Source file src/net/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  // +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
     7  
     8  // Read system DNS config from /etc/resolv.conf
     9  
    10  package net
    11  
    12  import (
    13  	"internal/bytealg"
    14  	"os"
    15  	"sync/atomic"
    16  	"time"
    17  )
    18  
    19  var (
    20  	defaultNS   = []string{"127.0.0.1:53", "[::1]:53"}
    21  	getHostname = os.Hostname // variable for testing
    22  )
    23  
    24  type dnsConfig struct {
    25  	servers       []string      // server addresses (in host:port form) to use
    26  	search        []string      // rooted suffixes to append to local name
    27  	ndots         int           // number of dots in name to trigger absolute lookup
    28  	timeout       time.Duration // wait before giving up on a query, including retries
    29  	attempts      int           // lost packets before giving up on server
    30  	rotate        bool          // round robin among servers
    31  	unknownOpt    bool          // anything unknown was encountered
    32  	lookup        []string      // OpenBSD top-level database "lookup" order
    33  	err           error         // any error that occurs during open of resolv.conf
    34  	mtime         time.Time     // time of resolv.conf modification
    35  	soffset       uint32        // used by serverOffset
    36  	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
    37  	useTCP        bool          // force usage of TCP for DNS resolutions
    38  }
    39  
    40  // See resolv.conf(5) on a Linux machine.
    41  func dnsReadConfig(filename string) *dnsConfig {
    42  	conf := &dnsConfig{
    43  		ndots:    1,
    44  		timeout:  5 * time.Second,
    45  		attempts: 2,
    46  	}
    47  	file, err := open(filename)
    48  	if err != nil {
    49  		conf.servers = defaultNS
    50  		conf.search = dnsDefaultSearch()
    51  		conf.err = err
    52  		return conf
    53  	}
    54  	defer file.close()
    55  	if fi, err := file.file.Stat(); err == nil {
    56  		conf.mtime = fi.ModTime()
    57  	} else {
    58  		conf.servers = defaultNS
    59  		conf.search = dnsDefaultSearch()
    60  		conf.err = err
    61  		return conf
    62  	}
    63  	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
    64  		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
    65  			// comment.
    66  			continue
    67  		}
    68  		f := getFields(line)
    69  		if len(f) < 1 {
    70  			continue
    71  		}
    72  		switch f[0] {
    73  		case "nameserver": // add one name server
    74  			if len(f) > 1 && len(conf.servers) < 3 { // small, but the standard limit
    75  				// One more check: make sure server name is
    76  				// just an IP address. Otherwise we need DNS
    77  				// to look it up.
    78  				if parseIPv4(f[1]) != nil {
    79  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    80  				} else if ip, _ := parseIPv6Zone(f[1]); ip != nil {
    81  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    82  				}
    83  			}
    84  
    85  		case "domain": // set search path to just this domain
    86  			if len(f) > 1 {
    87  				conf.search = []string{ensureRooted(f[1])}
    88  			}
    89  
    90  		case "search": // set search path to given servers
    91  			conf.search = make([]string, len(f)-1)
    92  			for i := 0; i < len(conf.search); i++ {
    93  				conf.search[i] = ensureRooted(f[i+1])
    94  			}
    95  
    96  		case "options": // magic options
    97  			for _, s := range f[1:] {
    98  				switch {
    99  				case hasPrefix(s, "ndots:"):
   100  					n, _, _ := dtoi(s[6:])
   101  					if n < 0 {
   102  						n = 0
   103  					} else if n > 15 {
   104  						n = 15
   105  					}
   106  					conf.ndots = n
   107  				case hasPrefix(s, "timeout:"):
   108  					n, _, _ := dtoi(s[8:])
   109  					if n < 1 {
   110  						n = 1
   111  					}
   112  					conf.timeout = time.Duration(n) * time.Second
   113  				case hasPrefix(s, "attempts:"):
   114  					n, _, _ := dtoi(s[9:])
   115  					if n < 1 {
   116  						n = 1
   117  					}
   118  					conf.attempts = n
   119  				case s == "rotate":
   120  					conf.rotate = true
   121  				case s == "single-request" || s == "single-request-reopen":
   122  					// Linux option:
   123  					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
   124  					// "By default, glibc performs IPv4 and IPv6 lookups in parallel [...]
   125  					//  This option disables the behavior and makes glibc
   126  					//  perform the IPv6 and IPv4 requests sequentially."
   127  					conf.singleRequest = true
   128  				case s == "use-vc" || s == "usevc" || s == "tcp":
   129  					// Linux (use-vc), FreeBSD (usevc) and OpenBSD (tcp) option:
   130  					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
   131  					// "Sets RES_USEVC in _res.options.
   132  					//  This option forces the use of TCP for DNS resolutions."
   133  					// https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports
   134  					// https://man.openbsd.org/resolv.conf.5
   135  					conf.useTCP = true
   136  				default:
   137  					conf.unknownOpt = true
   138  				}
   139  			}
   140  
   141  		case "lookup":
   142  			// OpenBSD option:
   143  			// https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
   144  			// "the legal space-separated values are: bind, file, yp"
   145  			conf.lookup = f[1:]
   146  
   147  		default:
   148  			conf.unknownOpt = true
   149  		}
   150  	}
   151  	if len(conf.servers) == 0 {
   152  		conf.servers = defaultNS
   153  	}
   154  	if len(conf.search) == 0 {
   155  		conf.search = dnsDefaultSearch()
   156  	}
   157  	return conf
   158  }
   159  
   160  // serverOffset returns an offset that can be used to determine
   161  // indices of servers in c.servers when making queries.
   162  // When the rotate option is enabled, this offset increases.
   163  // Otherwise it is always 0.
   164  func (c *dnsConfig) serverOffset() uint32 {
   165  	if c.rotate {
   166  		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
   167  	}
   168  	return 0
   169  }
   170  
   171  func dnsDefaultSearch() []string {
   172  	hn, err := getHostname()
   173  	if err != nil {
   174  		// best effort
   175  		return nil
   176  	}
   177  	if i := bytealg.IndexByteString(hn, '.'); i >= 0 && i < len(hn)-1 {
   178  		return []string{ensureRooted(hn[i+1:])}
   179  	}
   180  	return nil
   181  }
   182  
   183  func hasPrefix(s, prefix string) bool {
   184  	return len(s) >= len(prefix) && s[:len(prefix)] == prefix
   185  }
   186  
   187  func ensureRooted(s string) string {
   188  	if len(s) > 0 && s[len(s)-1] == '.' {
   189  		return s
   190  	}
   191  	return s + "."
   192  }
   193  

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