udp.go raw

   1  //go:build !windows && !darwin
   2  // +build !windows,!darwin
   3  
   4  package dns
   5  
   6  import (
   7  	"net"
   8  
   9  	"golang.org/x/net/ipv4"
  10  	"golang.org/x/net/ipv6"
  11  )
  12  
  13  // This is the required size of the OOB buffer to pass to ReadMsgUDP.
  14  var udpOOBSize = func() int {
  15  	// We can't know whether we'll get an IPv4 control message or an
  16  	// IPv6 control message ahead of time. To get around this, we size
  17  	// the buffer equal to the largest of the two.
  18  
  19  	oob4 := ipv4.NewControlMessage(ipv4.FlagDst | ipv4.FlagInterface)
  20  	oob6 := ipv6.NewControlMessage(ipv6.FlagDst | ipv6.FlagInterface)
  21  
  22  	if len(oob4) > len(oob6) {
  23  		return len(oob4)
  24  	}
  25  
  26  	return len(oob6)
  27  }()
  28  
  29  // SessionUDP holds the remote address and the associated
  30  // out-of-band data.
  31  type SessionUDP struct {
  32  	raddr   *net.UDPAddr
  33  	context []byte
  34  }
  35  
  36  // RemoteAddr returns the remote network address.
  37  func (s *SessionUDP) RemoteAddr() net.Addr { return s.raddr }
  38  
  39  // ReadFromSessionUDP acts just like net.UDPConn.ReadFrom(), but returns a session object instead of a
  40  // net.UDPAddr.
  41  func ReadFromSessionUDP(conn *net.UDPConn, b []byte) (int, *SessionUDP, error) {
  42  	oob := make([]byte, udpOOBSize)
  43  	n, oobn, _, raddr, err := conn.ReadMsgUDP(b, oob)
  44  	if err != nil {
  45  		return n, nil, err
  46  	}
  47  	return n, &SessionUDP{raddr, oob[:oobn]}, err
  48  }
  49  
  50  // WriteToSessionUDP acts just like net.UDPConn.WriteTo(), but uses a *SessionUDP instead of a net.Addr.
  51  func WriteToSessionUDP(conn *net.UDPConn, b []byte, session *SessionUDP) (int, error) {
  52  	oob := correctSource(session.context)
  53  	n, _, err := conn.WriteMsgUDP(b, oob, session.raddr)
  54  	return n, err
  55  }
  56  
  57  func setUDPSocketOptions(conn *net.UDPConn) error {
  58  	// Try setting the flags for both families and ignore the errors unless they
  59  	// both error.
  60  	err6 := ipv6.NewPacketConn(conn).SetControlMessage(ipv6.FlagDst|ipv6.FlagInterface, true)
  61  	err4 := ipv4.NewPacketConn(conn).SetControlMessage(ipv4.FlagDst|ipv4.FlagInterface, true)
  62  	if err6 != nil && err4 != nil {
  63  		return err4
  64  	}
  65  	return nil
  66  }
  67  
  68  // parseDstFromOOB takes oob data and returns the destination IP.
  69  func parseDstFromOOB(oob []byte) net.IP {
  70  	// Start with IPv6 and then fallback to IPv4
  71  	// TODO(fastest963): Figure out a way to prefer one or the other. Looking at
  72  	// the lvl of the header for a 0 or 41 isn't cross-platform.
  73  	cm6 := new(ipv6.ControlMessage)
  74  	if cm6.Parse(oob) == nil && cm6.Dst != nil {
  75  		return cm6.Dst
  76  	}
  77  	cm4 := new(ipv4.ControlMessage)
  78  	if cm4.Parse(oob) == nil && cm4.Dst != nil {
  79  		return cm4.Dst
  80  	}
  81  	return nil
  82  }
  83  
  84  // correctSource takes oob data and returns new oob data with the Src equal to the Dst
  85  func correctSource(oob []byte) []byte {
  86  	dst := parseDstFromOOB(oob)
  87  	if dst == nil {
  88  		return nil
  89  	}
  90  	// If the dst is definitely an IPv6, then use ipv6's ControlMessage to
  91  	// respond otherwise use ipv4's because ipv6's marshal ignores ipv4
  92  	// addresses.
  93  	if dst.To4() == nil {
  94  		cm := new(ipv6.ControlMessage)
  95  		cm.Src = dst
  96  		oob = cm.Marshal()
  97  	} else {
  98  		cm := new(ipv4.ControlMessage)
  99  		cm.Src = dst
 100  		oob = cm.Marshal()
 101  	}
 102  	return oob
 103  }
 104