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  //
    22  // Do not remove or change the type signature.
    23  // See go.dev/issue/67401.
    24  //
    25  //go:linkname defaultNS
    26  var defaultNS = []string{"127.0.0.1:53", "[::1]:53"}
    27  
    28  var getHostname = os.Hostname // variable for testing
    29  
    30  type dnsConfig struct {
    31  	servers       []string      // server addresses (in host:port form) to use
    32  	search        []string      // rooted suffixes to append to local name
    33  	ndots         int           // number of dots in name to trigger absolute lookup
    34  	timeout       time.Duration // wait before giving up on a query, including retries
    35  	attempts      int           // lost packets before giving up on server
    36  	rotate        bool          // round robin among servers
    37  	unknownOpt    bool          // anything unknown was encountered
    38  	lookup        []string      // OpenBSD top-level database "lookup" order
    39  	err           error         // any error that occurs during open of resolv.conf
    40  	mtime         time.Time     // time of resolv.conf modification
    41  	soffset       uint32        // used by serverOffset
    42  	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
    43  	useTCP        bool          // force usage of TCP for DNS resolutions
    44  	trustAD       bool          // add AD flag to queries
    45  	noReload      bool          // do not check for config file updates
    46  }
    47  
    48  // serverOffset returns an offset that can be used to determine
    49  // indices of servers in c.servers when making queries.
    50  // When the rotate option is enabled, this offset increases.
    51  // Otherwise it is always 0.
    52  func (c *dnsConfig) serverOffset() uint32 {
    53  	if c.rotate {
    54  		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
    55  	}
    56  	return 0
    57  }
    58
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