Support for larger UDP DNS responses #30

As [RFC6891](https://tools.ietf.org/html/rfc6891) define, Extension Mechanisms for DNS(EDNS(0)) can make UDP DNS response packets reach max 65535 bytes and breakthrough the early [RFC1035](‘https://tools.ietf.org/html/rfc1035’) 512 limitation.

godns.conf

@@ -18,6 +18,8 @@ timeout = 5  # 5 seconds
 # Match the PR15, https://github.com/kenshinx/godns/pull/15
 interval = 200 # 200 milliseconds
 
+setedns0 = false #Support for larger UDP DNS responses
+
 [redis]
 host = "127.0.0.1"
 port = 6379

resolver.go

@@ -34,6 +34,10 @@ func (r *Resolver) Lookup(net string, req *dns.Msg) (message *dns.Msg, err error
 		WriteTimeout: r.Timeout(),
 	}
 
+	if net == "udp" && settings.ResolvConfig.SetEDNS0 {
+		req = req.SetEdns0(65535, true)
+	}
+
 	qname := req.Question[0].Name
 
 	res := make(chan *dns.Msg, 1)

settings.go

@@ -37,6 +37,7 @@ type ResolvSettings struct {
 	ResolvFile string `toml:"resolv-file"`
 	Timeout    int
 	Interval   int
+	SetEDNS0   bool
 }
 
 type DNSServerSettings struct {