Add dns reverse mapping

option/dns.go

@@ -13,6 +13,7 @@ type DNSOptions struct {
 	Servers        []DNSServerOptions `json:"servers,omitempty"`
 	Rules          []DNSRule          `json:"rules,omitempty"`
 	Final          string             `json:"final,omitempty"`
+	ReverseMapping bool               `json:"reverse_mapping,omitempty"`
 	DNSClientOptions
 }
 

route/router.go

@@ -89,6 +89,7 @@ type Router struct {
 	transports                         []dns.Transport
 	transportMap                       map[string]dns.Transport
 	transportDomainStrategy            map[dns.Transport]dns.DomainStrategy
+	dnsReverseMapping                  *DNSReverseMapping
 	interfaceFinder                    myInterfaceFinder
 	autoDetectInterface                bool
 	defaultInterface                   string
@@ -267,6 +268,10 @@ func NewRouter(
 	router.transportMap = transportMap
 	router.transportDomainStrategy = transportDomainStrategy
 
+	if dnsOptions.ReverseMapping {
+		router.dnsReverseMapping = NewDNSReverseMapping()
+	}
+
 	needInterfaceMonitor := platformInterface == nil && (options.AutoDetectInterface || common.Any(inbounds, func(inbound option.Inbound) bool {
 		return inbound.HTTPOptions.SetSystemProxy || inbound.MixedOptions.SetSystemProxy || inbound.TunOptions.AutoRoute
 	}))
@@ -627,6 +632,15 @@ func (r *Router) RouteConnection(ctx context.Context, conn net.Conn, metadata ad
 			buffer.Release()
 		}
 	}
+
+	if r.dnsReverseMapping != nil && metadata.Domain == "" {
+		domain, loaded := r.dnsReverseMapping.Query(metadata.Destination.Addr)
+		if loaded {
+			metadata.Domain = domain
+			r.logger.DebugContext(ctx, "found reserve mapped domain: ", metadata.Domain)
+		}
+	}
+
 	if metadata.Destination.IsFqdn() && dns.DomainStrategy(metadata.InboundOptions.DomainStrategy) != dns.DomainStrategyAsIS {
 		addresses, err := r.Lookup(adapter.WithContext(ctx, &metadata), metadata.Destination.Fqdn, dns.DomainStrategy(metadata.InboundOptions.DomainStrategy))
 		if err != nil {

route/router_dns.go

@@ -4,17 +4,39 @@ import (
 	"context"
 	"net/netip"
 	"strings"
+	"time"
 
 	"github.com/sagernet/sing-box/adapter"
 	C "github.com/sagernet/sing-box/constant"
 	"github.com/sagernet/sing-box/log"
 	"github.com/sagernet/sing-dns"
+	"github.com/sagernet/sing/common/cache"
 	E "github.com/sagernet/sing/common/exceptions"
 	F "github.com/sagernet/sing/common/format"
+	M "github.com/sagernet/sing/common/metadata"
 
 	mDNS "github.com/miekg/dns"
 )
 
+type DNSReverseMapping struct {
+	cache *cache.LruCache[netip.Addr, string]
+}
+
+func NewDNSReverseMapping() *DNSReverseMapping {
+	return &DNSReverseMapping{
+		cache: cache.New[netip.Addr, string](),
+	}
+}
+
+func (m *DNSReverseMapping) Save(address netip.Addr, domain string, ttl int) {
+	m.cache.StoreWithExpire(address, domain, time.Now().Add(time.Duration(ttl)*time.Second))
+}
+
+func (m *DNSReverseMapping) Query(address netip.Addr) (string, bool) {
+	domain, loaded := m.cache.Load(address)
+	return domain, loaded
+}
+
 func (r *Router) matchDNS(ctx context.Context) (context.Context, dns.Transport, dns.DomainStrategy) {
 	metadata := adapter.ContextFrom(ctx)
 	if metadata == nil {
@@ -69,6 +91,16 @@ func (r *Router) Exchange(ctx context.Context, message *mDNS.Msg) (*mDNS.Msg, er
 	if len(message.Question) > 0 && response != nil {
 		LogDNSAnswers(r.dnsLogger, ctx, message.Question[0].Name, response.Answer)
 	}
+	if r.dnsReverseMapping != nil && len(message.Question) > 0 && response != nil && len(response.Answer) > 0 {
+		for _, answer := range response.Answer {
+			switch record := answer.(type) {
+			case *mDNS.A:
+				r.dnsReverseMapping.Save(M.AddrFromIP(record.A), fqdnToDomain(record.Hdr.Name), int(record.Hdr.Ttl))
+			case *mDNS.AAAA:
+				r.dnsReverseMapping.Save(M.AddrFromIP(record.AAAA), fqdnToDomain(record.Hdr.Name), int(record.Hdr.Ttl))
+			}
+		}
+	}
 	return response, err
 }