqdn
+	}
+	if domainHost == "" {
+		return false
+	}
+	canonical := mDNS.CanonicalName(domainHost)
+	for _, transport := range r.transports {
+		if transport.PreferredDomain(canonical) {
+			return true
+		}
+	}
+	return false
+}
+
+func (r *PreferredByDNSItem) String() string {
+	description := "preferred_by="
+	pLen := len(r.transportTags)
+	if pLen == 1 {
+		description += F.ToString(r.transportTags[0])
+	} else {
+		description += "[" + strings.Join(F.MapToString(r.transportTags), " ") + "]"
+	}
+	return description
+}