// Copyright 2009 The Go Authors. All rights reserved. // Use of this source code is governed by a BSD-style // license that can be found in the LICENSE file. //go:build aix || darwin || dragonfly || freebsd || linux || netbsd || openbsd || solaris // Read system DNS config from /etc/resolv.conf package net import ( "internal/bytealg" "os" "sync/atomic" "time" ) var ( defaultNS = []string{"127.0.0.1:53", "[::1]:53"} getHostname = os.Hostname // variable for testing ) type dnsConfig struct { servers []string // server addresses (in host:port form) to use search []string // rooted suffixes to append to local name ndots int // number of dots in name to trigger absolute lookup timeout time.Duration // wait before giving up on a query, including retries attempts int // lost packets before giving up on server rotate bool // round robin among servers unknownOpt bool // anything unknown was encountered lookup []string // OpenBSD top-level database "lookup" order err error // any error that occurs during open of resolv.conf mtime time.Time // time of resolv.conf modification soffset uint32 // used by serverOffset singleRequest bool // use sequential A and AAAA queries instead of parallel queries useTCP bool // force usage of TCP for DNS resolutions } // See resolv.conf(5) on a Linux machine. func dnsReadConfig(filename string) *dnsConfig { conf := &dnsConfig{ ndots: 1, timeout: 5 * time.Second, attempts: 2, } file, err := open(filename) if err != nil { conf.servers = defaultNS conf.search = dnsDefaultSearch() conf.err = err return conf } defer file.close() if fi, err := file.file.Stat(); err == nil { conf.mtime = fi.ModTime() } else { conf.servers = defaultNS conf.search = dnsDefaultSearch() conf.err = err return conf } for line, ok := file.readLine(); ok; line, ok = file.readLine() { if len(line) > 0 && (line[0] == ';' || line[0] == '#') { // comment. continue } f := getFields(line) if len(f) < 1 { continue } switch f[0] { case "nameserver": // add one name server if len(f) > 1 && len(conf.servers) < 3 { // small, but the standard limit // One more check: make sure server name is // just an IP address. Otherwise we need DNS // to look it up. if parseIPv4(f[1]) != nil { conf.servers = append(conf.servers, JoinHostPort(f[1], "53")) } else if ip, _ := parseIPv6Zone(f[1]); ip != nil { conf.servers = append(conf.servers, JoinHostPort(f[1], "53")) } } case "domain": // set search path to just this domain if len(f) > 1 { conf.search = []string{ensureRooted(f[1])} } case "search": // set search path to given servers conf.search = make([]string, len(f)-1) for i := 0; i < len(conf.search); i++ { conf.search[i] = ensureRooted(f[i+1]) } case "options": // magic options for _, s := range f[1:] { switch { case hasPrefix(s, "ndots:"): n, _, _ := dtoi(s[6:]) if n < 0 { n = 0 } else if n > 15 { n = 15 } conf.ndots = n case hasPrefix(s, "timeout:"): n, _, _ := dtoi(s[8:]) if n < 1 { n = 1 } conf.timeout = time.Duration(n) * time.Second case hasPrefix(s, "attempts:"): n, _, _ := dtoi(s[9:]) if n < 1 { n = 1 } conf.attempts = n case s == "rotate": conf.rotate = true case s == "single-request" || s == "single-request-reopen": // Linux option: // http://man7.org/linux/man-pages/man5/resolv.conf.5.html // "By default, glibc performs IPv4 and IPv6 lookups in parallel [...] // This option disables the behavior and makes glibc // perform the IPv6 and IPv4 requests sequentially." conf.singleRequest = true case s == "use-vc" || s == "usevc" || s == "tcp": // Linux (use-vc), FreeBSD (usevc) and OpenBSD (tcp) option: // http://man7.org/linux/man-pages/man5/resolv.conf.5.html // "Sets RES_USEVC in _res.options. // This option forces the use of TCP for DNS resolutions." // https://www.freebsd.org/cgi/man.cgi?query=resolv.conf&sektion=5&manpath=freebsd-release-ports // https://man.openbsd.org/resolv.conf.5 conf.useTCP = true default: conf.unknownOpt = true } } case "lookup": // OpenBSD option: // https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5 // "the legal space-separated values are: bind, file, yp" conf.lookup = f[1:] default: conf.unknownOpt = true } } if len(conf.servers) == 0 { conf.servers = defaultNS } if len(conf.search) == 0 { conf.search = dnsDefaultSearch() } return conf } // serverOffset returns an offset that can be used to determine // indices of servers in c.servers when making queries. // When the rotate option is enabled, this offset increases. // Otherwise it is always 0. func (c *dnsConfig) serverOffset() uint32 { if c.rotate { return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start } return 0 } func dnsDefaultSearch() []string { hn, err := getHostname() if err != nil { // best effort return nil } if i := bytealg.IndexByteString(hn, '.'); i >= 0 && i < len(hn)-1 { return []string{ensureRooted(hn[i+1:])} } return nil } func hasPrefix(s, prefix string) bool { return len(s) >= len(prefix) && s[:len(prefix)] == prefix } func ensureRooted(s string) string { if len(s) > 0 && s[len(s)-1] == '.' { return s } return s + "." }