1 // Copyright 2022 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
4 5 package net
6 7 import (
8 "os"
9 "sync/atomic"
10 "time"
11 _ "unsafe"
12 )
13 14 // defaultNS is the default name servers to use in the absence of DNS configuration.
15 //
16 // defaultNS should be an internal detail,
17 // but widely used packages access it using linkname.
18 // Notable members of the hall of shame include:
19 // - github.com/pojntfx/hydrapp/hydrapp
20 // - github.com/mtibben/androiddnsfix
21 // - github.com/metacubex/mihomo
22 //
23 // Do not remove or change the type signature.
24 // See go.dev/issue/67401.
25 //
26 //go:linkname defaultNS
27 var defaultNS = [][]byte{"127.0.0.1:53", "[::1]:53"}
28 29 var getHostname = os.Hostname // variable for testing
30 31 type dnsConfig struct {
32 servers [][]byte // server addresses (in host:port form) to use
33 search [][]byte // rooted suffixes to append to local name
34 ndots int // number of dots in name to trigger absolute lookup
35 timeout time.Duration // wait before giving up on a query, including retries
36 attempts int // lost packets before giving up on server
37 rotate bool // round robin among servers
38 unknownOpt bool // anything unknown was encountered
39 lookup [][]byte // OpenBSD top-level database "lookup" order
40 err error // any error that occurs during open of resolv.conf
41 mtime time.Time // time of resolv.conf modification
42 soffset uint32 // used by serverOffset
43 singleRequest bool // use sequential A and AAAA queries instead of parallel queries
44 useTCP bool // force usage of TCP for DNS resolutions
45 trustAD bool // add AD flag to queries
46 noReload bool // do not check for config file updates
47 }
48 49 // serverOffset returns an offset that can be used to determine
50 // indices of servers in c.servers when making queries.
51 // When the rotate option is enabled, this offset increases.
52 // Otherwise it is always 0.
53 func (c *dnsConfig) serverOffset() uint32 {
54 if c.rotate {
55 return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
56 }
57 return 0
58 }
59