mirror of
https://github.com/xjasonlyu/tun2socks.git
synced 2025-10-07 01:33:15 +08:00
67 lines
2.2 KiB
Go
67 lines
2.2 KiB
Go
package dnsfallback
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import (
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"encoding/binary"
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"errors"
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"net"
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"github.com/xjasonlyu/tun2socks/common/dns"
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"github.com/xjasonlyu/tun2socks/core"
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)
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// UDP handler that intercepts DNS queries and replies with a truncated response (TC bit)
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// in order for the client to retry over TCP. This DNS/TCP fallback mechanism is
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// useful for proxy servers that do not support UDP.
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// Note that non-DNS UDP traffic is dropped.
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type udpHandler struct{}
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const (
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dnsHeaderLength = 12
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dnsMaskQr = uint8(0x80)
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dnsMaskTc = uint8(0x02)
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dnsMaskRcode = uint8(0x0F)
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)
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func NewUDPHandler() core.UDPConnHandler {
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return &udpHandler{}
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}
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func (h *udpHandler) Connect(conn core.UDPConn, udpAddr *net.UDPAddr) error {
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if udpAddr.Port != dns.CommonDnsPort {
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return errors.New("cannot handle non-DNS packet")
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}
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return nil
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}
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func (h *udpHandler) ReceiveTo(conn core.UDPConn, data []byte, addr *net.UDPAddr) error {
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if len(data) < dnsHeaderLength {
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return errors.New("received malformed DNS query")
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}
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// DNS Header
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// 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// | ID |
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// |QR| Opcode |AA|TC|RD|RA| Z | RCODE |
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// | QDCOUNT |
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// | ANCOUNT |
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// | NSCOUNT |
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// | ARCOUNT |
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// +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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// Set response and truncated bits
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data[2] |= dnsMaskQr | dnsMaskTc
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// Set response code to 'no error'.
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data[3] &= ^dnsMaskRcode
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// Set ANCOUNT to QDCOUNT. This is technically incorrect, since the response does not
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// include an answer. However, without it some DNS clients (i.e. Windows 7) do not retry
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// over TCP.
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var qdcount = binary.BigEndian.Uint16(data[4:6])
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binary.BigEndian.PutUint16(data[6:], qdcount)
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_, err := conn.WriteFrom(data, addr)
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return err
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}
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