Source file src/pkg/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		"bytes"
     9		"fmt"
    10		"os"
    11		"rand"
    12		"sort"
    13	)
    14	
    15	// DNSError represents a DNS lookup error.
    16	type DNSError struct {
    17		Error     string // description of the error
    18		Name      string // name looked for
    19		Server    string // server used
    20		IsTimeout bool
    21	}
    22	
    23	func (e *DNSError) String() string {
    24		if e == nil {
    25			return "<nil>"
    26		}
    27		s := "lookup " + e.Name
    28		if e.Server != "" {
    29			s += " on " + e.Server
    30		}
    31		s += ": " + e.Error
    32		return s
    33	}
    34	
    35	func (e *DNSError) Timeout() bool   { return e.IsTimeout }
    36	func (e *DNSError) Temporary() bool { return e.IsTimeout }
    37	
    38	const noSuchHost = "no such host"
    39	
    40	// reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
    41	// address addr suitable for rDNS (PTR) record lookup or an error if it fails
    42	// to parse the IP address.
    43	func reverseaddr(addr string) (arpa string, err os.Error) {
    44		ip := ParseIP(addr)
    45		if ip == nil {
    46			return "", &DNSError{Error: "unrecognized address", Name: addr}
    47		}
    48		if ip.To4() != nil {
    49			return fmt.Sprintf("%d.%d.%d.%d.in-addr.arpa.", ip[15], ip[14], ip[13], ip[12]), nil
    50		}
    51		// Must be IPv6
    52		var buf bytes.Buffer
    53		// Add it, in reverse, to the buffer
    54		for i := len(ip) - 1; i >= 0; i-- {
    55			s := fmt.Sprintf("%02x", ip[i])
    56			buf.WriteByte(s[1])
    57			buf.WriteByte('.')
    58			buf.WriteByte(s[0])
    59			buf.WriteByte('.')
    60		}
    61		// Append "ip6.arpa." and return (buf already has the final .)
    62		return buf.String() + "ip6.arpa.", nil
    63	}
    64	
    65	// Find answer for name in dns message.
    66	// On return, if err == nil, addrs != nil.
    67	func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs []dnsRR, err os.Error) {
    68		addrs = make([]dnsRR, 0, len(dns.answer))
    69	
    70		if dns.rcode == dnsRcodeNameError && dns.recursion_available {
    71			return "", nil, &DNSError{Error: noSuchHost, Name: name}
    72		}
    73		if dns.rcode != dnsRcodeSuccess {
    74			// None of the error codes make sense
    75			// for the query we sent.  If we didn't get
    76			// a name error and we didn't get success,
    77			// the server is behaving incorrectly.
    78			return "", nil, &DNSError{Error: "server misbehaving", Name: name, Server: server}
    79		}
    80	
    81		// Look for the name.
    82		// Presotto says it's okay to assume that servers listed in
    83		// /etc/resolv.conf are recursive resolvers.
    84		// We asked for recursion, so it should have included
    85		// all the answers we need in this one packet.
    86	Cname:
    87		for cnameloop := 0; cnameloop < 10; cnameloop++ {
    88			addrs = addrs[0:0]
    89			for _, rr := range dns.answer {
    90				if _, justHeader := rr.(*dnsRR_Header); justHeader {
    91					// Corrupt record: we only have a
    92					// header. That header might say it's
    93					// of type qtype, but we don't
    94					// actually have it. Skip.
    95					continue
    96				}
    97				h := rr.Header()
    98				if h.Class == dnsClassINET && h.Name == name {
    99					switch h.Rrtype {
   100					case qtype:
   101						addrs = append(addrs, rr)
   102					case dnsTypeCNAME:
   103						// redirect to cname
   104						name = rr.(*dnsRR_CNAME).Cname
   105						continue Cname
   106					}
   107				}
   108			}
   109			if len(addrs) == 0 {
   110				return "", nil, &DNSError{Error: noSuchHost, Name: name, Server: server}
   111			}
   112			return name, addrs, nil
   113		}
   114	
   115		return "", nil, &DNSError{Error: "too many redirects", Name: name, Server: server}
   116	}
   117	
   118	func isDomainName(s string) bool {
   119		// See RFC 1035, RFC 3696.
   120		if len(s) == 0 {
   121			return false
   122		}
   123		if len(s) > 255 {
   124			return false
   125		}
   126		if s[len(s)-1] != '.' { // simplify checking loop: make name end in dot
   127			s += "."
   128		}
   129	
   130		last := byte('.')
   131		ok := false // ok once we've seen a letter
   132		partlen := 0
   133		for i := 0; i < len(s); i++ {
   134			c := s[i]
   135			switch {
   136			default:
   137				return false
   138			case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
   139				ok = true
   140				partlen++
   141			case '0' <= c && c <= '9':
   142				// fine
   143				partlen++
   144			case c == '-':
   145				// byte before dash cannot be dot
   146				if last == '.' {
   147					return false
   148				}
   149				partlen++
   150			case c == '.':
   151				// byte before dot cannot be dot, dash
   152				if last == '.' || last == '-' {
   153					return false
   154				}
   155				if partlen > 63 || partlen == 0 {
   156					return false
   157				}
   158				partlen = 0
   159			}
   160			last = c
   161		}
   162	
   163		return ok
   164	}
   165	
   166	// An SRV represents a single DNS SRV record.
   167	type SRV struct {
   168		Target   string
   169		Port     uint16
   170		Priority uint16
   171		Weight   uint16
   172	}
   173	
   174	// byPriorityWeight sorts SRV records by ascending priority and weight.
   175	type byPriorityWeight []*SRV
   176	
   177	func (s byPriorityWeight) Len() int { return len(s) }
   178	
   179	func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
   180	
   181	func (s byPriorityWeight) Less(i, j int) bool {
   182		return s[i].Priority < s[j].Priority ||
   183			(s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
   184	}
   185	
   186	// shuffleByWeight shuffles SRV records by weight using the algorithm
   187	// described in RFC 2782.  
   188	func (addrs byPriorityWeight) shuffleByWeight() {
   189		sum := 0
   190		for _, addr := range addrs {
   191			sum += int(addr.Weight)
   192		}
   193		for sum > 0 && len(addrs) > 1 {
   194			s := 0
   195			n := rand.Intn(sum + 1)
   196			for i := range addrs {
   197				s += int(addrs[i].Weight)
   198				if s >= n {
   199					if i > 0 {
   200						t := addrs[i]
   201						copy(addrs[1:i+1], addrs[0:i])
   202						addrs[0] = t
   203					}
   204					break
   205				}
   206			}
   207			sum -= int(addrs[0].Weight)
   208			addrs = addrs[1:]
   209		}
   210	}
   211	
   212	// sort reorders SRV records as specified in RFC 2782.
   213	func (addrs byPriorityWeight) sort() {
   214		sort.Sort(addrs)
   215		i := 0
   216		for j := 1; j < len(addrs); j++ {
   217			if addrs[i].Priority != addrs[j].Priority {
   218				addrs[i:j].shuffleByWeight()
   219				i = j
   220			}
   221		}
   222		addrs[i:].shuffleByWeight()
   223	}
   224	
   225	// An MX represents a single DNS MX record.
   226	type MX struct {
   227		Host string
   228		Pref uint16
   229	}
   230	
   231	// byPref implements sort.Interface to sort MX records by preference
   232	type byPref []*MX
   233	
   234	func (s byPref) Len() int { return len(s) }
   235	
   236	func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
   237	
   238	func (s byPref) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
   239	
   240	// sort reorders MX records as specified in RFC 5321.
   241	func (s byPref) sort() {
   242		for i := range s {
   243			j := rand.Intn(i + 1)
   244			s[i], s[j] = s[j], s[i]
   245		}
   246		sort.Sort(s)
   247	}
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Source file src/pkg/net/dnsclient.go
