dnsclient.go

Documentation: net

     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  // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
    13  // address addr suitable for rDNS (PTR) record lookup or an error if it fails
    14  // to parse the IP address.
    15  func reverseaddr(addr string) (arpa string, err error) {
    16  	ip := ParseIP(addr)
    17  	if ip == nil {
    18  		return "", &DNSError{Err: "unrecognized address", Name: addr}
    19  	}
    20  	if ip.To4() != nil {
    21  		return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
    22  	}
    23  	// Must be IPv6
    24  	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
    25  	// Add it, in reverse, to the buffer
    26  	for i := len(ip) - 1; i >= 0; i-- {
    27  		v := ip[i]
    28  		buf = append(buf, hexDigit[v&0xF])
    29  		buf = append(buf, '.')
    30  		buf = append(buf, hexDigit[v>>4])
    31  		buf = append(buf, '.')
    32  	}
    33  	// Append "ip6.arpa." and return (buf already has the final .)
    34  	buf = append(buf, "ip6.arpa."...)
    35  	return string(buf), nil
    36  }
    37  
    38  // Find answer for name in dns message.
    39  // On return, if err == nil, addrs != nil.
    40  func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs []dnsRR, err error) {
    41  	addrs = make([]dnsRR, 0, len(dns.answer))
    42  
    43  	if dns.rcode == dnsRcodeNameError {
    44  		return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
    45  	}
    46  	if dns.rcode != dnsRcodeSuccess {
    47  		// None of the error codes make sense
    48  		// for the query we sent. If we didn't get
    49  		// a name error and we didn't get success,
    50  		// the server is behaving incorrectly or
    51  		// having temporary trouble.
    52  		err := &DNSError{Err: "server misbehaving", Name: name, Server: server}
    53  		if dns.rcode == dnsRcodeServerFailure {
    54  			err.IsTemporary = true
    55  		}
    56  		return "", nil, err
    57  	}
    58  
    59  	// Look for the name.
    60  	// Presotto says it's okay to assume that servers listed in
    61  	// /etc/resolv.conf are recursive resolvers.
    62  	// We asked for recursion, so it should have included
    63  	// all the answers we need in this one packet.
    64  Cname:
    65  	for cnameloop := 0; cnameloop < 10; cnameloop++ {
    66  		addrs = addrs[0:0]
    67  		for _, rr := range dns.answer {
    68  			if _, justHeader := rr.(*dnsRR_Header); justHeader {
    69  				// Corrupt record: we only have a
    70  				// header. That header might say it's
    71  				// of type qtype, but we don't
    72  				// actually have it. Skip.
    73  				continue
    74  			}
    75  			h := rr.Header()
    76  			if h.Class == dnsClassINET && equalASCIILabel(h.Name, name) {
    77  				switch h.Rrtype {
    78  				case qtype:
    79  					addrs = append(addrs, rr)
    80  				case dnsTypeCNAME:
    81  					// redirect to cname
    82  					name = rr.(*dnsRR_CNAME).Cname
    83  					continue Cname
    84  				}
    85  			}
    86  		}
    87  		if len(addrs) == 0 {
    88  			return "", nil, &DNSError{Err: errNoSuchHost.Error(), Name: name, Server: server}
    89  		}
    90  		return name, addrs, nil
    91  	}
    92  
    93  	return "", nil, &DNSError{Err: "too many redirects", Name: name, Server: server}
    94  }
    95  
    96  func equalASCIILabel(x, y string) bool {
    97  	if len(x) != len(y) {
    98  		return false
    99  	}
   100  	for i := 0; i < len(x); i++ {
   101  		a := x[i]
   102  		b := y[i]
   103  		if 'A' <= a && a <= 'Z' {
   104  			a += 0x20
   105  		}
   106  		if 'A' <= b && b <= 'Z' {
   107  			b += 0x20
   108  		}
   109  		if a != b {
   110  			return false
   111  		}
   112  	}
   113  	return true
   114  }
   115  
   116  // isDomainName checks if a string is a presentation-format domain name
   117  // (currently restricted to hostname-compatible "preferred name" LDH labels and
   118  // SRV-like "underscore labels"; see golang.org/issue/12421).
   119  func isDomainName(s string) bool {
   120  	// See RFC 1035, RFC 3696.
   121  	// Presentation format has dots before every label except the first, and the
   122  	// terminal empty label is optional here because we assume fully-qualified
   123  	// (absolute) input. We must therefore reserve space for the first and last
   124  	// labels' length octets in wire format, where they are necessary and the
   125  	// maximum total length is 255.
   126  	// So our _effective_ maximum is 253, but 254 is not rejected if the last
   127  	// character is a dot.
   128  	l := len(s)
   129  	if l == 0 || l > 254 || l == 254 && s[l-1] != '.' {
   130  		return false
   131  	}
   132  
   133  	last := byte('.')
   134  	ok := false // Ok once we've seen a letter.
   135  	partlen := 0
   136  	for i := 0; i < len(s); i++ {
   137  		c := s[i]
   138  		switch {
   139  		default:
   140  			return false
   141  		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
   142  			ok = true
   143  			partlen++
   144  		case '0' <= c && c <= '9':
   145  			// fine
   146  			partlen++
   147  		case c == '-':
   148  			// Byte before dash cannot be dot.
   149  			if last == '.' {
   150  				return false
   151  			}
   152  			partlen++
   153  		case c == '.':
   154  			// Byte before dot cannot be dot, dash.
   155  			if last == '.' || last == '-' {
   156  				return false
   157  			}
   158  			if partlen > 63 || partlen == 0 {
   159  				return false
   160  			}
   161  			partlen = 0
   162  		}
   163  		last = c
   164  	}
   165  	if last == '-' || partlen > 63 {
   166  		return false
   167  	}
   168  
   169  	return ok
   170  }
   171  
   172  // absDomainName returns an absolute domain name which ends with a
   173  // trailing dot to match pure Go reverse resolver and all other lookup
   174  // routines.
   175  // See golang.org/issue/12189.
   176  // But we don't want to add dots for local names from /etc/hosts.
   177  // It's hard to tell so we settle on the heuristic that names without dots
   178  // (like "localhost" or "myhost") do not get trailing dots, but any other
   179  // names do.
   180  func absDomainName(b []byte) string {
   181  	hasDots := false
   182  	for _, x := range b {
   183  		if x == '.' {
   184  			hasDots = true
   185  			break
   186  		}
   187  	}
   188  	if hasDots && b[len(b)-1] != '.' {
   189  		b = append(b, '.')
   190  	}
   191  	return string(b)
   192  }
   193  
   194  // An SRV represents a single DNS SRV record.
   195  type SRV struct {
   196  	Target   string
   197  	Port     uint16
   198  	Priority uint16
   199  	Weight   uint16
   200  }
   201  
   202  // byPriorityWeight sorts SRV records by ascending priority and weight.
   203  type byPriorityWeight []*SRV
   204  
   205  func (s byPriorityWeight) Len() int { return len(s) }
   206  func (s byPriorityWeight) Less(i, j int) bool {
   207  	return s[i].Priority < s[j].Priority || (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
   208  }
   209  func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
   210  
   211  // shuffleByWeight shuffles SRV records by weight using the algorithm
   212  // described in RFC 2782.
   213  func (addrs byPriorityWeight) shuffleByWeight() {
   214  	sum := 0
   215  	for _, addr := range addrs {
   216  		sum += int(addr.Weight)
   217  	}
   218  	for sum > 0 && len(addrs) > 1 {
   219  		s := 0
   220  		n := rand.Intn(sum)
   221  		for i := range addrs {
   222  			s += int(addrs[i].Weight)
   223  			if s > n {
   224  				if i > 0 {
   225  					addrs[0], addrs[i] = addrs[i], addrs[0]
   226  				}
   227  				break
   228  			}
   229  		}
   230  		sum -= int(addrs[0].Weight)
   231  		addrs = addrs[1:]
   232  	}
   233  }
   234  
   235  // sort reorders SRV records as specified in RFC 2782.
   236  func (addrs byPriorityWeight) sort() {
   237  	sort.Sort(addrs)
   238  	i := 0
   239  	for j := 1; j < len(addrs); j++ {
   240  		if addrs[i].Priority != addrs[j].Priority {
   241  			addrs[i:j].shuffleByWeight()
   242  			i = j
   243  		}
   244  	}
   245  	addrs[i:].shuffleByWeight()
   246  }
   247  
   248  // An MX represents a single DNS MX record.
   249  type MX struct {
   250  	Host string
   251  	Pref uint16
   252  }
   253  
   254  // byPref implements sort.Interface to sort MX records by preference
   255  type byPref []*MX
   256  
   257  func (s byPref) Len() int           { return len(s) }
   258  func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
   259  func (s byPref) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
   260  
   261  // sort reorders MX records as specified in RFC 5321.
   262  func (s byPref) sort() {
   263  	for i := range s {
   264  		j := rand.Intn(i + 1)
   265  		s[i], s[j] = s[j], s[i]
   266  	}
   267  	sort.Sort(s)
   268  }
   269  
   270  // An NS represents a single DNS NS record.
   271  type NS struct {
   272  	Host string
   273  }
   274
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