github.com/mh-cbon/go@v0.0.0-20160603070303-9e112a3fe4c0/src/net/dnsconfig_unix.go (about)

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