coredns/plugin/cache/item.go

package cache

import (
	"time"

	"github.com/coredns/coredns/plugin/cache/freq"
	"github.com/miekg/dns"
)

type item struct {
	Rcode              int
	AuthenticatedData  bool
	RecursionAvailable bool
	Answer             []dns.RR
	Ns                 []dns.RR
	Extra              []dns.RR

	origTTL uint32
	stored  time.Time

	*freq.Freq
}

func newItem(m *dns.Msg, now time.Time, d time.Duration) *item {
	i := new(item)
	i.Rcode = m.Rcode
	i.AuthenticatedData = m.AuthenticatedData
	i.RecursionAvailable = m.RecursionAvailable
	i.Answer = m.Answer
	i.Ns = m.Ns
	i.Extra = make([]dns.RR, len(m.Extra))
	// Don't copy OPT records as these are hop-by-hop.
	j := 0
	for _, e := range m.Extra {
		if e.Header().Rrtype == dns.TypeOPT {
			continue
		}
		i.Extra[j] = e
		j++
	}
	i.Extra = i.Extra[:j]

	i.origTTL = uint32(d.Seconds())
	i.stored = now.UTC()

	i.Freq = new(freq.Freq)

	return i
}

// toMsg turns i into a message, it tailors the reply to m.
// The Authoritative bit should be set to 0, but some client stub resolver implementations, most notably,
// on some legacy systems(e.g. ubuntu 14.04 with glib version 2.20), low-level glibc function `getaddrinfo`
// useb by Python/Ruby/etc.. will discard answers that do not have this bit set.
// So we're forced to always set this to 1; regardless if the answer came from the cache or not.
// On newer systems(e.g. ubuntu 16.04 with glib version 2.23), this issue is resolved.
// So we may set this bit back to 0 in the future ?
func (i *item) toMsg(m *dns.Msg, now time.Time, do bool) *dns.Msg {
	m1 := new(dns.Msg)
	m1.SetReply(m)

	// Set this to true as some DNS clients discard the *entire* packet when it's non-authoritative.
	// This is probably not according to spec, but the bit itself is not super useful as this point, so
	// just set it to true.
	m1.Authoritative = true
	m1.AuthenticatedData = i.AuthenticatedData
	if !do {
		m1.AuthenticatedData = false // when DNSSEC was not wanted, it can't be authenticated data.
	}
	m1.RecursionAvailable = i.RecursionAvailable
	m1.Rcode = i.Rcode

	m1.Answer = make([]dns.RR, len(i.Answer))
	m1.Ns = make([]dns.RR, len(i.Ns))
	m1.Extra = make([]dns.RR, len(i.Extra))

	ttl := uint32(i.ttl(now))
	j := 0
	for _, r := range i.Answer {
		if !do && isDNSSEC(r) {
			continue
		}
		m1.Answer[j] = dns.Copy(r)
		m1.Answer[j].Header().Ttl = ttl
		j++
	}
	m1.Answer = m1.Answer[:j]
	j = 0
	for _, r := range i.Ns {
		if !do && isDNSSEC(r) {
			continue
		}
		m1.Ns[j] = dns.Copy(r)
		m1.Ns[j].Header().Ttl = ttl
		j++
	}
	m1.Ns = m1.Ns[:j]
	// newItem skips OPT records, so we can just use i.Extra as is.
	j = 0
	for _, r := range i.Extra {
		if !do && isDNSSEC(r) {
			continue
		}
		m1.Extra[j] = dns.Copy(r)
		m1.Extra[j].Header().Ttl = ttl
		j++
	}
	m1.Extra = m1.Extra[:j]
	return m1
}

func (i *item) ttl(now time.Time) int {
	ttl := int(i.origTTL) - int(now.UTC().Sub(i.stored).Seconds())
	return ttl
}

// isDNSSEC returns true if r is a DNSSEC record. NSEC,NSEC3,DS and RRSIG/SIG
// are DNSSEC records. DNSKEYs is not in this list on the assumption that the
// client explictly asked for it.
func isDNSSEC(r dns.RR) bool {
	switch r.Header().Rrtype {
	case dns.TypeNSEC:
		return true
	case dns.TypeNSEC3:
		return true
	case dns.TypeDS:
		return true
	case dns.TypeRRSIG:
		return true
	case dns.TypeSIG:
		return true
	}
	return false
}