// Build writes a configuration file to path containing a "nameserver" entry
// for every element in dns, a "search" entry for every element in
// dnsSearch, and an "options" entry for every element in dnsOptions.
func Build(path string, dns, dnsSearch, dnsOptions []string) (*File, error) {
	content := bytes.NewBuffer(nil)
	if len(dnsSearch) > 0 {
		if searchString := strings.Join(dnsSearch, " "); strings.Trim(searchString, " ") != "." {
			if _, err := content.WriteString("search " + searchString + "\n"); err != nil {
				return nil, err
			}
		}
	}
	for _, dns := range dns {
		if _, err := content.WriteString("nameserver " + dns + "\n"); err != nil {
			return nil, err
		}
	}
	if len(dnsOptions) > 0 {
		if optsString := strings.Join(dnsOptions, " "); strings.Trim(optsString, " ") != "" {
			if _, err := content.WriteString("options " + optsString + "\n"); err != nil {
				return nil, err
			}
		}
	}

	hash, err := ioutils.HashData(bytes.NewReader(content.Bytes()))
	if err != nil {
		return nil, err
	}

	return &File{Content: content.Bytes(), Hash: hash}, ioutil.WriteFile(path, content.Bytes(), 0644)
}

// GetSpecific returns the contents of the user specified resolv.conf file and its hash
func GetSpecific(path string) (*File, error) {
	resolv, err := ioutil.ReadFile(path)
	if err != nil {
		return nil, err
	}
	hash, err := ioutils.HashData(bytes.NewReader(resolv))
	if err != nil {
		return nil, err
	}
	return &File{Content: resolv, Hash: hash}, nil
}

// Get returns the contents of /etc/resolv.conf and its hash
func Get() (*File, error) {
	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
	if err != nil {
		return nil, err
	}
	hash, err := ioutils.HashData(bytes.NewReader(resolv))
	if err != nil {
		return nil, err
	}
	return &File{Content: resolv, Hash: hash}, nil
}

// set up the watch on the host's /etc/resolv.conf so that we can update container's
// live resolv.conf when the network changes on the host
func (daemon *Daemon) setupResolvconfWatcher() error {

	watcher, err := fsnotify.NewWatcher()
	if err != nil {
		return err
	}

	//this goroutine listens for the events on the watch we add
	//on the resolv.conf file on the host
	go func() {
		for {
			select {
			case event := <-watcher.Events:
				if event.Name == "/etc/resolv.conf" &&
					(event.Op&(fsnotify.Write|fsnotify.Create) != 0) {
					// verify a real change happened before we go further--a file write may have happened
					// without an actual change to the file
					updatedResolvConf, newResolvConfHash, err := resolvconf.GetIfChanged()
					if err != nil {
						logrus.Debugf("Error retrieving updated host resolv.conf: %v", err)
					} else if updatedResolvConf != nil {
						// because the new host resolv.conf might have localhost nameservers..
						updatedResolvConf, modified := resolvconf.FilterResolvDns(updatedResolvConf, daemon.config.Bridge.EnableIPv6)
						if modified {
							// changes have occurred during localhost cleanup: generate an updated hash
							newHash, err := ioutils.HashData(bytes.NewReader(updatedResolvConf))
							if err != nil {
								logrus.Debugf("Error generating hash of new resolv.conf: %v", err)
							} else {
								newResolvConfHash = newHash
							}
						}
						logrus.Debug("host network resolv.conf changed--walking container list for updates")
						contList := daemon.containers.List()
						for _, container := range contList {
							if err := container.updateResolvConf(updatedResolvConf, newResolvConfHash); err != nil {
								logrus.Debugf("Error on resolv.conf update check for container ID: %s: %v", container.ID, err)
							}
						}
					}
				}
			case err := <-watcher.Errors:
				logrus.Debugf("host resolv.conf notify error: %v", err)
			}
		}
	}()

	if err := watcher.Add("/etc"); err != nil {
		return err
	}
	return nil
}

func TestGet(t *testing.T) {
	resolvConfUtils, err := Get()
	if err != nil {
		t.Fatal(err)
	}
	resolvConfSystem, err := ioutil.ReadFile("/etc/resolv.conf")
	if err != nil {
		t.Fatal(err)
	}
	if string(resolvConfUtils.Content) != string(resolvConfSystem) {
		t.Fatalf("/etc/resolv.conf and GetResolvConf have different content.")
	}
	hashSystem, err := ioutils.HashData(bytes.NewReader(resolvConfSystem))
	if err != nil {
		t.Fatal(err)
	}
	if resolvConfUtils.Hash != hashSystem {
		t.Fatalf("/etc/resolv.conf and GetResolvConf have different hashes.")
	}
}

// GetIfChanged retrieves the host /etc/resolv.conf file, checks against the last hash
// and, if modified since last check, returns the bytes and new hash.
// This feature is used by the resolv.conf updater for containers
func GetIfChanged() ([]byte, string, error) {
	lastModified.Lock()
	defer lastModified.Unlock()

	resolv, err := ioutil.ReadFile("/etc/resolv.conf")
	if err != nil {
		return nil, "", err
	}
	newHash, err := ioutils.HashData(bytes.NewReader(resolv))
	if err != nil {
		return nil, "", err
	}
	if lastModified.sha256 != newHash {
		lastModified.sha256 = newHash
		lastModified.contents = resolv
		return resolv, newHash, nil
	}
	// nothing changed, so return no data
	return nil, "", nil
}

// FilterResolvDNS cleans up the config in resolvConf.  It has two main jobs:
// 1. It looks for localhost (127.*|::1) entries in the provided
//    resolv.conf, removing local nameserver entries, and, if the resulting
//    cleaned config has no defined nameservers left, adds default DNS entries
// 2. Given the caller provides the enable/disable state of IPv6, the filter
//    code will remove all IPv6 nameservers if it is not enabled for containers
//
func FilterResolvDNS(resolvConf []byte, ipv6Enabled bool) (*File, error) {
	cleanedResolvConf := localhostNSRegexp.ReplaceAll(resolvConf, []byte{})
	// if IPv6 is not enabled, also clean out any IPv6 address nameserver
	if !ipv6Enabled {
		cleanedResolvConf = nsIPv6Regexp.ReplaceAll(cleanedResolvConf, []byte{})
	}
	// if the resulting resolvConf has no more nameservers defined, add appropriate
	// default DNS servers for IPv4 and (optionally) IPv6
	if len(GetNameservers(cleanedResolvConf, types.IP)) == 0 {
		logrus.Infof("No non-localhost DNS nameservers are left in resolv.conf. Using default external servers: %v", defaultIPv4Dns)
		dns := defaultIPv4Dns
		if ipv6Enabled {
			logrus.Infof("IPv6 enabled; Adding default IPv6 external servers: %v", defaultIPv6Dns)
			dns = append(dns, defaultIPv6Dns...)
		}
		cleanedResolvConf = append(cleanedResolvConf, []byte("\n"+strings.Join(dns, "\n"))...)
	}
	hash, err := ioutils.HashData(bytes.NewReader(cleanedResolvConf))
	if err != nil {
		return nil, err
	}
	return &File{Content: cleanedResolvConf, Hash: hash}, nil
}

// called when the host's resolv.conf changes to check whether container's resolv.conf
// is unchanged by the container "user" since container start: if unchanged, the
// container's resolv.conf will be updated to match the host's new resolv.conf
func (container *Container) updateResolvConf(updatedResolvConf []byte, newResolvHash string) error {

	if container.ResolvConfPath == "" {
		return nil
	}
	if container.Running {
		//set a marker in the hostConfig to update on next start/restart
		container.UpdateDns = true
		return nil
	}

	resolvHashFile := container.ResolvConfPath + ".hash"

	//read the container's current resolv.conf and compute the hash
	resolvBytes, err := ioutil.ReadFile(container.ResolvConfPath)
	if err != nil {
		return err
	}
	curHash, err := ioutils.HashData(bytes.NewReader(resolvBytes))
	if err != nil {
		return err
	}

	//read the hash from the last time we wrote resolv.conf in the container
	hashBytes, err := ioutil.ReadFile(resolvHashFile)
	if err != nil {
		if !os.IsNotExist(err) {
			return err
		}
		// backwards compat: if no hash file exists, this container pre-existed from
		// a Docker daemon that didn't contain this update feature. Given we can't know
		// if the user has modified the resolv.conf since container start time, safer
		// to just never update the container's resolv.conf during it's lifetime which
		// we can control by setting hashBytes to an empty string
		hashBytes = []byte("")
	}

	//if the user has not modified the resolv.conf of the container since we wrote it last
	//we will replace it with the updated resolv.conf from the host
	if string(hashBytes) == curHash {
		logrus.Debugf("replacing %q with updated host resolv.conf", container.ResolvConfPath)

		// for atomic updates to these files, use temporary files with os.Rename:
		dir := path.Dir(container.ResolvConfPath)
		tmpHashFile, err := ioutil.TempFile(dir, "hash")
		if err != nil {
			return err
		}
		tmpResolvFile, err := ioutil.TempFile(dir, "resolv")
		if err != nil {
			return err
		}

		// write the updates to the temp files
		if err = ioutil.WriteFile(tmpHashFile.Name(), []byte(newResolvHash), 0644); err != nil {
			return err
		}
		if err = ioutil.WriteFile(tmpResolvFile.Name(), updatedResolvConf, 0644); err != nil {
			return err
		}

		// rename the temp files for atomic replace
		if err = os.Rename(tmpHashFile.Name(), resolvHashFile); err != nil {
			return err
		}
		return os.Rename(tmpResolvFile.Name(), container.ResolvConfPath)
	}
	return nil
}

func (container *Container) setupContainerDns() error {
	if container.ResolvConfPath != "" {
		// check if this is an existing container that needs DNS update:
		if container.UpdateDns {
			// read the host's resolv.conf, get the hash and call updateResolvConf
			logrus.Debugf("Check container (%s) for update to resolv.conf - UpdateDns flag was set", container.ID)
			latestResolvConf, latestHash := resolvconf.GetLastModified()

			// clean container resolv.conf re: localhost nameservers and IPv6 NS (if IPv6 disabled)
			updatedResolvConf, modified := resolvconf.FilterResolvDns(latestResolvConf, container.daemon.config.Bridge.EnableIPv6)
			if modified {
				// changes have occurred during resolv.conf localhost cleanup: generate an updated hash
				newHash, err := ioutils.HashData(bytes.NewReader(updatedResolvConf))
				if err != nil {
					return err
				}
				latestHash = newHash
			}

			if err := container.updateResolvConf(updatedResolvConf, latestHash); err != nil {
				return err
			}
			// successful update of the restarting container; set the flag off
			container.UpdateDns = false
		}
		return nil
	}

	var (
		config = container.hostConfig
		daemon = container.daemon
	)

	resolvConf, err := resolvconf.Get()
	if err != nil {
		return err
	}
	container.ResolvConfPath, err = container.GetRootResourcePath("resolv.conf")
	if err != nil {
		return err
	}

	if config.NetworkMode != "host" {
		// check configurations for any container/daemon dns settings
		if len(config.Dns) > 0 || len(daemon.config.Dns) > 0 || len(config.DnsSearch) > 0 || len(daemon.config.DnsSearch) > 0 {
			var (
				dns       = resolvconf.GetNameservers(resolvConf)
				dnsSearch = resolvconf.GetSearchDomains(resolvConf)
			)
			if len(config.Dns) > 0 {
				dns = config.Dns
			} else if len(daemon.config.Dns) > 0 {
				dns = daemon.config.Dns
			}
			if len(config.DnsSearch) > 0 {
				dnsSearch = config.DnsSearch
			} else if len(daemon.config.DnsSearch) > 0 {
				dnsSearch = daemon.config.DnsSearch
			}
			return resolvconf.Build(container.ResolvConfPath, dns, dnsSearch)
		}

		// replace any localhost/127.*, and remove IPv6 nameservers if IPv6 disabled in daemon
		resolvConf, _ = resolvconf.FilterResolvDns(resolvConf, daemon.config.Bridge.EnableIPv6)
	}
	//get a sha256 hash of the resolv conf at this point so we can check
	//for changes when the host resolv.conf changes (e.g. network update)
	resolvHash, err := ioutils.HashData(bytes.NewReader(resolvConf))
	if err != nil {
		return err
	}
	resolvHashFile := container.ResolvConfPath + ".hash"
	if err = ioutil.WriteFile(resolvHashFile, []byte(resolvHash), 0644); err != nil {
		return err
	}
	return ioutil.WriteFile(container.ResolvConfPath, resolvConf, 0644)
}

func (sb *sandbox) updateDNS(ipv6Enabled bool) error {
	var oldHash []byte
	hashFile := sb.config.resolvConfHashFile

	resolvConf, err := ioutil.ReadFile(sb.config.resolvConfPath)
	if err != nil {
		if !os.IsNotExist(err) {
			return err
		}
	} else {
		oldHash, err = ioutil.ReadFile(hashFile)
		if err != nil {
			if !os.IsNotExist(err) {
				return err
			}

			oldHash = []byte{}
		}
	}

	curHash, err := ioutils.HashData(bytes.NewReader(resolvConf))
	if err != nil {
		return err
	}

	if string(oldHash) != "" && curHash != string(oldHash) {
		// Seems the user has changed the container resolv.conf since the last time
		// we checked so return without doing anything.
		log.Infof("Skipping update of resolv.conf file with ipv6Enabled: %t because file was touched by user", ipv6Enabled)
		return nil
	}

	// replace any localhost/127.* and remove IPv6 nameservers if IPv6 disabled.
	resolvConf, _ = resolvconf.FilterResolvDNS(resolvConf, ipv6Enabled)

	newHash, err := ioutils.HashData(bytes.NewReader(resolvConf))
	if err != nil {
		return err
	}

	// for atomic updates to these files, use temporary files with os.Rename:
	dir := path.Dir(sb.config.resolvConfPath)
	tmpHashFile, err := ioutil.TempFile(dir, "hash")
	if err != nil {
		return err
	}
	tmpResolvFile, err := ioutil.TempFile(dir, "resolv")
	if err != nil {
		return err
	}

	// Change the perms to filePerm (0644) since ioutil.TempFile creates it by default as 0600
	if err := os.Chmod(tmpResolvFile.Name(), filePerm); err != nil {
		return err
	}

	// write the updates to the temp files
	if err = ioutil.WriteFile(tmpHashFile.Name(), []byte(newHash), filePerm); err != nil {
		return err
	}
	if err = ioutil.WriteFile(tmpResolvFile.Name(), resolvConf, filePerm); err != nil {
		return err
	}

	// rename the temp files for atomic replace
	if err = os.Rename(tmpHashFile.Name(), hashFile); err != nil {
		return err
	}
	return os.Rename(tmpResolvFile.Name(), sb.config.resolvConfPath)
}

func (ep *endpoint) updateDNS(resolvConf []byte) error {
	ep.Lock()
	container := ep.container
	network := ep.network
	ep.Unlock()

	if container == nil {
		return ErrNoContainer{}
	}

	oldHash := []byte{}
	hashFile := container.config.resolvConfPath + ".hash"

	resolvBytes, err := ioutil.ReadFile(container.config.resolvConfPath)
	if err != nil {
		if !os.IsNotExist(err) {
			return err
		}
	} else {
		oldHash, err = ioutil.ReadFile(hashFile)
		if err != nil {
			if !os.IsNotExist(err) {
				return err
			}

			oldHash = []byte{}
		}
	}

	curHash, err := ioutils.HashData(bytes.NewReader(resolvBytes))
	if err != nil {
		return err
	}

	if string(oldHash) != "" && curHash != string(oldHash) {
		// Seems the user has changed the container resolv.conf since the last time
		// we checked so return without doing anything.
		return nil
	}

	// replace any localhost/127.* and remove IPv6 nameservers if IPv6 disabled.
	resolvConf, _ = resolvconf.FilterResolvDNS(resolvConf, network.enableIPv6)

	newHash, err := ioutils.HashData(bytes.NewReader(resolvConf))
	if err != nil {
		return err
	}

	// for atomic updates to these files, use temporary files with os.Rename:
	dir := path.Dir(container.config.resolvConfPath)
	tmpHashFile, err := ioutil.TempFile(dir, "hash")
	if err != nil {
		return err
	}
	tmpResolvFile, err := ioutil.TempFile(dir, "resolv")
	if err != nil {
		return err
	}

	// Change the perms to 0644 since ioutil.TempFile creates it by default as 0600
	if err := os.Chmod(tmpResolvFile.Name(), 0644); err != nil {
		return err
	}

	// write the updates to the temp files
	if err = ioutil.WriteFile(tmpHashFile.Name(), []byte(newHash), 0644); err != nil {
		return err
	}
	if err = ioutil.WriteFile(tmpResolvFile.Name(), resolvConf, 0644); err != nil {
		return err
	}

	// rename the temp files for atomic replace
	if err = os.Rename(tmpHashFile.Name(), hashFile); err != nil {
		return err
	}
	return os.Rename(tmpResolvFile.Name(), container.config.resolvConfPath)
}