/*
   Copyright The containerd Authors.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

/*
   Portions from https://github.com/moby/moby/blob/6014c1e29dc34dffa77fb5749cc3281c1b4854ac/libnetwork/resolvconf/resolvconf.go
   Copyright (C) Docker/Moby authors.
   Licensed under the Apache License, Version 2.0
   NOTICE: https://github.com/moby/moby/blob/6014c1e29dc34dffa77fb5749cc3281c1b4854ac/NOTICE
*/

// Package resolvconf provides utility code to query and update DNS configuration in /etc/resolv.conf
// originally from https://github.com/moby/moby/blob/6014c1e29dc34dffa77fb5749cc3281c1b4854ac/libnetwork/resolvconf/resolvconf.go
package resolvconf

import (
	"bytes"
	"crypto/sha256"
	"encoding/hex"
	"io"
	"os"
	"regexp"
	"strings"
	"sync"

	"github.com/containerd/log"
)

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"
)

// constants for the IP address type
const (
	IP = iota // IPv4 and IPv6
	IPv4
	IPv6
)

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 := os.ReadFile(defaultPath)
		if err != nil {
			// silencing error as it will resurface at next calls trying to read defaultPath
			return
		}
		ns := GetNameservers(candidateResolvConf, IP)
		if len(ns) == 1 && ns[0] == "127.0.0.53" {
			pathAfterSystemdDetection = alternatePath
			log.L.Debugf("detected 127.0.0.53 nameserver, assuming systemd-resolved, so using resolv.conf: %s", alternatePath)
		}
	})
	return pathAfterSystemdDetection
}

const (
	// ipLocalhost is a regex pattern for IPv4 or IPv6 loopback range.
	ipLocalhost  = `((127\.([0-9]{1,3}\.){2}[0-9]{1,3})|(::1)$)`
	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+)?`
)

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"}

	localhostNSRegexp = regexp.MustCompile(`(?m)^nameserver\s+` + 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 := os.ReadFile(path)
	if err != nil {
		return nil, err
	}
	hash, err := 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 := os.ReadFile(Path())
	if err != nil {
		return nil, err
	}
	newHash, err := 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, IP)) == 0 {
		log.L.Infof("No non-localhost DNS nameservers are left in resolv.conf. Using default external servers: %v", defaultIPv4Dns)
		dns := defaultIPv4Dns
		if ipv6Enabled {
			log.L.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 := 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 == IP {
			ns = nsRegexp.FindSubmatch(line)
		} else if kind == IPv4 {
			ns = nsIPv4Regexpmatch.FindSubmatch(line)
		} else if kind == 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, 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 := hashData(bytes.NewReader(content.Bytes()))
	if err != nil {
		return nil, err
	}

	err = os.WriteFile(path, content.Bytes(), 0o644)
	if err != nil {
		return nil, err
	}

	// os.WriteFile relies on syscall.Open. Unless there are ACLs, the effective mode of the file will be matched
	// against the current process umask.
	// See https://www.man7.org/linux/man-pages/man2/open.2.html for details.
	// Since we must make sure that these files are world readable, explicitly chmod them here.
	return &File{Content: content.Bytes(), Hash: hash}, os.Chmod(path, 0o644)
}

func hashData(src io.Reader) (string, error) {
	h := sha256.New()
	if _, err := io.Copy(h, src); err != nil {
		return "", err
	}
	return "sha256:" + hex.EncodeToString(h.Sum(nil)), nil
}
