fix: dns rfc compliant ttl

discovery/dns/dns.go

@@ -50,6 +50,57 @@ const (
 	namespace = "prometheus"
 )
 
+// DNSCache stores DNS responses based on their TTL values.
+type DNSCache struct {
+	mu    sync.RWMutex
+	cache map[string]*dnsCacheEntry
+}
+
+type dnsCacheEntry struct {
+	msg      *dns.Msg
+	expiry   time.Time
+}
+
+func NewDNSCache() *DNSCache {
+	return &DNSCache{
+		cache: make(map[string]*dnsCacheEntry),
+	}
+}
+
+func (c *DNSCache) Get(name string) (*dns.Msg, bool) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	entry, ok := c.cache[name]
+	if !ok || time.Now().After(entry.expiry) {
+		return nil, false
+	}
+	return entry.msg, true
+}
+
+func (c *DNSCache) Set(name string, msg *dns.Msg) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	ttl := getMinimumTTL(msg)
+	entry := &dnsCacheEntry{
+		msg:    msg,
+		expiry: time.Now().Add(ttl),
+	}
+	c.cache[name] = entry
+}
+
+func getMinimumTTL(msg *dns.Msg) time.Duration {
+	minTTL := uint32(^uint32(0)) // Set to maximum value
+	for _, record := range append(msg.Answer, msg.Ns...) {
+		hdr := record.Header()
+		if hdr.Ttl < minTTL {
+			minTTL = hdr.Ttl
+		}
+	}
+	return time.Duration(minTTL) * time.Second
+}
+
 // DefaultSDConfig is the default DNS SD configuration.
 var DefaultSDConfig = SDConfig{
 	RefreshInterval: model.Duration(30 * time.Second),
@@ -68,20 +119,16 @@ type SDConfig struct {
 	Port            int            `yaml:"port"` // Ignored for SRV records
 }
 
-// NewDiscovererMetrics implements discovery.Config.
 func (*SDConfig) NewDiscovererMetrics(reg prometheus.Registerer, rmi discovery.RefreshMetricsInstantiator) discovery.DiscovererMetrics {
 	return newDiscovererMetrics(reg, rmi)
 }
 
-// Name returns the name of the Config.
 func (*SDConfig) Name() string { return "dns" }
 
-// NewDiscoverer returns a Discoverer for the Config.
 func (c *SDConfig) NewDiscoverer(opts discovery.DiscovererOptions) (discovery.Discoverer, error) {
 	return NewDiscovery(*c, opts.Logger, opts.Metrics)
 }
 
-// UnmarshalYAML implements the yaml.Unmarshaler interface.
 func (c *SDConfig) UnmarshalYAML(unmarshal func(interface{}) error) error {
 	*c = DefaultSDConfig
 	type plain SDConfig
@@ -113,6 +160,7 @@ type Discovery struct {
 	qtype   uint16
 	logger  log.Logger
 	metrics *dnsMetrics
+	cache   *DNSCache
 
 	lookupFn func(name string, qtype uint16, logger log.Logger) (*dns.Msg, error)
 }
@@ -148,6 +196,7 @@ func NewDiscovery(conf SDConfig, logger log.Logger, metrics discovery.Discoverer
 		logger:   logger,
 		lookupFn: lookupWithSearchPath,
 		metrics:  m,
+		cache:    NewDNSCache(),
 	}
 
 	d.Discovery = refresh.NewDiscovery(
@@ -192,6 +241,22 @@ func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error) {
 }
 
 func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targetgroup.Group) error {
+	cachedMsg, found := d.cache.Get(name)
+	if found {
+		d.metrics.dnsSDLookupsCount.Inc()
+		tg, err := d.dnsMsgToTargetGroup(ctx, name, cachedMsg)
+		if err != nil {
+			d.metrics.dnsSDLookupFailuresCount.Inc()
+			return err
+		}
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case ch <- tg:
+		}
+		return nil
+	}
+
 	response, err := d.lookupFn(name, d.qtype, d.logger)
 	d.metrics.dnsSDLookupsCount.Inc()
 	if err != nil {
@@ -199,12 +264,27 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 		return err
 	}
 
+	d.cache.Set(name, response)
+	tg, err := d.dnsMsgToTargetGroup(ctx, name, response)
+	if err != nil {
+		d.metrics.dnsSDLookupFailuresCount.Inc()
+		return err
+	}
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case ch <- tg:
+	}
+	return nil
+}
+
+func (d *Discovery) dnsMsgToTargetGroup(ctx context.Context, name string, msg *dns.Msg) (*targetgroup.Group, error) {
 	tg := &targetgroup.Group{}
 	hostPort := func(a string, p int) model.LabelValue {
 		return model.LabelValue(net.JoinHostPort(a, strconv.Itoa(p)))
 	}
 
-	for _, record := range response.Answer {
+	for _, record := range msg.Answer {
 		var target, dnsSrvRecordTarget, dnsSrvRecordPort, dnsMxRecordTarget, dnsNsRecordTarget model.LabelValue
 
 		switch addr := record.(type) {
@@ -212,21 +292,18 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 			dnsSrvRecordTarget = model.LabelValue(addr.Target)
 			dnsSrvRecordPort = model.LabelValue(strconv.Itoa(int(addr.Port)))
 
-			// Remove the final dot from rooted DNS names to make them look more usual.
 			addr.Target = strings.TrimRight(addr.Target, ".")
 
 			target = hostPort(addr.Target, int(addr.Port))
 		case *dns.MX:
 			dnsMxRecordTarget = model.LabelValue(addr.Mx)
 
-			// Remove the final dot from rooted DNS names to make them look more usual.
 			addr.Mx = strings.TrimRight(addr.Mx, ".")
 
 			target = hostPort(addr.Mx, d.port)
 		case *dns.NS:
 			dnsNsRecordTarget = model.LabelValue(addr.Ns)
 
-			// Remove the final dot from rooted DNS names to make them look more usual.
 			addr.Ns = strings.TrimRight(addr.Ns, ".")
 
 			target = hostPort(addr.Ns, d.port)
@@ -235,7 +312,6 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 		case *dns.AAAA:
 			target = hostPort(addr.AAAA.String(), d.port)
 		case *dns.CNAME:
-			// CNAME responses can occur with "Type: A" dns_sd_config requests.
 			continue
 		default:
 			level.Warn(d.logger).Log("msg", "Invalid record", "record", record)
@@ -252,13 +328,7 @@ func (d *Discovery) refreshOne(ctx context.Context, name string, ch chan<- *targ
 	}
 
 	tg.Source = name
-	select {
-	case <-ctx.Done():
-		return ctx.Err()
-	case ch <- tg:
-	}
-
-	return nil
+	return tg, nil
 }
 
 // lookupWithSearchPath tries to get an answer for various permutations of
@@ -301,23 +371,15 @@ func lookupWithSearchPath(name string, qtype uint16, logger log.Logger) (*dns.Ms
 
 		switch {
 		case err != nil:
-			// We can't go home yet, because a later name
-			// may give us a valid, successful answer.  However
-			// we can no longer say "this name definitely doesn't
-			// exist", because we did not get that answer for
-			// at least one name.
 			allResponsesValid = false
 		case response.Rcode == dns.RcodeSuccess:
-			// Outcome 1: GOLD!
 			return response, nil
 		}
 	}
 
 	if allResponsesValid {
-		// Outcome 2: everyone says NXDOMAIN, that's good enough for me.
 		return &dns.Msg{}, nil
 	}
-	// Outcome 3: boned.
 	return nil, fmt.Errorf("could not resolve %q: all servers responded with errors to at least one search domain", name)
 }
 
@@ -325,18 +387,6 @@ func lookupWithSearchPath(name string, qtype uint16, logger log.Logger) (*dns.Ms
 // name.  If a viable answer is received from a server, then it is
 // immediately returned, otherwise the other servers in the config are
 // tried, and if none of them return a viable answer, an error is returned.
-//
-// A "viable answer" is one which indicates either:
-//
-//  1. "yes, I know that name, and here are its records of the requested type"
-//     (RCODE==SUCCESS, ANCOUNT > 0);
-//  2. "yes, I know that name, but it has no records of the requested type"
-//     (RCODE==SUCCESS, ANCOUNT==0); or
-//  3. "I know that name doesn't exist" (RCODE==NXDOMAIN).
-//
-// A non-viable answer is "anything else", which encompasses both various
-// system-level problems (like network timeouts) and also
-// valid-but-unexpected DNS responses (SERVFAIL, REFUSED, etc).
 func lookupFromAnyServer(name string, qtype uint16, conf *dns.ClientConfig, logger log.Logger) (*dns.Msg, error) {
 	client := &dns.Client{}
 
@@ -349,7 +399,6 @@ func lookupFromAnyServer(name string, qtype uint16, conf *dns.ClientConfig, logg
 		}
 
 		if msg.Rcode == dns.RcodeSuccess || msg.Rcode == dns.RcodeNameError {
-			// We have our answer.  Time to go home.
 			return msg, nil
 		}
 	}