mirror of
https://github.com/autc04/Retro68.git
synced 2024-09-08 20:55:01 +00:00
538 lines
14 KiB
Go
538 lines
14 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// +build darwin dragonfly freebsd linux netbsd openbsd solaris
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// DNS client: see RFC 1035.
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// Has to be linked into package net for Dial.
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// TODO(rsc):
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// Could potentially handle many outstanding lookups faster.
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// Could have a small cache.
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// Random UDP source port (net.Dial should do that for us).
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// Random request IDs.
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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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"sync"
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"time"
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)
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// A dnsDialer provides dialing suitable for DNS queries.
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type dnsDialer interface {
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dialDNS(string, string) (dnsConn, error)
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}
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var testHookDNSDialer = func(d time.Duration) dnsDialer { return &Dialer{Timeout: d} }
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// A dnsConn represents a DNS transport endpoint.
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type dnsConn interface {
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io.Closer
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SetDeadline(time.Time) error
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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 := testHookDNSDialer(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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return nil, err
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}
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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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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) (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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timeout := time.Duration(cfg.timeout) * time.Second
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var lastErr error
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for i := 0; i < cfg.attempts; i++ {
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for _, server := range cfg.servers {
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server = JoinHostPort(server, "53")
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msg, err := exchange(server, name, qtype, timeout)
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if err != nil {
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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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if nerr, ok := err.(Error); ok && nerr.Timeout() {
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lastErr.(*DNSError).IsTimeout = true
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}
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continue
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}
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cname, rrs, err := answer(name, server, msg, qtype)
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// If answer errored for rcodes dnsRcodeSuccess or dnsRcodeNameError,
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// it means the response in msg was not useful and trying another
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// server probably won't help. Return now in those cases.
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// TODO: indicate this in a more obvious way, such as a field on DNSError?
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if err == nil || msg.rcode == dnsRcodeSuccess || msg.rcode == dnsRcodeNameError {
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return cname, rrs, err
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}
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lastErr = err
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}
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}
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return "", nil, lastErr
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}
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// addrRecordList converts and returns a list of IP addresses from DNS
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// address records (both A and AAAA). Other record types are ignored.
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func addrRecordList(rrs []dnsRR) []IPAddr {
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addrs := make([]IPAddr, 0, 4)
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for _, rr := range rrs {
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switch rr := rr.(type) {
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case *dnsRR_A:
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addrs = append(addrs, IPAddr{IP: IPv4(byte(rr.A>>24), byte(rr.A>>16), byte(rr.A>>8), byte(rr.A))})
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case *dnsRR_AAAA:
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ip := make(IP, IPv6len)
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copy(ip, rr.AAAA[:])
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addrs = append(addrs, IPAddr{IP: ip})
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}
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}
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return addrs
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}
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// A resolverConfig represents a DNS stub resolver configuration.
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type resolverConfig struct {
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initOnce sync.Once // guards init of resolverConfig
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// ch is used as a semaphore that only allows one lookup at a
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// time to recheck resolv.conf.
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ch chan struct{} // guards lastChecked and modTime
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lastChecked time.Time // last time resolv.conf was checked
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modTime time.Time // time of resolv.conf modification
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mu sync.RWMutex // protects dnsConfig
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dnsConfig *dnsConfig // parsed resolv.conf structure used in lookups
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}
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var resolvConf resolverConfig
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// init initializes conf and is only called via conf.initOnce.
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func (conf *resolverConfig) init() {
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// Set dnsConfig, modTime, and lastChecked so we don't parse
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// resolv.conf twice the first time.
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conf.dnsConfig = systemConf().resolv
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if conf.dnsConfig == nil {
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conf.dnsConfig = dnsReadConfig("/etc/resolv.conf")
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}
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if fi, err := os.Stat("/etc/resolv.conf"); err == nil {
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conf.modTime = fi.ModTime()
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}
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conf.lastChecked = time.Now()
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// Prepare ch so that only one update of resolverConfig may
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// run at once.
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conf.ch = make(chan struct{}, 1)
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}
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// tryUpdate tries to update conf with the named resolv.conf file.
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// The name variable only exists for testing. It is otherwise always
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// "/etc/resolv.conf".
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func (conf *resolverConfig) tryUpdate(name string) {
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conf.initOnce.Do(conf.init)
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// Ensure only one update at a time checks resolv.conf.
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if !conf.tryAcquireSema() {
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return
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}
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defer conf.releaseSema()
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now := time.Now()
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if conf.lastChecked.After(now.Add(-5 * time.Second)) {
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return
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}
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conf.lastChecked = now
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if fi, err := os.Stat(name); err == nil {
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if fi.ModTime().Equal(conf.modTime) {
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return
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}
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conf.modTime = fi.ModTime()
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} else {
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// If modTime wasn't set prior, assume nothing has changed.
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if conf.modTime.IsZero() {
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return
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}
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conf.modTime = time.Time{}
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}
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dnsConf := dnsReadConfig(name)
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conf.mu.Lock()
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conf.dnsConfig = dnsConf
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conf.mu.Unlock()
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}
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func (conf *resolverConfig) tryAcquireSema() bool {
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select {
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case conf.ch <- struct{}{}:
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return true
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default:
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return false
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}
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}
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func (conf *resolverConfig) releaseSema() {
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<-conf.ch
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}
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func lookup(name string, qtype uint16) (cname string, rrs []dnsRR, err error) {
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if !isDomainName(name) {
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return "", nil, &DNSError{Err: "invalid domain name", Name: name}
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}
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resolvConf.tryUpdate("/etc/resolv.conf")
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resolvConf.mu.RLock()
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conf := resolvConf.dnsConfig
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resolvConf.mu.RUnlock()
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for _, fqdn := range conf.nameList(name) {
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cname, rrs, err = tryOneName(conf, fqdn, qtype)
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if err == nil {
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break
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}
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}
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if err, ok := err.(*DNSError); ok {
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// Show original name passed to lookup, not suffixed one.
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// In general we might have tried many suffixes; showing
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// just one is misleading. See also golang.org/issue/6324.
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err.Name = name
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}
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return
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}
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// nameList returns a list of names for sequential DNS queries.
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func (conf *dnsConfig) nameList(name string) []string {
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// If name is rooted (trailing dot), try only that name.
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rooted := len(name) > 0 && name[len(name)-1] == '.'
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if rooted {
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return []string{name}
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}
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// Build list of search choices.
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names := make([]string, 0, 1+len(conf.search))
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// If name has enough dots, try unsuffixed first.
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if count(name, '.') >= conf.ndots {
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names = append(names, name+".")
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}
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// Try suffixes.
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for _, suffix := range conf.search {
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suffixed := name + "." + suffix
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if suffixed[len(suffixed)-1] != '.' {
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suffixed += "."
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}
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names = append(names, suffixed)
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}
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// Try unsuffixed, if not tried first above.
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if count(name, '.') < conf.ndots {
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names = append(names, name+".")
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}
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return names
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}
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// hostLookupOrder specifies the order of LookupHost lookup strategies.
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// It is basically a simplified representation of nsswitch.conf.
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// "files" means /etc/hosts.
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type hostLookupOrder int
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const (
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// hostLookupCgo means defer to cgo.
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hostLookupCgo hostLookupOrder = iota
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hostLookupFilesDNS // files first
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hostLookupDNSFiles // dns first
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hostLookupFiles // only files
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hostLookupDNS // only DNS
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)
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var lookupOrderName = map[hostLookupOrder]string{
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hostLookupCgo: "cgo",
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hostLookupFilesDNS: "files,dns",
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hostLookupDNSFiles: "dns,files",
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hostLookupFiles: "files",
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hostLookupDNS: "dns",
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}
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func (o hostLookupOrder) String() string {
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if s, ok := lookupOrderName[o]; ok {
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return s
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}
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return "hostLookupOrder=" + itoa(int(o)) + "??"
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}
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// goLookupHost is the native Go implementation of LookupHost.
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// Used only if cgoLookupHost refuses to handle the request
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// (that is, only if cgoLookupHost is the stub in cgo_stub.go).
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// Normally we let cgo use the C library resolver instead of
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// depending on our lookup code, so that Go and C get the same
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// answers.
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func goLookupHost(name string) (addrs []string, err error) {
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return goLookupHostOrder(name, hostLookupFilesDNS)
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}
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func goLookupHostOrder(name string, order hostLookupOrder) (addrs []string, err error) {
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if order == hostLookupFilesDNS || order == hostLookupFiles {
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// Use entries from /etc/hosts if they match.
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addrs = lookupStaticHost(name)
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if len(addrs) > 0 || order == hostLookupFiles {
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return
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}
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}
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ips, err := goLookupIPOrder(name, order)
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if err != nil {
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return
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}
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addrs = make([]string, 0, len(ips))
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for _, ip := range ips {
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addrs = append(addrs, ip.String())
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}
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return
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}
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// lookup entries from /etc/hosts
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func goLookupIPFiles(name string) (addrs []IPAddr) {
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for _, haddr := range lookupStaticHost(name) {
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haddr, zone := splitHostZone(haddr)
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if ip := ParseIP(haddr); ip != nil {
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addr := IPAddr{IP: ip, Zone: zone}
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addrs = append(addrs, addr)
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}
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}
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sortByRFC6724(addrs)
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return
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}
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// goLookupIP is the native Go implementation of LookupIP.
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// The libc versions are in cgo_*.go.
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func goLookupIP(name string) (addrs []IPAddr, err error) {
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return goLookupIPOrder(name, hostLookupFilesDNS)
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}
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func goLookupIPOrder(name string, order hostLookupOrder) (addrs []IPAddr, err error) {
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if order == hostLookupFilesDNS || order == hostLookupFiles {
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addrs = goLookupIPFiles(name)
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if len(addrs) > 0 || order == hostLookupFiles {
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return addrs, nil
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}
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}
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if !isDomainName(name) {
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return nil, &DNSError{Err: "invalid domain name", Name: name}
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}
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resolvConf.tryUpdate("/etc/resolv.conf")
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resolvConf.mu.RLock()
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conf := resolvConf.dnsConfig
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resolvConf.mu.RUnlock()
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type racer struct {
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fqdn string
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rrs []dnsRR
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error
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}
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lane := make(chan racer, 1)
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qtypes := [...]uint16{dnsTypeA, dnsTypeAAAA}
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var lastErr error
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for _, fqdn := range conf.nameList(name) {
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for _, qtype := range qtypes {
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go func(qtype uint16) {
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_, rrs, err := tryOneName(conf, fqdn, qtype)
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lane <- racer{fqdn, rrs, err}
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}(qtype)
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}
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for range qtypes {
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racer := <-lane
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if racer.error != nil {
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// Prefer error for original name.
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if lastErr == nil || racer.fqdn == name+"." {
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lastErr = racer.error
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}
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continue
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}
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addrs = append(addrs, addrRecordList(racer.rrs)...)
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}
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if len(addrs) > 0 {
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break
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}
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}
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if lastErr, ok := lastErr.(*DNSError); ok {
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// Show original name passed to lookup, not suffixed one.
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// In general we might have tried many suffixes; showing
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// just one is misleading. See also golang.org/issue/6324.
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lastErr.Name = name
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}
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sortByRFC6724(addrs)
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if len(addrs) == 0 {
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if order == hostLookupDNSFiles {
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addrs = goLookupIPFiles(name)
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}
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if len(addrs) == 0 && lastErr != nil {
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return nil, lastErr
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}
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}
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return addrs, nil
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}
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|
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// goLookupCNAME is the native Go implementation of LookupCNAME.
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// Used only if cgoLookupCNAME refuses to handle the request
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// (that is, only if cgoLookupCNAME is the stub in cgo_stub.go).
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// Normally we let cgo use the C library resolver instead of
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// depending on our lookup code, so that Go and C get the same
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// answers.
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func goLookupCNAME(name string) (cname string, err error) {
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_, rrs, err := lookup(name, dnsTypeCNAME)
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if err != nil {
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return
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}
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cname = rrs[0].(*dnsRR_CNAME).Cname
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return
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}
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// goLookupPTR is the native Go implementation of LookupAddr.
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|
// Used only if cgoLookupPTR refuses to handle the request (that is,
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|
// only if cgoLookupPTR is the stub in cgo_stub.go).
|
|
// Normally we let cgo use the C library resolver instead of depending
|
|
// on our lookup code, so that Go and C get the same answers.
|
|
func goLookupPTR(addr string) ([]string, error) {
|
|
names := lookupStaticAddr(addr)
|
|
if len(names) > 0 {
|
|
return names, nil
|
|
}
|
|
arpa, err := reverseaddr(addr)
|
|
if err != nil {
|
|
return nil, err
|
|
}
|
|
_, rrs, err := lookup(arpa, dnsTypePTR)
|
|
if err != nil {
|
|
return nil, err
|
|
}
|
|
ptrs := make([]string, len(rrs))
|
|
for i, rr := range rrs {
|
|
ptrs[i] = rr.(*dnsRR_PTR).Ptr
|
|
}
|
|
return ptrs, nil
|
|
}
|