libgo/go/net/dnsclient.go

   1 // Copyright 2009 The Go Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style
   3 // license that can be found in the LICENSE file.
   4
   5 package net
   6
   7 import (
   8         "math/rand"
   9         "sort"
  10 )
  11
  12 // DNSError represents a DNS lookup error.
  13 type DNSError struct {
  14         Err       string // description of the error
  15         Name      string // name looked for
  16         Server    string // server used
  17         IsTimeout bool
  18 }
  19
  20 func (e *DNSError) Error() string {
  21         if e == nil {
  22                 return "<nil>"
  23         }
  24         s := "lookup " + e.Name
  25         if e.Server != "" {
  26                 s += " on " + e.Server
  27         }
  28         s += ": " + e.Err
  29         return s
  30 }
  31
  32 func (e *DNSError) Timeout() bool   { return e.IsTimeout }
  33 func (e *DNSError) Temporary() bool { return e.IsTimeout }
  34
  35 const noSuchHost = "no such host"
  36
  37 // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
  38 // address addr suitable for rDNS (PTR) record lookup or an error if it fails
  39 // to parse the IP address.
  40 func reverseaddr(addr string) (arpa string, err error) {
  41         ip := ParseIP(addr)
  42         if ip == nil {
  43                 return "", &DNSError{Err: "unrecognized address", Name: addr}
  44         }
  45         if ip.To4() != nil {
  46                 return itoa(int(ip[15])) + "." + itoa(int(ip[14])) + "." + itoa(int(ip[13])) + "." +
  47                         itoa(int(ip[12])) + ".in-addr.arpa.", nil
  48         }
  49         // Must be IPv6
  50         buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
  51         // Add it, in reverse, to the buffer
  52         for i := len(ip) - 1; i >= 0; i-- {
  53                 v := ip[i]
  54                 buf = append(buf, hexDigit[v&0xF])
  55                 buf = append(buf, '.')
  56                 buf = append(buf, hexDigit[v>>4])
  57                 buf = append(buf, '.')
  58         }
  59         // Append "ip6.arpa." and return (buf already has the final .)
  60         buf = append(buf, "ip6.arpa."...)
  61         return string(buf), nil
  62 }
  63
  64 // Find answer for name in dns message.
  65 // On return, if err == nil, addrs != nil.
  66 func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs []dnsRR, err error) {
  67         addrs = make([]dnsRR, 0, len(dns.answer))
  68
  69         if dns.rcode == dnsRcodeNameError && dns.recursion_available {
  70                 return "", nil, &DNSError{Err: noSuchHost, Name: name}
  71         }
  72         if dns.rcode != dnsRcodeSuccess {
  73                 // None of the error codes make sense
  74                 // for the query we sent.  If we didn't get
  75                 // a name error and we didn't get success,
  76                 // the server is behaving incorrectly.
  77                 return "", nil, &DNSError{Err: "server misbehaving", Name: name, Server: server}
  78         }
  79
  80         // Look for the name.
  81         // Presotto says it's okay to assume that servers listed in
  82         // /etc/resolv.conf are recursive resolvers.
  83         // We asked for recursion, so it should have included
  84         // all the answers we need in this one packet.
  85 Cname:
  86         for cnameloop := 0; cnameloop < 10; cnameloop++ {
  87                 addrs = addrs[0:0]
  88                 for _, rr := range dns.answer {
  89                         if _, justHeader := rr.(*dnsRR_Header); justHeader {
  90                                 // Corrupt record: we only have a
  91                                 // header. That header might say it's
  92                                 // of type qtype, but we don't
  93                                 // actually have it. Skip.
  94                                 continue
  95                         }
  96                         h := rr.Header()
  97                         if h.Class == dnsClassINET && h.Name == name {
  98                                 switch h.Rrtype {
  99                                 case qtype:
 100                                         addrs = append(addrs, rr)
 101                                 case dnsTypeCNAME:
 102                                         // redirect to cname
 103                                         name = rr.(*dnsRR_CNAME).Cname
 104                                         continue Cname
 105                                 }
 106                         }
 107                 }
 108                 if len(addrs) == 0 {
 109                         return "", nil, &DNSError{Err: noSuchHost, Name: name, Server: server}
 110                 }
 111                 return name, addrs, nil
 112         }
 113
 114         return "", nil, &DNSError{Err: "too many redirects", Name: name, Server: server}
 115 }
 116
 117 func isDomainName(s string) bool {
 118         // See RFC 1035, RFC 3696.
 119         if len(s) == 0 {
 120                 return false
 121         }
 122         if len(s) > 255 {
 123                 return false
 124         }
 125
 126         last := byte('.')
 127         ok := false // Ok once we've seen a letter.
 128         partlen := 0
 129         for i := 0; i < len(s); i++ {
 130                 c := s[i]
 131                 switch {
 132                 default:
 133                         return false
 134                 case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
 135                         ok = true
 136                         partlen++
 137                 case '0' <= c && c <= '9':
 138                         // fine
 139                         partlen++
 140                 case c == '-':
 141                         // Byte before dash cannot be dot.
 142                         if last == '.' {
 143                                 return false
 144                         }
 145                         partlen++
 146                 case c == '.':
 147                         // Byte before dot cannot be dot, dash.
 148                         if last == '.' || last == '-' {
 149                                 return false
 150                         }
 151                         if partlen > 63 || partlen == 0 {
 152                                 return false
 153                         }
 154                         partlen = 0
 155                 }
 156                 last = c
 157         }
 158         if last == '-' || partlen > 63 {
 159                 return false
 160         }
 161
 162         return ok
 163 }
 164
 165 // An SRV represents a single DNS SRV record.
 166 type SRV struct {
 167         Target   string
 168         Port     uint16
 169         Priority uint16
 170         Weight   uint16
 171 }
 172
 173 // byPriorityWeight sorts SRV records by ascending priority and weight.
 174 type byPriorityWeight []*SRV
 175
 176 func (s byPriorityWeight) Len() int { return len(s) }
 177
 178 func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
 179
 180 func (s byPriorityWeight) Less(i, j int) bool {
 181         return s[i].Priority < s[j].Priority ||
 182                 (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
 183 }
 184
 185 // shuffleByWeight shuffles SRV records by weight using the algorithm
 186 // described in RFC 2782.
 187 func (addrs byPriorityWeight) shuffleByWeight() {
 188         sum := 0
 189         for _, addr := range addrs {
 190                 sum += int(addr.Weight)
 191         }
 192         for sum > 0 && len(addrs) > 1 {
 193                 s := 0
 194                 n := rand.Intn(sum)
 195                 for i := range addrs {
 196                         s += int(addrs[i].Weight)
 197                         if s > n {
 198                                 if i > 0 {
 199                                         t := addrs[i]
 200                                         copy(addrs[1:i+1], addrs[0:i])
 201                                         addrs[0] = t
 202                                 }
 203                                 break
 204                         }
 205                 }
 206                 sum -= int(addrs[0].Weight)
 207                 addrs = addrs[1:]
 208         }
 209 }
 210
 211 // sort reorders SRV records as specified in RFC 2782.
 212 func (addrs byPriorityWeight) sort() {
 213         sort.Sort(addrs)
 214         i := 0
 215         for j := 1; j < len(addrs); j++ {
 216                 if addrs[i].Priority != addrs[j].Priority {
 217                         addrs[i:j].shuffleByWeight()
 218                         i = j
 219                 }
 220         }
 221         addrs[i:].shuffleByWeight()
 222 }
 223
 224 // An MX represents a single DNS MX record.
 225 type MX struct {
 226         Host string
 227         Pref uint16
 228 }
 229
 230 // byPref implements sort.Interface to sort MX records by preference
 231 type byPref []*MX
 232
 233 func (s byPref) Len() int { return len(s) }
 234
 235 func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
 236
 237 func (s byPref) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
 238
 239 // sort reorders MX records as specified in RFC 5321.
 240 func (s byPref) sort() {
 241         for i := range s {
 242                 j := rand.Intn(i + 1)
 243                 s[i], s[j] = s[j], s[i]
 244         }
 245         sort.Sort(s)
 246 }
 247
 248 // An NS represents a single DNS NS record.
 249 type NS struct {
 250         Host string
 251 }