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