144 lines
2.2 KiB
Go
144 lines
2.2 KiB
Go
package main
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import (
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"bufio"
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"bytes"
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"encoding/binary"
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"errors"
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"fmt"
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"net"
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"os"
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"strings"
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"time"
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)
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const (
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ARecord = 0x0001
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INClass = 0x0001
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)
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const DNSPort = 53
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const Timeout = 2
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type dnsConn struct {
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net.Conn
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}
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type header struct {
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Id, Flags uint16
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Qdcount uint16
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Ancount uint16
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Nscount uint16
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Arcount uint16
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}
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type answer struct {
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Ntype byte
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Ptr uint16
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Qtype uint16
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Class uint16
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Ttl uint32
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Addrlen uint16
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Addr uint32
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}
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func findNsAddr() (string, error) {
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conf := []string{
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"/etc/resolv.conf",
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}
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for _, i := range conf {
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f, err := os.Open(i)
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if err != nil {
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return "", err
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}
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defer f.Close()
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scanner := bufio.NewScanner(f)
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for scanner.Scan() {
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s := scanner.Text()
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if s[0] == '#' {
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continue
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}
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args := strings.Split(s, " ")
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if args[0] == "nameserver" && len(args) > 1 {
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return args[1], nil
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}
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}
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}
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return "", errors.New("nameserver not found")
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}
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func read(buf *bytes.Buffer, data any) error {
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return binary.Read(buf, binary.BigEndian, data)
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}
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func write(buf *bytes.Buffer, data any) error {
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return binary.Write(buf, binary.BigEndian, data)
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}
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func writeName(buf *bytes.Buffer, name string) (n int) {
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for _, i := range strings.Split(name, ".") {
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buf.WriteByte(byte(len(i)))
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n++
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buf.WriteString(i)
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n += len(i)
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}
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buf.WriteByte(0)
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return
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}
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func dial(server string) (dnsConn, error) {
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c, err := net.Dial("udp", fmt.Sprintf("%s:%d", server, DNSPort))
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if err == nil {
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c.SetDeadline(time.Now().Add(Timeout * time.Second))
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}
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return dnsConn{c}, err
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}
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func (c dnsConn) sendQuery(name string) (answer, error) {
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buf := &bytes.Buffer{}
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ah := &header{}
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a := &answer{}
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h := header{
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0x4c3f,
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1 << 8, // recursion
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0x0001, // single question
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0x0000,
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0x0000,
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0x0000,
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}
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write(buf, h)
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nc := writeName(buf, name)
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write(buf, []uint16{ARecord, INClass})
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if _, err := c.Write(buf.Bytes()); err != nil {
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return *a, err
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}
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b := make([]byte, 64)
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if _, err := c.Read(b); err != nil {
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return *a, err
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}
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abuf := bytes.NewBuffer(b)
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read(abuf, ah)
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abuf.Next(nc + 4)
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read(abuf, a)
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if a.Addrlen != 4 {
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return *a, errors.New("invalid name")
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}
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return *a, nil
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}
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