summaryrefslogtreecommitdiffstats
path: root/src/net/conf.go
blob: f1bbfedad06cb5ba749e2b47ef60f9ceecab196b (plain)
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
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
// Copyright 2015 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.

// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris

package net

import (
	"internal/bytealg"
	"os"
	"runtime"
	"sync"
	"syscall"
)

// conf represents a system's network configuration.
type conf struct {
	// forceCgoLookupHost forces CGO to always be used, if available.
	forceCgoLookupHost bool

	netGo  bool // go DNS resolution forced
	netCgo bool // cgo DNS resolution forced

	// machine has an /etc/mdns.allow file
	hasMDNSAllow bool

	goos          string // the runtime.GOOS, to ease testing
	dnsDebugLevel int

	nss    *nssConf
	resolv *dnsConfig
}

var (
	confOnce sync.Once // guards init of confVal via initConfVal
	confVal  = &conf{goos: runtime.GOOS}
)

// systemConf returns the machine's network configuration.
func systemConf() *conf {
	confOnce.Do(initConfVal)
	return confVal
}

func initConfVal() {
	dnsMode, debugLevel := goDebugNetDNS()
	confVal.dnsDebugLevel = debugLevel
	confVal.netGo = netGo || dnsMode == "go"
	confVal.netCgo = netCgo || dnsMode == "cgo"

	if confVal.dnsDebugLevel > 0 {
		defer func() {
			switch {
			case confVal.netGo:
				if netGo {
					println("go package net: built with netgo build tag; using Go's DNS resolver")
				} else {
					println("go package net: GODEBUG setting forcing use of Go's resolver")
				}
			case confVal.forceCgoLookupHost:
				println("go package net: using cgo DNS resolver")
			default:
				println("go package net: dynamic selection of DNS resolver")
			}
		}()
	}

	// Darwin pops up annoying dialog boxes if programs try to do
	// their own DNS requests. So always use cgo instead, which
	// avoids that.
	if runtime.GOOS == "darwin" || runtime.GOOS == "ios" {
		confVal.forceCgoLookupHost = true
		return
	}

	// If any environment-specified resolver options are specified,
	// force cgo. Note that LOCALDOMAIN can change behavior merely
	// by being specified with the empty string.
	_, localDomainDefined := syscall.Getenv("LOCALDOMAIN")
	if os.Getenv("RES_OPTIONS") != "" ||
		os.Getenv("HOSTALIASES") != "" ||
		confVal.netCgo ||
		localDomainDefined {
		confVal.forceCgoLookupHost = true
		return
	}

	// OpenBSD apparently lets you override the location of resolv.conf
	// with ASR_CONFIG. If we notice that, defer to libc.
	if runtime.GOOS == "openbsd" && os.Getenv("ASR_CONFIG") != "" {
		confVal.forceCgoLookupHost = true
		return
	}

	if runtime.GOOS != "openbsd" {
		confVal.nss = parseNSSConfFile("/etc/nsswitch.conf")
	}

	confVal.resolv = dnsReadConfig("/etc/resolv.conf")
	if confVal.resolv.err != nil && !os.IsNotExist(confVal.resolv.err) &&
		!os.IsPermission(confVal.resolv.err) {
		// If we can't read the resolv.conf file, assume it
		// had something important in it and defer to cgo.
		// libc's resolver might then fail too, but at least
		// it wasn't our fault.
		confVal.forceCgoLookupHost = true
	}

	if _, err := os.Stat("/etc/mdns.allow"); err == nil {
		confVal.hasMDNSAllow = true
	}
}

// canUseCgo reports whether calling cgo functions is allowed
// for non-hostname lookups.
func (c *conf) canUseCgo() bool {
	return c.hostLookupOrder(nil, "") == hostLookupCgo
}

// hostLookupOrder determines which strategy to use to resolve hostname.
// The provided Resolver is optional. nil means to not consider its options.
func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrder) {
	if c.dnsDebugLevel > 1 {
		defer func() {
			print("go package net: hostLookupOrder(", hostname, ") = ", ret.String(), "\n")
		}()
	}
	fallbackOrder := hostLookupCgo
	if c.netGo || r.preferGo() {
		fallbackOrder = hostLookupFilesDNS
	}
	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {
		return fallbackOrder
	}
	if bytealg.IndexByteString(hostname, '\\') != -1 || bytealg.IndexByteString(hostname, '%') != -1 {
		// Don't deal with special form hostnames with backslashes
		// or '%'.
		return fallbackOrder
	}

	// OpenBSD is unique and doesn't use nsswitch.conf.
	// It also doesn't support mDNS.
	if c.goos == "openbsd" {
		// OpenBSD's resolv.conf manpage says that a non-existent
		// resolv.conf means "lookup" defaults to only "files",
		// without DNS lookups.
		if os.IsNotExist(c.resolv.err) {
			return hostLookupFiles
		}
		lookup := c.resolv.lookup
		if len(lookup) == 0 {
			// https://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man5/resolv.conf.5
			// "If the lookup keyword is not used in the
			// system's resolv.conf file then the assumed
			// order is 'bind file'"
			return hostLookupDNSFiles
		}
		if len(lookup) < 1 || len(lookup) > 2 {
			return fallbackOrder
		}
		switch lookup[0] {
		case "bind":
			if len(lookup) == 2 {
				if lookup[1] == "file" {
					return hostLookupDNSFiles
				}
				return fallbackOrder
			}
			return hostLookupDNS
		case "file":
			if len(lookup) == 2 {
				if lookup[1] == "bind" {
					return hostLookupFilesDNS
				}
				return fallbackOrder
			}
			return hostLookupFiles
		default:
			return fallbackOrder
		}
	}

	// Canonicalize the hostname by removing any trailing dot.
	if stringsHasSuffix(hostname, ".") {
		hostname = hostname[:len(hostname)-1]
	}
	if stringsHasSuffixFold(hostname, ".local") {
		// Per RFC 6762, the ".local" TLD is special. And
		// because Go's native resolver doesn't do mDNS or
		// similar local resolution mechanisms, assume that
		// libc might (via Avahi, etc) and use cgo.
		return fallbackOrder
	}

	nss := c.nss
	srcs := nss.sources["hosts"]
	// If /etc/nsswitch.conf doesn't exist or doesn't specify any
	// sources for "hosts", assume Go's DNS will work fine.
	if os.IsNotExist(nss.err) || (nss.err == nil && len(srcs) == 0) {
		if c.goos == "solaris" {
			// illumos defaults to "nis [NOTFOUND=return] files"
			return fallbackOrder
		}
		return hostLookupFilesDNS
	}
	if nss.err != nil {
		// We failed to parse or open nsswitch.conf, so
		// conservatively assume we should use cgo if it's
		// available.
		return fallbackOrder
	}

	var mdnsSource, filesSource, dnsSource bool
	var first string
	for _, src := range srcs {
		if src.source == "myhostname" {
			if isLocalhost(hostname) || isGateway(hostname) {
				return fallbackOrder
			}
			hn, err := getHostname()
			if err != nil || stringsEqualFold(hostname, hn) {
				return fallbackOrder
			}
			continue
		}
		if src.source == "files" || src.source == "dns" {
			if !src.standardCriteria() {
				return fallbackOrder // non-standard; let libc deal with it.
			}
			if src.source == "files" {
				filesSource = true
			} else if src.source == "dns" {
				dnsSource = true
			}
			if first == "" {
				first = src.source
			}
			continue
		}
		if stringsHasPrefix(src.source, "mdns") {
			// e.g. "mdns4", "mdns4_minimal"
			// We already returned true before if it was *.local.
			// libc wouldn't have found a hit on this anyway.
			mdnsSource = true
			continue
		}
		// Some source we don't know how to deal with.
		return fallbackOrder
	}

	// We don't parse mdns.allow files. They're rare. If one
	// exists, it might list other TLDs (besides .local) or even
	// '*', so just let libc deal with it.
	if mdnsSource && c.hasMDNSAllow {
		return fallbackOrder
	}

	// Cases where Go can handle it without cgo and C thread
	// overhead.
	switch {
	case filesSource && dnsSource:
		if first == "files" {
			return hostLookupFilesDNS
		} else {
			return hostLookupDNSFiles
		}
	case filesSource:
		return hostLookupFiles
	case dnsSource:
		return hostLookupDNS
	}

	// Something weird. Let libc deal with it.
	return fallbackOrder
}

// goDebugNetDNS parses the value of the GODEBUG "netdns" value.
// The netdns value can be of the form:
//    1       // debug level 1
//    2       // debug level 2
//    cgo     // use cgo for DNS lookups
//    go      // use go for DNS lookups
//    cgo+1   // use cgo for DNS lookups + debug level 1
//    1+cgo   // same
//    cgo+2   // same, but debug level 2
// etc.
func goDebugNetDNS() (dnsMode string, debugLevel int) {
	goDebug := goDebugString("netdns")
	parsePart := func(s string) {
		if s == "" {
			return
		}
		if '0' <= s[0] && s[0] <= '9' {
			debugLevel, _, _ = dtoi(s)
		} else {
			dnsMode = s
		}
	}
	if i := bytealg.IndexByteString(goDebug, '+'); i != -1 {
		parsePart(goDebug[:i])
		parsePart(goDebug[i+1:])
		return
	}
	parsePart(goDebug)
	return
}

// isLocalhost reports whether h should be considered a "localhost"
// name for the myhostname NSS module.
func isLocalhost(h string) bool {
	return stringsEqualFold(h, "localhost") || stringsEqualFold(h, "localhost.localdomain") || stringsHasSuffixFold(h, ".localhost") || stringsHasSuffixFold(h, ".localhost.localdomain")
}

// isGateway reports whether h should be considered a "gateway"
// name for the myhostname NSS module.
func isGateway(h string) bool {
	return stringsEqualFold(h, "gateway")
}