Add config option to tweak the DNS cache TTL

config/config.go

@@ -185,6 +185,9 @@ type ServiceConfig struct {
 	// ClientTLS is used to configure the http default transport
 	// with TLS parameters
 	ClientTLS *ClientTLS `mapstructure:"client_tls"`
+
+	// DNSCacheTTL is the duration of the cached data for the DNS lookups
+	DNSCacheTTL time.Duration `mapstructure:"dns_cache_ttl"`
 }
 
 // AsyncAgent defines the configuration of a single subscriber/consumer to be initialized

config/config_test.go

@@ -211,7 +211,7 @@ func TestConfig_init(t *testing.T) {
 		t.Error(err.Error())
 	}
 
-	if hash != "dEnC/Sn/mHYtt5vj+DkI0Ib22D/1liL8zx/WHpT2ajY=" {
+	if hash != "mIfuGTgHS91DJtAE6KMVl2kcluxCzc9n3f6fi0YgWs8=" {
 		t.Errorf("unexpected hash: %s", hash)
 	}
 }

config/parser.go

@@ -165,6 +165,7 @@ type parseableServiceConfig struct {
 	TLS                   *parseableTLS              `json:"tls,omitempty"`
 	ClientTLS             *parseableClientTLS        `json:"client_tls,omitempty"`
 	UseH2C                bool                       `json:"use_h2c,omitempty"`
+	DNSCacheTTL           string                     `json:"dns_cache_ttl"`
 }
 
 func (p *parseableServiceConfig) normalize() ServiceConfig {
@@ -197,6 +198,7 @@ func (p *parseableServiceConfig) normalize() ServiceConfig {
 		OutputEncoding:        p.OutputEncoding,
 		Plugin:                p.Plugin,
 		UseH2C:                p.UseH2C,
+		DNSCacheTTL:           parseDuration(p.DNSCacheTTL),
 	}
 	if p.TLS != nil {
 		cfg.TLS = &TLS{

sd/dnssrv/subscriber.go

@@ -18,14 +18,25 @@ import (
 
 // Namespace is the key for the dns sd module
 const Namespace = "dns"
+const DefaultTTL = 30 * time.Second
+const MinTTL = time.Second
 
 // Register registers the dns sd subscriber factory under the name defined by Namespace
 func Register() error {
 	return sd.GetRegister().Register(Namespace, SubscriberFactory)
 }
 
 // TTL is the duration of the cached data
-var TTL = 30 * time.Second
+var TTL = DefaultTTL
+
+func SetTTL(d time.Duration) {
+	if d < MinTTL {
+		// in case the TTL is less than the minimum, we leave what is
+		// already set.
+		return
+	}
+	TTL = d
+}
 
 // DefaultLookup is the function used for the DNS resolution
 var DefaultLookup = net.LookupSRV