Improve local DNS server on darwin

We mistakenly believed that `libresolv`'s `search` function worked correctly in NetworkExtension, but it seems only `getaddrinfo` does.

This commit changes the behavior of the `local` DNS server in NetworkExtension to prefer DHCP, falling back to `getaddrinfo` if DHCP servers are unavailable.

It's worth noting that `prefer_go` does not disable DHCP since it respects Dial Fields, but `getaddrinfo` does the opposite. The new behavior only applies to NetworkExtension, not to all scenarios (primarily command-line binaries) as it did previously.

In addition, this commit also improves the DHCP DNS server to use the same robust query logic as `local`.

dns/transport/local/local_darwin.go

@@ -0,0 +1,134 @@
+//go:build darwin
+
+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/dns/transport/hosts"
+	"github.com/sagernet/sing-box/log"
+	"github.com/sagernet/sing-box/option"
+	"github.com/sagernet/sing/common"
+	E "github.com/sagernet/sing/common/exceptions"
+	"github.com/sagernet/sing/common/logger"
+	M "github.com/sagernet/sing/common/metadata"
+	N "github.com/sagernet/sing/common/network"
+	"github.com/sagernet/sing/service"
+
+	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.TransportAdapter
+	ctx           context.Context
+	logger        logger.ContextLogger
+	hosts         *hosts.File
+	dialer        N.Dialer
+	preferGo      bool
+	fallback      bool
+	dhcpTransport dhcpTransport
+	resolver      net.Resolver
+}
+
+type dhcpTransport interface {
+	adapter.DNSTransport
+	Fetch() ([]M.Socksaddr, error)
+	Exchange0(ctx context.Context, message *mDNS.Msg, servers []M.Socksaddr) (*mDNS.Msg, error)
+}
+
+func NewTransport(ctx context.Context, logger log.ContextLogger, tag string, options option.LocalDNSServerOptions) (adapter.DNSTransport, error) {
+	transportDialer, err := dns.NewLocalDialer(ctx, options)
+	if err != nil {
+		return nil, err
+	}
+	transportAdapter := dns.NewTransportAdapterWithLocalOptions(C.DNSTypeLocal, tag, options)
+	return &Transport{
+		TransportAdapter: transportAdapter,
+		ctx:              ctx,
+		logger:           logger,
+		hosts:            hosts.NewFile(hosts.DefaultPath),
+		dialer:           transportDialer,
+		preferGo:         options.PreferGo,
+	}, nil
+}
+
+func (t *Transport) Start(stage adapter.StartStage) error {
+	if stage != adapter.StartStateStart {
+		return nil
+	}
+	inboundManager := service.FromContext[adapter.InboundManager](t.ctx)
+	for _, inbound := range inboundManager.Inbounds() {
+		if inbound.Type() == C.TypeTun {
+			t.fallback = true
+			break
+		}
+	}
+	if t.fallback {
+		t.dhcpTransport = newDHCPTransport(t.TransportAdapter, log.ContextWithOverrideLevel(t.ctx, log.LevelDebug), t.dialer, t.logger)
+		if t.dhcpTransport != nil {
+			err := t.dhcpTransport.Start(stage)
+			if err != nil {
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+func (t *Transport) Close() error {
+	return common.Close(
+		t.dhcpTransport,
+	)
+}
+
+func (t *Transport) Exchange(ctx context.Context, message *mDNS.Msg) (*mDNS.Msg, error) {
+	question := message.Question[0]
+	if question.Qtype == mDNS.TypeA || question.Qtype == mDNS.TypeAAAA {
+		addresses := t.hosts.Lookup(dns.FqdnToDomain(question.Name))
+		if len(addresses) > 0 {
+			return dns.FixedResponse(message.Id, question, addresses, C.DefaultDNSTTL), nil
+		}
+	}
+	if !t.fallback {
+		return t.exchange(ctx, message, question.Name)
+	}
+	if t.dhcpTransport != nil {
+		dhcpTransports, _ := t.dhcpTransport.Fetch()
+		if len(dhcpTransports) > 0 {
+			return t.dhcpTransport.Exchange0(ctx, message, dhcpTransports)
+		}
+	}
+	if t.preferGo {
+		// Assuming the user knows what they are doing, we still execute the query which will fail.
+		return t.exchange(ctx, message, 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 := t.resolver.LookupNetIP(ctx, network, question.Name)
+		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
+	}
+	return nil, E.New("only A and AAAA queries are supported on Apple platforms when using TUN and DHCP unavailable.")
+}