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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package net
import (
"errors"
"internal/bytealg"
"io/fs"
"net/netip"
"sync"
"time"
)
const cacheMaxAge = 5 * time.Second
func parseLiteralIP(addr string) string {
ip, err := netip.ParseAddr(addr)
if err != nil {
return ""
}
return ip.String()
}
type byName struct {
addrs []string
canonicalName string
}
// hosts contains known host entries.
var hosts struct {
sync.Mutex
// Key for the list of literal IP addresses must be a host
// name. It would be part of DNS labels, a FQDN or an absolute
// FQDN.
// For now the key is converted to lower case for convenience.
byName map[string]byName
// Key for the list of host names must be a literal IP address
// including IPv6 address with zone identifier.
// We don't support old-classful IP address notation.
byAddr map[string][]string
expire time.Time
path string
mtime time.Time
size int64
}
func readHosts() {
now := time.Now()
hp := hostsFilePath
if now.Before(hosts.expire) && hosts.path == hp && len(hosts.byName) > 0 {
return
}
mtime, size, err := stat(hp)
if err == nil && hosts.path == hp && hosts.mtime.Equal(mtime) && hosts.size == size {
hosts.expire = now.Add(cacheMaxAge)
return
}
hs := make(map[string]byName)
is := make(map[string][]string)
file, err := open(hp)
if err != nil {
if !errors.Is(err, fs.ErrNotExist) && !errors.Is(err, fs.ErrPermission) {
return
}
}
if file != nil {
defer file.close()
for line, ok := file.readLine(); ok; line, ok = file.readLine() {
if i := bytealg.IndexByteString(line, '#'); i >= 0 {
// Discard comments.
line = line[0:i]
}
f := getFields(line)
if len(f) < 2 {
continue
}
addr := parseLiteralIP(f[0])
if addr == "" {
continue
}
var canonical string
for i := 1; i < len(f); i++ {
name := absDomainName(f[i])
h := []byte(f[i])
lowerASCIIBytes(h)
key := absDomainName(string(h))
if i == 1 {
canonical = key
}
is[addr] = append(is[addr], name)
if v, ok := hs[key]; ok {
hs[key] = byName{
addrs: append(v.addrs, addr),
canonicalName: v.canonicalName,
}
continue
}
hs[key] = byName{
addrs: []string{addr},
canonicalName: canonical,
}
}
}
}
// Update the data cache.
hosts.expire = now.Add(cacheMaxAge)
hosts.path = hp
hosts.byName = hs
hosts.byAddr = is
hosts.mtime = mtime
hosts.size = size
}
// lookupStaticHost looks up the addresses and the canonical name for the given host from /etc/hosts.
func lookupStaticHost(host string) ([]string, string) {
hosts.Lock()
defer hosts.Unlock()
readHosts()
if len(hosts.byName) != 0 {
if hasUpperCase(host) {
lowerHost := []byte(host)
lowerASCIIBytes(lowerHost)
host = string(lowerHost)
}
if byName, ok := hosts.byName[absDomainName(host)]; ok {
ipsCp := make([]string, len(byName.addrs))
copy(ipsCp, byName.addrs)
return ipsCp, byName.canonicalName
}
}
return nil, ""
}
// lookupStaticAddr looks up the hosts for the given address from /etc/hosts.
func lookupStaticAddr(addr string) []string {
hosts.Lock()
defer hosts.Unlock()
readHosts()
addr = parseLiteralIP(addr)
if addr == "" {
return nil
}
if len(hosts.byAddr) != 0 {
if hosts, ok := hosts.byAddr[addr]; ok {
hostsCp := make([]string, len(hosts))
copy(hostsCp, hosts)
return hostsCp
}
}
return nil
}
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