Add DHCP DNS server support

include/dhcp.go

@@ -0,0 +1,5 @@
+//go:build with_dhcp
+
+package include
+
+import _ "github.com/sagernet/sing-box/transport/dhcp"

include/dhcp_stub.go

@@ -0,0 +1,18 @@
+//go:build !with_dhcp
+
+package include
+
+import (
+	"context"
+
+	"github.com/sagernet/sing-dns"
+	E "github.com/sagernet/sing/common/exceptions"
+	"github.com/sagernet/sing/common/logger"
+	N "github.com/sagernet/sing/common/network"
+)
+
+func init() {
+	dns.RegisterTransport([]string{"dhcp"}, func(ctx context.Context, logger logger.ContextLogger, dialer N.Dialer, link string) (dns.Transport, error) {
+		return nil, E.New(`DHCP is not included in this build, rebuild with -tags with_dhcp`)
+	})
+}