// Embedded DNS server has to be enabled for this sandbox. Rebuild the container's
// resolv.conf by doing the following
// - Save the external name servers in resolv.conf in the sandbox
// - Add only the embedded server's IP to container's resolv.conf
// - If the embedded server needs any resolv.conf options add it to the current list
func (sb *sandbox) rebuildDNS() error {
	currRC, err := resolvconf.GetSpecific(sb.config.resolvConfPath)
	if err != nil {
		return err
	}

	// localhost entries have already been filtered out from the list
	// retain only the v4 servers in sb for forwarding the DNS queries
	sb.extDNS = resolvconf.GetNameservers(currRC.Content, types.IPv4)

	var (
		dnsList        = []string{sb.resolver.NameServer()}
		dnsOptionsList = resolvconf.GetOptions(currRC.Content)
		dnsSearchList  = resolvconf.GetSearchDomains(currRC.Content)
	)

	// external v6 DNS servers has to be listed in resolv.conf
	dnsList = append(dnsList, resolvconf.GetNameservers(currRC.Content, types.IPv6)...)

	// If the user config and embedded DNS server both have ndots option set,
	// remember the user's config so that unqualified names not in the docker
	// domain can be dropped.
	resOptions := sb.resolver.ResolverOptions()

dnsOpt:
	for _, resOpt := range resOptions {
		if strings.Contains(resOpt, "ndots") {
			for _, option := range dnsOptionsList {
				if strings.Contains(option, "ndots") {
					parts := strings.Split(option, ":")
					if len(parts) != 2 {
						return fmt.Errorf("invalid ndots option %v", option)
					}
					if num, err := strconv.Atoi(parts[1]); err != nil {
						return fmt.Errorf("invalid number for ndots option %v", option)
					} else if num > 0 {
						sb.ndotsSet = true
						break dnsOpt
					}
				}
			}
		}
	}

	dnsOptionsList = append(dnsOptionsList, resOptions...)

	_, err = resolvconf.Build(sb.config.resolvConfPath, dnsList, dnsSearchList, dnsOptionsList)
	return err
}

// Embedded DNS server has to be enabled for this sandbox. Rebuild the container's
// resolv.conf by doing the follwing
// - Save the external name servers in resolv.conf in the sandbox
// - Add only the embedded server's IP to container's resolv.conf
// - If the embedded server needs any resolv.conf options add it to the current list
func (sb *sandbox) rebuildDNS() error {
	currRC, err := resolvconf.GetSpecific(sb.config.resolvConfPath)
	if err != nil {
		return err
	}

	// localhost entries have already been filtered out from the list
	sb.extDNS = resolvconf.GetNameservers(currRC.Content)

	var (
		dnsList        = []string{sb.resolver.NameServer()}
		dnsOptionsList = resolvconf.GetOptions(currRC.Content)
		dnsSearchList  = resolvconf.GetSearchDomains(currRC.Content)
	)

	// Resolver returns the options in the format resolv.conf expects
	dnsOptionsList = append(dnsOptionsList, sb.resolver.ResolverOptions()...)

	dir := path.Dir(sb.config.resolvConfPath)
	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
	}

	_, err = resolvconf.Build(tmpResolvFile.Name(), dnsList, dnsSearchList, dnsOptionsList)
	if err != nil {
		return err
	}

	return os.Rename(tmpResolvFile.Name(), sb.config.resolvConfPath)
}

func (sb *sandbox) updateDNS(ipv6Enabled bool) error {
	var (
		currHash string
		hashFile = sb.config.resolvConfHashFile
	)

	if len(sb.config.dnsList) > 0 || len(sb.config.dnsSearchList) > 0 || len(sb.config.dnsOptionsList) > 0 {
		return nil
	}

	currRC, err := resolvconf.GetSpecific(sb.config.resolvConfPath)
	if err != nil {
		if !os.IsNotExist(err) {
			return err
		}
	} else {
		h, err := ioutil.ReadFile(hashFile)
		if err != nil {
			if !os.IsNotExist(err) {
				return err
			}
		} else {
			currHash = string(h)
		}
	}

	if currHash != "" && currHash != currRC.Hash {
		// 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.
	newRC, err := resolvconf.FilterResolvDNS(currRC.Content, ipv6Enabled)
	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(newRC.Hash), filePerm); err != nil {
		return err
	}
	if err = ioutil.WriteFile(tmpResolvFile.Name(), newRC.Content, 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)
}