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
|
// 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 !windows
// Read system DNS config from /etc/resolv.conf
package net
import (
"internal/bytealg"
"net/netip"
"time"
)
// 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 _, err := netip.ParseAddr(f[1]); err == 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, 0, len(f)-1)
for i := 1; i < len(f); i++ {
name := ensureRooted(f[i])
if name == "." {
continue
}
conf.search = append(conf.search, name)
}
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
case s == "trust-ad":
conf.trustAD = true
case s == "edns0":
// We use EDNS by default.
// Ignore this option.
case s == "no-reload":
conf.noReload = 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
}
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 + "."
}
|
