// 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.Write ||
						event.Op&fsnotify.Create == fsnotify.Create) {
					// 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 {
						log.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.EnableIPv6)
						if modified {
							// changes have occurred during localhost cleanup: generate an updated hash
							newHash, err := utils.HashData(bytes.NewReader(updatedResolvConf))
							if err != nil {
								log.Debugf("Error generating hash of new resolv.conf: %v", err)
							} else {
								newResolvConfHash = newHash
							}
						}
						log.Debugf("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 {
								log.Debugf("Error on resolv.conf update check for container ID: %s: %v", container.ID, err)
							}
						}
					}
				}
			case err := <-watcher.Errors:
				log.Debugf("host resolv.conf notify error: %v", err)
			}
		}
	}()

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

// 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 := utils.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
}

// 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 := utils.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 {
		log.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
			log.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.EnableIPv6)
			if modified {
				// changes have occurred during resolv.conf localhost cleanup: generate an updated hash
				newHash, err := utils.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.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 := utils.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)
}