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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  	"internal/itoa"
     9  	"sort"
    10  
    11  	"golang.org/x/net/dns/dnsmessage"
    12  )
    13  
    14  // provided by runtime
    15  func fastrand() uint32
    16  
    17  func randInt() int {
    18  	x, y := fastrand(), fastrand()    // 32-bit halves
    19  	u := uint(x)<<31 ^ uint(int32(y)) // full uint, even on 64-bit systems; avoid 32-bit shift on 32-bit systems
    20  	i := int(u >> 1)                  // clear sign bit, even on 32-bit systems
    21  	return i
    22  }
    23  
    24  func randIntn(n int) int {
    25  	return randInt() % n
    26  }
    27  
    28  // reverseaddr returns the in-addr.arpa. or ip6.arpa. hostname of the IP
    29  // address addr suitable for rDNS (PTR) record lookup or an error if it fails
    30  // to parse the IP address.
    31  func reverseaddr(addr string) (arpa string, err error) {
    32  	ip := ParseIP(addr)
    33  	if ip == nil {
    34  		return "", &DNSError{Err: "unrecognized address", Name: addr}
    35  	}
    36  	if ip.To4() != nil {
    37  		return itoa.Uitoa(uint(ip[15])) + "." + itoa.Uitoa(uint(ip[14])) + "." + itoa.Uitoa(uint(ip[13])) + "." + itoa.Uitoa(uint(ip[12])) + ".in-addr.arpa.", nil
    38  	}
    39  	// Must be IPv6
    40  	buf := make([]byte, 0, len(ip)*4+len("ip6.arpa."))
    41  	// Add it, in reverse, to the buffer
    42  	for i := len(ip) - 1; i >= 0; i-- {
    43  		v := ip[i]
    44  		buf = append(buf, hexDigit[v&0xF],
    45  			'.',
    46  			hexDigit[v>>4],
    47  			'.')
    48  	}
    49  	// Append "ip6.arpa." and return (buf already has the final .)
    50  	buf = append(buf, "ip6.arpa."...)
    51  	return string(buf), nil
    52  }
    53  
    54  func equalASCIIName(x, y dnsmessage.Name) bool {
    55  	if x.Length != y.Length {
    56  		return false
    57  	}
    58  	for i := 0; i < int(x.Length); i++ {
    59  		a := x.Data[i]
    60  		b := y.Data[i]
    61  		if 'A' <= a && a <= 'Z' {
    62  			a += 0x20
    63  		}
    64  		if 'A' <= b && b <= 'Z' {
    65  			b += 0x20
    66  		}
    67  		if a != b {
    68  			return false
    69  		}
    70  	}
    71  	return true
    72  }
    73  
    74  // isDomainName checks if a string is a presentation-format domain name
    75  // (currently restricted to hostname-compatible "preferred name" LDH labels and
    76  // SRV-like "underscore labels"; see golang.org/issue/12421).
    77  func isDomainName(s string) bool {
    78  	// See RFC 1035, RFC 3696.
    79  	// Presentation format has dots before every label except the first, and the
    80  	// terminal empty label is optional here because we assume fully-qualified
    81  	// (absolute) input. We must therefore reserve space for the first and last
    82  	// labels' length octets in wire format, where they are necessary and the
    83  	// maximum total length is 255.
    84  	// So our _effective_ maximum is 253, but 254 is not rejected if the last
    85  	// character is a dot.
    86  	l := len(s)
    87  	if l == 0 || l > 254 || l == 254 && s[l-1] != '.' {
    88  		return false
    89  	}
    90  
    91  	last := byte('.')
    92  	nonNumeric := false // true once we've seen a letter or hyphen
    93  	partlen := 0
    94  	for i := 0; i < len(s); i++ {
    95  		c := s[i]
    96  		switch {
    97  		default:
    98  			return false
    99  		case 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z' || c == '_':
   100  			nonNumeric = true
   101  			partlen++
   102  		case '0' <= c && c <= '9':
   103  			// fine
   104  			partlen++
   105  		case c == '-':
   106  			// Byte before dash cannot be dot.
   107  			if last == '.' {
   108  				return false
   109  			}
   110  			partlen++
   111  			nonNumeric = true
   112  		case c == '.':
   113  			// Byte before dot cannot be dot, dash.
   114  			if last == '.' || last == '-' {
   115  				return false
   116  			}
   117  			if partlen > 63 || partlen == 0 {
   118  				return false
   119  			}
   120  			partlen = 0
   121  		}
   122  		last = c
   123  	}
   124  	if last == '-' || partlen > 63 {
   125  		return false
   126  	}
   127  
   128  	return nonNumeric
   129  }
   130  
   131  // absDomainName returns an absolute domain name which ends with a
   132  // trailing dot to match pure Go reverse resolver and all other lookup
   133  // routines.
   134  // See golang.org/issue/12189.
   135  // But we don't want to add dots for local names from /etc/hosts.
   136  // It's hard to tell so we settle on the heuristic that names without dots
   137  // (like "localhost" or "myhost") do not get trailing dots, but any other
   138  // names do.
   139  func absDomainName(b []byte) string {
   140  	hasDots := false
   141  	for _, x := range b {
   142  		if x == '.' {
   143  			hasDots = true
   144  			break
   145  		}
   146  	}
   147  	if hasDots && b[len(b)-1] != '.' {
   148  		b = append(b, '.')
   149  	}
   150  	return string(b)
   151  }
   152  
   153  // An SRV represents a single DNS SRV record.
   154  type SRV struct {
   155  	Target   string
   156  	Port     uint16
   157  	Priority uint16
   158  	Weight   uint16
   159  }
   160  
   161  // byPriorityWeight sorts SRV records by ascending priority and weight.
   162  type byPriorityWeight []*SRV
   163  
   164  func (s byPriorityWeight) Len() int { return len(s) }
   165  func (s byPriorityWeight) Less(i, j int) bool {
   166  	return s[i].Priority < s[j].Priority || (s[i].Priority == s[j].Priority && s[i].Weight < s[j].Weight)
   167  }
   168  func (s byPriorityWeight) Swap(i, j int) { s[i], s[j] = s[j], s[i] }
   169  
   170  // shuffleByWeight shuffles SRV records by weight using the algorithm
   171  // described in RFC 2782.
   172  func (addrs byPriorityWeight) shuffleByWeight() {
   173  	sum := 0
   174  	for _, addr := range addrs {
   175  		sum += int(addr.Weight)
   176  	}
   177  	for sum > 0 && len(addrs) > 1 {
   178  		s := 0
   179  		n := randIntn(sum)
   180  		for i := range addrs {
   181  			s += int(addrs[i].Weight)
   182  			if s > n {
   183  				if i > 0 {
   184  					addrs[0], addrs[i] = addrs[i], addrs[0]
   185  				}
   186  				break
   187  			}
   188  		}
   189  		sum -= int(addrs[0].Weight)
   190  		addrs = addrs[1:]
   191  	}
   192  }
   193  
   194  // sort reorders SRV records as specified in RFC 2782.
   195  func (addrs byPriorityWeight) sort() {
   196  	sort.Sort(addrs)
   197  	i := 0
   198  	for j := 1; j < len(addrs); j++ {
   199  		if addrs[i].Priority != addrs[j].Priority {
   200  			addrs[i:j].shuffleByWeight()
   201  			i = j
   202  		}
   203  	}
   204  	addrs[i:].shuffleByWeight()
   205  }
   206  
   207  // An MX represents a single DNS MX record.
   208  type MX struct {
   209  	Host string
   210  	Pref uint16
   211  }
   212  
   213  // byPref implements sort.Interface to sort MX records by preference
   214  type byPref []*MX
   215  
   216  func (s byPref) Len() int           { return len(s) }
   217  func (s byPref) Less(i, j int) bool { return s[i].Pref < s[j].Pref }
   218  func (s byPref) Swap(i, j int)      { s[i], s[j] = s[j], s[i] }
   219  
   220  // sort reorders MX records as specified in RFC 5321.
   221  func (s byPref) sort() {
   222  	for i := range s {
   223  		j := randIntn(i + 1)
   224  		s[i], s[j] = s[j], s[i]
   225  	}
   226  	sort.Sort(s)
   227  }
   228  
   229  // An NS represents a single DNS NS record.
   230  type NS struct {
   231  	Host string
   232  }
   233  

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