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package cache
import (
"context"
"math"
"time"
"github.com/coredns/coredns/plugin"
"github.com/coredns/coredns/plugin/metrics"
"github.com/coredns/coredns/request"
"github.com/miekg/dns"
)
// ServeDNS implements the plugin.Handler interface.
func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
rc := r.Copy() // We potentially modify r, to prevent other plugins from seeing this (r is a pointer), copy r into rc.
state := request.Request{W: w, Req: rc}
do := state.Do()
zone := plugin.Zones(c.Zones).Matches(state.Name())
if zone == "" {
return plugin.NextOrFailure(c.Name(), c.Next, ctx, w, rc)
}
now := c.now().UTC()
server := metrics.WithServer(ctx)
// On cache miss, if the request has the OPT record and the DO bit set we leave the message as-is. If there isn't a DO bit
// set we will modify the request to _add_ one. This means we will always do DNSSEC lookups on cache misses.
// When writing to cache, any DNSSEC RRs in the response are written to cache with the response.
// When sending a response to a non-DNSSEC client, we remove DNSSEC RRs from the response. We use a 2048 buffer size, which is
// less than 4096 (and older default) and more than 1024 which may be too small. We might need to tweaks this
// value to be smaller still to prevent UDP fragmentation?
ttl := 0
i := c.getIgnoreTTL(now, state, server)
if i != nil {
ttl = i.ttl(now)
}
if i == nil {
crr := &ResponseWriter{ResponseWriter: w, Cache: c, state: state, server: server, do: do}
return c.doRefresh(ctx, state, crr)
}
if ttl < 0 {
servedStale.WithLabelValues(server).Inc()
// Adjust the time to get a 0 TTL in the reply built from a stale item.
now = now.Add(time.Duration(ttl) * time.Second)
cw := newPrefetchResponseWriter(server, state, c)
go c.doPrefetch(ctx, state, cw, i, now)
} else if c.shouldPrefetch(i, now) {
cw := newPrefetchResponseWriter(server, state, c)
go c.doPrefetch(ctx, state, cw, i, now)
}
resp := i.toMsg(r, now, do)
w.WriteMsg(resp)
return dns.RcodeSuccess, nil
}
func (c *Cache) doPrefetch(ctx context.Context, state request.Request, cw *ResponseWriter, i *item, now time.Time) {
cachePrefetches.WithLabelValues(cw.server).Inc()
c.doRefresh(ctx, state, cw)
// When prefetching we loose the item i, and with it the frequency
// that we've gathered sofar. See we copy the frequencies info back
// into the new item that was stored in the cache.
if i1 := c.exists(state); i1 != nil {
i1.Freq.Reset(now, i.Freq.Hits())
}
}
func (c *Cache) doRefresh(ctx context.Context, state request.Request, cw *ResponseWriter) (int, error) {
if !state.Do() {
setDo(state.Req)
}
return plugin.NextOrFailure(c.Name(), c.Next, ctx, cw, state.Req)
}
func (c *Cache) shouldPrefetch(i *item, now time.Time) bool {
if c.prefetch <= 0 {
return false
}
i.Freq.Update(c.duration, now)
threshold := int(math.Ceil(float64(c.percentage) / 100 * float64(i.origTTL)))
return i.Freq.Hits() >= c.prefetch && i.ttl(now) <= threshold
}
// Name implements the Handler interface.
func (c *Cache) Name() string { return "cache" }
func (c *Cache) get(now time.Time, state request.Request, server string) (*item, bool) {
k := hash(state.Name(), state.QType())
if i, ok := c.ncache.Get(k); ok && i.(*item).ttl(now) > 0 {
cacheHits.WithLabelValues(server, Denial).Inc()
return i.(*item), true
}
if i, ok := c.pcache.Get(k); ok && i.(*item).ttl(now) > 0 {
cacheHits.WithLabelValues(server, Success).Inc()
return i.(*item), true
}
cacheMisses.WithLabelValues(server).Inc()
return nil, false
}
// getIgnoreTTL unconditionally returns an item if it exists in the cache.
func (c *Cache) getIgnoreTTL(now time.Time, state request.Request, server string) *item {
k := hash(state.Name(), state.QType())
if i, ok := c.ncache.Get(k); ok {
ttl := i.(*item).ttl(now)
if ttl > 0 || (c.staleUpTo > 0 && -ttl < int(c.staleUpTo.Seconds())) {
cacheHits.WithLabelValues(server, Denial).Inc()
return i.(*item)
}
}
if i, ok := c.pcache.Get(k); ok {
ttl := i.(*item).ttl(now)
if ttl > 0 || (c.staleUpTo > 0 && -ttl < int(c.staleUpTo.Seconds())) {
cacheHits.WithLabelValues(server, Success).Inc()
return i.(*item)
}
}
cacheMisses.WithLabelValues(server).Inc()
return nil
}
func (c *Cache) exists(state request.Request) *item {
k := hash(state.Name(), state.QType())
if i, ok := c.ncache.Get(k); ok {
return i.(*item)
}
if i, ok := c.pcache.Get(k); ok {
return i.(*item)
}
return nil
}
// setDo sets the DO bit and UDP buffer size in the message m.
func setDo(m *dns.Msg) {
o := m.IsEdns0()
if o != nil {
o.SetDo()
o.SetUDPSize(defaultUDPBufSize)
return
}
o = &dns.OPT{Hdr: dns.RR_Header{Name: ".", Rrtype: dns.TypeOPT}}
o.SetDo()
o.SetUDPSize(defaultUDPBufSize)
m.Extra = append(m.Extra, o)
}
// defaultUDPBufsize is the bufsize the cache plugin uses on outgoing requests that don't
// have an OPT RR.
const defaultUDPBufSize = 2048
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