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
5package net
6
7import (
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
27var defaultNS = []string{"127.0.0.1:53", "[::1]:53"}
28
29var getHostname = os.Hostname // variable for testing
30
31type dnsConfig struct {
32	servers       []string      // server addresses (in host:port form) to use
33	search        []string      // 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        []string      // 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.
53func (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