dnsconfig_unix.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  // +build darwin dragonfly freebsd linux netbsd openbsd solaris
     6  
     7  // Read system DNS config from /etc/resolv.conf
     8  
     9  package net
    10  
    11  import (
    12  	"os"
    13  	"sync/atomic"
    14  	"time"
    15  )
    16  
    17  var (
    18  	defaultNS   = []string{"127.0.0.1:53", "[::1]:53"}
    19  	getHostname = os.Hostname // variable for testing
    20  )
    21  
    22  type dnsConfig struct {
    23  	servers    []string      // server addresses (in host:port form) to use
    24  	search     []string      // rooted suffixes to append to local name
    25  	ndots      int           // number of dots in name to trigger absolute lookup
    26  	timeout    time.Duration // wait before giving up on a query, including retries
    27  	attempts   int           // lost packets before giving up on server
    28  	rotate     bool          // round robin among servers
    29  	unknownOpt bool          // anything unknown was encountered
    30  	lookup     []string      // OpenBSD top-level database "lookup" order
    31  	err        error         // any error that occurs during open of resolv.conf
    32  	mtime      time.Time     // time of resolv.conf modification
    33  	soffset    uint32        // used by serverOffset
    34  }
    35  
    36  // See resolv.conf(5) on a Linux machine.
    37  func dnsReadConfig(filename string) *dnsConfig {
    38  	conf := &dnsConfig{
    39  		ndots:    1,
    40  		timeout:  5 * time.Second,
    41  		attempts: 2,
    42  	}
    43  	file, err := open(filename)
    44  	if err != nil {
    45  		conf.servers = defaultNS
    46  		conf.search = dnsDefaultSearch()
    47  		conf.err = err
    48  		return conf
    49  	}
    50  	defer file.close()
    51  	if fi, err := file.file.Stat(); err == nil {
    52  		conf.mtime = fi.ModTime()
    53  	} else {
    54  		conf.servers = defaultNS
    55  		conf.search = dnsDefaultSearch()
    56  		conf.err = err
    57  		return conf
    58  	}
    59  	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
    60  		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
    61  			// comment.
    62  			continue
    63  		}
    64  		f := getFields(line)
    65  		if len(f) < 1 {
    66  			continue
    67  		}
    68  		switch f[0] {
    69  		case "nameserver": // add one name server
    70  			if len(f) > 1 && len(conf.servers) < 3 { // small, but the standard limit
    71  				// One more check: make sure server name is
    72  				// just an IP address. Otherwise we need DNS
    73  				// to look it up.
    74  				if parseIPv4(f[1]) != nil {
    75  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    76  				} else if ip, _ := parseIPv6(f[1], true); ip != nil {
    77  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    78  				}
    79  			}
    80  
    81  		case "domain": // set search path to just this domain
    82  			if len(f) > 1 {
    83  				conf.search = []string{ensureRooted(f[1])}
    84  			}
    85  
    86  		case "search": // set search path to given servers
    87  			conf.search = make([]string, len(f)-1)
    88  			for i := 0; i < len(conf.search); i++ {
    89  				conf.search[i] = ensureRooted(f[i+1])
    90  			}
    91  
    92  		case "options": // magic options
    93  			for _, s := range f[1:] {
    94  				switch {
    95  				case hasPrefix(s, "ndots:"):
    96  					n, _, _ := dtoi(s[6:])
    97  					if n < 0 {
    98  						n = 0
    99  					} else if n > 15 {
   100  						n = 15
   101  					}
   102  					conf.ndots = n
   103  				case hasPrefix(s, "timeout:"):
   104  					n, _, _ := dtoi(s[8:])
   105  					if n < 1 {
   106  						n = 1
   107  					}
   108  					conf.timeout = time.Duration(n) * time.Second
   109  				case hasPrefix(s, "attempts:"):
   110  					n, _, _ := dtoi(s[9:])
   111  					if n < 1 {
   112  						n = 1
   113  					}
   114  					conf.attempts = n
   115  				case s == "rotate":
   116  					conf.rotate = true
   117  				default:
   118  					conf.unknownOpt = true
   119  				}
   120  			}
   121  
   122  		case "lookup":
   123  			// OpenBSD option:
   124  			// http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
   125  			// "the legal space-separated values are: bind, file, yp"
   126  			conf.lookup = f[1:]
   127  
   128  		default:
   129  			conf.unknownOpt = true
   130  		}
   131  	}
   132  	if len(conf.servers) == 0 {
   133  		conf.servers = defaultNS
   134  	}
   135  	if len(conf.search) == 0 {
   136  		conf.search = dnsDefaultSearch()
   137  	}
   138  	return conf
   139  }
   140  
   141  // serverOffset returns an offset that can be used to determine
   142  // indices of servers in c.servers when making queries.
   143  // When the rotate option is enabled, this offset increases.
   144  // Otherwise it is always 0.
   145  func (c *dnsConfig) serverOffset() uint32 {
   146  	if c.rotate {
   147  		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
   148  	}
   149  	return 0
   150  }
   151  
   152  func dnsDefaultSearch() []string {
   153  	hn, err := getHostname()
   154  	if err != nil {
   155  		// best effort
   156  		return nil
   157  	}
   158  	if i := byteIndex(hn, '.'); i >= 0 && i < len(hn)-1 {
   159  		return []string{ensureRooted(hn[i+1:])}
   160  	}
   161  	return nil
   162  }
   163  
   164  func hasPrefix(s, prefix string) bool {
   165  	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
   166  }
   167  
   168  func ensureRooted(s string) string {
   169  	if len(s) > 0 && s[len(s)-1] == '.' {
   170  		return s
   171  	}
   172  	return s + "."
   173  }
   174
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