Fix panic when parsing resolv.conf

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

@@ -1,10 +1,12 @@
-// +build !windows,!plan9
+// +build !windows
 
 package dns
 
 import (
 	"net"
-	"syscall"
+
+	"golang.org/x/net/ipv4"
+	"golang.org/x/net/ipv6"
 )
 
 // SessionUDP holds the remote address and the associated
@@ -17,29 +19,6 @@ type SessionUDP struct {
 // RemoteAddr returns the remote network address.
 func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
 
-// setUDPSocketOptions sets the UDP socket options.
-// This function is implemented on a per platform basis. See udp_*.go for more details
-func setUDPSocketOptions(conn *net.UDPConn) error {
-	sa, err := getUDPSocketName(conn)
-	if err != nil {
-		return err
-	}
-	switch sa.(type) {
-	case *syscall.SockaddrInet6:
-		v6only, err := getUDPSocketOptions6Only(conn)
-		if err != nil {
-			return err
-		}
-		setUDPSocketOptions6(conn)
-		if !v6only {
-			setUDPSocketOptions4(conn)
-		}
-	case *syscall.SockaddrInet4:
-		setUDPSocketOptions4(conn)
-	}
-	return nil
-}
-
 // 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) {
@@ -51,8 +30,60 @@ func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
 	return n, &SessionUDP{raddr, oob[:oobn]}, err
 }
 
-// WriteToSessionUDP acts just like net.UDPConn.WritetTo(), but uses a *SessionUDP instead of a net.Addr.
+// 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) {
-	n, _, err := conn.WriteMsgUDP(b, session.context, session.raddr)
+	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.
+	var dst net.IP
+	cm6 := new(ipv6.ControlMessage)
+	if cm6.Parse(oob) == nil {
+		dst = cm6.Dst
+	}
+	if dst == nil {
+		cm4 := new(ipv4.ControlMessage)
+		if cm4.Parse(oob) == nil {
+			dst = cm4.Dst
+		}
+	}
+	return dst
+}
+
+// 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
+}