dnsconfig.go

     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 = []string{"127.0.0.1:53", "[::1]:53"}
    28  
    29  var getHostname = os.Hostname // variable for testing
    30  
    31  type 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.
    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
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