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 !windows
     6  
     7  // Read system DNS config from /etc/resolv.conf
     8  
     9  package net
    10  
    11  import (
    12  	"internal/bytealg"
    13  	"internal/stringslite"
    14  	"net/netip"
    15  	"time"
    16  )
    17  
    18  // See resolv.conf(5) on a Linux machine.
    19  func dnsReadConfig(filename string) *dnsConfig {
    20  	conf := &dnsConfig{
    21  		ndots:    1,
    22  		timeout:  5 * time.Second,
    23  		attempts: 2,
    24  	}
    25  	file, err := open(filename)
    26  	if err != nil {
    27  		conf.servers = defaultNS
    28  		conf.search = dnsDefaultSearch()
    29  		conf.err = err
    30  		return conf
    31  	}
    32  	defer file.close()
    33  	if fi, err := file.file.Stat(); err == nil {
    34  		conf.mtime = fi.ModTime()
    35  	} else {
    36  		conf.servers = defaultNS
    37  		conf.search = dnsDefaultSearch()
    38  		conf.err = err
    39  		return conf
    40  	}
    41  	for line, ok := file.readLine(); ok; line, ok = file.readLine() {
    42  		if len(line) > 0 && (line[0] == ';' || line[0] == '#') {
    43  			// comment.
    44  			continue
    45  		}
    46  		f := getFields(line)
    47  		if len(f) < 1 {
    48  			continue
    49  		}
    50  		switch f[0] {
    51  		case "nameserver": // add one name server
    52  			if len(f) > 1 && len(conf.servers) < 3 { // small, but the standard limit
    53  				// One more check: make sure server name is
    54  				// just an IP address. Otherwise we need DNS
    55  				// to look it up.
    56  				if _, err := netip.ParseAddr(f[1]); err == nil {
    57  					conf.servers = append(conf.servers, JoinHostPort(f[1], "53"))
    58  				}
    59  			}
    60  
    61  		case "domain": // set search path to just this domain
    62  			if len(f) > 1 {
    63  				conf.search = []string{ensureRooted(f[1])}
    64  			}
    65  
    66  		case "search": // set search path to given servers
    67  			conf.search = make([]string, 0, len(f)-1)
    68  			for i := 1; i < len(f); i++ {
    69  				name := ensureRooted(f[i])
    70  				if name == "." {
    71  					continue
    72  				}
    73  				conf.search = append(conf.search, name)
    74  			}
    75  
    76  		case "options": // magic options
    77  			for _, s := range f[1:] {
    78  				switch {
    79  				case stringslite.HasPrefix(s, "ndots:"):
    80  					n, _, _ := dtoi(s[6:])
    81  					if n < 0 {
    82  						n = 0
    83  					} else if n > 15 {
    84  						n = 15
    85  					}
    86  					conf.ndots = n
    87  				case stringslite.HasPrefix(s, "timeout:"):
    88  					n, _, _ := dtoi(s[8:])
    89  					if n < 1 {
    90  						n = 1
    91  					}
    92  					conf.timeout = time.Duration(n) * time.Second
    93  				case stringslite.HasPrefix(s, "attempts:"):
    94  					n, _, _ := dtoi(s[9:])
    95  					if n < 1 {
    96  						n = 1
    97  					}
    98  					conf.attempts = n
    99  				case s == "rotate":
   100  					conf.rotate = true
   101  				case s == "single-request" || s == "single-request-reopen":
   102  					// Linux option:
   103  					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
   104  					// "By default, glibc performs IPv4 and IPv6 lookups in parallel [...]
   105  					//  This option disables the behavior and makes glibc
   106  					//  perform the IPv6 and IPv4 requests sequentially."
   107  					conf.singleRequest = true
   108  				case s == "use-vc" || s == "usevc" || s == "tcp":
   109  					// Linux (use-vc), FreeBSD (usevc) and OpenBSD (tcp) option:
   110  					// http://man7.org/linux/man-pages/man5/resolv.conf.5.html
   111  					// "Sets RES_USEVC in _res.options.
   112  					//  This option forces the use of TCP for DNS resolutions."
   113  					// https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports
   114  					// https://man.openbsd.org/resolv.conf.5
   115  					conf.useTCP = true
   116  				case s == "trust-ad":
   117  					conf.trustAD = true
   118  				case s == "edns0":
   119  					// We use EDNS by default.
   120  					// Ignore this option.
   121  				case s == "no-reload":
   122  					conf.noReload = true
   123  				default:
   124  					conf.unknownOpt = true
   125  				}
   126  			}
   127  
   128  		case "lookup":
   129  			// OpenBSD option:
   130  			// https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
   131  			// "the legal space-separated values are: bind, file, yp"
   132  			conf.lookup = f[1:]
   133  
   134  		default:
   135  			conf.unknownOpt = true
   136  		}
   137  	}
   138  	if len(conf.servers) == 0 {
   139  		conf.servers = defaultNS
   140  	}
   141  	if len(conf.search) == 0 {
   142  		conf.search = dnsDefaultSearch()
   143  	}
   144  	return conf
   145  }
   146  
   147  func dnsDefaultSearch() []string {
   148  	hn, err := getHostname()
   149  	if err != nil {
   150  		// best effort
   151  		return nil
   152  	}
   153  	if i := bytealg.IndexByteString(hn, '.'); i >= 0 && i < len(hn)-1 {
   154  		return []string{ensureRooted(hn[i+1:])}
   155  	}
   156  	return nil
   157  }
   158  
   159  func ensureRooted(s string) string {
   160  	if len(s) > 0 && s[len(s)-1] == '.' {
   161  		return s
   162  	}
   163  	return s + "."
   164  }
   165
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