diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 1b282a799d..cc773b6382 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -9,6 +9,7 @@ You're always welcome to submit your PR straight away and start the discussion (
 If you have questions about the contribution process or discuss specific issues, please interact with the community using the following resources.
 
 - [GitHub discussions](https://github.com/apache/incubator-pekko/discussions): for questions and general discussion.
+- [Pekko user mailing list](https://lists.apache.org/list.html?user@pekko.apache.org): for Pekko usage discussions.
 - [Pekko dev mailing list](https://lists.apache.org/list.html?dev@pekko.apache.org): for Pekko development discussions.
 - [GitHub issues](https://github.com/apache/incubator-pekko/issues): for bug reports and feature requests. Please search the existing issues before creating new ones. If you are unsure whether you have found a bug, consider asking in GitHub discussions or the mailing list first.
 
diff --git a/actor-testkit-typed/src/main/resources/reference.conf b/actor-testkit-typed/src/main/resources/reference.conf
index 1da447d66e..3c22d322dc 100644
--- a/actor-testkit-typed/src/main/resources/reference.conf
+++ b/actor-testkit-typed/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-##################################################
+# SPDX-License-Identifier: Apache-2.0
+
+###################################################
 # Pekko Actor Typed Testkit Reference Config File #
-##################################################
+###################################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/actor-testkit-typed/src/test/resources/application-test.conf b/actor-testkit-typed/src/test/resources/application-test.conf
index b52f74104b..f8672895aa 100644
--- a/actor-testkit-typed/src/test/resources/application-test.conf
+++ b/actor-testkit-typed/src/test/resources/application-test.conf
@@ -1,2 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
+
 # used by ActorTestKitSpec
 test.from-application-test = yes
diff --git a/actor-testkit-typed/src/test/resources/application.conf b/actor-testkit-typed/src/test/resources/application.conf
index c557fa2abc..594856dc47 100644
--- a/actor-testkit-typed/src/test/resources/application.conf
+++ b/actor-testkit-typed/src/test/resources/application.conf
@@ -1,2 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
+
 # used by ActorTestKitSpec
 test.from-application = yes
diff --git a/actor-tests/src/test/resources/reference.conf b/actor-tests/src/test/resources/reference.conf
index 211720948f..ec176bb311 100644
--- a/actor-tests/src/test/resources/reference.conf
+++ b/actor-tests/src/test/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko {
   # for the org.apache.pekko.actor.ExtensionSpec
   library-extensions += "org.apache.pekko.actor.InstanceCountingExtension"
diff --git a/actor-typed-tests/src/test/resources/mailbox-config-sample.conf b/actor-typed-tests/src/test/resources/mailbox-config-sample.conf
index ff2a831288..e3ba94a0c2 100644
--- a/actor-typed-tests/src/test/resources/mailbox-config-sample.conf
+++ b/actor-typed-tests/src/test/resources/mailbox-config-sample.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 my-app {
   my-special-mailbox {
     mailbox-type = "org.apache.pekko.dispatch.SingleConsumerOnlyUnboundedMailbox"
diff --git a/actor-typed/src/main/resources/reference.conf b/actor-typed/src/main/resources/reference.conf
index 4afc8bb7d3..d564d6070b 100644
--- a/actor-typed/src/main/resources/reference.conf
+++ b/actor-typed/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.actor.typed {
 
   # List FQCN of `org.apache.pekko.actor.typed.ExtensionId`s which shall be loaded at actor system startup.
diff --git a/actor/src/main/resources/reference.conf b/actor/src/main/resources/reference.conf
index 2cfe4fdf98..53212baf92 100644
--- a/actor/src/main/resources/reference.conf
+++ b/actor/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-####################################
+# SPDX-License-Identifier: Apache-2.0
+
+#####################################
 # Pekko Actor Reference Config File #
-####################################
+########################3############
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/cluster-metrics/src/main/resources/reference.conf b/cluster-metrics/src/main/resources/reference.conf
index 4a13bbda14..510746d3fc 100644
--- a/cluster-metrics/src/main/resources/reference.conf
+++ b/cluster-metrics/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-##############################################
+# SPDX-License-Identifier: Apache-2.0
+
+###############################################
 # Pekko Cluster Metrics Reference Config File #
-##############################################
+###############################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits in your application.conf in order to override these settings.
diff --git a/cluster-sharding-typed/src/main/resources/reference.conf b/cluster-sharding-typed/src/main/resources/reference.conf
index 1fa23e5cab..0d83b70871 100644
--- a/cluster-sharding-typed/src/main/resources/reference.conf
+++ b/cluster-sharding-typed/src/main/resources/reference.conf
@@ -1,3 +1,4 @@
+# SPDX-License-Identifier: Apache-2.0
 
 # //#sharding-ext-config
 # //#number-of-shards
diff --git a/cluster-sharding/src/main/resources/reference.conf b/cluster-sharding/src/main/resources/reference.conf
index 13c625ee67..9ca318a365 100644
--- a/cluster-sharding/src/main/resources/reference.conf
+++ b/cluster-sharding/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-###############################################
+# SPDX-License-Identifier: Apache-2.0
+
+################################################
 # Pekko Cluster Sharding Reference Config File #
-###############################################
+#######################3########################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/cluster-sharding/src/test/resources/adaptivity-trace.conf b/cluster-sharding/src/test/resources/adaptivity-trace.conf
index 00baa07c23..a869217f29 100644
--- a/cluster-sharding/src/test/resources/adaptivity-trace.conf
+++ b/cluster-sharding/src/test/resources/adaptivity-trace.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run a joined trace for testing adaptivity, switching between a recency-biased R3 Corda trace and
 # a frequency-biased LIRS loop trace.
diff --git a/cluster-sharding/src/test/resources/arc-trace-database.conf b/cluster-sharding/src/test/resources/arc-trace-database.conf
index ceb1778f26..76d81f81d3 100644
--- a/cluster-sharding/src/test/resources/arc-trace-database.conf
+++ b/cluster-sharding/src/test/resources/arc-trace-database.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the "database" trace (DS1) from the authors of the ARC algorithm.
 #
diff --git a/cluster-sharding/src/test/resources/arc-trace-search.conf b/cluster-sharding/src/test/resources/arc-trace-search.conf
index eee0a30594..fca0bcf227 100644
--- a/cluster-sharding/src/test/resources/arc-trace-search.conf
+++ b/cluster-sharding/src/test/resources/arc-trace-search.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the merged "search" trace (MergeS) from the authors of the ARC algorithm.
 #
diff --git a/cluster-sharding/src/test/resources/lirs-trace-glimpse.conf b/cluster-sharding/src/test/resources/lirs-trace-glimpse.conf
index 8993d434f1..44701c3445 100644
--- a/cluster-sharding/src/test/resources/lirs-trace-glimpse.conf
+++ b/cluster-sharding/src/test/resources/lirs-trace-glimpse.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the "glimpse" trace (gli.trace) from the authors of the LIRS algorithm.
 #
diff --git a/cluster-sharding/src/test/resources/lirs-trace-multi.conf b/cluster-sharding/src/test/resources/lirs-trace-multi.conf
index c04b49781e..fe8feb6f36 100644
--- a/cluster-sharding/src/test/resources/lirs-trace-multi.conf
+++ b/cluster-sharding/src/test/resources/lirs-trace-multi.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the "multi3" trace (multi3.trace) from the authors of the LIRS algorithm.
 #
diff --git a/cluster-sharding/src/test/resources/lirs-trace-postgres.conf b/cluster-sharding/src/test/resources/lirs-trace-postgres.conf
index 5010c0f579..415b01d4cf 100644
--- a/cluster-sharding/src/test/resources/lirs-trace-postgres.conf
+++ b/cluster-sharding/src/test/resources/lirs-trace-postgres.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the "postgres" trace (ps.trace) from the authors of the LIRS algorithm.
 #
diff --git a/cluster-sharding/src/test/resources/lirs2-trace-w106.conf b/cluster-sharding/src/test/resources/lirs2-trace-w106.conf
index 2f43db116f..3e608eb9b2 100644
--- a/cluster-sharding/src/test/resources/lirs2-trace-w106.conf
+++ b/cluster-sharding/src/test/resources/lirs2-trace-w106.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the "w106" binary trace (w106.trace) from the authors of the LIRS2 algorithm.
 #
diff --git a/cluster-sharding/src/test/resources/reference.conf b/cluster-sharding/src/test/resources/reference.conf
index d43bd1a978..efa3d6e733 100644
--- a/cluster-sharding/src/test/resources/reference.conf
+++ b/cluster-sharding/src/test/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.cluster.sharding {
   passivation.simulator {
     runs = []
diff --git a/cluster-sharding/src/test/resources/synthetic-loop.conf b/cluster-sharding/src/test/resources/synthetic-loop.conf
index e39868228a..f7164ffc15 100644
--- a/cluster-sharding/src/test/resources/synthetic-loop.conf
+++ b/cluster-sharding/src/test/resources/synthetic-loop.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run with synthetically generated access events with a looping scan through ids.
 #
diff --git a/cluster-sharding/src/test/resources/synthetic-zipfian.conf b/cluster-sharding/src/test/resources/synthetic-zipfian.conf
index abbdfc2567..7d79cb7278 100644
--- a/cluster-sharding/src/test/resources/synthetic-zipfian.conf
+++ b/cluster-sharding/src/test/resources/synthetic-zipfian.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run with synthetically generated access events with a scrambled zipfian distribution.
 #
diff --git a/cluster-sharding/src/test/resources/text-moby-dick.conf b/cluster-sharding/src/test/resources/text-moby-dick.conf
index 49c1f36a69..84631fafce 100644
--- a/cluster-sharding/src/test/resources/text-moby-dick.conf
+++ b/cluster-sharding/src/test/resources/text-moby-dick.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run against the text of Moby Dick, for a natural zipfian-like distribution.
 #
diff --git a/cluster-sharding/src/test/resources/wikipedia-trace-2018.conf b/cluster-sharding/src/test/resources/wikipedia-trace-2018.conf
index 712f7c5b0f..9edeeee7a0 100644
--- a/cluster-sharding/src/test/resources/wikipedia-trace-2018.conf
+++ b/cluster-sharding/src/test/resources/wikipedia-trace-2018.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #
 # Run the 14-day Wikipedia trace from 2018 (wiki2018.tr) from the LRB simulator.
 #
diff --git a/cluster-tools/src/main/resources/reference.conf b/cluster-tools/src/main/resources/reference.conf
index b6cc8e9388..1548c9a823 100644
--- a/cluster-tools/src/main/resources/reference.conf
+++ b/cluster-tools/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-############################################
+# SPDX-License-Identifier: Apache-2.0
+
+#############################################
 # Pekko Cluster Tools Reference Config File #
-############################################
+#############################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/cluster-typed/src/main/resources/reference.conf b/cluster-typed/src/main/resources/reference.conf
index b49cf3a699..ddfdd555f5 100644
--- a/cluster-typed/src/main/resources/reference.conf
+++ b/cluster-typed/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-############################################
+# SPDX-License-Identifier: Apache-2.0
+
+#############################################
 # Pekko Cluster Typed Reference Config File #
-############################################
+#############################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/cluster/src/main/resources/reference.conf b/cluster/src/main/resources/reference.conf
index a5793c2737..a0e403dccd 100644
--- a/cluster/src/main/resources/reference.conf
+++ b/cluster/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-######################################
+# SPDX-License-Identifier: Apache-2.0
+
+#######################################
 # Pekko Cluster Reference Config File #
-######################################
+#######################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/coordination/src/main/resources/reference.conf b/coordination/src/main/resources/reference.conf
index 502c2554d1..9e151d129d 100644
--- a/coordination/src/main/resources/reference.conf
+++ b/coordination/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.coordination {
 
   # Defaults for any lease implementation that doesn't include these properties
diff --git a/discovery/src/main/resources/reference.conf b/discovery/src/main/resources/reference.conf
index 00a78f456b..05f1fc2ae9 100644
--- a/discovery/src/main/resources/reference.conf
+++ b/discovery/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 ######################################################
 # Pekko Discovery Config                              #
 ######################################################
diff --git a/distributed-data/src/main/resources/reference.conf b/distributed-data/src/main/resources/reference.conf
index fc8a98b825..f7a5c5b5ad 100644
--- a/distributed-data/src/main/resources/reference.conf
+++ b/distributed-data/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-##############################################
+# SPDX-License-Identifier: Apache-2.0
+
+###############################################
 # Pekko Distributed DataReference Config File #
-##############################################
+###############################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/docs/src/test/resources/application.conf b/docs/src/test/resources/application.conf
index cadf8a256b..c5843ebfb7 100644
--- a/docs/src/test/resources/application.conf
+++ b/docs/src/test/resources/application.conf
@@ -1 +1,3 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.loggers = ["org.apache.pekko.testkit.TestEventListener"]
diff --git a/multi-node-testkit/src/main/resources/reference.conf b/multi-node-testkit/src/main/resources/reference.conf
index c2d1401093..f78bf8c8ca 100644
--- a/multi-node-testkit/src/main/resources/reference.conf
+++ b/multi-node-testkit/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-#############################################
+# SPDX-License-Identifier: Apache-2.0
+
+##############################################
 # Pekko Remote Testing Reference Config File #
-#############################################
+##############################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/persistence-query/src/main/resources/reference.conf b/persistence-query/src/main/resources/reference.conf
index 2a0a9809a0..2dfff044c1 100644
--- a/persistence-query/src/main/resources/reference.conf
+++ b/persistence-query/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #######################################################
 # Pekko Persistence Query Reference Configuration File #
 #######################################################
diff --git a/persistence-testkit/src/main/resources/reference.conf b/persistence-testkit/src/main/resources/reference.conf
index d43bc305a0..9e94ad060e 100644
--- a/persistence-testkit/src/main/resources/reference.conf
+++ b/persistence-testkit/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-##################################################
+# SPDX-License-Identifier: Apache-2.0
+
+###################################################
 # Pekko Persistence Testkit Reference Config File #
-##################################################
+###################################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/persistence-testkit/src/test/resources/application.conf b/persistence-testkit/src/test/resources/application.conf
index 0a0043059b..0cddb1c841 100644
--- a/persistence-testkit/src/test/resources/application.conf
+++ b/persistence-testkit/src/test/resources/application.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.persistence.testkit {
   events {
     assert-timeout = 500ms
diff --git a/persistence-typed/src/main/resources/reference.conf b/persistence-typed/src/main/resources/reference.conf
index 5f2c6d3564..1d0b7c4237 100644
--- a/persistence-typed/src/main/resources/reference.conf
+++ b/persistence-typed/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.actor {
 
   serialization-identifiers."org.apache.pekko.persistence.typed.serialization.ReplicatedEventSourcingSerializer" = 40
diff --git a/persistence/src/main/resources/reference.conf b/persistence/src/main/resources/reference.conf
index e332cf913e..435341de45 100644
--- a/persistence/src/main/resources/reference.conf
+++ b/persistence/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 ###########################################################
 # Pekko Persistence Extension Reference Configuration File #
 ###########################################################
diff --git a/project/CopyrightHeader.scala b/project/CopyrightHeader.scala
index 4b58ada295..6d2012a1c5 100644
--- a/project/CopyrightHeader.scala
+++ b/project/CopyrightHeader.scala
@@ -37,7 +37,17 @@ trait CopyrightHeader extends AutoPlugin {
             HeaderFileType("template") -> cStyleComment)))
     }
 
-  override def projectSettings: Seq[Def.Setting[_]] = headerMappingSettings
+  private def confHeaderMappingSettings: Seq[Def.Setting[_]] =
+    Seq(Compile, Test).flatMap { config =>
+      inConfig(config)(
+        Seq(
+          headerLicense := Some(HeaderLicense.Custom(apacheSpdxHeader)),
+          headerMappings := headerMappings.value ++ Map(
+            HeaderFileType.conf -> hashLineComment)))
+    }
+
+  override def projectSettings: Seq[Def.Setting[_]] =
+    Def.settings(headerMappingSettings, confHeaderMappingSettings, additional)
 
   def additional: Seq[Def.Setting[_]] =
     Def.settings(Compile / compile := {
@@ -61,41 +71,58 @@ trait CopyrightHeader extends AutoPlugin {
       |This file is part of the Apache Pekko project, derived from Akka.
       |""".stripMargin
 
+  val apacheSpdxHeader: String = "SPDX-License-Identifier: Apache-2.0"
+
   val cStyleComment = HeaderCommentStyle.cStyleBlockComment.copy(commentCreator = new CommentCreator() {
 
     override def apply(text: String, existingText: Option[String]): String = {
       val formatted = existingText match {
-        case Some(existedText) if isValidCopyrightAnnotated(existedText) =>
-          existedText
-        case Some(existedText) if isOnlyLightbendOrEpflCopyrightAnnotated(existedText) =>
-          HeaderCommentStyle.cStyleBlockComment.commentCreator(text, existingText) + NewLine * 2 + existedText
-        case Some(existedText) =>
-          throw new IllegalStateException(s"Unable to detect copyright for header:[${existedText}]")
+        case Some(currentText) if isValidCopyrightAnnotated(currentText) =>
+          currentText
+        case Some(currentText) if isOnlyLightbendOrEpflCopyrightAnnotated(currentText) =>
+          HeaderCommentStyle.cStyleBlockComment.commentCreator(text, existingText) + NewLine * 2 + currentText
+        case Some(currentText) =>
+          throw new IllegalStateException(s"Unable to detect copyright for header:[${currentText}]")
         case None =>
           HeaderCommentStyle.cStyleBlockComment.commentCreator(text, existingText)
       }
       formatted.trim
     }
-
-    private def isApacheCopyrighted(text: String): Boolean =
-      StringUtils.containsIgnoreCase(text, "licensed to the apache software foundation (asf)") ||
-      StringUtils.containsIgnoreCase(text, "www.apache.org/licenses/license-2.0")
-
-    private def isLAMPCopyrighted(text: String): Boolean =
-      StringUtils.containsIgnoreCase(text, "lamp/epfl")
-
-    private def isLightbendCopyrighted(text: String): Boolean =
-      StringUtils.containsIgnoreCase(text, "lightbend inc.")
-
-    private def isValidCopyrightAnnotated(text: String): Boolean = {
-      isApacheCopyrighted(text)
-    }
-
-    private def isOnlyLightbendOrEpflCopyrightAnnotated(text: String): Boolean = {
-      (isLightbendCopyrighted(text) || isLAMPCopyrighted(text)) && !isApacheCopyrighted(text)
-    }
-
   })
+
+  val hashLineComment = HeaderCommentStyle.hashLineComment.copy(commentCreator = new CommentCreator() {
+
+    override def apply(text: String, existingText: Option[String]): String = {
+      val formatted = existingText match {
+        case Some(currentText) if isApacheCopyrighted(currentText) =>
+          currentText
+        case Some(currentText) =>
+          HeaderCommentStyle.hashLineComment.commentCreator(text, existingText) + NewLine * 2 + currentText
+        case None =>
+          HeaderCommentStyle.hashLineComment.commentCreator(text, existingText)
+      }
+      formatted.trim
+    }
+  })
+
+  private def isApacheCopyrighted(text: String): Boolean =
+    StringUtils.containsIgnoreCase(text, "licensed to the apache software foundation (asf)") ||
+    StringUtils.containsIgnoreCase(text, "www.apache.org/licenses/license-2.0") ||
+    StringUtils.contains(text, "Apache-2.0")
+
+  private def isLAMPCopyrighted(text: String): Boolean =
+    StringUtils.containsIgnoreCase(text, "lamp/epfl")
+
+  private def isLightbendCopyrighted(text: String): Boolean =
+    StringUtils.containsIgnoreCase(text, "lightbend inc.")
+
+  private def isValidCopyrightAnnotated(text: String): Boolean = {
+    isApacheCopyrighted(text)
+  }
+
+  private def isOnlyLightbendOrEpflCopyrightAnnotated(text: String): Boolean = {
+    (isLightbendCopyrighted(text) || isLAMPCopyrighted(text)) && !isApacheCopyrighted(text)
+  }
 }
 
 object CopyrightHeader extends CopyrightHeader
diff --git a/remote/src/main/resources/reference.conf b/remote/src/main/resources/reference.conf
index dbaab55fa7..1aec87aee8 100644
--- a/remote/src/main/resources/reference.conf
+++ b/remote/src/main/resources/reference.conf
@@ -1,7 +1,9 @@
+# SPDX-License-Identifier: Apache-2.0
+
 #//#shared
-#####################################
+######################################
 # Pekko Remote Reference Config File #
-#####################################
+######################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/serialization-jackson/src/main/resources/reference.conf b/serialization-jackson/src/main/resources/reference.conf
index 6a070923c9..7877a2bdcc 100644
--- a/serialization-jackson/src/main/resources/reference.conf
+++ b/serialization-jackson/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-##########################################
+# SPDX-License-Identifier: Apache-2.0
+
+###########################################
 # Pekko Serialization Jackson Config File #
-##########################################
+###########################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/serialization-jackson/src/test/resources/reference.conf b/serialization-jackson/src/test/resources/reference.conf
index 9964158067..66ebe312d1 100644
--- a/serialization-jackson/src/test/resources/reference.conf
+++ b/serialization-jackson/src/test/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko {
   actor {
     serialization-bindings {
diff --git a/stream-testkit/src/main/resources/reference.conf b/stream-testkit/src/main/resources/reference.conf
index 74e194fcf3..dc46e379dd 100644
--- a/stream-testkit/src/main/resources/reference.conf
+++ b/stream-testkit/src/main/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko.stream.testkit {
   all-stages-stopped-timeout = 5 s
 }
diff --git a/stream-testkit/src/test/resources/reference.conf b/stream-testkit/src/test/resources/reference.conf
index 5712cfbe60..990b427747 100644
--- a/stream-testkit/src/test/resources/reference.conf
+++ b/stream-testkit/src/test/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 # The StreamTestDefaultMailbox verifies that stream actors are using the dispatcher defined in ActorMaterializerSettings.
 #
 # All stream tests should use the dedicated `pekko.test.stream-dispatcher` or disable this validation by defining:
diff --git a/stream-tests/src/test/resources/reference.conf b/stream-tests/src/test/resources/reference.conf
index 0a6c83f766..b493f66610 100644
--- a/stream-tests/src/test/resources/reference.conf
+++ b/stream-tests/src/test/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko {
   loggers = ["org.apache.pekko.testkit.TestEventListener"]
   actor {
diff --git a/stream/src/main/resources/reference.conf b/stream/src/main/resources/reference.conf
index 445e1fb653..acf624baa9 100644
--- a/stream/src/main/resources/reference.conf
+++ b/stream/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-#####################################
+# SPDX-License-Identifier: Apache-2.0
+
+######################################
 # Pekko Stream Reference Config File #
-#####################################
+######################################
 
 # eager creation of the system wide materializer
 pekko.library-extensions += "org.apache.pekko.stream.SystemMaterializer$"
diff --git a/testkit/src/main/resources/reference.conf b/testkit/src/main/resources/reference.conf
index 3d52d83590..8fa48deff0 100644
--- a/testkit/src/main/resources/reference.conf
+++ b/testkit/src/main/resources/reference.conf
@@ -1,6 +1,8 @@
-######################################
+# SPDX-License-Identifier: Apache-2.0
+
+#######################################
 # Pekko Testkit Reference Config File #
-######################################
+#######################################
 
 # This is the reference config file that contains all the default settings.
 # Make your edits/overrides in your application.conf.
diff --git a/testkit/src/test/resources/reference.conf b/testkit/src/test/resources/reference.conf
index b85ab5e054..f5a7a86f23 100644
--- a/testkit/src/test/resources/reference.conf
+++ b/testkit/src/test/resources/reference.conf
@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: Apache-2.0
+
 pekko {
   # Configures MetricsKit
   test.metrics {