195 lines
6.8 KiB
Go
195 lines
6.8 KiB
Go
// Package resolvconf provides utility code to query and update DNS configuration in /etc/resolv.conf
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package resolvconf
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import (
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"bytes"
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"io/ioutil"
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"regexp"
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"strings"
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"sync"
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"github.com/Sirupsen/logrus"
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"github.com/docker/docker/pkg/ioutils"
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)
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var (
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// Note: the default IPv4 & IPv6 resolvers are set to Google's Public DNS
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defaultIPv4Dns = []string{"nameserver 8.8.8.8", "nameserver 8.8.4.4"}
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defaultIPv6Dns = []string{"nameserver 2001:4860:4860::8888", "nameserver 2001:4860:4860::8844"}
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ipv4NumBlock = `(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)`
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ipv4Address = `(` + ipv4NumBlock + `\.){3}` + ipv4NumBlock
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// This is not an IPv6 address verifier as it will accept a super-set of IPv6, and also
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// will *not match* IPv4-Embedded IPv6 Addresses (RFC6052), but that and other variants
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// -- e.g. other link-local types -- either won't work in containers or are unnecessary.
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// For readability and sufficiency for Docker purposes this seemed more reasonable than a
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// 1000+ character regexp with exact and complete IPv6 validation
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ipv6Address = `([0-9A-Fa-f]{0,4}:){2,7}([0-9A-Fa-f]{0,4})`
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ipLocalhost = `((127\.([0-9]{1,3}.){2}[0-9]{1,3})|(::1))`
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localhostIPRegexp = regexp.MustCompile(ipLocalhost)
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localhostNSRegexp = regexp.MustCompile(`(?m)^nameserver\s+` + ipLocalhost + `\s*\n*`)
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nsIPv6Regexp = regexp.MustCompile(`(?m)^nameserver\s+` + ipv6Address + `\s*\n*`)
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nsRegexp = regexp.MustCompile(`^\s*nameserver\s*((` + ipv4Address + `)|(` + ipv6Address + `))\s*$`)
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searchRegexp = regexp.MustCompile(`^\s*search\s*(([^\s]+\s*)*)$`)
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)
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var lastModified struct {
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sync.Mutex
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sha256 string
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contents []byte
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}
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// Get returns the contents of /etc/resolv.conf
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func Get() ([]byte, error) {
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resolv, err := ioutil.ReadFile("/etc/resolv.conf")
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if err != nil {
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return nil, err
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}
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return resolv, nil
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}
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// GetIfChanged retrieves the host /etc/resolv.conf file, checks against the last hash
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// and, if modified since last check, returns the bytes and new hash.
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// This feature is used by the resolv.conf updater for containers
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func GetIfChanged() ([]byte, string, error) {
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lastModified.Lock()
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defer lastModified.Unlock()
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resolv, err := ioutil.ReadFile("/etc/resolv.conf")
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if err != nil {
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return nil, "", err
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}
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newHash, err := ioutils.HashData(bytes.NewReader(resolv))
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if err != nil {
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return nil, "", err
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}
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if lastModified.sha256 != newHash {
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lastModified.sha256 = newHash
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lastModified.contents = resolv
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return resolv, newHash, nil
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}
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// nothing changed, so return no data
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return nil, "", nil
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}
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// GetLastModified retrieves the last used contents and hash of the host resolv.conf.
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// Used by containers updating on restart
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func GetLastModified() ([]byte, string) {
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lastModified.Lock()
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defer lastModified.Unlock()
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return lastModified.contents, lastModified.sha256
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}
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// FilterResolvDns cleans up the config in resolvConf. It has two main jobs:
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// 1. It looks for localhost (127.*|::1) entries in the provided
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// resolv.conf, removing local nameserver entries, and, if the resulting
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// cleaned config has no defined nameservers left, adds default DNS entries
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// 2. Given the caller provides the enable/disable state of IPv6, the filter
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// code will remove all IPv6 nameservers if it is not enabled for containers
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//
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// It returns a boolean to notify the caller if changes were made at all
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func FilterResolvDns(resolvConf []byte, ipv6Enabled bool) ([]byte, bool) {
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changed := false
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cleanedResolvConf := localhostNSRegexp.ReplaceAll(resolvConf, []byte{})
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// if IPv6 is not enabled, also clean out any IPv6 address nameserver
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if !ipv6Enabled {
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cleanedResolvConf = nsIPv6Regexp.ReplaceAll(cleanedResolvConf, []byte{})
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}
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// if the resulting resolvConf has no more nameservers defined, add appropriate
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// default DNS servers for IPv4 and (optionally) IPv6
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if len(GetNameservers(cleanedResolvConf)) == 0 {
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logrus.Infof("No non-localhost DNS nameservers are left in resolv.conf. Using default external servers : %v", defaultIPv4Dns)
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dns := defaultIPv4Dns
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if ipv6Enabled {
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logrus.Infof("IPv6 enabled; Adding default IPv6 external servers : %v", defaultIPv6Dns)
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dns = append(dns, defaultIPv6Dns...)
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}
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cleanedResolvConf = append(cleanedResolvConf, []byte("\n"+strings.Join(dns, "\n"))...)
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}
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if !bytes.Equal(resolvConf, cleanedResolvConf) {
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changed = true
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}
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return cleanedResolvConf, changed
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}
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// getLines parses input into lines and strips away comments.
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func getLines(input []byte, commentMarker []byte) [][]byte {
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lines := bytes.Split(input, []byte("\n"))
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var output [][]byte
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for _, currentLine := range lines {
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var commentIndex = bytes.Index(currentLine, commentMarker)
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if commentIndex == -1 {
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output = append(output, currentLine)
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} else {
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output = append(output, currentLine[:commentIndex])
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}
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}
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return output
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}
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// IsLocalhost returns true if ip matches the localhost IP regular expression.
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// Used for determining if nameserver settings are being passed which are
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// localhost addresses
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func IsLocalhost(ip string) bool {
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return localhostIPRegexp.MatchString(ip)
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}
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// GetNameservers returns nameservers (if any) listed in /etc/resolv.conf
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func GetNameservers(resolvConf []byte) []string {
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nameservers := []string{}
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for _, line := range getLines(resolvConf, []byte("#")) {
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var ns = nsRegexp.FindSubmatch(line)
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if len(ns) > 0 {
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nameservers = append(nameservers, string(ns[1]))
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}
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}
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return nameservers
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}
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// GetNameserversAsCIDR returns nameservers (if any) listed in
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// /etc/resolv.conf as CIDR blocks (e.g., "1.2.3.4/32")
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// This function's output is intended for net.ParseCIDR
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func GetNameserversAsCIDR(resolvConf []byte) []string {
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nameservers := []string{}
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for _, nameserver := range GetNameservers(resolvConf) {
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nameservers = append(nameservers, nameserver+"/32")
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}
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return nameservers
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}
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// GetSearchDomains returns search domains (if any) listed in /etc/resolv.conf
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// If more than one search line is encountered, only the contents of the last
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// one is returned.
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func GetSearchDomains(resolvConf []byte) []string {
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domains := []string{}
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for _, line := range getLines(resolvConf, []byte("#")) {
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match := searchRegexp.FindSubmatch(line)
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if match == nil {
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continue
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}
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domains = strings.Fields(string(match[1]))
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}
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return domains
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}
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// Build writes a configuration file to path containing a "nameserver" entry
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// for every element in dns, and a "search" entry for every element in
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// dnsSearch.
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func Build(path string, dns, dnsSearch []string) error {
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content := bytes.NewBuffer(nil)
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for _, dns := range dns {
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if _, err := content.WriteString("nameserver " + dns + "\n"); err != nil {
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return err
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}
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}
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if len(dnsSearch) > 0 {
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if searchString := strings.Join(dnsSearch, " "); strings.Trim(searchString, " ") != "." {
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if _, err := content.WriteString("search " + searchString + "\n"); err != nil {
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return err
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}
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}
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}
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return ioutil.WriteFile(path, content.Bytes(), 0644)
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}
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