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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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