package dnsforward

import (
	"encoding/binary"
	"net"
	"strings"
	"time"

	"github.com/AdguardTeam/AdGuardHome/internal/dhcpd"
	"github.com/AdguardTeam/AdGuardHome/internal/filtering"
	"github.com/AdguardTeam/dnsproxy/proxy"
	"github.com/AdguardTeam/golibs/log"
	"github.com/AdguardTeam/golibs/netutil"
	"github.com/AdguardTeam/golibs/stringutil"
	"github.com/miekg/dns"
)

// To transfer information between modules
type dnsContext struct {
	proxyCtx *proxy.DNSContext

	// setts are the filtering settings for the client.
	setts *filtering.Settings

	result *filtering.Result
	// origResp is the response received from upstream.  It is set when the
	// response is modified by filters.
	origResp *dns.Msg

	// unreversedReqIP stores an IP address obtained from PTR request if it
	// parsed successfully and belongs to one of locally-served IP ranges as per
	// RFC 6303.
	unreversedReqIP net.IP

	// err is the error returned from a processing function.
	err error

	// clientID is the clientID from DoH, DoQ, or DoT, if provided.
	clientID string

	// origQuestion is the question received from the client.  It is set
	// when the request is modified by rewrites.
	origQuestion dns.Question

	// startTime is the time at which the processing of the request has started.
	startTime time.Time

	// protectionEnabled shows if the filtering is enabled, and if the
	// server's DNS filter is ready.
	protectionEnabled bool

	// responseFromUpstream shows if the response is received from the
	// upstream servers.
	responseFromUpstream bool

	// responseAD shows if the response had the AD bit set.
	responseAD bool

	// isLocalClient shows if client's IP address is from locally-served
	// network.
	isLocalClient bool
}

// resultCode is the result of a request processing function.
type resultCode int

const (
	// resultCodeSuccess is returned when a handler performed successfully,
	// and the next handler must be called.
	resultCodeSuccess resultCode = iota
	// resultCodeFinish is returned when a handler performed successfully,
	// and the processing of the request must be stopped.
	resultCodeFinish
	// resultCodeError is returned when a handler failed, and the processing
	// of the request must be stopped.
	resultCodeError
)

// handleDNSRequest filters the incoming DNS requests and writes them to the query log
func (s *Server) handleDNSRequest(_ *proxy.Proxy, d *proxy.DNSContext) error {
	ctx := &dnsContext{
		proxyCtx:  d,
		result:    &filtering.Result{},
		startTime: time.Now(),
	}

	type modProcessFunc func(ctx *dnsContext) (rc resultCode)

	// Since (*dnsforward.Server).handleDNSRequest(...) is used as
	// proxy.(Config).RequestHandler, there is no need for additional index
	// out of range checking in any of the following functions, because the
	// (*proxy.Proxy).handleDNSRequest method performs it before calling the
	// appropriate handler.
	mods := []modProcessFunc{
		s.processRecursion,
		s.processInitial,
		s.processDetermineLocal,
		s.processInternalHosts,
		s.processRestrictLocal,
		s.processInternalIPAddrs,
		s.processFilteringBeforeRequest,
		s.processLocalPTR,
		s.processUpstream,
		s.processFilteringAfterResponse,
		s.ipset.process,
		s.processQueryLogsAndStats,
	}
	for _, process := range mods {
		r := process(ctx)
		switch r {
		case resultCodeSuccess:
			// continue: call the next filter

		case resultCodeFinish:
			return nil

		case resultCodeError:
			return ctx.err
		}
	}

	if d.Res != nil {
		d.Res.Compress = true // some devices require DNS message compression
	}
	return nil
}

// processRecursion checks the incoming request and halts it's handling if s
// have tried to resolve it recently.
func (s *Server) processRecursion(dctx *dnsContext) (rc resultCode) {
	pctx := dctx.proxyCtx

	if msg := pctx.Req; msg != nil && s.recDetector.check(*msg) {
		log.Debug("recursion detected resolving %q", msg.Question[0].Name)
		pctx.Res = s.genNXDomain(pctx.Req)

		return resultCodeFinish

	}

	return resultCodeSuccess
}

// processInitial terminates the following processing for some requests if
// needed and enriches the ctx with some client-specific information.
//
// TODO(e.burkov):  Decompose into less general processors.
func (s *Server) processInitial(ctx *dnsContext) (rc resultCode) {
	d := ctx.proxyCtx
	if s.conf.AAAADisabled && d.Req.Question[0].Qtype == dns.TypeAAAA {
		_ = proxy.CheckDisabledAAAARequest(d, true)
		return resultCodeFinish
	}

	if s.conf.OnDNSRequest != nil {
		s.conf.OnDNSRequest(d)
	}

	// disable Mozilla DoH
	// https://support.mozilla.org/en-US/kb/canary-domain-use-application-dnsnet
	if (d.Req.Question[0].Qtype == dns.TypeA || d.Req.Question[0].Qtype == dns.TypeAAAA) &&
		d.Req.Question[0].Name == "use-application-dns.net." {
		d.Res = s.genNXDomain(d.Req)
		return resultCodeFinish
	}

	// Get the client's ID if any.  It should be performed before getting
	// client-specific filtering settings.
	var key [8]byte
	binary.BigEndian.PutUint64(key[:], d.RequestID)
	ctx.clientID = string(s.clientIDCache.Get(key[:]))

	// Get the client-specific filtering settings.
	ctx.protectionEnabled = s.conf.ProtectionEnabled
	ctx.setts = s.getClientRequestFilteringSettings(ctx)

	return resultCodeSuccess
}

func (s *Server) setTableHostToIP(t hostToIPTable) {
	s.tableHostToIPLock.Lock()
	defer s.tableHostToIPLock.Unlock()

	s.tableHostToIP = t
}

func (s *Server) setTableIPToHost(t *netutil.IPMap) {
	s.tableIPToHostLock.Lock()
	defer s.tableIPToHostLock.Unlock()

	s.tableIPToHost = t
}

func (s *Server) onDHCPLeaseChanged(flags int) {
	var err error

	add := true
	switch flags {
	case dhcpd.LeaseChangedAdded,
		dhcpd.LeaseChangedAddedStatic,
		dhcpd.LeaseChangedRemovedStatic:
		// Go on.
	case dhcpd.LeaseChangedRemovedAll:
		add = false
	default:
		return
	}

	var hostToIP hostToIPTable
	var ipToHost *netutil.IPMap
	if add {
		ll := s.dhcpServer.Leases(dhcpd.LeasesAll)

		hostToIP = make(hostToIPTable, len(ll))
		ipToHost = netutil.NewIPMap(len(ll))

		for _, l := range ll {
			// TODO(a.garipov): Remove this after we're finished with the client
			// hostname validations in the DHCP server code.
			err = netutil.ValidateDomainName(l.Hostname)
			if err != nil {
				log.Debug(
					"dns: skipping invalid hostname %q from dhcp: %s",
					l.Hostname,
					err,
				)
			}

			lowhost := strings.ToLower(l.Hostname)

			ipToHost.Set(l.IP, lowhost)

			ip := make(net.IP, 4)
			copy(ip, l.IP.To4())
			hostToIP[lowhost] = ip
		}

		log.Debug("dns: added %d A/PTR entries from DHCP", ipToHost.Len())
	}

	s.setTableHostToIP(hostToIP)
	s.setTableIPToHost(ipToHost)
}

// processDetermineLocal determines if the client's IP address is from
// locally-served network and saves the result into the context.
func (s *Server) processDetermineLocal(dctx *dnsContext) (rc resultCode) {
	rc = resultCodeSuccess

	var ip net.IP
	if ip, _ = netutil.IPAndPortFromAddr(dctx.proxyCtx.Addr); ip == nil {
		return rc
	}

	dctx.isLocalClient = s.subnetDetector.IsLocallyServedNetwork(ip)

	return rc
}

// hostToIP tries to get an IP leased by DHCP and returns the copy of address
// since the data inside the internal table may be changed while request
// processing.  It's safe for concurrent use.
func (s *Server) hostToIP(host string) (ip net.IP, ok bool) {
	s.tableHostToIPLock.Lock()
	defer s.tableHostToIPLock.Unlock()

	if s.tableHostToIP == nil {
		return nil, false
	}

	var ipFromTable net.IP
	ipFromTable, ok = s.tableHostToIP[host]
	if !ok {
		return nil, false
	}

	ip = make(net.IP, len(ipFromTable))
	copy(ip, ipFromTable)

	return ip, true
}

// processInternalHosts respond to A requests if the target hostname is known to
// the server.
//
// TODO(a.garipov): Adapt to AAAA as well.
func (s *Server) processInternalHosts(dctx *dnsContext) (rc resultCode) {
	if !s.dhcpServer.Enabled() {
		return resultCodeSuccess
	}

	req := dctx.proxyCtx.Req
	q := req.Question[0]

	// Go on processing the AAAA request despite the fact that we don't support
	// it yet.  The expected behavior here is to respond with an empty answer
	// and not NXDOMAIN.
	if q.Qtype != dns.TypeA && q.Qtype != dns.TypeAAAA {
		return resultCodeSuccess
	}

	reqHost := strings.ToLower(q.Name)
	// TODO(a.garipov): Move everything related to DHCP local domain to the DHCP
	// server.
	host := strings.TrimSuffix(reqHost, s.localDomainSuffix)
	if host == reqHost {
		return resultCodeSuccess
	}

	d := dctx.proxyCtx
	if !dctx.isLocalClient {
		log.Debug("dns: %q requests for internal host", d.Addr)
		d.Res = s.genNXDomain(req)

		// Do not even put into query log.
		return resultCodeFinish
	}

	ip, ok := s.hostToIP(host)
	if !ok {
		// TODO(e.burkov): Inspect special cases when user want to apply some
		// rules handled by other processors to the hosts with TLD.
		d.Res = s.genNXDomain(req)

		return resultCodeFinish
	}

	log.Debug("dns: internal record: %s -> %s", q.Name, ip)

	resp := s.makeResponse(req)
	if q.Qtype == dns.TypeA {
		a := &dns.A{
			Hdr: s.hdr(req, dns.TypeA),
			A:   ip,
		}
		resp.Answer = append(resp.Answer, a)
	}
	dctx.proxyCtx.Res = resp

	return resultCodeSuccess
}

// processRestrictLocal responds with NXDOMAIN to PTR requests for IP addresses
// in locally-served network from external clients.
func (s *Server) processRestrictLocal(ctx *dnsContext) (rc resultCode) {
	d := ctx.proxyCtx
	req := d.Req
	q := req.Question[0]
	if q.Qtype != dns.TypePTR {
		// No need for restriction.
		return resultCodeSuccess
	}

	ip, err := netutil.IPFromReversedAddr(q.Name)
	if err != nil {
		log.Debug("dns: parsing reversed addr: %s", err)

		// DNS-Based Service Discovery uses PTR records having not an ARPA
		// format of the domain name in question.  Those shouldn't be
		// invalidated.  See http://www.dns-sd.org/ServerStaticSetup.html and
		// RFC 2782.
		name := strings.TrimSuffix(q.Name, ".")
		if err = netutil.ValidateSRVDomainName(name); err != nil {
			log.Debug("dns: validating service domain: %s", err)

			return resultCodeError
		}

		log.Debug("dns: request is for a service domain")

		return resultCodeSuccess
	}

	// Restrict an access to local addresses for external clients.  We also
	// assume that all the DHCP leases we give are locally-served or at least
	// don't need to be inaccessible externally.
	if !s.subnetDetector.IsLocallyServedNetwork(ip) {
		log.Debug("dns: addr %s is not from locally-served network", ip)

		return resultCodeSuccess
	}

	if !ctx.isLocalClient {
		log.Debug("dns: %q requests an internal ip", d.Addr)
		d.Res = s.genNXDomain(req)

		// Do not even put into query log.
		return resultCodeFinish
	}

	// Do not perform unreversing ever again.
	ctx.unreversedReqIP = ip

	// There is no need to filter request from external addresses since this
	// code is only executed when the request is for locally-served ARPA
	// hostname so disable redundant filters.
	ctx.setts.ParentalEnabled = false
	ctx.setts.SafeBrowsingEnabled = false
	ctx.setts.SafeSearchEnabled = false
	ctx.setts.ServicesRules = nil

	// Nothing to restrict.
	return resultCodeSuccess
}

// ipToHost tries to get a hostname leased by DHCP.  It's safe for concurrent
// use.
func (s *Server) ipToHost(ip net.IP) (host string, ok bool) {
	s.tableIPToHostLock.Lock()
	defer s.tableIPToHostLock.Unlock()

	if s.tableIPToHost == nil {
		return "", false
	}

	var v interface{}
	v, ok = s.tableIPToHost.Get(ip)
	if !ok {
		return "", false
	}

	if host, ok = v.(string); !ok {
		log.Error("dns: bad type %T in tableIPToHost for %s", v, ip)

		return "", false
	}

	return host, true
}

// Respond to PTR requests if the target IP is leased by our DHCP server and the
// requestor is inside the local network.
func (s *Server) processInternalIPAddrs(ctx *dnsContext) (rc resultCode) {
	d := ctx.proxyCtx
	if d.Res != nil {
		return resultCodeSuccess
	}

	ip := ctx.unreversedReqIP
	if ip == nil {
		return resultCodeSuccess
	}

	host, ok := s.ipToHost(ip)
	if !ok {
		return resultCodeSuccess
	}

	log.Debug("dns: reverse-lookup: %s -> %s", ip, host)

	req := d.Req
	resp := s.makeResponse(req)
	ptr := &dns.PTR{
		Hdr: dns.RR_Header{
			Name:   req.Question[0].Name,
			Rrtype: dns.TypePTR,
			Ttl:    s.conf.BlockedResponseTTL,
			Class:  dns.ClassINET,
		},
		Ptr: dns.Fqdn(host),
	}
	resp.Answer = append(resp.Answer, ptr)
	d.Res = resp

	return resultCodeSuccess
}

// processLocalPTR responds to PTR requests if the target IP is detected to be
// inside the local network and the query was not answered from DHCP.
func (s *Server) processLocalPTR(ctx *dnsContext) (rc resultCode) {
	d := ctx.proxyCtx
	if d.Res != nil {
		return resultCodeSuccess
	}

	ip := ctx.unreversedReqIP
	if ip == nil {
		return resultCodeSuccess
	}

	s.serverLock.RLock()
	defer s.serverLock.RUnlock()

	if !s.subnetDetector.IsLocallyServedNetwork(ip) {
		return resultCodeSuccess
	}

	if s.conf.UsePrivateRDNS {
		s.recDetector.add(*d.Req)
		if err := s.localResolvers.Resolve(d); err != nil {
			ctx.err = err

			return resultCodeError
		}
	}

	if d.Res == nil {
		d.Res = s.genNXDomain(d.Req)

		// Do not even put into query log.
		return resultCodeFinish
	}

	return resultCodeSuccess
}

// Apply filtering logic
func (s *Server) processFilteringBeforeRequest(ctx *dnsContext) (rc resultCode) {
	if ctx.proxyCtx.Res != nil {
		// Go on since the response is already set.
		return resultCodeSuccess
	}

	s.serverLock.RLock()
	defer s.serverLock.RUnlock()

	if s.dnsFilter == nil {
		return resultCodeSuccess
	}

	var err error
	if ctx.result, err = s.filterDNSRequest(ctx); err != nil {
		ctx.err = err

		return resultCodeError
	}

	return resultCodeSuccess
}

// ipStringFromAddr extracts an IP address string from net.Addr.
func ipStringFromAddr(addr net.Addr) (ipStr string) {
	if ip, _ := netutil.IPAndPortFromAddr(addr); ip != nil {
		return ip.String()
	}

	return ""
}

// processUpstream passes request to upstream servers and handles the response.
func (s *Server) processUpstream(dctx *dnsContext) (rc resultCode) {
	pctx := dctx.proxyCtx
	if pctx.Res != nil {
		// The response has already been set.
		return resultCodeSuccess
	}

	if pctx.Addr != nil && s.conf.GetCustomUpstreamByClient != nil {
		// Use the clientID first, since it has a higher priority.
		id := stringutil.Coalesce(dctx.clientID, ipStringFromAddr(pctx.Addr))
		upsConf, err := s.conf.GetCustomUpstreamByClient(id)
		if err != nil {
			log.Error("dns: getting custom upstreams for client %s: %s", id, err)
		} else if upsConf != nil {
			log.Debug("dns: using custom upstreams for client %s", id)
			pctx.CustomUpstreamConfig = upsConf
		}
	}

	req := pctx.Req
	origReqAD := false
	if s.conf.EnableDNSSEC {
		if req.AuthenticatedData {
			origReqAD = true
		} else {
			req.AuthenticatedData = true
		}
	}

	// Process the request further since it wasn't filtered.
	prx := s.proxy()
	if prx == nil {
		dctx.err = srvClosedErr

		return resultCodeError
	}

	if dctx.err = prx.Resolve(pctx); dctx.err != nil {
		return resultCodeError
	}

	dctx.responseFromUpstream = true
	dctx.responseAD = pctx.Res.AuthenticatedData

	if s.conf.EnableDNSSEC && !origReqAD {
		pctx.Req.AuthenticatedData = false
		pctx.Res.AuthenticatedData = false
	}

	return resultCodeSuccess
}

// Apply filtering logic after we have received response from upstream servers
func (s *Server) processFilteringAfterResponse(ctx *dnsContext) (rc resultCode) {
	d := ctx.proxyCtx

	switch res := ctx.result; res.Reason {
	case filtering.NotFilteredAllowList:
		// Go on.
	case
		filtering.Rewritten,
		filtering.RewrittenRule:

		if len(ctx.origQuestion.Name) == 0 {
			// origQuestion is set in case we get only CNAME without IP from
			// rewrites table.
			break
		}

		d.Req.Question[0], d.Res.Question[0] = ctx.origQuestion, ctx.origQuestion
		if len(d.Res.Answer) > 0 {
			answer := append([]dns.RR{s.genAnswerCNAME(d.Req, res.CanonName)}, d.Res.Answer...)
			d.Res.Answer = answer
		}
	default:
		// Check the response only if the it's from an upstream.  Don't check
		// the response if the protection is disabled since dnsrewrite rules
		// aren't applied to it anyway.
		if !ctx.protectionEnabled || !ctx.responseFromUpstream || s.dnsFilter == nil {
			break
		}

		origResp := d.Res
		result, err := s.filterDNSResponse(ctx)
		if err != nil {
			ctx.err = err

			return resultCodeError
		}

		if result != nil {
			ctx.result = result
			ctx.origResp = origResp
		}
	}

	if ctx.result == nil {
		ctx.result = &filtering.Result{}
	}

	return resultCodeSuccess
}