Add Fake DNS support (#309)

Co-authored-by: Xiaokang Wang <xiaokangwang@outlook.com>
Co-authored-by: loyalsoldier <10487845+Loyalsoldier@users.noreply.github.com>
Co-authored-by: kslr <kslrwang@gmail.com>

app/dns/fakedns/fake.go

@@ -0,0 +1,136 @@
+// +build !confonly
+
+package fakedns
+
+import (
+	"context"
+	"math"
+	"math/big"
+	gonet "net"
+	"time"
+
+	"github.com/xtls/xray-core/common"
+	"github.com/xtls/xray-core/common/cache"
+	"github.com/xtls/xray-core/common/net"
+	"github.com/xtls/xray-core/features/dns"
+)
+
+type Holder struct {
+	domainToIP cache.Lru
+	ipRange *gonet.IPNet
+
+	config *FakeDnsPool
+}
+
+func (*Holder) Type() interface{} {
+	return (*dns.FakeDNSEngine)(nil)
+}
+
+func (fkdns *Holder) Start() error {
+	return fkdns.initializeFromConfig()
+}
+
+func (fkdns *Holder) Close() error {
+	fkdns.domainToIP = nil
+	fkdns.ipRange = nil
+	return nil
+}
+
+func NewFakeDNSHolder() (*Holder, error) {
+	var fkdns *Holder
+	var err error
+
+	if fkdns, err = NewFakeDNSHolderConfigOnly(nil); err != nil {
+		return nil, newError("Unable to create Fake Dns Engine").Base(err).AtError()
+	}
+	err = fkdns.initialize("240.0.0.0/8", 65535)
+	if err != nil {
+		return nil, err
+	}
+	return fkdns, nil
+}
+
+func NewFakeDNSHolderConfigOnly(conf *FakeDnsPool) (*Holder, error) {
+	return &Holder{nil, nil, conf}, nil
+}
+
+func (fkdns *Holder) initializeFromConfig() error {
+	return fkdns.initialize(fkdns.config.IpPool, int(fkdns.config.LruSize))
+}
+
+func (fkdns *Holder) initialize(ipPoolCidr string, lruSize int) error {
+	var ipRange *gonet.IPNet
+	var err error
+
+	if _, ipRange, err = gonet.ParseCIDR(ipPoolCidr); err != nil {
+		return newError("Unable to parse CIDR for Fake DNS IP assignment").Base(err).AtError()
+	}
+
+	ones, bits := ipRange.Mask.Size()
+	rooms := bits - ones
+	if math.Log2(float64(lruSize)) >= float64(rooms) {
+		return newError("LRU size is bigger than subnet size").AtError()
+	}
+	fkdns.domainToIP = cache.NewLru(lruSize)
+	fkdns.ipRange = ipRange
+	return nil
+}
+
+// GetFakeIPForDomain check and generate a fake IP for a domain name
+func (fkdns *Holder) GetFakeIPForDomain(domain string) []net.Address {
+	if v, ok := fkdns.domainToIP.Get(domain); ok {
+		return []net.Address{v.(net.Address)}
+	}
+	var currentTimeMillis = uint64(time.Now().UnixNano() / 1e6)
+	ones, bits := fkdns.ipRange.Mask.Size()
+	rooms := bits - ones
+	if rooms < 64 {
+		currentTimeMillis %= (uint64(1) << rooms)
+	}
+	var bigIntIP = big.NewInt(0).SetBytes(fkdns.ipRange.IP)
+	bigIntIP = bigIntIP.Add(bigIntIP, new(big.Int).SetUint64(currentTimeMillis))
+	var ip net.Address
+	for {
+		ip = net.IPAddress(bigIntIP.Bytes())
+
+		// if we run for a long time, we may go back to beginning and start seeing the IP in use
+		if _, ok := fkdns.domainToIP.PeekKeyFromValue(ip); !ok {
+			break
+		}
+
+		bigIntIP = bigIntIP.Add(bigIntIP, big.NewInt(1))
+		if !fkdns.ipRange.Contains(bigIntIP.Bytes()) {
+			bigIntIP = big.NewInt(0).SetBytes(fkdns.ipRange.IP)
+		}
+	}
+	fkdns.domainToIP.Put(domain, ip)
+	return []net.Address{ip}
+}
+
+// GetDomainFromFakeDNS check if an IP is a fake IP and have corresponding domain name
+func (fkdns *Holder) GetDomainFromFakeDNS(ip net.Address) string {
+	if !ip.Family().IsIP() || !fkdns.ipRange.Contains(ip.IP()) {
+		return ""
+	}
+	if k, ok := fkdns.domainToIP.GetKeyFromValue(ip); ok {
+		return k.(string)
+	}
+	newError("A fake ip request to ", ip, ", however there is no matching domain name in fake DNS").AtInfo().WriteToLog()
+	return ""
+}
+
+// GetFakeIPRange return fake IP range from configuration
+func (fkdns *Holder) GetFakeIPRange() *gonet.IPNet {
+	return fkdns.ipRange
+}
+
+func init() {
+	common.Must(common.RegisterConfig((*FakeDnsPool)(nil), func(ctx context.Context, config interface{}) (interface{}, error) {
+		var f *Holder
+		var err error
+		if f, err = NewFakeDNSHolderConfigOnly(config.(*FakeDnsPool)); err != nil {
+			return nil, err
+		}
+		return f, nil
+	}))
+}