Accept TTL values of 0 in CachePolicy #26427

As per RFC spec https://www.ietf.org/rfc/rfc1035.txt (Section 3.2.1)

akka-actor/src/main/scala/akka/io/dns/CachePolicy.scala

@@ -15,7 +15,8 @@ object CachePolicy {
   case object Forever extends CachePolicy
 
   final class Ttl private (val value: FiniteDuration) extends CachePolicy {
-    if (value <= Duration.Zero) throw new IllegalArgumentException(s"TTL values must be a positive value.")
+    if (value < Duration.Zero)
+      throw new IllegalArgumentException(s"TTL values must be a positive value (zero included).")
     import akka.util.JavaDurationConverters._
     def getValue: java.time.Duration = value.asJava
 
@@ -32,10 +33,16 @@ object CachePolicy {
   object Ttl {
     def unapply(ttl: Ttl): Option[FiniteDuration] = Some(ttl.value)
     def fromPositive(value: FiniteDuration): Ttl = {
+      if (value <= Duration.Zero)
+        throw new IllegalArgumentException(
+          s"Positive TTL values must be a strictly positive value. Use Ttl.never for zero.")
       new Ttl(value)
     }
     def fromPositive(value: java.time.Duration): Ttl = fromPositive(value.asScala)
 
+    // DNS RFC states that zero values are interpreted to mean that the RR should not be cached
+    val never: Ttl = new Ttl(0.seconds)
+
     // There's places where only a Ttl makes sense (DNS RFC says TTL is a positive 32 bit integer)
     // but we know the value can be cached effectively forever (e.g. the Lookup name was the actual IP already)
     val effectivelyForever: Ttl = fromPositive(Int.MaxValue.seconds)

akka-actor/src/main/scala/akka/io/dns/DnsResourceRecords.scala

@@ -138,7 +138,10 @@ private[dns] object ResourceRecord {
     val recType = it.getShort
     val recClass = it.getShort
     // According to https://www.ietf.org/rfc/rfc1035.txt: "TTL: positive values of a signed 32 bit number."
-    val ttl = Ttl.fromPositive(it.getInt.seconds)
+    val ttl = (it.getInt: @switch) match {
+      case 0       => Ttl.never
+      case nonZero => Ttl.fromPositive(nonZero.seconds)
+    }
     val rdLength = it.getShort
     val data = it.clone().take(rdLength)
     it.drop(rdLength)