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		"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		if s[len(s)-1] != '.' { // simplify checking loop: make name end in dot
   126			s += "."
   127		}
   128	
   129		last := byte('.')
   130		ok := false // ok once we've seen a letter
   131		partlen := 0
   132		for i := 0; i < len(s); i++ {
   133			c := s[i]
   134			switch {
   135			default:
   136				return false
   137			case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
   138				ok = true
   139				partlen++
   140			case '0' <= c && c <= '9':
   141				// fine
   142				partlen++
   143			case c == '-':
   144				// byte before dash cannot be dot
   145				if last == '.' {
   146					return false
   147				}
   148				partlen++
   149			case c == '.':
   150				// byte before dot cannot be dot, dash
   151				if last == '.' || last == '-' {
   152					return false
   153				}
   154				if partlen > 63 || partlen == 0 {
   155					return false
   156				}
   157				partlen = 0
   158			}
   159			last = c
   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 + 1)
   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	}