package nameserver

import (
	"context"
	"net"
	"strings"

	"github.com/safing/portbase/modules/subsystems"

	"github.com/safing/portbase/log"
	"github.com/safing/portbase/modules"
	"github.com/safing/portmaster/detection/dga"
	"github.com/safing/portmaster/firewall"
	"github.com/safing/portmaster/network"
	"github.com/safing/portmaster/network/environment"
	"github.com/safing/portmaster/network/netutils"
	"github.com/safing/portmaster/resolver"

	"github.com/miekg/dns"
)

var (
	module       *modules.Module
	dnsServer    *dns.Server
	mtDNSRequest = "dns request"

	listenAddress = "0.0.0.0:53"
	ipv4Localhost = net.IPv4(127, 0, 0, 1)
	localhostRRs  []dns.RR
)

func init() {
	module = modules.Register("nameserver", prep, start, stop, "core", "resolver", "network")
	subsystems.Register(
		"dns",
		"Secure DNS",
		"DNS resolver with scoping and DNS-over-TLS",
		module,
		"config:dns/",
		nil,
	)
}

func prep() error {
	localhostIPv4, err := dns.NewRR("localhost. 17 IN A 127.0.0.1")
	if err != nil {
		return err
	}

	localhostIPv6, err := dns.NewRR("localhost. 17 IN AAAA ::1")
	if err != nil {
		return err
	}

	localhostRRs = []dns.RR{localhostIPv4, localhostIPv6}

	return nil
}

func start() error {
	dnsServer = &dns.Server{Addr: listenAddress, Net: "udp"}
	dns.HandleFunc(".", handleRequestAsMicroTask)

	module.StartServiceWorker("dns resolver", 0, func(ctx context.Context) error {
		err := dnsServer.ListenAndServe()
		if err != nil {
			// check if we are shutting down
			if module.IsStopping() {
				return nil
			}
			// is something blocking our port?
			checkErr := checkForConflictingService()
			if checkErr != nil {
				return checkErr
			}
		}
		return err
	})

	return nil
}

func stop() error {
	if dnsServer != nil {
		return dnsServer.Shutdown()
	}
	return nil
}

func returnNXDomain(w dns.ResponseWriter, query *dns.Msg) {
	m := new(dns.Msg)
	m.SetRcode(query, dns.RcodeNameError)
	_ = w.WriteMsg(m)
}

func returnServerFailure(w dns.ResponseWriter, query *dns.Msg) {
	m := new(dns.Msg)
	m.SetRcode(query, dns.RcodeServerFailure)
	_ = w.WriteMsg(m)
}

func handleRequestAsMicroTask(w dns.ResponseWriter, query *dns.Msg) {
	err := module.RunMicroTask(&mtDNSRequest, func(ctx context.Context) error {
		return handleRequest(ctx, w, query)
	})
	if err != nil {
		log.Warningf("intel: failed to handle dns request: %s", err)
	}
}

func handleRequest(ctx context.Context, w dns.ResponseWriter, query *dns.Msg) error {
	// return with server failure if offline
	if environment.GetOnlineStatus() == environment.StatusOffline {
		returnServerFailure(w, query)
		return nil
	}

	// only process first question, that's how everyone does it.
	question := query.Question[0]
	q := &resolver.Query{
		FQDN:  question.Name,
		QType: dns.Type(question.Qtype),
	}

	// check class
	if question.Qclass != dns.ClassINET {
		// we only serve IN records, return nxdomain
		returnNXDomain(w, query)
		return nil
	}

	// handle request for localhost
	if strings.HasSuffix(q.FQDN, "localhost.") {
		m := new(dns.Msg)
		m.SetReply(query)
		m.Answer = localhostRRs
		_ = w.WriteMsg(m)
		return nil
	}

	// get addresses
	remoteAddr, ok := w.RemoteAddr().(*net.UDPAddr)
	if !ok {
		log.Warningf("nameserver: could not get remote address of request for %s%s, ignoring", q.FQDN, q.QType)
		return nil
	}
	if !remoteAddr.IP.Equal(ipv4Localhost) {
		// if request is not coming from 127.0.0.1, check if it's really local

		localAddr, ok := w.RemoteAddr().(*net.UDPAddr)
		if !ok {
			log.Warningf("nameserver: could not get local address of request for %s%s, ignoring", q.FQDN, q.QType)
			return nil
		}

		// ignore external request
		if !remoteAddr.IP.Equal(localAddr.IP) {
			log.Warningf("nameserver: external request for %s%s, ignoring", q.FQDN, q.QType)
			return nil
		}
	}

	// check if valid domain name
	if !netutils.IsValidFqdn(q.FQDN) {
		log.Debugf("nameserver: domain name %s is invalid, returning nxdomain", q.FQDN)
		returnNXDomain(w, query)
		return nil
	}

	// start tracer
	ctx, tracer := log.AddTracer(ctx)
	tracer.Tracef("nameserver: handling new request for %s%s from %s:%d", q.FQDN, q.QType, remoteAddr.IP, remoteAddr.Port)

	// TODO: if there are 3 request for the same domain/type in a row, delete all caches of that domain

	// get connection
	comm, err := network.GetCommunicationByDNSRequest(ctx, remoteAddr.IP, uint16(remoteAddr.Port), q.FQDN)
	if err != nil {
		tracer.Errorf("nameserver: could not identify process of %s:%d, returning nxdomain: %s", remoteAddr.IP, remoteAddr.Port, err)
		returnNXDomain(w, query)
		return nil
	}
	defer func() {
		go comm.SaveIfNeeded()
	}()

	// save security level to query
	q.SecurityLevel = comm.Process().Profile().SecurityLevel()

	// check for possible DNS tunneling / data transmission
	// TODO: improve this
	lms := dga.LmsScoreOfDomain(q.FQDN)
	// log.Tracef("nameserver: domain %s has lms score of %f", fqdn, lms)
	if lms < 10 {
		tracer.Warningf("nameserver: possible data tunnel by %s: %s has lms score of %f, returning nxdomain", comm.Process(), q.FQDN, lms)
		returnNXDomain(w, query)
		return nil
	}

	// check profile before we even get intel and rr
	firewall.DecideOnCommunicationBeforeDNS(comm)
	comm.Lock()
	comm.SaveWhenFinished()
	comm.Unlock()

	if comm.GetVerdict() == network.VerdictBlock || comm.GetVerdict() == network.VerdictDrop {
		tracer.Infof("nameserver: %s denied before intel, returning nxdomain", comm)
		returnNXDomain(w, query)
		return nil
	}

	// resolve
	rrCache, err := resolver.Resolve(ctx, q)
	if err != nil {
		// TODO: analyze nxdomain requests, malware could be trying DGA-domains
		tracer.Warningf("nameserver: %s requested %s%s: %s", comm.Process(), q.FQDN, q.QType, err)
		returnNXDomain(w, query)
		return nil
	}

	// filter DNS response
	rrCache = firewall.FilterDNSResponse(comm, q, rrCache)
	if rrCache == nil {
		tracer.Infof("nameserver: %s implicitly denied by filtering the dns response, returning nxdomain", comm)
		returnNXDomain(w, query)
		return nil
	}

	// save IP addresses to IPInfo
	for _, rr := range append(rrCache.Answer, rrCache.Extra...) {
		switch v := rr.(type) {
		case *dns.A:
			ipInfo, err := resolver.GetIPInfo(v.A.String())
			if err != nil {
				ipInfo = &resolver.IPInfo{
					IP:      v.A.String(),
					Domains: []string{q.FQDN},
				}
				_ = ipInfo.Save()
			} else {
				added := ipInfo.AddDomain(q.FQDN)
				if added {
					_ = ipInfo.Save()
				}
			}
		case *dns.AAAA:
			ipInfo, err := resolver.GetIPInfo(v.AAAA.String())
			if err != nil {
				ipInfo = &resolver.IPInfo{
					IP:      v.AAAA.String(),
					Domains: []string{q.FQDN},
				}
				_ = ipInfo.Save()
			} else {
				added := ipInfo.AddDomain(q.FQDN)
				if added {
					_ = ipInfo.Save()
				}
			}
		}
	}

	// reply to query
	m := new(dns.Msg)
	m.SetReply(query)
	m.Answer = rrCache.Answer
	m.Ns = rrCache.Ns
	m.Extra = rrCache.Extra
	_ = w.WriteMsg(m)
	tracer.Debugf("nameserver: returning response %s%s to %s", q.FQDN, q.QType, comm.Process())

	return nil
}