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// Package resolvconf provides utility code to query and update DNS configuration in /etc/resolv.conf
package resolvconf
import (
"bytes"
"io/ioutil"
"regexp"
"strings"
"sync"
"github.com/docker/docker/pkg/ioutils"
"github.com/docker/libnetwork/resolvconf/dns"
"github.com/docker/libnetwork/types"
"github.com/sirupsen/logrus"
)
const (
// defaultPath is the default path to the resolv.conf that contains information to resolve DNS. See Path().
defaultPath = "/etc/resolv.conf"
// alternatePath is a path different from defaultPath, that may be used to resolve DNS. See Path().
alternatePath = "/run/systemd/resolve/resolv.conf"
)
var (
detectSystemdResolvConfOnce sync.Once
pathAfterSystemdDetection = defaultPath
)
// Path returns the path to the resolv.conf file that libnetwork should use.
//
// When /etc/resolv.conf contains 127.0.0.53 as the only nameserver, then
// it is assumed systemd-resolved manages DNS. Because inside the container 127.0.0.53
// is not a valid DNS server, Path() returns /run/systemd/resolve/resolv.conf
// which is the resolv.conf that systemd-resolved generates and manages.
// Otherwise Path() returns /etc/resolv.conf.
//
// Errors are silenced as they will inevitably resurface at future open/read calls.
//
// More information at https://www.freedesktop.org/software/systemd/man/systemd-resolved.service.html#/etc/resolv.conf
func Path() string {
detectSystemdResolvConfOnce.Do(func() {
candidateResolvConf, err := ioutil.ReadFile(defaultPath)
if err != nil {
// silencing error as it will resurface at next calls trying to read defaultPath
return
}
ns := GetNameservers(candidateResolvConf, types.IP)
if len(ns) == 1 && ns[0] == "127.0.0.53" {
pathAfterSystemdDetection = alternatePath
logrus.Infof("detected 127.0.0.53 nameserver, assuming systemd-resolved, so using resolv.conf: %s", alternatePath)
}
})
return pathAfterSystemdDetection
}
var (
// Note: the default IPv4 & IPv6 resolvers are set to Google's Public DNS
defaultIPv4Dns = []string{"nameserver 8.8.8.8", "nameserver 8.8.4.4"}
defaultIPv6Dns = []string{"nameserver 2001:4860:4860::8888", "nameserver 2001:4860:4860::8844"}
ipv4NumBlock = `(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)`
ipv4Address = `(` + ipv4NumBlock + `\.){3}` + ipv4NumBlock
// This is not an IPv6 address verifier as it will accept a super-set of IPv6, and also
// will *not match* IPv4-Embedded IPv6 Addresses (RFC6052), but that and other variants
// -- e.g. other link-local types -- either won't work in containers or are unnecessary.
// For readability and sufficiency for Docker purposes this seemed more reasonable than a
// 1000+ character regexp with exact and complete IPv6 validation
ipv6Address = `([0-9A-Fa-f]{0,4}:){2,7}([0-9A-Fa-f]{0,4})(%\w+)?`
localhostNSRegexp = regexp.MustCompile(`(?m)^nameserver\s+` + dns.IPLocalhost + `\s*\n*`)
nsIPv6Regexp = regexp.MustCompile(`(?m)^nameserver\s+` + ipv6Address + `\s*\n*`)
nsRegexp = regexp.MustCompile(`^\s*nameserver\s*((` + ipv4Address + `)|(` + ipv6Address + `))\s*$`)
nsIPv6Regexpmatch = regexp.MustCompile(`^\s*nameserver\s*((` + ipv6Address + `))\s*$`)
nsIPv4Regexpmatch = regexp.MustCompile(`^\s*nameserver\s*((` + ipv4Address + `))\s*$`)
searchRegexp = regexp.MustCompile(`^\s*search\s*(([^\s]+\s*)*)$`)
optionsRegexp = regexp.MustCompile(`^\s*options\s*(([^\s]+\s*)*)$`)
)
var lastModified struct {
sync.Mutex
sha256 string
contents []byte
}
// File contains the resolv.conf content and its hash
type File struct {
Content []byte
Hash string
}
// Get returns the contents of /etc/resolv.conf and its hash
func Get() (*File, error) {
return GetSpecific(Path())
}
// 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
}
// 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() (*File, error) {
lastModified.Lock()
defer lastModified.Unlock()
resolv, err := ioutil.ReadFile(Path())
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 &File{Content: resolv, Hash: newHash}, nil
}
// nothing changed, so return no data
return nil, nil
}
// GetLastModified retrieves the last used contents and hash of the host resolv.conf.
// Used by containers updating on restart
func GetLastModified() *File {
lastModified.Lock()
defer lastModified.Unlock()
return &File{Content: lastModified.contents, Hash: lastModified.sha256}
}
// 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
}
// getLines parses input into lines and strips away comments.
func getLines(input []byte, commentMarker []byte) [][]byte {
lines := bytes.Split(input, []byte("\n"))
var output [][]byte
for _, currentLine := range lines {
var commentIndex = bytes.Index(currentLine, commentMarker)
if commentIndex == -1 {
output = append(output, currentLine)
} else {
output = append(output, currentLine[:commentIndex])
}
}
return output
}
// GetNameservers returns nameservers (if any) listed in /etc/resolv.conf
func GetNameservers(resolvConf []byte, kind int) []string {
nameservers := []string{}
for _, line := range getLines(resolvConf, []byte("#")) {
var ns [][]byte
if kind == types.IP {
ns = nsRegexp.FindSubmatch(line)
} else if kind == types.IPv4 {
ns = nsIPv4Regexpmatch.FindSubmatch(line)
} else if kind == types.IPv6 {
ns = nsIPv6Regexpmatch.FindSubmatch(line)
}
if len(ns) > 0 {
nameservers = append(nameservers, string(ns[1]))
}
}
return nameservers
}
// GetNameserversAsCIDR returns nameservers (if any) listed in
// /etc/resolv.conf as CIDR blocks (e.g., "1.2.3.4/32")
// This function's output is intended for net.ParseCIDR
func GetNameserversAsCIDR(resolvConf []byte) []string {
nameservers := []string{}
for _, nameserver := range GetNameservers(resolvConf, types.IP) {
var address string
// If IPv6, strip zone if present
if strings.Contains(nameserver, ":") {
address = strings.Split(nameserver, "%")[0] + "/128"
} else {
address = nameserver + "/32"
}
nameservers = append(nameservers, address)
}
return nameservers
}
// GetSearchDomains returns search domains (if any) listed in /etc/resolv.conf
// If more than one search line is encountered, only the contents of the last
// one is returned.
func GetSearchDomains(resolvConf []byte) []string {
domains := []string{}
for _, line := range getLines(resolvConf, []byte("#")) {
match := searchRegexp.FindSubmatch(line)
if match == nil {
continue
}
domains = strings.Fields(string(match[1]))
}
return domains
}
// GetOptions returns options (if any) listed in /etc/resolv.conf
// If more than one options line is encountered, only the contents of the last
// one is returned.
func GetOptions(resolvConf []byte) []string {
options := []string{}
for _, line := range getLines(resolvConf, []byte("#")) {
match := optionsRegexp.FindSubmatch(line)
if match == nil {
continue
}
options = strings.Fields(string(match[1]))
}
return options
}
// 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)
}