// Package nscheck asks other name servers about a zone over DNS:
//
//   - Serials: is every name server of a zone serving the serial this
//     server serves? For a primary zone those are its secondaries (the NS
//     records and also-notify targets); for a secondary zone, its
//     primaries.
//   - Delegation: does the parent zone (e.g. "lol." for adbraun.lol)
//     delegate the zone to the name servers, with the glue addresses, that
//     Jarl has? Only meaningful for zones on the internet, so it only runs
//     when asked and reports a missing delegation as information, not as a
//     problem.
//
// It only sends ordinary non-recursive SOA and NS queries.
package nscheck

import (
	"context"
	"errors"
	"fmt"
	"net"
	"net/netip"
	"sort"
	"strings"
	"sync"
	"syscall"
	"time"

	"github.com/miekg/dns"
)

// Key is a TSIG key to sign queries with, for servers that pick the view
// (or allow the query) by key.
type Key struct {
	Name      string
	Algorithm string // e.g. hmac-sha256
	Secret    string // base64
}

// Target is a server to ask.
type Target struct {
	Name      string   // host name (FQDN), "" when only an address is known
	Addresses []string // known addresses; nil: look the name up
	Source    string   // why it is asked: "NS record", "also-notify", "primaries", ...
	Port      string   // "" for the default
	Key       *Key
}

// Result is one server's answer.
type Result struct {
	Name    string `json:"name,omitempty"`
	Address string `json:"address,omitempty"`
	Port    string `json:"port,omitempty"`
	Source  string `json:"source"`
	// Status is one of:
	//   self               this server (not queried)
	//   in_sync            serves the same serial
	//   answered           serves the zone (no serial here to compare with)
	//   behind, ahead      serves an older or newer serial (RFC 1982)
	//   not_authoritative  answers, but doesn't serve the zone
	//   refused            refuses the query (allow-query, or its view
	//                      doesn't have the zone for this server)
	//   unreachable        no answer in time
	//   no_route           this host has no route to the address (e.g.
	//                      IPv6 on a host without it); not a problem
	//   no_address         the name has no address
	//   error              anything else (see Error)
	Status string  `json:"status"`
	Serial *uint32 `json:"serial,omitempty"`
	// Diff is Serial minus the reference serial (negative: behind).
	Diff  int64  `json:"diff,omitempty"`
	RTTMs int64  `json:"rtt_ms,omitempty"`
	Error string `json:"error,omitempty"`
}

// OK reports whether the result is good news.
func (r *Result) OK() bool {
	switch r.Status {
	case "in_sync", "answered", "self", "no_route":
		return true
	}
	return false
}

// Checker sends the queries.
type Checker struct {
	// Timeout per query (default 2s; two tries per address).
	Timeout time.Duration
	// Port is the DNS port (default 53; tests use another).
	Port string
	// Resolver is the recursive resolver used to look up names
	// (host:port); default: the first nameserver in /etc/resolv.conf.
	Resolver string
	// IsLocal reports whether an address belongs to this host (default:
	// its interface addresses and loopback).
	IsLocal func(netip.Addr) bool
}

func (c *Checker) timeout() time.Duration {
	if c.Timeout > 0 {
		return c.Timeout
	}
	return 2 * time.Second
}

func (c *Checker) port() string {
	if c.Port != "" {
		return c.Port
	}
	return "53"
}

func (c *Checker) isLocal(a netip.Addr) bool {
	if c.IsLocal != nil {
		return c.IsLocal(a)
	}
	return IsLocalAddr(a)
}

// IsLocalAddr reports whether a is a loopback address or one of this
// host's interface addresses.
func IsLocalAddr(a netip.Addr) bool {
	if a.IsLoopback() {
		return true
	}
	addrs, err := net.InterfaceAddrs()
	if err != nil {
		return false
	}
	for _, ia := range addrs {
		if n, ok := ia.(*net.IPNet); ok {
			if b, ok := netip.AddrFromSlice(n.IP); ok && b.Unmap() == a.Unmap() {
				return true
			}
		}
	}
	return false
}

// SystemResolver returns the first nameserver in /etc/resolv.conf as
// host:port.
func SystemResolver() (string, error) {
	cc, err := dns.ClientConfigFromFile("/etc/resolv.conf")
	if err != nil {
		return "", fmt.Errorf("reading /etc/resolv.conf: %w", err)
	}
	if len(cc.Servers) == 0 {
		return "", errors.New("no nameserver in /etc/resolv.conf")
	}
	return net.JoinHostPort(cc.Servers[0], cc.Port), nil
}

func (c *Checker) resolver() (string, error) {
	if c.Resolver != "" {
		return c.Resolver, nil
	}
	return SystemResolver()
}

// exchange sends one query to addr (an IP address) with a retry, and
// over TCP when the UDP answer is truncated.
func (c *Checker) exchange(ctx context.Context, m *dns.Msg, addr string, key *Key) (*dns.Msg, time.Duration, error) {
	return c.exchangeHostPort(ctx, m, net.JoinHostPort(addr, c.port()), key)
}

func (c *Checker) exchangeHostPort(ctx context.Context, m *dns.Msg, hostport string, key *Key) (*dns.Msg, time.Duration, error) {
	cl := &dns.Client{Timeout: c.timeout()}
	if key != nil {
		name := dns.Fqdn(key.Name)
		cl.TsigSecret = map[string]string{name: key.Secret}
		m.SetTsig(name, dns.Fqdn(key.Algorithm), 300, time.Now().Unix())
	}
	var resp *dns.Msg
	var rtt time.Duration
	var err error
	for try := 0; try < 2; try++ {
		resp, rtt, err = cl.ExchangeContext(ctx, m, hostport)
		if err == nil || ctx.Err() != nil {
			break
		}
	}
	if err == nil && resp.Truncated {
		cl.Net = "tcp"
		resp, rtt, err = cl.ExchangeContext(ctx, m, hostport)
	}
	return resp, rtt, err
}

// lookup returns a name's IPv4 and IPv6 addresses through the resolver.
func (c *Checker) lookup(ctx context.Context, name string) ([]string, error) {
	res, err := c.resolver()
	if err != nil {
		return nil, err
	}
	var out []string
	var firstErr error
	for _, t := range []uint16{dns.TypeA, dns.TypeAAAA} {
		m := new(dns.Msg)
		m.SetQuestion(dns.Fqdn(name), t)
		resp, _, err := c.exchangeHostPort(ctx, m, res, nil)
		if err != nil {
			if firstErr == nil {
				firstErr = err
			}
			continue
		}
		for _, rr := range resp.Answer {
			switch v := rr.(type) {
			case *dns.A:
				out = append(out, v.A.String())
			case *dns.AAAA:
				out = append(out, v.AAAA.String())
			}
		}
	}
	if len(out) == 0 && firstErr != nil {
		return nil, firstErr
	}
	return out, nil
}

// soa asks addr for zone's SOA and fills in r (whose Name, Address and
// Source are set).
func (c *Checker) soa(ctx context.Context, zone string, key *Key, ref *uint32, r *Result) {
	m := new(dns.Msg)
	m.SetQuestion(dns.Fqdn(zone), dns.TypeSOA)
	m.RecursionDesired = false
	port := r.Port
	if port == "" {
		port = c.port()
	}
	resp, rtt, err := c.exchangeHostPort(ctx, m, net.JoinHostPort(r.Address, port), key)
	if err != nil {
		var ne net.Error
		switch {
		case errors.Is(err, syscall.ENETUNREACH):
			// Not the server's fault: typically IPv6 on a host without it.
			r.Status, r.Error = "no_route", "this server has no route to that address"
		case errors.As(err, &ne) && ne.Timeout(), errors.Is(err, context.DeadlineExceeded):
			r.Status, r.Error = "unreachable", "no answer within "+c.timeout().String()
		default:
			r.Status, r.Error = "unreachable", err.Error()
		}
		return
	}
	r.RTTMs = rtt.Milliseconds()
	switch {
	case resp.Rcode == dns.RcodeRefused:
		r.Status = "refused"
		return
	case resp.Rcode != dns.RcodeSuccess:
		r.Status, r.Error = "error", "answered "+dns.RcodeToString[resp.Rcode]
		return
	case !resp.Authoritative:
		r.Status = "not_authoritative"
		return
	}
	for _, rr := range resp.Answer {
		if s, ok := rr.(*dns.SOA); ok && strings.EqualFold(s.Hdr.Name, dns.Fqdn(zone)) {
			serial := s.Serial
			r.Serial = &serial
			r.Status = "answered"
			if ref != nil {
				r.Status = "in_sync"
				r.Diff = int64(int32(serial - *ref))
				switch {
				case r.Diff < 0:
					r.Status = "behind"
				case r.Diff > 0:
					r.Status = "ahead"
				}
			}
			return
		}
	}
	r.Status, r.Error = "error", "the answer has no SOA record"
}

// Serials asks each target for zone's SOA serial and compares it with
// ref, the serial this server serves (nil when unknown). Targets with
// local addresses are this server and aren't asked. Results are sorted by
// name and address.
func (c *Checker) Serials(ctx context.Context, zone string, ref *uint32, targets []Target) []Result {
	var mu sync.Mutex
	var wg sync.WaitGroup
	var out []Result
	add := func(r Result) {
		mu.Lock()
		out = append(out, r)
		mu.Unlock()
	}
	seen := map[string]bool{}
	for _, t := range targets {
		addrs := t.Addresses
		if addrs == nil && t.Name != "" {
			found, err := c.lookup(ctx, t.Name)
			if err != nil || len(found) == 0 {
				msg := "no A or AAAA record"
				if err != nil {
					msg = "looking up its address: " + err.Error()
				}
				add(Result{Name: t.Name, Source: t.Source, Status: "no_address", Error: msg})
				continue
			}
			addrs = found
		}
		for _, a := range addrs {
			ip, err := netip.ParseAddr(a)
			if err != nil || seen[ip.String()+"#"+t.Port] {
				continue
			}
			seen[ip.String()+"#"+t.Port] = true
			r := Result{Name: t.Name, Address: ip.String(), Port: t.Port, Source: t.Source}
			if c.isLocal(ip) {
				r.Status = "self"
				if ref != nil {
					s := *ref
					r.Serial = &s
				}
				add(r)
				continue
			}
			wg.Add(1)
			go func(r Result, key *Key) {
				defer wg.Done()
				c.soa(ctx, zone, key, ref, &r)
				add(r)
			}(r, t.Key)
		}
	}
	wg.Wait()
	sort.Slice(out, func(i, j int) bool {
		if out[i].Name != out[j].Name {
			return out[i].Name < out[j].Name
		}
		return out[i].Address < out[j].Address
	})
	return out
}