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Source file src/net/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  	"internal/x/net/dns/dnsmessage"
    12  )
    13  
    14  // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
    15  // address addr suitable for rDNS (PTR) record lookup or an error if it fails
    16  // to parse the IP address.
    17  func reverseaddr(addr string) (arpa string, err error) {
    18  	ip := ParseIP(addr)
    19  	if ip == nil {
    20  		return "", &DNSError{Err: "unrecognized address", Name: addr}
    21  	}
    22  	if ip.To4() != nil {
    23  		return uitoa(uint(ip[15])) + "." + uitoa(uint(ip[14])) + "." + uitoa(uint(ip[13])) + "." + uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
    24  	}
    25  	// Must be IPv6
    26  	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
    27  	// Add it, in reverse, to the buffer
    28  	for i := len(ip) - 1; i >= 0; i-- {
    29  		v := ip[i]
    30  		buf = append(buf, hexDigit[v&0xF],
    31  			'.',
    32  			hexDigit[v>>4],
    33  			'.')
    34  	}
    35  	// Append "ip6.arpa." and return (buf already has the final .)
    36  	buf = append(buf, "ip6.arpa."...)
    37  	return string(buf), nil
    38  }
    39  
    40  func equalASCIIName(x, y dnsmessage.Name) bool {
    41  	if x.Length != y.Length {
    42  		return false
    43  	}
    44  	for i := 0; i < int(x.Length); i++ {
    45  		a := x.Data[i]
    46  		b := y.Data[i]
    47  		if 'A' <= a && a <= 'Z' {
    48  			a += 0x20
    49  		}
    50  		if 'A' <= b && b <= 'Z' {
    51  			b += 0x20
    52  		}
    53  		if a != b {
    54  			return false
    55  		}
    56  	}
    57  	return true
    58  }
    59  
    60  // isDomainName checks if a string is a presentation-format domain name
    61  // (currently restricted to hostname-compatible "preferred name" LDH labels and
    62  // SRV-like "underscore labels"; see golang.org/issue/12421).
    63  func isDomainName(s string) bool {
    64  	// See RFC 1035, RFC 3696.
    65  	// Presentation format has dots before every label except the first, and the
    66  	// terminal empty label is optional here because we assume fully-qualified
    67  	// (absolute) input. We must therefore reserve space for the first and last
    68  	// labels' length octets in wire format, where they are necessary and the
    69  	// maximum total length is 255.
    70  	// So our _effective_ maximum is 253, but 254 is not rejected if the last
    71  	// character is a dot.
    72  	l := len(s)
    73  	if l == 0 || l > 254 || l == 254 && s[l-1] != '.' {
    74  		return false
    75  	}
    76  
    77  	last := byte('.')
    78  	nonNumeric := false // true once we've seen a letter or hyphen
    79  	partlen := 0
    80  	for i := 0; i < len(s); i++ {
    81  		c := s[i]
    82  		switch {
    83  		default:
    84  			return false
    85  		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
    86  			nonNumeric = true
    87  			partlen++
    88  		case '0' <= c && c <= '9':
    89  			// fine
    90  			partlen++
    91  		case c == '-':
    92  			// Byte before dash cannot be dot.
    93  			if last == '.' {
    94  				return false
    95  			}
    96  			partlen++
    97  			nonNumeric = true
    98  		case c == '.':
    99  			// Byte before dot cannot be dot, dash.
   100  			if last == '.' || last == '-' {
   101  				return false
   102  			}
   103  			if partlen > 63 || partlen == 0 {
   104  				return false
   105  			}
   106  			partlen = 0
   107  		}
   108  		last = c
   109  	}
   110  	if last == '-' || partlen > 63 {
   111  		return false
   112  	}
   113  
   114  	return nonNumeric
   115  }
   116  
   117  // absDomainName returns an absolute domain name which ends with a
   118  // trailing dot to match pure Go reverse resolver and all other lookup
   119  // routines.
   120  // See golang.org/issue/12189.
   121  // But we don't want to add dots for local names from /etc/hosts.
   122  // It's hard to tell so we settle on the heuristic that names without dots
   123  // (like "localhost" or "myhost") do not get trailing dots, but any other
   124  // names do.
   125  func absDomainName(b []byte) string {
   126  	hasDots := false
   127  	for _, x := range b {
   128  		if x == '.' {
   129  			hasDots = true
   130  			break
   131  		}
   132  	}
   133  	if hasDots && b[len(b)-1] != '.' {
   134  		b = append(b, '.')
   135  	}
   136  	return string(b)
   137  }
   138  
   139  // An SRV represents a single DNS SRV record.
   140  type SRV struct {
   141  	Target   string
   142  	Port     uint16
   143  	Priority uint16
   144  	Weight   uint16
   145  }
   146  
   147  // byPriorityWeight sorts SRV records by ascending priority and weight.
   148  type byPriorityWeight []*SRV
   149  
   150  func (s byPriorityWeight) Len() int { return len(s) }
   151  func (s byPriorityWeight) Less(i, j int) bool {
   152  	return s[i].Priority < s[j].Priority || (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
   153  }
   154  func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
   155  
   156  // shuffleByWeight shuffles SRV records by weight using the algorithm
   157  // described in RFC 2782.
   158  func (addrs byPriorityWeight) shuffleByWeight() {
   159  	sum := 0
   160  	for _, addr := range addrs {
   161  		sum += int(addr.Weight)
   162  	}
   163  	for sum > 0 && len(addrs) > 1 {
   164  		s := 0
   165  		n := rand.Intn(sum)
   166  		for i := range addrs {
   167  			s += int(addrs[i].Weight)
   168  			if s > n {
   169  				if i > 0 {
   170  					addrs[0], addrs[i] = addrs[i], addrs[0]
   171  				}
   172  				break
   173  			}
   174  		}
   175  		sum -= int(addrs[0].Weight)
   176  		addrs = addrs[1:]
   177  	}
   178  }
   179  
   180  // sort reorders SRV records as specified in RFC 2782.
   181  func (addrs byPriorityWeight) sort() {
   182  	sort.Sort(addrs)
   183  	i := 0
   184  	for j := 1; j < len(addrs); j++ {
   185  		if addrs[i].Priority != addrs[j].Priority {
   186  			addrs[i:j].shuffleByWeight()
   187  			i = j
   188  		}
   189  	}
   190  	addrs[i:].shuffleByWeight()
   191  }
   192  
   193  // An MX represents a single DNS MX record.
   194  type MX struct {
   195  	Host string
   196  	Pref uint16
   197  }
   198  
   199  // byPref implements sort.Interface to sort MX records by preference
   200  type byPref []*MX
   201  
   202  func (s byPref) Len() int           { return len(s) }
   203  func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
   204  func (s byPref) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
   205  
   206  // sort reorders MX records as specified in RFC 5321.
   207  func (s byPref) sort() {
   208  	for i := range s {
   209  		j := rand.Intn(i + 1)
   210  		s[i], s[j] = s[j], s[i]
   211  	}
   212  	sort.Sort(s)
   213  }
   214  
   215  // An NS represents a single DNS NS record.
   216  type NS struct {
   217  	Host string
   218  }
   219  

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