dnsconfig_unix.go

     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  //go:build aix || 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  	"internal/bytealg"
    13  	"os"
    14  	"sync/atomic"
    15  	"time"
    16  )
    17  
    18  var (
    19  	defaultNS   = []string{"127.0.0.1:53", "[::1]:53"}
    20  	getHostname = os.Hostname // variable for testing
    21  )
    22  
    23  type dnsConfig struct {
    24  	servers       []string      // server addresses (in host:port form) to use
    25  	search        []string      // rooted suffixes to append to local name
    26  	ndots         int           // number of dots in name to trigger absolute lookup
    27  	timeout       time.Duration // wait before giving up on a query, including retries
    28  	attempts      int           // lost packets before giving up on server
    29  	rotate        bool          // round robin among servers
    30  	unknownOpt    bool          // anything unknown was encountered
    31  	lookup        []string      // OpenBSD top-level database "lookup" order
    32  	err           error         // any error that occurs during open of resolv.conf
    33  	mtime         time.Time     // time of resolv.conf modification
    34  	soffset       uint32        // used by serverOffset
    35  	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
    36  	useTCP        bool          // force usage of TCP for DNS resolutions
    37  }
    38  
    39  // See resolv.conf(5) on a Linux machine.
    40  func dnsReadConfig(filename string) *dnsConfig {
    41  	conf := &dnsConfig{
    42  		ndots:    1,
    43  		timeout:  5 * time.Second,
    44  		attempts: 2,
    45  	}
    46  	file, err := open(filename)
    47  	if err != nil {
    48  		conf.servers = defaultNS
    49  		conf.search = dnsDefaultSearch()
    50  		conf.err = err
    51  		return conf
    52  	}
    53  	defer file.close()
    54  	if fi, err := file.file.Stat(); err == nil {
    55  		conf.mtime = fi.ModTime()
    56  	} else {
    57  		conf.servers = defaultNS
    58  		conf.search = dnsDefaultSearch()
    59  		conf.err = err
    60  		return conf
    61  	}
    62  	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
    63  		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
    64  			// comment.
    65  			continue
    66  		}
    67  		f := getFields(line)
    68  		if len(f) < 1 {
    69  			continue
    70  		}
    71  		switch f[0] {
    72  		case "nameserver": // add one name server
    73  			if len(f) > 1 && len(conf.servers) < 3 { // small, but the standard limit
    74  				// One more check: make sure server name is
    75  				// just an IP address. Otherwise we need DNS
    76  				// to look it up.
    77  				if parseIPv4(f[1]) != nil {
    78  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    79  				} else if ip, _ := parseIPv6Zone(f[1]); ip != nil {
    80  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    81  				}
    82  			}
    83  
    84  		case "domain": // set search path to just this domain
    85  			if len(f) > 1 {
    86  				conf.search = []string{ensureRooted(f[1])}
    87  			}
    88  
    89  		case "search": // set search path to given servers
    90  			conf.search = make([]string, len(f)-1)
    91  			for i := 0; i < len(conf.search); i++ {
    92  				conf.search[i] = ensureRooted(f[i+1])
    93  			}
    94  
    95  		case "options": // magic options
    96  			for _, s := range f[1:] {
    97  				switch {
    98  				case hasPrefix(s, "ndots:"):
    99  					n, _, _ := dtoi(s[6:])
   100  					if n < 0 {
   101  						n = 0
   102  					} else if n > 15 {
   103  						n = 15
   104  					}
   105  					conf.ndots = n
   106  				case hasPrefix(s, "timeout:"):
   107  					n, _, _ := dtoi(s[8:])
   108  					if n < 1 {
   109  						n = 1
   110  					}
   111  					conf.timeout = time.Duration(n) * time.Second
   112  				case hasPrefix(s, "attempts:"):
   113  					n, _, _ := dtoi(s[9:])
   114  					if n < 1 {
   115  						n = 1
   116  					}
   117  					conf.attempts = n
   118  				case s == "rotate":
   119  					conf.rotate = true
   120  				case s == "single-request" || s == "single-request-reopen":
   121  					// Linux option:
   122  					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
   123  					// "By default, glibc performs IPv4 and IPv6 lookups in parallel [...]
   124  					//  This option disables the behavior and makes glibc
   125  					//  perform the IPv6 and IPv4 requests sequentially."
   126  					conf.singleRequest = true
   127  				case s == "use-vc" || s == "usevc" || s == "tcp":
   128  					// Linux (use-vc), FreeBSD (usevc) and OpenBSD (tcp) option:
   129  					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
   130  					// "Sets RES_USEVC in _res.options.
   131  					//  This option forces the use of TCP for DNS resolutions."
   132  					// https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports
   133  					// https://man.openbsd.org/resolv.conf.5
   134  					conf.useTCP = true
   135  				default:
   136  					conf.unknownOpt = true
   137  				}
   138  			}
   139  
   140  		case "lookup":
   141  			// OpenBSD option:
   142  			// https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
   143  			// "the legal space-separated values are: bind, file, yp"
   144  			conf.lookup = f[1:]
   145  
   146  		default:
   147  			conf.unknownOpt = true
   148  		}
   149  	}
   150  	if len(conf.servers) == 0 {
   151  		conf.servers = defaultNS
   152  	}
   153  	if len(conf.search) == 0 {
   154  		conf.search = dnsDefaultSearch()
   155  	}
   156  	return conf
   157  }
   158  
   159  // serverOffset returns an offset that can be used to determine
   160  // indices of servers in c.servers when making queries.
   161  // When the rotate option is enabled, this offset increases.
   162  // Otherwise it is always 0.
   163  func (c *dnsConfig) serverOffset() uint32 {
   164  	if c.rotate {
   165  		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
   166  	}
   167  	return 0
   168  }
   169  
   170  func dnsDefaultSearch() []string {
   171  	hn, err := getHostname()
   172  	if err != nil {
   173  		// best effort
   174  		return nil
   175  	}
   176  	if i := bytealg.IndexByteString(hn, '.'); i >= 0 && i < len(hn)-1 {
   177  		return []string{ensureRooted(hn[i+1:])}
   178  	}
   179  	return nil
   180  }
   181  
   182  func hasPrefix(s, prefix string) bool {
   183  	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
   184  }
   185  
   186  func ensureRooted(s string) string {
   187  	if len(s) > 0 && s[len(s)-1] == '.' {
   188  		return s
   189  	}
   190  	return s + "."
   191  }
   192
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