package proxy

import (
	"context"
	"crypto/tls"
	"encoding/base64"
	"fmt"
	"io"
	"log/slog"
	"net"
	"net/http"
	"net/netip"
	"net/url"
	"strings"

	"github.com/AdguardTeam/dnsproxy/internal/bootstrap"
	"github.com/AdguardTeam/golibs/httphdr"
	"github.com/AdguardTeam/golibs/ioutil"
	"github.com/AdguardTeam/golibs/logutil/slogutil"
	"github.com/miekg/dns"
	"github.com/quic-go/quic-go"
	"github.com/quic-go/quic-go/http3"
	"golang.org/x/net/http2"
)

// listenHTTP creates instances of TLS listeners that will be used to run an
// HTTP/1.1 and HTTP/2 servers.  It returns the address the listener actually
// listens to.  addr must not be nil.
func (p *Proxy) listenHTTP(
	ctx context.Context,
	addr *net.TCPAddr,
) (ln net.Listener, tcpAddr *net.TCPAddr, err error) {
	var tcpListen *net.TCPListener
	err = p.bindWithRetry(ctx, func() (listenErr error) {
		tcpListen, listenErr = net.ListenTCP(bootstrap.NetworkTCP, addr)

		// Don't wrap the error since it's informative enough as is.
		return listenErr
	})
	if err != nil {
		return nil, nil, fmt.Errorf("tcp listener: %w", err)
	}

	laddr := tcpListen.Addr()
	tcpAddr, ok := laddr.(*net.TCPAddr)
	if !ok {
		return nil, nil, fmt.Errorf("bad listener address type: %T", laddr)
	}

	p.logger.InfoContext(ctx, "listening to https", "addr", tcpAddr)

	tlsConfig := p.tlsConf.Clone()
	tlsConfig.NextProtos = []string{http2.NextProtoTLS, "http/1.1"}

	tlsListen := tls.NewListener(tcpListen, tlsConfig)

	return tlsListen, tcpAddr, nil
}

// listenH3 creates instances of QUIC listeners that will be used for running
// an HTTP/3 server.  addr must not be nil.
func (p *Proxy) listenH3(
	ctx context.Context,
	addr *net.UDPAddr,
) (ln *quic.EarlyListener, err error) {
	tlsConfig := p.tlsConf.Clone()
	tlsConfig.NextProtos = []string{"h3"}
	quicListen, err := quic.ListenAddrEarly(addr.String(), tlsConfig, newServerDoH3Config())
	if err != nil {
		return nil, fmt.Errorf("quic listener: %w", err)
	}

	p.logger.InfoContext(ctx, "listening to h3", "addr", quicListen.Addr())

	return quicListen, nil
}

// newServerDoH3Config creates *quic.Config populated with the default settings.
// This function is supposed to be used for the DoH3 server only.
func newServerDoH3Config() (conf *quic.Config) {
	return &quic.Config{
		MaxIdleTimeout: maxQUICIdleTimeout,
		// Enable 0-RTT by default for all connections on the server-side.
		Allow0RTT: true,
	}
}

// initHTTPSListeners creates TCP/UDP listeners and HTTP/H3 servers.
func (p *Proxy) initHTTPSListeners(ctx context.Context) (err error) {
	httpConf := p.httpConf
	if httpConf == nil {
		p.logger.DebugContext(ctx, "no https configuration provided")

		return nil
	}

	mux := http.NewServeMux()
	p.routeDoH(mux)

	p.httpsServer = &http.Server{
		Handler:           mux,
		ReadTimeout:       httpConf.ReadTimeout,
		ReadHeaderTimeout: httpConf.ReadTimeout,
		WriteTimeout:      httpConf.WriteTimeout,
	}

	if httpConf.HTTP3Enabled {
		p.h3Server = &http3.Server{
			Handler: mux,
		}
	}

	for _, addr := range httpConf.ListenAddresses {
		p.logger.InfoContext(ctx, "creating an https server")

		ln, tcpAddr, lErr := p.listenHTTP(ctx, net.TCPAddrFromAddrPort(addr))
		if lErr != nil {
			return fmt.Errorf("failed to start https server on %s: %w", addr, lErr)
		}

		p.httpsListen = append(p.httpsListen, ln)

		if httpConf.HTTP3Enabled {
			// HTTP/3 server listens to the same pair IP:port as the one HTTP/2
			// server listens to.
			udpAddr := &net.UDPAddr{IP: tcpAddr.IP, Port: tcpAddr.Port}

			var quicListen *quic.EarlyListener
			quicListen, err = p.listenH3(ctx, udpAddr)
			if err != nil {
				return fmt.Errorf("failed to start h3 server on %s: %w", udpAddr, err)
			}

			p.h3Listen = append(p.h3Listen, quicListen)
		}
	}

	return nil
}

// newDoHReq returns new DNS request parsed from the given HTTP request.  In
// case of invalid request returns nil and the suitable status code for an HTTP
// error response.  r and l must not be nil.
func newDoHReq(
	ctx context.Context,
	r *http.Request,
	l *slog.Logger,
) (req *dns.Msg, statusCode int) {
	var buf []byte
	var err error

	switch r.Method {
	case http.MethodGet:
		dnsParam := r.URL.Query().Get("dns")
		buf, err = base64.RawURLEncoding.DecodeString(dnsParam)
		if len(buf) == 0 || err != nil {
			l.DebugContext(
				ctx,
				"parsing dns request from http get param",
				"param_name", dnsParam,
				slogutil.KeyError, err,
			)

			return nil, http.StatusBadRequest
		}
	case http.MethodPost:
		contentType := r.Header.Get(httphdr.ContentType)
		if contentType != "application/dns-message" {
			l.DebugContext(ctx, "unsupported media type", "content_type", contentType)

			return nil, http.StatusUnsupportedMediaType
		}

		limitBody := ioutil.LimitReader(r.Body, dns.MaxMsgSize)
		buf, err = io.ReadAll(limitBody)
		if err != nil {
			l.DebugContext(ctx, "reading http request body", slogutil.KeyError, err)

			return nil, http.StatusBadRequest
		}

		defer slogutil.CloseAndLog(ctx, l, r.Body, slog.LevelDebug)
	default:
		l.DebugContext(ctx, "bad http method", "method", r.Method)

		return nil, http.StatusMethodNotAllowed
	}

	req = &dns.Msg{}
	if err = req.Unpack(buf); err != nil {
		l.DebugContext(ctx, "unpacking http msg", slogutil.KeyError, err)

		return nil, http.StatusBadRequest
	}

	return req, http.StatusOK
}

// ServeHTTP is the [http.Handler] implementation that handles DoH queries.
//
// It returns:
//
//   - http.StatusNotFound if the request is not encrypted and proxy is not
//     configured to accept unencrypted requests,
//   - http.StatusBadRequest if there is no DNS request data,
//   - http.StatusUnsupportedMediaType if request content type is not
//     "application/dns-message",
//   - http.StatusMethodNotAllowed if request method is not GET or POST.
func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	ctx := r.Context()
	ctx, cancel := p.reqCtx.New(ctx)
	defer cancel()

	p.logger.DebugContext(ctx, "incoming https request", "url", r.URL)

	if !p.httpConf.InsecureEnabled && r.TLS == nil {
		statusCode := http.StatusNotFound
		http.Error(w, http.StatusText(statusCode), statusCode)

		return
	}

	raddr, prx, err := remoteAddr(r, p.logger)
	if err != nil {
		p.logger.DebugContext(ctx, "getting real ip", slogutil.KeyError, err)
	}

	if !p.checkBasicAuth(w, r, raddr) {
		return
	}

	req, statusCode := newDoHReq(ctx, r, p.logger)
	if req == nil {
		http.Error(w, http.StatusText(statusCode), statusCode)

		return
	}

	if prx.IsValid() {
		l := p.logger.With("addr", prx)

		l.DebugContext(ctx, "request came from proxy server")

		if !p.trustedProxies.Contains(prx.Addr()) {
			l.DebugContext(ctx, "proxy is not trusted, using original remote addr")

			// So the address of the proxy itself is used, as the remote address
			// parsed from headers cannot be trusted.
			//
			// TODO(e.burkov): Do not parse headers in this case.
			raddr = prx
		}
	}

	d := p.newDNSContext(ProtoHTTPS, req, raddr)
	d.HTTPRequest = r
	d.HTTPResponseWriter = w

	err = p.handleDNSRequest(ctx, d)
	if err != nil {
		p.logger.DebugContext(ctx, "handling dns request", "proto", d.Proto, slogutil.KeyError, err)
	}
}

// checkBasicAuth checks the basic authorization data, if necessary, and if the
// data isn't valid, it writes an error.  shouldHandle is false if the request
// has been denied.  w, r and p.httpConf must not be nil.
func (p *Proxy) checkBasicAuth(
	w http.ResponseWriter,
	r *http.Request,
	raddr netip.AddrPort,
) (shouldHandle bool) {
	ui := p.httpConf.Userinfo
	if ui == nil {
		return true
	}

	user, pass, _ := r.BasicAuth()
	if matchesUserinfo(ui, user, pass) {
		return true
	}

	p.logger.Error("basic auth failed", "user", user, "raddr", raddr)

	h := w.Header()
	h.Set(httphdr.WWWAuthenticate, `Basic realm="DNS", charset="UTF-8"`)
	http.Error(w, "Authorization required", http.StatusUnauthorized)

	return false
}

// matchesUserinfo returns false if user and pass don't match userinfo.
// userinfo must not be nil.
func matchesUserinfo(userinfo *url.Userinfo, user, pass string) (ok bool) {
	requiredPassword, _ := userinfo.Password()

	return user == userinfo.Username() && pass == requiredPassword
}

// respondHTTPS writes a response to the DoH client.  d must not be nil.
func (p *Proxy) respondHTTPS(d *DNSContext) (err error) {
	resp := d.Res
	w := d.HTTPResponseWriter

	if resp == nil {
		// Indicate the response's absence via a http.StatusInternalServerError.
		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)

		return nil
	}

	bytes, err := resp.Pack()
	if err != nil {
		http.Error(w, http.StatusText(http.StatusInternalServerError), http.StatusInternalServerError)

		return fmt.Errorf("packing message: %w", err)
	}

	if srvHeader := p.httpConf.ServerHeader; srvHeader != "" {
		w.Header().Set(httphdr.Server, srvHeader)
	}

	w.Header().Set(httphdr.ContentType, "application/dns-message")
	_, err = w.Write(bytes)

	return err
}

// realIPFromHdrs extracts the actual client's IP address from the first
// suitable r's header.  It returns an error if r doesn't contain any
// information about real client's IP address.  r must not be nil.  Current
// headers priority is:
//
//  1. [httphdr.CFConnectingIP]
//  2. [httphdr.TrueClientIP]
//  3. [httphdr.XRealIP]
//  4. [httphdr.XForwardedFor]
func realIPFromHdrs(r *http.Request) (realIP netip.Addr, err error) {
	for _, h := range []string{
		httphdr.CFConnectingIP,
		httphdr.TrueClientIP,
		httphdr.XRealIP,
	} {
		realIP, err = netip.ParseAddr(strings.TrimSpace(r.Header.Get(h)))
		if err == nil {
			return realIP, nil
		}
	}

	xff := r.Header.Get(httphdr.XForwardedFor)
	firstComma := strings.IndexByte(xff, ',')
	if firstComma > 0 {
		xff = xff[:firstComma]
	}

	return netip.ParseAddr(strings.TrimSpace(xff))
}

// remoteAddr returns the real client's address and the IP address of the latest
// proxy server if any.  r and l must not be nil.
func remoteAddr(r *http.Request, l *slog.Logger) (addr, prx netip.AddrPort, err error) {
	host, err := netip.ParseAddrPort(r.RemoteAddr)
	if err != nil {
		return netip.AddrPort{}, netip.AddrPort{}, err
	}

	realIP, err := realIPFromHdrs(r)
	if err != nil {
		l.Debug("getting ip address from http request", slogutil.KeyError, err)

		return host, netip.AddrPort{}, nil
	}

	l.Debug("using ip address from http request", "addr", realIP)

	// TODO(a.garipov): Add port if we can get it from headers like X-Real-Port,
	// X-Forwarded-Port, etc.
	addr = netip.AddrPortFrom(realIP, 0)

	return addr, host, nil
}