hosts.go

     1  // Copyright 2009 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  	"errors"
     9  	"internal/bytealg"
    10  	"io/fs"
    11  	"net/netip"
    12  	"sync"
    13  	"time"
    14  )
    15  
    16  const cacheMaxAge = 5 * time.Second
    17  
    18  func parseLiteralIP(addr string) string {
    19  	ip, err := netip.ParseAddr(addr)
    20  	if err != nil {
    21  		return ""
    22  	}
    23  	return ip.String()
    24  }
    25  
    26  type byName struct {
    27  	addrs         []string
    28  	canonicalName string
    29  }
    30  
    31  // hosts contains known host entries.
    32  var hosts struct {
    33  	sync.Mutex
    34  
    35  	// Key for the list of literal IP addresses must be a host
    36  	// name. It would be part of DNS labels, a FQDN or an absolute
    37  	// FQDN.
    38  	// For now the key is converted to lower case for convenience.
    39  	byName map[string]byName
    40  
    41  	// Key for the list of host names must be a literal IP address
    42  	// including IPv6 address with zone identifier.
    43  	// We don't support old-classful IP address notation.
    44  	byAddr map[string][]string
    45  
    46  	expire time.Time
    47  	path   string
    48  	mtime  time.Time
    49  	size   int64
    50  }
    51  
    52  func readHosts() {
    53  	now := time.Now()
    54  	hp := hostsFilePath
    55  
    56  	if now.Before(hosts.expire) && hosts.path == hp && len(hosts.byName) > 0 {
    57  		return
    58  	}
    59  	mtime, size, err := stat(hp)
    60  	if err == nil && hosts.path == hp && hosts.mtime.Equal(mtime) && hosts.size == size {
    61  		hosts.expire = now.Add(cacheMaxAge)
    62  		return
    63  	}
    64  
    65  	hs := make(map[string]byName)
    66  	is := make(map[string][]string)
    67  
    68  	file, err := open(hp)
    69  	if err != nil {
    70  		if !errors.Is(err, fs.ErrNotExist) && !errors.Is(err, fs.ErrPermission) {
    71  			return
    72  		}
    73  	}
    74  
    75  	if file != nil {
    76  		defer file.close()
    77  		for line, ok := file.readLine(); ok; line, ok = file.readLine() {
    78  			if i := bytealg.IndexByteString(line, '#'); i >= 0 {
    79  				// Discard comments.
    80  				line = line[0:i]
    81  			}
    82  			f := getFields(line)
    83  			if len(f) < 2 {
    84  				continue
    85  			}
    86  			addr := parseLiteralIP(f[0])
    87  			if addr == "" {
    88  				continue
    89  			}
    90  
    91  			var canonical string
    92  			for i := 1; i < len(f); i++ {
    93  				name := absDomainName(f[i])
    94  				h := []byte(f[i])
    95  				lowerASCIIBytes(h)
    96  				key := absDomainName(string(h))
    97  
    98  				if i == 1 {
    99  					canonical = key
   100  				}
   101  
   102  				is[addr] = append(is[addr], name)
   103  
   104  				if v, ok := hs[key]; ok {
   105  					hs[key] = byName{
   106  						addrs:         append(v.addrs, addr),
   107  						canonicalName: v.canonicalName,
   108  					}
   109  					continue
   110  				}
   111  
   112  				hs[key] = byName{
   113  					addrs:         []string{addr},
   114  					canonicalName: canonical,
   115  				}
   116  			}
   117  		}
   118  	}
   119  	// Update the data cache.
   120  	hosts.expire = now.Add(cacheMaxAge)
   121  	hosts.path = hp
   122  	hosts.byName = hs
   123  	hosts.byAddr = is
   124  	hosts.mtime = mtime
   125  	hosts.size = size
   126  }
   127  
   128  // lookupStaticHost looks up the addresses and the canonical name for the given host from /etc/hosts.
   129  func lookupStaticHost(host string) ([]string, string) {
   130  	hosts.Lock()
   131  	defer hosts.Unlock()
   132  	readHosts()
   133  	if len(hosts.byName) != 0 {
   134  		if hasUpperCase(host) {
   135  			lowerHost := []byte(host)
   136  			lowerASCIIBytes(lowerHost)
   137  			host = string(lowerHost)
   138  		}
   139  		if byName, ok := hosts.byName[absDomainName(host)]; ok {
   140  			ipsCp := make([]string, len(byName.addrs))
   141  			copy(ipsCp, byName.addrs)
   142  			return ipsCp, byName.canonicalName
   143  		}
   144  	}
   145  	return nil, ""
   146  }
   147  
   148  // lookupStaticAddr looks up the hosts for the given address from /etc/hosts.
   149  func lookupStaticAddr(addr string) []string {
   150  	hosts.Lock()
   151  	defer hosts.Unlock()
   152  	readHosts()
   153  	addr = parseLiteralIP(addr)
   154  	if addr == "" {
   155  		return nil
   156  	}
   157  	if len(hosts.byAddr) != 0 {
   158  		if hosts, ok := hosts.byAddr[addr]; ok {
   159  			hostsCp := make([]string, len(hosts))
   160  			copy(hostsCp, hosts)
   161  			return hostsCp
   162  		}
   163  	}
   164  	return nil
   165  }
   166
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