Source file src/pkg/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	// +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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