1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
|
// 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 + "."
}
|
