// +skip_license_check

/*
This file contains portions of code directly taken from the 'xenolf/lego' project.
A copy of the license for this code can be found in the file named LICENSE in
this directory.
*/

package util

import (
	"fmt"
	"net"
	"strings"
	"sync"
	"time"

	"github.com/golang/glog"
	"github.com/miekg/dns"
)

type preCheckDNSFunc func(fqdn, value string, nameservers []string,
	useAuthoritative bool) (bool, error)

var (
	// PreCheckDNS checks DNS propagation before notifying ACME that
	// the DNS challenge is ready.
	PreCheckDNS preCheckDNSFunc = checkDNSPropagation

	fqdnToZoneLock sync.RWMutex
	fqdnToZone     = map[string]string{}
)

const defaultResolvConf = "/etc/resolv.conf"

var defaultNameservers = []string{
	"8.8.8.8:53",
	"8.8.4.4:53",
}

var RecursiveNameservers = getNameservers(defaultResolvConf, defaultNameservers)

// DNSTimeout is used to override the default DNS timeout of 10 seconds.
var DNSTimeout = 10 * time.Second

// getNameservers attempts to get systems nameservers before falling back to the defaults
func getNameservers(path string, defaults []string) []string {
	config, err := dns.ClientConfigFromFile(path)
	if err != nil || len(config.Servers) == 0 {
		return defaults
	}

	systemNameservers := []string{}
	for _, server := range config.Servers {
		// ensure all servers have a port number
		if _, _, err := net.SplitHostPort(server); err != nil {
			systemNameservers = append(systemNameservers, net.JoinHostPort(server, "53"))
		} else {
			systemNameservers = append(systemNameservers, server)
		}
	}
	return systemNameservers
}

// Update FQDN with CNAME if any
func updateDomainWithCName(r *dns.Msg, fqdn string) string {
	for _, rr := range r.Answer {
		if cn, ok := rr.(*dns.CNAME); ok {
			if cn.Hdr.Name == fqdn {
				glog.Infof("Updating FQDN: %s with it's CNAME: %s", fqdn, cn.Target)
				fqdn = cn.Target
				break
			}
		}
	}

	return fqdn
}

// checkDNSPropagation checks if the expected TXT record has been propagated to all authoritative nameservers.
func checkDNSPropagation(fqdn, value string, nameservers []string,
	useAuthoritative bool) (bool, error) {
	// Initial attempt to resolve at the recursive NS
	r, err := dnsQuery(fqdn, dns.TypeTXT, nameservers, true)
	if err != nil {
		return false, err
	}
	if r.Rcode == dns.RcodeSuccess {
		fqdn = updateDomainWithCName(r, fqdn)
	}

	if !useAuthoritative {
		return checkAuthoritativeNss(fqdn, value, nameservers)
	}

	authoritativeNss, err := lookupNameservers(fqdn, nameservers)
	if err != nil {
		return false, err
	}

	for i, ans := range authoritativeNss {
		authoritativeNss[i] = net.JoinHostPort(ans, "53")
	}
	return checkAuthoritativeNss(fqdn, value, authoritativeNss)
}

// checkAuthoritativeNss queries each of the given nameservers for the expected TXT record.
func checkAuthoritativeNss(fqdn, value string, nameservers []string) (bool, error) {
	for _, ns := range nameservers {
		r, err := dnsQuery(fqdn, dns.TypeTXT, []string{ns}, true)
		if err != nil {
			return false, err
		}

		// NXDomain response is not really an error, just waiting for propagation to happen
		if !(r.Rcode == dns.RcodeSuccess || r.Rcode == dns.RcodeNameError) {
			return false, fmt.Errorf("NS %s returned %s for %s", ns, dns.RcodeToString[r.Rcode], fqdn)
		}

		glog.V(6).Infof("Looking up TXT records for %q", fqdn)
		var found bool
		for _, rr := range r.Answer {
			if txt, ok := rr.(*dns.TXT); ok {
				if strings.Join(txt.Txt, "") == value {
					found = true
					break
				}
			}
		}

		if !found {
			return false, nil
		}
	}

	return true, nil
}

// dnsQuery will query a nameserver, iterating through the supplied servers as it retries
// The nameserver should include a port, to facilitate testing where we talk to a mock dns server.
func dnsQuery(fqdn string, rtype uint16, nameservers []string, recursive bool) (in *dns.Msg, err error) {
	m := new(dns.Msg)
	m.SetQuestion(fqdn, rtype)
	m.SetEdns0(4096, false)

	if !recursive {
		m.RecursionDesired = false
	}

	// Will retry the request based on the number of servers (n+1)
	for i := 1; i <= len(nameservers)+1; i++ {
		ns := nameservers[i%len(nameservers)]
		udp := &dns.Client{Net: "udp", Timeout: DNSTimeout}
		in, _, err = udp.Exchange(m, ns)

		if err == dns.ErrTruncated ||
			(err != nil && strings.HasPrefix(err.Error(), "read udp") && strings.HasSuffix(err.Error(), "i/o timeout")) {
			glog.V(6).Infof("UDP dns lookup failed, retrying with TCP: %v", err)
			tcp := &dns.Client{Net: "tcp", Timeout: DNSTimeout}
			// If the TCP request succeeds, the err will reset to nil
			in, _, err = tcp.Exchange(m, ns)
		}

		if err == nil {
			break
		}
	}
	return
}

// lookupNameservers returns the authoritative nameservers for the given fqdn.
func lookupNameservers(fqdn string, nameservers []string) ([]string, error) {
	var authoritativeNss []string

	zone, err := FindZoneByFqdn(fqdn, nameservers)
	if err != nil {
		return nil, fmt.Errorf("Could not determine the zone: %v", err)
	}

	r, err := dnsQuery(zone, dns.TypeNS, nameservers, true)
	if err != nil {
		return nil, err
	}

	for _, rr := range r.Answer {
		if ns, ok := rr.(*dns.NS); ok {
			authoritativeNss = append(authoritativeNss, strings.ToLower(ns.Ns))
		}
	}

	if len(authoritativeNss) > 0 {
		return authoritativeNss, nil
	}
	return nil, fmt.Errorf("Could not determine authoritative nameservers")
}

// FindZoneByFqdn determines the zone apex for the given fqdn by recursing up the
// domain labels until the nameserver returns a SOA record in the answer section.
func FindZoneByFqdn(fqdn string, nameservers []string) (string, error) {
	fqdnToZoneLock.RLock()
	// Do we have it cached?
	if zone, ok := fqdnToZone[fqdn]; ok {
		fqdnToZoneLock.RUnlock()
		return zone, nil
	}
	fqdnToZoneLock.RUnlock()

	labelIndexes := dns.Split(fqdn)
	for _, index := range labelIndexes {
		domain := fqdn[index:]

		in, err := dnsQuery(domain, dns.TypeSOA, nameservers, true)
		if err != nil {
			return "", err
		}

		// Any response code other than NOERROR and NXDOMAIN is treated as error
		if in.Rcode != dns.RcodeNameError && in.Rcode != dns.RcodeSuccess {
			return "", fmt.Errorf("Unexpected response code '%s' for %s",
				dns.RcodeToString[in.Rcode], domain)
		}

		// Check if we got a SOA RR in the answer section
		if in.Rcode == dns.RcodeSuccess {

			// CNAME records cannot/should not exist at the root of a zone.
			// So we skip a domain when a CNAME is found.
			if dnsMsgContainsCNAME(in) {
				continue
			}

			for _, ans := range in.Answer {
				if soa, ok := ans.(*dns.SOA); ok {
					fqdnToZoneLock.Lock()
					defer fqdnToZoneLock.Unlock()

					zone := soa.Hdr.Name
					fqdnToZone[fqdn] = zone
					return zone, nil
				}
			}
		}
	}

	return "", fmt.Errorf("Could not find the start of authority")
}

// dnsMsgContainsCNAME checks for a CNAME answer in msg
func dnsMsgContainsCNAME(msg *dns.Msg) bool {
	for _, ans := range msg.Answer {
		if _, ok := ans.(*dns.CNAME); ok {
			return true
		}
	}
	return false
}

// ToFqdn converts the name into a fqdn appending a trailing dot.
func ToFqdn(name string) string {
	n := len(name)
	if n == 0 || name[n-1] == '.' {
		return name
	}
	return name + "."
}

// UnFqdn converts the fqdn into a name removing the trailing dot.
func UnFqdn(name string) string {
	n := len(name)
	if n != 0 && name[n-1] == '.' {
		return name[:n-1]
	}
	return name
}

// WaitFor polls the given function 'f', once every 'interval', up to 'timeout'.
func WaitFor(timeout, interval time.Duration, f func() (bool, error)) error {
	var lastErr string
	timeup := time.After(timeout)
	for {
		select {
		case <-timeup:
			return fmt.Errorf("Time limit exceeded. Last error: %s", lastErr)
		default:
		}

		stop, err := f()
		if stop {
			return nil
		}
		if err != nil {
			lastErr = err.Error()
		}

		time.Sleep(interval)
	}
}