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- // 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 || hurd || 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 + "."
- }
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