Add fallback local DNS server for iOS

dns/transport/local/local.go

@@ -19,10 +19,6 @@ import (
 	mDNS "github.com/miekg/dns"
 )
 
-func RegisterTransport(registry *dns.TransportRegistry) {
-	dns.RegisterTransport[option.LocalDNSServerOptions](registry, C.DNSTypeLocal, NewTransport)
-}
-
 var _ adapter.DNSTransport = (*Transport)(nil)
 
 type Transport struct {

dns/transport/local/local_fallback.go

@@ -0,0 +1,201 @@
+package local
+
+import (
+	"context"
+	"errors"
+	"net"
+
+	"github.com/sagernet/sing-box/adapter"
+	C "github.com/sagernet/sing-box/constant"
+	"github.com/sagernet/sing-box/dns"
+	"github.com/sagernet/sing-box/experimental/libbox/platform"
+	"github.com/sagernet/sing-box/log"
+	"github.com/sagernet/sing-box/option"
+	E "github.com/sagernet/sing/common/exceptions"
+	"github.com/sagernet/sing/service"
+
+	mDNS "github.com/miekg/dns"
+)
+
+func RegisterTransport(registry *dns.TransportRegistry) {
+	dns.RegisterTransport[option.LocalDNSServerOptions](registry, C.DNSTypeLocal, NewFallbackTransport)
+}
+
+type FallbackTransport struct {
+	adapter.DNSTransport
+	ctx      context.Context
+	fallback bool
+	resolver net.Resolver
+}
+
+func NewFallbackTransport(ctx context.Context, logger log.ContextLogger, tag string, options option.LocalDNSServerOptions) (adapter.DNSTransport, error) {
+	transport, err := NewTransport(ctx, logger, tag, options)
+	if err != nil {
+		return nil, err
+	}
+	return &FallbackTransport{
+		DNSTransport: transport,
+		ctx:          ctx,
+	}, nil
+}
+
+func (f *FallbackTransport) Start(stage adapter.StartStage) error {
+	if stage != adapter.StartStateStart {
+		return nil
+	}
+	platformInterface := service.FromContext[platform.Interface](f.ctx)
+	if platformInterface == nil {
+		return nil
+	}
+	inboundManager := service.FromContext[adapter.InboundManager](f.ctx)
+	for _, inbound := range inboundManager.Inbounds() {
+		if inbound.Type() == C.TypeTun {
+			// platform tun hijacks DNS, so we can only use cgo resolver here
+			f.fallback = true
+			break
+		}
+	}
+	return nil
+}
+
+func (f *FallbackTransport) Exchange(ctx context.Context, message *mDNS.Msg) (*mDNS.Msg, error) {
+	if f.fallback {
+		return f.DNSTransport.Exchange(ctx, message)
+	}
+	question := message.Question[0]
+	domain := dns.FqdnToDomain(question.Name)
+	if question.Qtype == mDNS.TypeA || question.Qtype == mDNS.TypeAAAA {
+		var network string
+		if question.Qtype == mDNS.TypeA {
+			network = "ip4"
+		} else {
+			network = "ip6"
+		}
+		addresses, err := f.resolver.LookupNetIP(ctx, network, domain)
+		if err != nil {
+			var dnsError *net.DNSError
+			if errors.As(err, &dnsError) && dnsError.IsNotFound {
+				return nil, dns.RcodeRefused
+			}
+			return nil, err
+		}
+		return dns.FixedResponse(message.Id, question, addresses, C.DefaultDNSTTL), nil
+	} else if question.Qtype == mDNS.TypeNS {
+		records, err := f.resolver.LookupNS(ctx, domain)
+		if err != nil {
+			var dnsError *net.DNSError
+			if errors.As(err, &dnsError) && dnsError.IsNotFound {
+				return nil, dns.RcodeRefused
+			}
+			return nil, err
+		}
+		response := &mDNS.Msg{
+			MsgHdr: mDNS.MsgHdr{
+				Id:       message.Id,
+				Rcode:    mDNS.RcodeSuccess,
+				Response: true,
+			},
+			Question: []mDNS.Question{question},
+		}
+		for _, record := range records {
+			response.Answer = append(response.Answer, &mDNS.NS{
+				Hdr: mDNS.RR_Header{
+					Name:   question.Name,
+					Rrtype: mDNS.TypeNS,
+					Class:  mDNS.ClassINET,
+					Ttl:    C.DefaultDNSTTL,
+				},
+				Ns: record.Host,
+			})
+		}
+		return response, nil
+	} else if question.Qtype == mDNS.TypeCNAME {
+		cname, err := f.resolver.LookupCNAME(ctx, domain)
+		if err != nil {
+			var dnsError *net.DNSError
+			if errors.As(err, &dnsError) && dnsError.IsNotFound {
+				return nil, dns.RcodeRefused
+			}
+			return nil, err
+		}
+		return &mDNS.Msg{
+			MsgHdr: mDNS.MsgHdr{
+				Id:       message.Id,
+				Rcode:    mDNS.RcodeSuccess,
+				Response: true,
+			},
+			Question: []mDNS.Question{question},
+			Answer: []mDNS.RR{
+				&mDNS.CNAME{
+					Hdr: mDNS.RR_Header{
+						Name:   question.Name,
+						Rrtype: mDNS.TypeCNAME,
+						Class:  mDNS.ClassINET,
+						Ttl:    C.DefaultDNSTTL,
+					},
+					Target: cname,
+				},
+			},
+		}, nil
+	} else if question.Qtype == mDNS.TypeTXT {
+		records, err := f.resolver.LookupTXT(ctx, domain)
+		if err != nil {
+			var dnsError *net.DNSError
+			if errors.As(err, &dnsError) && dnsError.IsNotFound {
+				return nil, dns.RcodeRefused
+			}
+			return nil, err
+		}
+		return &mDNS.Msg{
+			MsgHdr: mDNS.MsgHdr{
+				Id:       message.Id,
+				Rcode:    mDNS.RcodeSuccess,
+				Response: true,
+			},
+			Question: []mDNS.Question{question},
+			Answer: []mDNS.RR{
+				&mDNS.TXT{
+					Hdr: mDNS.RR_Header{
+						Name:   question.Name,
+						Rrtype: mDNS.TypeCNAME,
+						Class:  mDNS.ClassINET,
+						Ttl:    C.DefaultDNSTTL,
+					},
+					Txt: records,
+				},
+			},
+		}, nil
+	} else if question.Qtype == mDNS.TypeMX {
+		records, err := f.resolver.LookupMX(ctx, domain)
+		if err != nil {
+			var dnsError *net.DNSError
+			if errors.As(err, &dnsError) && dnsError.IsNotFound {
+				return nil, dns.RcodeRefused
+			}
+			return nil, err
+		}
+		response := &mDNS.Msg{
+			MsgHdr: mDNS.MsgHdr{
+				Id:       message.Id,
+				Rcode:    mDNS.RcodeSuccess,
+				Response: true,
+			},
+			Question: []mDNS.Question{question},
+		}
+		for _, record := range records {
+			response.Answer = append(response.Answer, &mDNS.MX{
+				Hdr: mDNS.RR_Header{
+					Name:   question.Name,
+					Rrtype: mDNS.TypeA,
+					Class:  mDNS.ClassINET,
+					Ttl:    C.DefaultDNSTTL,
+				},
+				Preference: record.Pref,
+				Mx:         record.Host,
+			})
+		}
+		return response, nil
+	} else {
+		return nil, E.New("only A, AAAA, NS, CNAME, TXT, MX queries are supported on current platform when using TUN, please switch to a fixed DNS server.")
+	}
+}

dns/transport/local/resolv_windows.go

@@ -69,9 +69,6 @@ func dnsReadConfig(_ string) *dnsConfig {
 	return conf
 }
 
-//go:linkname defaultNS net.defaultNS
-var defaultNS []string
-
 func adapterAddresses() ([]*windows.IpAdapterAddresses, error) {
 	var b []byte
 	l := uint32(15000) // recommended initial size

dns/transport/predefined.go

@@ -79,5 +79,5 @@ func (t *PredefinedTransport) Exchange(ctx context.Context, message *mDNS.Msg) (
 			}
 		}
 	}
-	return nil, dns.RCodeNameError
+	return nil, dns.RcodeNameError
 }