DNS: Support returning upstream TTL to clients (#4526)

Closes https://github.com/XTLS/Xray-core/issues/4527

features/dns/client.go

@@ -21,7 +21,7 @@ type Client interface {
 	features.Feature
 
 	// LookupIP returns IP address for the given domain. IPs may contain IPv4 and/or IPv6 addresses.
-	LookupIP(domain string, option IPOption) ([]net.IP, error)
+	LookupIP(domain string, option IPOption) ([]net.IP, uint32, error)
 }
 
 type HostsLookup interface {

features/dns/localdns/client.go

@@ -20,10 +20,10 @@ func (*Client) Start() error { return nil }
 func (*Client) Close() error { return nil }
 
 // LookupIP implements Client.
-func (*Client) LookupIP(host string, option dns.IPOption) ([]net.IP, error) {
+func (*Client) LookupIP(host string, option dns.IPOption) ([]net.IP, uint32, error) {
 	ips, err := net.LookupIP(host)
 	if err != nil {
-		return nil, err
+		return nil, 0, err
 	}
 	parsedIPs := make([]net.IP, 0, len(ips))
 	ipv4 := make([]net.IP, 0, len(ips))
@@ -40,21 +40,22 @@ func (*Client) LookupIP(host string, option dns.IPOption) ([]net.IP, error) {
 			ipv6 = append(ipv6, ip)
 		}
 	}
+	// Local DNS ttl is 600
 	switch {
 	case option.IPv4Enable && option.IPv6Enable:
 		if len(parsedIPs) > 0 {
-			return parsedIPs, nil
+			return parsedIPs, 600, nil
 		}
 	case option.IPv4Enable:
 		if len(ipv4) > 0 {
-			return ipv4, nil
+			return ipv4, 600, nil
 		}
 	case option.IPv6Enable:
 		if len(ipv6) > 0 {
-			return ipv6, nil
+			return ipv6, 600, nil
 		}
 	}
-	return nil, dns.ErrEmptyResponse
+	return nil, 0, dns.ErrEmptyResponse
 }
 
 // New create a new dns.Client that queries localhost for DNS.

features/routing/dns/context.go

@@ -23,7 +23,7 @@ func (ctx *ResolvableContext) GetTargetIPs() []net.IP {
 	}
 
 	if domain := ctx.GetTargetDomain(); len(domain) != 0 {
-		ips, err := ctx.dnsClient.LookupIP(domain, dns.IPOption{
+		ips, _, err := ctx.dnsClient.LookupIP(domain, dns.IPOption{
 			IPv4Enable: true,
 			IPv6Enable: true,
 			FakeEnable: false,