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  //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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