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net: separate DNS transport from DNS query-response interaction
Before fixing issue 6579 this CL separates DNS transport from DNS message interaction to make it easier to add builtin DNS resolver control logic. Update #6579 LGTM=alex, kevlar R=golang-codereviews, alex, gobot, iant, minux, kevlar CC=golang-codereviews https://golang.org/cl/101220044
This commit is contained in:
Mikio Hara
parent
f2f17c0ff2
commit
48e7533783
@ -16,6 +16,7 @@
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package net
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import (
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"errors"
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"io"
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"math/rand"
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"os"
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@ -23,118 +24,187 @@ import (
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"time"
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)
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// Send a request on the connection and hope for a reply.
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// Up to cfg.attempts attempts.
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func exchange(cfg *dnsConfig, c Conn, name string, qtype uint16) (*dnsMsg, error) {
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_, useTCP := c.(*TCPConn)
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if len(name) >= 256 {
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return nil, &DNSError{Err: "name too long", Name: name}
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}
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out := new(dnsMsg)
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out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
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out.question = []dnsQuestion{
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{name, qtype, dnsClassINET},
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}
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out.recursion_desired = true
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msg, ok := out.Pack()
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if !ok {
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return nil, &DNSError{Err: "internal error - cannot pack message", Name: name}
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}
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if useTCP {
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mlen := uint16(len(msg))
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msg = append([]byte{byte(mlen >> 8), byte(mlen)}, msg...)
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}
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for attempt := 0; attempt < cfg.attempts; attempt++ {
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n, err := c.Write(msg)
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if err != nil {
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return nil, err
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}
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// A dnsConn represents a DNS transport endpoint.
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type dnsConn interface {
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Conn
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if cfg.timeout == 0 {
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c.SetReadDeadline(noDeadline)
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} else {
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c.SetReadDeadline(time.Now().Add(time.Duration(cfg.timeout) * time.Second))
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}
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buf := make([]byte, 2000)
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if useTCP {
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n, err = io.ReadFull(c, buf[:2])
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if err != nil {
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if e, ok := err.(Error); ok && e.Timeout() {
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continue
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}
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}
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mlen := int(buf[0])<<8 | int(buf[1])
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if mlen > len(buf) {
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buf = make([]byte, mlen)
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}
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n, err = io.ReadFull(c, buf[:mlen])
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} else {
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n, err = c.Read(buf)
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}
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// readDNSResponse reads a DNS response message from the DNS
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// transport endpoint and returns the received DNS response
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// message.
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readDNSResponse() (*dnsMsg, error)
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// writeDNSQuery writes a DNS query message to the DNS
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// connection endpoint.
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writeDNSQuery(*dnsMsg) error
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}
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func (c *UDPConn) readDNSResponse() (*dnsMsg, error) {
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b := make([]byte, 512) // see RFC 1035
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n, err := c.Read(b)
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if err != nil {
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return nil, err
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}
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msg := &dnsMsg{}
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if !msg.Unpack(b[:n]) {
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return nil, errors.New("cannot unmarshal DNS message")
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}
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return msg, nil
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}
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func (c *UDPConn) writeDNSQuery(msg *dnsMsg) error {
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b, ok := msg.Pack()
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if !ok {
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return errors.New("cannot marshal DNS message")
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}
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if _, err := c.Write(b); err != nil {
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return err
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}
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return nil
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}
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func (c *TCPConn) readDNSResponse() (*dnsMsg, error) {
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b := make([]byte, 1280) // 1280 is a reasonable initial size for IP over Ethernet, see RFC 4035
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if _, err := io.ReadFull(c, b[:2]); err != nil {
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return nil, err
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}
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l := int(b[0])<<8 | int(b[1])
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if l > len(b) {
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b = make([]byte, l)
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}
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n, err := io.ReadFull(c, b[:l])
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if err != nil {
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return nil, err
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}
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msg := &dnsMsg{}
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if !msg.Unpack(b[:n]) {
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return nil, errors.New("cannot unmarshal DNS message")
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}
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return msg, nil
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}
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func (c *TCPConn) writeDNSQuery(msg *dnsMsg) error {
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b, ok := msg.Pack()
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if !ok {
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return errors.New("cannot marshal DNS message")
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}
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l := uint16(len(b))
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b = append([]byte{byte(l >> 8), byte(l)}, b...)
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if _, err := c.Write(b); err != nil {
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return err
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}
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return nil
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}
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func (d *Dialer) dialDNS(network, server string) (dnsConn, error) {
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switch network {
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case "tcp", "tcp4", "tcp6", "udp", "udp4", "udp6":
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default:
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return nil, UnknownNetworkError(network)
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}
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// Calling Dial here is scary -- we have to be sure not to
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// dial a name that will require a DNS lookup, or Dial will
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// call back here to translate it. The DNS config parser has
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// already checked that all the cfg.servers[i] are IP
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// addresses, which Dial will use without a DNS lookup.
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c, err := d.Dial(network, server)
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if err != nil {
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return nil, err
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}
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switch network {
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case "tcp", "tcp4", "tcp6":
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return c.(*TCPConn), nil
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case "udp", "udp4", "udp6":
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return c.(*UDPConn), nil
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}
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panic("unreachable")
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}
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// exchange sends a query on the connection and hopes for a response.
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func exchange(server, name string, qtype uint16, timeout time.Duration) (*dnsMsg, error) {
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d := Dialer{Timeout: timeout}
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out := dnsMsg{
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dnsMsgHdr: dnsMsgHdr{
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recursion_desired: true,
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},
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question: []dnsQuestion{
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{name, qtype, dnsClassINET},
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},
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}
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for _, network := range []string{"udp", "tcp"} {
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c, err := d.dialDNS(network, server)
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if err != nil {
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if e, ok := err.(Error); ok && e.Timeout() {
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continue
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}
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return nil, err
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}
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buf = buf[:n]
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in := new(dnsMsg)
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if !in.Unpack(buf) || in.id != out.id {
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defer c.Close()
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if timeout > 0 {
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c.SetDeadline(time.Now().Add(timeout))
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}
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out.id = uint16(rand.Int()) ^ uint16(time.Now().UnixNano())
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if err := c.writeDNSQuery(&out); err != nil {
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return nil, err
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}
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in, err := c.readDNSResponse()
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if err != nil {
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return nil, err
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}
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if in.id != out.id {
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return nil, errors.New("DNS message ID mismatch")
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}
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if in.truncated { // see RFC 5966
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continue
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}
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return in, nil
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}
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var server string
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if a := c.RemoteAddr(); a != nil {
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server = a.String()
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}
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return nil, &DNSError{Err: "no answer from server", Name: name, Server: server, IsTimeout: true}
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return nil, errors.New("no answer from DNS server")
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}
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// Do a lookup for a single name, which must be rooted
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// (otherwise answer will not find the answers).
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func tryOneName(cfg *dnsConfig, name string, qtype uint16) (cname string, addrs []dnsRR, err error) {
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func tryOneName(cfg *dnsConfig, name string, qtype uint16) (string, []dnsRR, error) {
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if len(cfg.servers) == 0 {
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return "", nil, &DNSError{Err: "no DNS servers", Name: name}
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}
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for i := 0; i < len(cfg.servers); i++ {
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// Calling Dial here is scary -- we have to be sure
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// not to dial a name that will require a DNS lookup,
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// or Dial will call back here to translate it.
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// The DNS config parser has already checked that
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// all the cfg.servers[i] are IP addresses, which
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// Dial will use without a DNS lookup.
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server := cfg.servers[i] + ":53"
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c, cerr := Dial("udp", server)
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if cerr != nil {
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err = cerr
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continue
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if len(name) >= 256 {
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return "", nil, &DNSError{Err: "DNS name too long", Name: name}
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}
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timeout := time.Duration(cfg.timeout) * time.Second
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var lastErr error
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for _, server := range cfg.servers {
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server += ":53"
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lastErr = &DNSError{
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Err: "no answer from DNS server",
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Name: name,
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Server: server,
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IsTimeout: true,
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}
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msg, merr := exchange(cfg, c, name, qtype)
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c.Close()
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if merr != nil {
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err = merr
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continue
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}
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if msg.truncated { // see RFC 5966
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c, cerr = Dial("tcp", server)
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if cerr != nil {
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err = cerr
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continue
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for i := 0; i < cfg.attempts; i++ {
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msg, err := exchange(server, name, qtype, timeout)
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if err != nil {
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if nerr, ok := err.(Error); ok && nerr.Timeout() {
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lastErr = &DNSError{
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Err: nerr.Error(),
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Name: name,
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Server: server,
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IsTimeout: true,
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}
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continue
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}
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lastErr = &DNSError{
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Err: err.Error(),
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Name: name,
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Server: server,
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}
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break
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}
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msg, merr = exchange(cfg, c, name, qtype)
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c.Close()
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if merr != nil {
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err = merr
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continue
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cname, addrs, err := answer(name, server, msg, qtype)
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if err == nil || err.(*DNSError).Err == noSuchHost {
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return cname, addrs, err
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}
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}
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cname, addrs, err = answer(name, server, msg, qtype)
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if err == nil || err.(*DNSError).Err == noSuchHost {
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break
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lastErr = err
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}
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}
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return
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return "", nil, lastErr
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}
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func convertRR_A(records []dnsRR) []IP {
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@ -16,19 +16,79 @@ import (
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"time"
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)
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func TestTCPLookup(t *testing.T) {
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var dnsTransportFallbackTests = []struct {
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server string
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name string
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qtype uint16
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timeout int
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rcode int
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}{
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// Querying "com." with qtype=255 usually makes an answer
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// which requires more than 512 bytes.
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{"8.8.8.8:53", "com.", dnsTypeALL, 2, dnsRcodeSuccess},
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{"8.8.4.4:53", "com.", dnsTypeALL, 4, dnsRcodeSuccess},
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}
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func TestDNSTransportFallback(t *testing.T) {
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if testing.Short() || !*testExternal {
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t.Skip("skipping test to avoid external network")
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}
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c, err := Dial("tcp", "8.8.8.8:53")
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if err != nil {
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t.Fatalf("Dial failed: %v", err)
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for _, tt := range dnsTransportFallbackTests {
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timeout := time.Duration(tt.timeout) * time.Second
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msg, err := exchange(tt.server, tt.name, tt.qtype, timeout)
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if err != nil {
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t.Error(err)
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continue
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}
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switch msg.rcode {
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case tt.rcode, dnsRcodeServerFailure:
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default:
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t.Errorf("got %v from %v; want %v", msg.rcode, tt.server, tt.rcode)
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continue
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}
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}
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defer c.Close()
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cfg := &dnsConfig{timeout: 10, attempts: 3}
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_, err = exchange(cfg, c, "com.", dnsTypeALL)
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if err != nil {
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t.Fatalf("exchange failed: %v", err)
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}
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// See RFC 6761 for further information about the reserved, pseudo
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// domain names.
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var specialDomainNameTests = []struct {
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name string
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qtype uint16
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rcode int
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}{
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// Name resoltion APIs and libraries should not recongnize the
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// followings as special.
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{"1.0.168.192.in-addr.arpa.", dnsTypePTR, dnsRcodeNameError},
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{"test.", dnsTypeALL, dnsRcodeNameError},
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{"example.com.", dnsTypeALL, dnsRcodeSuccess},
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// Name resoltion APIs and libraries should recongnize the
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// followings as special and should not send any queries.
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// Though, we test those names here for verifying nagative
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// answers at DNS query-response interaction level.
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{"localhost.", dnsTypeALL, dnsRcodeNameError},
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{"invalid.", dnsTypeALL, dnsRcodeNameError},
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}
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func TestSpecialDomainName(t *testing.T) {
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if testing.Short() || !*testExternal {
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t.Skip("skipping test to avoid external network")
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}
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server := "8.8.8.8:53"
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for _, tt := range specialDomainNameTests {
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msg, err := exchange(server, tt.name, tt.qtype, 0)
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if err != nil {
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t.Error(err)
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continue
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}
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switch msg.rcode {
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case tt.rcode, dnsRcodeServerFailure:
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default:
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t.Errorf("got %v from %v; want %v", msg.rcode, server, tt.rcode)
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continue
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}
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}
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}
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