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