Add health check for DNS

vendor/github.com/miekg/dns/udp.go

@@ -0,0 +1,102 @@
+// +build !windows
+
+package dns
+
+import (
+	"net"
+
+	"golang.org/x/net/ipv4"
+	"golang.org/x/net/ipv6"
+)
+
+// This is the required size of the OOB buffer to pass to ReadMsgUDP.
+var udpOOBSize = func() int {
+	// We can't know whether we'll get an IPv4 control message or an
+	// IPv6 control message ahead of time. To get around this, we size
+	// the buffer equal to the largest of the two.
+
+	oob4 := ipv4.NewControlMessage(ipv4.FlagDst | ipv4.FlagInterface)
+	oob6 := ipv6.NewControlMessage(ipv6.FlagDst | ipv6.FlagInterface)
+
+	if len(oob4) > len(oob6) {
+		return len(oob4)
+	}
+
+	return len(oob6)
+}()
+
+// SessionUDP holds the remote address and the associated
+// out-of-band data.
+type SessionUDP struct {
+	raddr   *net.UDPAddr
+	context []byte
+}
+
+// RemoteAddr returns the remote network address.
+func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
+
+// ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
+// net.UDPAddr.
+func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
+	oob := make([]byte, udpOOBSize)
+	n, oobn, _, raddr, err := conn.ReadMsgUDP(b, oob)
+	if err != nil {
+		return n, nil, err
+	}
+	return n, &SessionUDP{raddr, oob[:oobn]}, err
+}
+
+// WriteToSessionUDP acts just like net.UDPConn.WriteTo(), but uses a *SessionUDP instead of a net.Addr.
+func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, error) {
+	oob := correctSource(session.context)
+	n, _, err := conn.WriteMsgUDP(b, oob, session.raddr)
+	return n, err
+}
+
+func setUDPSocketOptions(conn *net.UDPConn) error {
+	// Try setting the flags for both families and ignore the errors unless they
+	// both error.
+	err6 := ipv6.NewPacketConn(conn).SetControlMessage(ipv6.FlagDst|ipv6.FlagInterface, true)
+	err4 := ipv4.NewPacketConn(conn).SetControlMessage(ipv4.FlagDst|ipv4.FlagInterface, true)
+	if err6 != nil && err4 != nil {
+		return err4
+	}
+	return nil
+}
+
+// parseDstFromOOB takes oob data and returns the destination IP.
+func parseDstFromOOB(oob []byte) net.IP {
+	// Start with IPv6 and then fallback to IPv4
+	// TODO(fastest963): Figure out a way to prefer one or the other. Looking at
+	// the lvl of the header for a 0 or 41 isn't cross-platform.
+	cm6 := new(ipv6.ControlMessage)
+	if cm6.Parse(oob) == nil && cm6.Dst != nil {
+		return cm6.Dst
+	}
+	cm4 := new(ipv4.ControlMessage)
+	if cm4.Parse(oob) == nil && cm4.Dst != nil {
+		return cm4.Dst
+	}
+	return nil
+}
+
+// correctSource takes oob data and returns new oob data with the Src equal to the Dst
+func correctSource(oob []byte) []byte {
+	dst := parseDstFromOOB(oob)
+	if dst == nil {
+		return nil
+	}
+	// If the dst is definitely an IPv6, then use ipv6's ControlMessage to
+	// respond otherwise use ipv4's because ipv6's marshal ignores ipv4
+	// addresses.
+	if dst.To4() == nil {
+		cm := new(ipv6.ControlMessage)
+		cm.Src = dst
+		oob = cm.Marshal()
+	} else {
+		cm := new(ipv4.ControlMessage)
+		cm.Src = dst
+		oob = cm.Marshal()
+	}
+	return oob
+}