// Package dnsfilter implements a DNS filter.
package dnsfilter

import (
	"fmt"
	"io/ioutil"
	"net"
	"net/http"
	"os"
	"runtime"
	"runtime/debug"
	"strings"
	"sync"

	"github.com/AdguardTeam/AdGuardHome/internal/util"
	"github.com/AdguardTeam/dnsproxy/upstream"
	"github.com/AdguardTeam/golibs/cache"
	"github.com/AdguardTeam/golibs/log"
	"github.com/AdguardTeam/urlfilter"
	"github.com/AdguardTeam/urlfilter/filterlist"
	"github.com/AdguardTeam/urlfilter/rules"
	"github.com/miekg/dns"
)

// ServiceEntry - blocked service array element
type ServiceEntry struct {
	Name  string
	Rules []*rules.NetworkRule
}

// RequestFilteringSettings is custom filtering settings
type RequestFilteringSettings struct {
	FilteringEnabled    bool
	SafeSearchEnabled   bool
	SafeBrowsingEnabled bool
	ParentalEnabled     bool

	ClientName string
	ClientIP   string
	ClientTags []string

	ServicesRules []ServiceEntry
}

// Config allows you to configure DNS filtering with New() or just change variables directly.
type Config struct {
	ParentalEnabled     bool   `yaml:"parental_enabled"`
	SafeSearchEnabled   bool   `yaml:"safesearch_enabled"`
	SafeBrowsingEnabled bool   `yaml:"safebrowsing_enabled"`
	ResolverAddress     string `yaml:"-"` // DNS server address

	SafeBrowsingCacheSize uint `yaml:"safebrowsing_cache_size"` // (in bytes)
	SafeSearchCacheSize   uint `yaml:"safesearch_cache_size"`   // (in bytes)
	ParentalCacheSize     uint `yaml:"parental_cache_size"`     // (in bytes)
	CacheTime             uint `yaml:"cache_time"`              // Element's TTL (in minutes)

	Rewrites []RewriteEntry `yaml:"rewrites"`

	// Names of services to block (globally).
	// Per-client settings can override this configuration.
	BlockedServices []string `yaml:"blocked_services"`

	// IP-hostname pairs taken from system configuration (e.g. /etc/hosts) files
	AutoHosts *util.AutoHosts `yaml:"-"`

	// Called when the configuration is changed by HTTP request
	ConfigModified func() `yaml:"-"`

	// Register an HTTP handler
	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request)) `yaml:"-"`
}

// LookupStats store stats collected during safebrowsing or parental checks
type LookupStats struct {
	Requests   uint64 // number of HTTP requests that were sent
	CacheHits  uint64 // number of lookups that didn't need HTTP requests
	Pending    int64  // number of currently pending HTTP requests
	PendingMax int64  // maximum number of pending HTTP requests
}

// Stats store LookupStats for safebrowsing, parental and safesearch
type Stats struct {
	Safebrowsing LookupStats
	Parental     LookupStats
	Safesearch   LookupStats
}

// Parameters to pass to filters-initializer goroutine
type filtersInitializerParams struct {
	allowFilters []Filter
	blockFilters []Filter
}

// Dnsfilter holds added rules and performs hostname matches against the rules
type Dnsfilter struct {
	rulesStorage         *filterlist.RuleStorage
	filteringEngine      *urlfilter.DNSEngine
	rulesStorageWhite    *filterlist.RuleStorage
	filteringEngineWhite *urlfilter.DNSEngine
	engineLock           sync.RWMutex

	parentalServer       string // access via methods
	safeBrowsingServer   string // access via methods
	parentalUpstream     upstream.Upstream
	safeBrowsingUpstream upstream.Upstream

	Config   // for direct access by library users, even a = assignment
	confLock sync.RWMutex

	// Channel for passing data to filters-initializer goroutine
	filtersInitializerChan chan filtersInitializerParams
	filtersInitializerLock sync.Mutex
}

// Filter represents a filter list
type Filter struct {
	ID       int64  // auto-assigned when filter is added (see nextFilterID)
	Data     []byte `yaml:"-"` // List of rules divided by '\n'
	FilePath string `yaml:"-"` // Path to a filtering rules file
}

// Reason holds an enum detailing why it was filtered or not filtered
type Reason int

const (
	// reasons for not filtering

	// NotFilteredNotFound - host was not find in any checks, default value for result
	NotFilteredNotFound Reason = iota
	// NotFilteredWhiteList - the host is explicitly whitelisted
	NotFilteredWhiteList
	// NotFilteredError - there was a transitive error during check
	NotFilteredError

	// reasons for filtering

	// FilteredBlackList - the host was matched to be advertising host
	FilteredBlackList
	// FilteredSafeBrowsing - the host was matched to be malicious/phishing
	FilteredSafeBrowsing
	// FilteredParental - the host was matched to be outside of parental control settings
	FilteredParental
	// FilteredInvalid - the request was invalid and was not processed
	FilteredInvalid
	// FilteredSafeSearch - the host was replaced with safesearch variant
	FilteredSafeSearch
	// FilteredBlockedService - the host is blocked by "blocked services" settings
	FilteredBlockedService

	// ReasonRewrite - rewrite rule was applied
	ReasonRewrite

	// RewriteEtcHosts - rewrite by /etc/hosts rule
	RewriteEtcHosts
)

var reasonNames = []string{
	"NotFilteredNotFound",
	"NotFilteredWhiteList",
	"NotFilteredError",

	"FilteredBlackList",
	"FilteredSafeBrowsing",
	"FilteredParental",
	"FilteredInvalid",
	"FilteredSafeSearch",
	"FilteredBlockedService",

	"Rewrite",
	"RewriteEtcHosts",
}

func (r Reason) String() string {
	if uint(r) >= uint(len(reasonNames)) {
		return ""
	}
	return reasonNames[r]
}

// GetConfig - get configuration
func (d *Dnsfilter) GetConfig() RequestFilteringSettings {
	c := RequestFilteringSettings{}
	// d.confLock.RLock()
	c.SafeSearchEnabled = d.Config.SafeSearchEnabled
	c.SafeBrowsingEnabled = d.Config.SafeBrowsingEnabled
	c.ParentalEnabled = d.Config.ParentalEnabled
	// d.confLock.RUnlock()
	return c
}

// WriteDiskConfig - write configuration
func (d *Dnsfilter) WriteDiskConfig(c *Config) {
	d.confLock.Lock()
	*c = d.Config
	c.Rewrites = rewriteArrayDup(d.Config.Rewrites)
	// BlockedServices
	d.confLock.Unlock()
}

// SetFilters - set new filters (synchronously or asynchronously)
// When filters are set asynchronously, the old filters continue working until the new filters are ready.
//  In this case the caller must ensure that the old filter files are intact.
func (d *Dnsfilter) SetFilters(blockFilters, allowFilters []Filter, async bool) error {
	if async {
		params := filtersInitializerParams{
			allowFilters: allowFilters,
			blockFilters: blockFilters,
		}

		d.filtersInitializerLock.Lock() // prevent multiple writers from adding more than 1 task
		// remove all pending tasks
		stop := false
		for !stop {
			select {
			case <-d.filtersInitializerChan:
				//
			default:
				stop = true
			}
		}

		d.filtersInitializerChan <- params
		d.filtersInitializerLock.Unlock()
		return nil
	}

	err := d.initFiltering(allowFilters, blockFilters)
	if err != nil {
		log.Error("Can't initialize filtering subsystem: %s", err)
		return err
	}

	return nil
}

// Starts initializing new filters by signal from channel
func (d *Dnsfilter) filtersInitializer() {
	for {
		params := <-d.filtersInitializerChan
		err := d.initFiltering(params.allowFilters, params.blockFilters)
		if err != nil {
			log.Error("Can't initialize filtering subsystem: %s", err)
			continue
		}
	}
}

// Close - close the object
func (d *Dnsfilter) Close() {
	d.engineLock.Lock()
	defer d.engineLock.Unlock()
	d.reset()
}

func (d *Dnsfilter) reset() {
	if d.rulesStorage != nil {
		_ = d.rulesStorage.Close()
	}
	if d.rulesStorageWhite != nil {
		d.rulesStorageWhite.Close()
	}
}

type dnsFilterContext struct {
	stats             Stats
	safebrowsingCache cache.Cache
	parentalCache     cache.Cache
	safeSearchCache   cache.Cache
}

var gctx dnsFilterContext // global dnsfilter context

// Result holds state of hostname check
type Result struct {
	IsFiltered bool   `json:",omitempty"` // True if the host name is filtered
	Reason     Reason `json:",omitempty"` // Reason for blocking / unblocking
	Rule       string `json:",omitempty"` // Original rule text
	IP         net.IP `json:",omitempty"` // Not nil only in the case of a hosts file syntax
	FilterID   int64  `json:",omitempty"` // Filter ID the rule belongs to

	// for ReasonRewrite:
	CanonName string `json:",omitempty"` // CNAME value

	// for RewriteEtcHosts:
	ReverseHosts []string `json:",omitempty"`

	// for ReasonRewrite & RewriteEtcHosts:
	IPList []net.IP `json:",omitempty"` // list of IP addresses

	// for FilteredBlockedService:
	ServiceName string `json:",omitempty"` // Name of the blocked service
}

// Matched can be used to see if any match at all was found, no matter filtered or not
func (r Reason) Matched() bool {
	return r != NotFilteredNotFound
}

// CheckHostRules tries to match the host against filtering rules only
func (d *Dnsfilter) CheckHostRules(host string, qtype uint16, setts *RequestFilteringSettings) (Result, error) {
	if !setts.FilteringEnabled {
		return Result{}, nil
	}

	return d.matchHost(host, qtype, *setts)
}

// CheckHost tries to match the host against filtering rules,
// then safebrowsing and parental if they are enabled
func (d *Dnsfilter) CheckHost(host string, qtype uint16, setts *RequestFilteringSettings) (Result, error) {
	// sometimes DNS clients will try to resolve ".", which is a request to get root servers
	if host == "" {
		return Result{Reason: NotFilteredNotFound}, nil
	}
	host = strings.ToLower(host)

	var result Result
	var err error

	// first - check rewrites, they have the highest priority
	result = d.processRewrites(host, qtype)
	if result.Reason == ReasonRewrite {
		return result, nil
	}

	// Now check the hosts file -- do we have any rules for it?
	// just like DNS rewrites, it has higher priority than filtering rules.
	if d.Config.AutoHosts != nil {
		matched, err := d.checkAutoHosts(host, qtype, &result)
		if matched {
			return result, err
		}
	}

	// Then check the filter lists.
	// if request is blocked -- it should be blocked.
	// if it is whitelisted -- we should do nothing with it anymore.
	if setts.FilteringEnabled {
		result, err = d.matchHost(host, qtype, *setts)
		if err != nil {
			return result, err
		}
		if result.Reason.Matched() {
			return result, nil
		}
	}

	// are there any blocked services?
	if len(setts.ServicesRules) != 0 {
		result = matchBlockedServicesRules(host, setts.ServicesRules)
		if result.Reason.Matched() {
			return result, nil
		}
	}

	// browsing security web service
	if setts.SafeBrowsingEnabled {
		result, err = d.checkSafeBrowsing(host)
		if err != nil {
			log.Info("SafeBrowsing: failed: %v", err)
			return Result{}, nil
		}
		if result.Reason.Matched() {
			return result, nil
		}
	}

	// parental control web service
	if setts.ParentalEnabled {
		result, err = d.checkParental(host)
		if err != nil {
			log.Printf("Parental: failed: %v", err)
			return Result{}, nil
		}
		if result.Reason.Matched() {
			return result, nil
		}
	}

	// apply safe search if needed
	if setts.SafeSearchEnabled {
		result, err = d.checkSafeSearch(host)
		if err != nil {
			log.Info("SafeSearch: failed: %v", err)
			return Result{}, nil
		}

		if result.Reason.Matched() {
			return result, nil
		}
	}

	return Result{}, nil
}

func (d *Dnsfilter) checkAutoHosts(host string, qtype uint16, result *Result) (matched bool, err error) {
	ips := d.Config.AutoHosts.Process(host, qtype)
	if ips != nil {
		result.Reason = RewriteEtcHosts
		result.IPList = ips

		return true, nil
	}

	revHosts := d.Config.AutoHosts.ProcessReverse(host, qtype)
	if len(revHosts) != 0 {
		result.Reason = RewriteEtcHosts

		// TODO(a.garipov): Optimize this with a buffer.
		result.ReverseHosts = make([]string, len(revHosts))
		for i := range revHosts {
			result.ReverseHosts[i] = revHosts[i] + "."
		}

		return true, nil
	}

	return false, nil
}

// Process rewrites table
// . Find CNAME for a domain name (exact match or by wildcard)
//  . if found and CNAME equals to domain name - this is an exception;  exit
//  . if found, set domain name to canonical name
//  . repeat for the new domain name (Note: we return only the last CNAME)
// . Find A or AAAA record for a domain name (exact match or by wildcard)
//  . if found, set IP addresses (IPv4 or IPv6 depending on qtype) in Result.IPList array
func (d *Dnsfilter) processRewrites(host string, qtype uint16) Result {
	var res Result

	d.confLock.RLock()
	defer d.confLock.RUnlock()

	rr := findRewrites(d.Rewrites, host)
	if len(rr) != 0 {
		res.Reason = ReasonRewrite
	}

	cnames := map[string]bool{}
	origHost := host
	for len(rr) != 0 && rr[0].Type == dns.TypeCNAME {
		log.Debug("Rewrite: CNAME for %s is %s", host, rr[0].Answer)

		if host == rr[0].Answer { // "host == CNAME" is an exception
			res.Reason = 0
			return res
		}

		host = rr[0].Answer
		_, ok := cnames[host]
		if ok {
			log.Info("Rewrite: breaking CNAME redirection loop: %s.  Question: %s", host, origHost)
			return res
		}
		cnames[host] = false
		res.CanonName = rr[0].Answer
		rr = findRewrites(d.Rewrites, host)
	}

	for _, r := range rr {
		if (r.Type == dns.TypeA && qtype == dns.TypeA) ||
			(r.Type == dns.TypeAAAA && qtype == dns.TypeAAAA) {

			if r.IP == nil { // IP exception
				res.Reason = 0
				return res
			}

			res.IPList = append(res.IPList, r.IP)
			log.Debug("Rewrite: A/AAAA for %s is %s", host, r.IP)
		}
	}

	return res
}

func matchBlockedServicesRules(host string, svcs []ServiceEntry) Result {
	req := rules.NewRequestForHostname(host)
	res := Result{}

	for _, s := range svcs {
		for _, rule := range s.Rules {
			if rule.Match(req) {
				res.Reason = FilteredBlockedService
				res.IsFiltered = true
				res.ServiceName = s.Name
				res.Rule = rule.Text()
				log.Debug("Blocked Services: matched rule: %s  host: %s  service: %s",
					res.Rule, host, s.Name)
				return res
			}
		}
	}
	return res
}

//
// Adding rule and matching against the rules
//

// fileExists returns true if file exists.
func fileExists(fn string) bool {
	_, err := os.Stat(fn)
	return err == nil
}

func createFilteringEngine(filters []Filter) (*filterlist.RuleStorage, *urlfilter.DNSEngine, error) {
	listArray := []filterlist.RuleList{}
	for _, f := range filters {
		var list filterlist.RuleList

		if f.ID == 0 {
			list = &filterlist.StringRuleList{
				ID:             0,
				RulesText:      string(f.Data),
				IgnoreCosmetic: true,
			}
		} else if !fileExists(f.FilePath) {
			list = &filterlist.StringRuleList{
				ID:             int(f.ID),
				IgnoreCosmetic: true,
			}
		} else if runtime.GOOS == "windows" {
			// On Windows we don't pass a file to urlfilter because
			// it's difficult to update this file while it's being
			// used.
			data, err := ioutil.ReadFile(f.FilePath)
			if err != nil {
				return nil, nil, fmt.Errorf("ioutil.ReadFile(): %s: %w", f.FilePath, err)
			}
			list = &filterlist.StringRuleList{
				ID:             int(f.ID),
				RulesText:      string(data),
				IgnoreCosmetic: true,
			}

		} else {
			var err error
			list, err = filterlist.NewFileRuleList(int(f.ID), f.FilePath, true)
			if err != nil {
				return nil, nil, fmt.Errorf("filterlist.NewFileRuleList(): %s: %w", f.FilePath, err)
			}
		}
		listArray = append(listArray, list)
	}

	rulesStorage, err := filterlist.NewRuleStorage(listArray)
	if err != nil {
		return nil, nil, fmt.Errorf("filterlist.NewRuleStorage(): %w", err)
	}
	filteringEngine := urlfilter.NewDNSEngine(rulesStorage)
	return rulesStorage, filteringEngine, nil
}

// Initialize urlfilter objects.
func (d *Dnsfilter) initFiltering(allowFilters, blockFilters []Filter) error {
	rulesStorage, filteringEngine, err := createFilteringEngine(blockFilters)
	if err != nil {
		return err
	}
	rulesStorageWhite, filteringEngineWhite, err := createFilteringEngine(allowFilters)
	if err != nil {
		return err
	}

	d.engineLock.Lock()
	d.reset()
	d.rulesStorage = rulesStorage
	d.filteringEngine = filteringEngine
	d.rulesStorageWhite = rulesStorageWhite
	d.filteringEngineWhite = filteringEngineWhite
	d.engineLock.Unlock()

	// Make sure that the OS reclaims memory as soon as possible
	debug.FreeOSMemory()
	log.Debug("initialized filtering engine")

	return nil
}

// matchHost is a low-level way to check only if hostname is filtered by rules,
// skipping expensive safebrowsing and parental lookups.
func (d *Dnsfilter) matchHost(host string, qtype uint16, setts RequestFilteringSettings) (Result, error) {
	d.engineLock.RLock()
	// Keep in mind that this lock must be held no just when calling Match()
	//  but also while using the rules returned by it.
	defer d.engineLock.RUnlock()

	ureq := urlfilter.DNSRequest{}
	ureq.Hostname = host
	ureq.ClientIP = setts.ClientIP
	ureq.ClientName = setts.ClientName
	ureq.SortedClientTags = setts.ClientTags

	if d.filteringEngineWhite != nil {
		rr, ok := d.filteringEngineWhite.MatchRequest(ureq)
		if ok {
			var rule rules.Rule
			if rr.NetworkRule != nil {
				rule = rr.NetworkRule
			} else if rr.HostRulesV4 != nil {
				rule = rr.HostRulesV4[0]
			} else if rr.HostRulesV6 != nil {
				rule = rr.HostRulesV6[0]
			}

			log.Debug("Filtering: found whitelist rule for host %q: %q  list_id: %d",
				host, rule.Text(), rule.GetFilterListID())
			res := makeResult(rule, NotFilteredWhiteList)
			return res, nil
		}
	}

	if d.filteringEngine == nil {
		return Result{}, nil
	}

	rr, ok := d.filteringEngine.MatchRequest(ureq)
	if !ok {
		return Result{}, nil
	}

	if rr.NetworkRule != nil {
		log.Debug("Filtering: found rule for host %q: %q  list_id: %d",
			host, rr.NetworkRule.Text(), rr.NetworkRule.GetFilterListID())
		reason := FilteredBlackList
		if rr.NetworkRule.Whitelist {
			reason = NotFilteredWhiteList
		}
		res := makeResult(rr.NetworkRule, reason)
		return res, nil
	}

	if qtype == dns.TypeA && rr.HostRulesV4 != nil {
		rule := rr.HostRulesV4[0] // note that we process only 1 matched rule
		log.Debug("Filtering: found rule for host %q: %q  list_id: %d",
			host, rule.Text(), rule.GetFilterListID())
		res := makeResult(rule, FilteredBlackList)
		res.IP = rule.IP.To4()
		return res, nil
	}

	if qtype == dns.TypeAAAA && rr.HostRulesV6 != nil {
		rule := rr.HostRulesV6[0] // note that we process only 1 matched rule
		log.Debug("Filtering: found rule for host %q: %q  list_id: %d",
			host, rule.Text(), rule.GetFilterListID())
		res := makeResult(rule, FilteredBlackList)
		res.IP = rule.IP
		return res, nil
	}

	if rr.HostRulesV4 != nil || rr.HostRulesV6 != nil {
		// Question Type doesn't match the host rules
		// Return the first matched host rule, but without an IP address
		var rule rules.Rule
		if rr.HostRulesV4 != nil {
			rule = rr.HostRulesV4[0]
		} else if rr.HostRulesV6 != nil {
			rule = rr.HostRulesV6[0]
		}
		log.Debug("Filtering: found rule for host %q: %q  list_id: %d",
			host, rule.Text(), rule.GetFilterListID())
		res := makeResult(rule, FilteredBlackList)
		res.IP = net.IP{}
		return res, nil
	}

	return Result{}, nil
}

// Construct Result object
func makeResult(rule rules.Rule, reason Reason) Result {
	res := Result{}
	res.FilterID = int64(rule.GetFilterListID())
	res.Rule = rule.Text()
	res.Reason = reason
	if reason == FilteredBlackList {
		res.IsFiltered = true
	}
	return res
}

// InitModule manually initializes blocked services map.
func InitModule() {
	initBlockedServices()
}

// New creates properly initialized DNS Filter that is ready to be used.
func New(c *Config, blockFilters []Filter) *Dnsfilter {
	if c != nil {
		cacheConf := cache.Config{
			EnableLRU: true,
		}

		if gctx.safebrowsingCache == nil {
			cacheConf.MaxSize = c.SafeBrowsingCacheSize
			gctx.safebrowsingCache = cache.New(cacheConf)
		}

		if gctx.safeSearchCache == nil {
			cacheConf.MaxSize = c.SafeSearchCacheSize
			gctx.safeSearchCache = cache.New(cacheConf)
		}

		if gctx.parentalCache == nil {
			cacheConf.MaxSize = c.ParentalCacheSize
			gctx.parentalCache = cache.New(cacheConf)
		}
	}

	d := new(Dnsfilter)

	err := d.initSecurityServices()
	if err != nil {
		log.Error("dnsfilter: initialize services: %s", err)
		return nil
	}

	if c != nil {
		d.Config = *c
		d.prepareRewrites()
	}

	bsvcs := []string{}
	for _, s := range d.BlockedServices {
		if !BlockedSvcKnown(s) {
			log.Debug("skipping unknown blocked-service %q", s)
			continue
		}
		bsvcs = append(bsvcs, s)
	}
	d.BlockedServices = bsvcs

	if blockFilters != nil {
		err := d.initFiltering(nil, blockFilters)
		if err != nil {
			log.Error("Can't initialize filtering subsystem: %s", err)
			d.Close()
			return nil
		}
	}

	return d
}

// Start - start the module:
// . start async filtering initializer goroutine
// . register web handlers
func (d *Dnsfilter) Start() {
	d.filtersInitializerChan = make(chan filtersInitializerParams, 1)
	go d.filtersInitializer()

	if d.Config.HTTPRegister != nil { // for tests
		d.registerSecurityHandlers()
		d.registerRewritesHandlers()
		d.registerBlockedServicesHandlers()
	}
}

//
// stats
//

// GetStats return dns filtering stats since startup.
func (d *Dnsfilter) GetStats() Stats {
	return gctx.stats
}