diff --git a/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy b/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy
index b29bb7a8cd3b7..399c4838d2b14 100644
--- a/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy
+++ b/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterConfiguration.groovy
@@ -68,7 +68,13 @@ class ClusterConfiguration {
      * In case of more than one node, this defaults to the number of nodes
      */
     @Input
-    Closure<Integer> minimumMasterNodes = { getNumNodes() > 1 ? getNumNodes() : -1 }
+    Closure<Integer> minimumMasterNodes = {
+        if (bwcVersion != null && bwcVersion.before("6.5.0")) {
+            return numNodes > 1 ? numNodes : -1
+        } else {
+            return numNodes > 1 ? numNodes.intdiv(2) + 1 : -1
+        }
+    }
 
     @Input
     String jvmArgs = "-Xms" + System.getProperty('tests.heap.size', '512m') +
@@ -104,6 +110,14 @@ class ClusterConfiguration {
         return seedNode.transportUri()
     }
 
+    /**
+     * A closure to call which returns a manually supplied list of unicast seed hosts.
+     */
+    @Input
+    Closure<List<String>> otherUnicastHostAddresses = {
+        Collections.emptyList()
+    }
+
     /**
      * A closure to call before the cluster is considered ready. The closure is passed the node info,
      * as well as a groovy AntBuilder, to enable running ant condition checks. The default wait
diff --git a/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy b/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy
index 52ed30880d374..6bc37d63b37b4 100644
--- a/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy
+++ b/buildSrc/src/main/groovy/org/elasticsearch/gradle/test/ClusterFormationTasks.groovy
@@ -122,8 +122,31 @@ class ClusterFormationTasks {
             }
             NodeInfo node = new NodeInfo(config, i, project, prefix, elasticsearchVersion, sharedDir)
             nodes.add(node)
-            Object dependsOn = startTasks.empty ? startDependencies : startTasks.get(0)
-            startTasks.add(configureNode(project, prefix, runner, dependsOn, node, config, distro, nodes.get(0)))
+            Closure<Map> writeConfigSetup
+            Object dependsOn
+            if (node.nodeVersion.onOrAfter("6.5.0")) {
+                writeConfigSetup = { Map esConfig ->
+                    // Don't force discovery provider if one is set by the test cluster specs already
+                    if (esConfig.containsKey('discovery.zen.hosts_provider') == false) {
+                        esConfig['discovery.zen.hosts_provider'] = 'file'
+                    }
+                    esConfig['discovery.zen.ping.unicast.hosts'] = []
+                    esConfig
+                }
+                dependsOn = startDependencies
+            } else {
+                dependsOn = startTasks.empty ? startDependencies : startTasks.get(0)
+                writeConfigSetup = { Map esConfig ->
+                    String unicastTransportUri = node.config.unicastTransportUri(nodes.get(0), node, project.createAntBuilder())
+                    if (unicastTransportUri == null) {
+                        esConfig['discovery.zen.ping.unicast.hosts'] = []
+                    } else {
+                        esConfig['discovery.zen.ping.unicast.hosts'] = "\"${unicastTransportUri}\""
+                    }
+                    esConfig
+                }
+            }
+            startTasks.add(configureNode(project, prefix, runner, dependsOn, node, config, distro, writeConfigSetup))
         }
 
         Task wait = configureWaitTask("${prefix}#wait", project, nodes, startTasks, config.nodeStartupWaitSeconds)
@@ -184,7 +207,7 @@ class ClusterFormationTasks {
      * @return a task which starts the node.
      */
     static Task configureNode(Project project, String prefix, Task runner, Object dependsOn, NodeInfo node, ClusterConfiguration config,
-                              Configuration distribution, NodeInfo seedNode) {
+                              Configuration distribution, Closure<Map> writeConfig) {
 
         // tasks are chained so their execution order is maintained
         Task setup = project.tasks.create(name: taskName(prefix, node, 'clean'), type: Delete, dependsOn: dependsOn) {
@@ -200,7 +223,7 @@ class ClusterFormationTasks {
         setup = configureCheckPreviousTask(taskName(prefix, node, 'checkPrevious'), project, setup, node)
         setup = configureStopTask(taskName(prefix, node, 'stopPrevious'), project, setup, node)
         setup = configureExtractTask(taskName(prefix, node, 'extract'), project, setup, node, distribution)
-        setup = configureWriteConfigTask(taskName(prefix, node, 'configure'), project, setup, node, seedNode)
+        setup = configureWriteConfigTask(taskName(prefix, node, 'configure'), project, setup, node, writeConfig)
         setup = configureCreateKeystoreTask(taskName(prefix, node, 'createKeystore'), project, setup, node)
         setup = configureAddKeystoreSettingTasks(prefix, project, setup, node)
         setup = configureAddKeystoreFileTasks(prefix, project, setup, node)
@@ -303,7 +326,7 @@ class ClusterFormationTasks {
     }
 
     /** Adds a task to write elasticsearch.yml for the given node configuration */
-    static Task configureWriteConfigTask(String name, Project project, Task setup, NodeInfo node, NodeInfo seedNode) {
+    static Task configureWriteConfigTask(String name, Project project, Task setup, NodeInfo node, Closure<Map> configFilter) {
         Map esConfig = [
                 'cluster.name'                 : node.clusterName,
                 'node.name'                    : "node-" + node.nodeNum,
@@ -317,7 +340,7 @@ class ClusterFormationTasks {
         if (minimumMasterNodes > 0) {
             esConfig['discovery.zen.minimum_master_nodes'] = minimumMasterNodes
         }
-        if (node.config.numNodes > 1) {
+        if (minimumMasterNodes > 1) {
             // don't wait for state.. just start up quickly
             // this will also allow new and old nodes in the BWC case to become the master
             esConfig['discovery.initial_state_timeout'] = '0s'
@@ -347,10 +370,7 @@ class ClusterFormationTasks {
 
         Task writeConfig = project.tasks.create(name: name, type: DefaultTask, dependsOn: setup)
         writeConfig.doFirst {
-            String unicastTransportUri = node.config.unicastTransportUri(seedNode, node, project.ant)
-            if (unicastTransportUri != null) {
-                esConfig['discovery.zen.ping.unicast.hosts'] = "\"${unicastTransportUri}\""
-            }
+            esConfig = configFilter.call(esConfig)
             File configFile = new File(node.pathConf, 'elasticsearch.yml')
             logger.info("Configuring ${configFile}")
             configFile.setText(esConfig.collect { key, value -> "${key}: ${value}" }.join('\n'), 'UTF-8')
@@ -681,6 +701,20 @@ class ClusterFormationTasks {
     static Task configureWaitTask(String name, Project project, List<NodeInfo> nodes, List<Task> startTasks, int waitSeconds) {
         Task wait = project.tasks.create(name: name, dependsOn: startTasks)
         wait.doLast {
+
+            Collection<String> unicastHosts = new HashSet<>()
+            nodes.forEach { node ->
+                unicastHosts.addAll(node.config.otherUnicastHostAddresses.call())
+                String unicastHost = node.config.unicastTransportUri(node, null, project.createAntBuilder())
+                if (unicastHost != null) {
+                    unicastHosts.addAll(Arrays.asList(unicastHost.split(",")))
+                }
+            }
+            String unicastHostsTxt = String.join("\n", unicastHosts)
+            nodes.forEach { node ->
+                node.pathConf.toPath().resolve("unicast_hosts.txt").setText(unicastHostsTxt)
+            }
+
             ant.waitfor(maxwait: "${waitSeconds}", maxwaitunit: 'second', checkevery: '500', checkeveryunit: 'millisecond', timeoutproperty: "failed${name}") {
                 or {
                     for (NodeInfo node : nodes) {
@@ -867,9 +901,10 @@ class ClusterFormationTasks {
                 outputPrintStream: outputStream,
                 messageOutputLevel: org.apache.tools.ant.Project.MSG_INFO)
 
-        project.ant.project.addBuildListener(listener)
-        Object retVal = command(project.ant)
-        project.ant.project.removeBuildListener(listener)
+        AntBuilder ant = project.createAntBuilder()
+        ant.project.addBuildListener(listener)
+        Object retVal = command(ant)
+        ant.project.removeBuildListener(listener)
         return retVal
     }
 
diff --git a/qa/rolling-upgrade/build.gradle b/qa/rolling-upgrade/build.gradle
index be8269ae941da..42c3c115d2f36 100644
--- a/qa/rolling-upgrade/build.gradle
+++ b/qa/rolling-upgrade/build.gradle
@@ -56,7 +56,6 @@ for (Version version : bwcVersions.wireCompatible) {
     mustRunAfter(precommit)
   }
 
-  Object extension = extensions.findByName("${baseName}#oldClusterTestCluster")
   configure(extensions.findByName("${baseName}#oldClusterTestCluster")) {
     bwcVersion = version
     numBwcNodes = 3
@@ -76,12 +75,12 @@ for (Version version : bwcVersions.wireCompatible) {
     systemProperty 'tests.upgrade_from_version', version.toString().replace('-SNAPSHOT', '')
   }
 
-  Closure configureUpgradeCluster = {String name, Task lastRunner, int stopNode, Closure unicastSeed ->
+  Closure configureUpgradeCluster = {String name, Task lastRunner, int stopNode, Closure getOtherUnicastHostAddresses ->
     configure(extensions.findByName("${baseName}#${name}")) {
       dependsOn lastRunner, "${baseName}#oldClusterTestCluster#node${stopNode}.stop"
       clusterName = 'rolling-upgrade'
-      unicastTransportUri = { seedNode, node, ant -> unicastSeed() }
-      minimumMasterNodes = { 3 }
+      otherUnicastHostAddresses = { getOtherUnicastHostAddresses() }
+      minimumMasterNodes = { 2 }
       /* Override the data directory so the new node always gets the node we
       * just stopped's data directory. */
       dataDir = { nodeNumber -> oldClusterTest.nodes[stopNode].dataDir }
@@ -95,7 +94,7 @@ for (Version version : bwcVersions.wireCompatible) {
 
   configureUpgradeCluster("oneThirdUpgradedTestCluster", oldClusterTestRunner, 0,
     // Use all running nodes as seed nodes so there is no race between pinging and the tests
-    { oldClusterTest.nodes.get(1).transportUri() + ',' + oldClusterTest.nodes.get(2).transportUri() })
+    { [oldClusterTest.nodes.get(1).transportUri(), oldClusterTest.nodes.get(2).transportUri()] })
 
   Task oneThirdUpgradedTestRunner = tasks.getByName("${baseName}#oneThirdUpgradedTestRunner")
   oneThirdUpgradedTestRunner.configure {
@@ -109,7 +108,7 @@ for (Version version : bwcVersions.wireCompatible) {
 
   configureUpgradeCluster("twoThirdsUpgradedTestCluster", oneThirdUpgradedTestRunner, 1,
     // Use all running nodes as seed nodes so there is no race between pinging and the tests
-    { oldClusterTest.nodes.get(2).transportUri() + ',' + oneThirdUpgradedTest.nodes.get(0).transportUri() })
+    { [oldClusterTest.nodes.get(2).transportUri(), oneThirdUpgradedTest.nodes.get(0).transportUri()] })
 
   Task twoThirdsUpgradedTestRunner = tasks.getByName("${baseName}#twoThirdsUpgradedTestRunner")
   twoThirdsUpgradedTestRunner.configure {
@@ -123,7 +122,7 @@ for (Version version : bwcVersions.wireCompatible) {
 
   configureUpgradeCluster("upgradedClusterTestCluster", twoThirdsUpgradedTestRunner, 2,
     // Use all running nodes as seed nodes so there is no race between pinging and the tests
-    { oneThirdUpgradedTest.nodes.get(0).transportUri() + ',' + twoThirdsUpgradedTest.nodes.get(0).transportUri() })
+    { [oneThirdUpgradedTest.nodes.get(0).transportUri(), twoThirdsUpgradedTest.nodes.get(0).transportUri()] })
 
   Task upgradedClusterTestRunner = tasks.getByName("${baseName}#upgradedClusterTestRunner")
   upgradedClusterTestRunner.configure {
diff --git a/x-pack/qa/rolling-upgrade/build.gradle b/x-pack/qa/rolling-upgrade/build.gradle
index 6e482b29415e8..f07aa7d7a7a8b 100644
--- a/x-pack/qa/rolling-upgrade/build.gradle
+++ b/x-pack/qa/rolling-upgrade/build.gradle
@@ -116,7 +116,6 @@ subprojects {
       mustRunAfter(precommit)
     }
 
-    Object extension = extensions.findByName("${baseName}#oldClusterTestCluster")
     configure(extensions.findByName("${baseName}#oldClusterTestCluster")) {
       dependsOn copyTestNodeKeystore
       if (version.before('6.3.0')) {
@@ -131,7 +130,6 @@ subprojects {
       bwcVersion = version
       numBwcNodes = 3
       numNodes = 3
-      minimumMasterNodes = { 3 }
       clusterName = 'rolling-upgrade'
       waitCondition = waitWithAuth
       setting 'xpack.monitoring.exporters._http.type', 'http'
@@ -184,13 +182,13 @@ subprojects {
       systemProperty 'tests.upgrade_from_version', version.toString().replace('-SNAPSHOT', '')
     }
 
-    Closure configureUpgradeCluster = {String name, Task lastRunner, int stopNode, Closure unicastSeed ->
+    Closure configureUpgradeCluster = {String name, Task lastRunner, int stopNode, Closure getOtherUnicastHostAddresses ->
       configure(extensions.findByName("${baseName}#${name}")) {
         dependsOn lastRunner, "${baseName}#oldClusterTestCluster#node${stopNode}.stop"
         setupCommand 'setupTestUser', 'bin/elasticsearch-users', 'useradd', 'test_user', '-p', 'x-pack-test-password', '-r', 'superuser'
         clusterName = 'rolling-upgrade'
-        unicastTransportUri = { seedNode, node, ant -> unicastSeed() }
-        minimumMasterNodes = { 3 }
+        otherUnicastHostAddresses = { getOtherUnicastHostAddresses() }
+        minimumMasterNodes = { 2 }
         /* Override the data directory so the new node always gets the node we
         * just stopped's data directory. */
         dataDir = { nodeNumber -> oldClusterTest.nodes[stopNode].dataDir }
@@ -226,7 +224,7 @@ subprojects {
 
     configureUpgradeCluster("oneThirdUpgradedTestCluster", oldClusterTestRunner, 0,
       // Use all running nodes as seed nodes so there is no race between pinging and the tests
-      { oldClusterTest.nodes.get(1).transportUri() + ',' + oldClusterTest.nodes.get(2).transportUri() })
+      { [oldClusterTest.nodes.get(1).transportUri(), oldClusterTest.nodes.get(2).transportUri()] })
 
     Task oneThirdUpgradedTestRunner = tasks.getByName("${baseName}#oneThirdUpgradedTestRunner")
     oneThirdUpgradedTestRunner.configure {
@@ -246,7 +244,7 @@ subprojects {
 
     configureUpgradeCluster("twoThirdsUpgradedTestCluster", oneThirdUpgradedTestRunner, 1,
       // Use all running nodes as seed nodes so there is no race between pinging and the tests
-      { oldClusterTest.nodes.get(2).transportUri() + ',' + oneThirdUpgradedTest.nodes.get(0).transportUri() })
+      { [oldClusterTest.nodes.get(2).transportUri(), oneThirdUpgradedTest.nodes.get(0).transportUri()] })
 
     Task twoThirdsUpgradedTestRunner = tasks.getByName("${baseName}#twoThirdsUpgradedTestRunner")
     twoThirdsUpgradedTestRunner.configure {
@@ -260,7 +258,7 @@ subprojects {
 
     configureUpgradeCluster("upgradedClusterTestCluster", twoThirdsUpgradedTestRunner, 2,
       // Use all running nodes as seed nodes so there is no race between pinging and the tests
-      { oneThirdUpgradedTest.nodes.get(0).transportUri() + ',' + twoThirdsUpgradedTest.nodes.get(0).transportUri() })
+      { [oneThirdUpgradedTest.nodes.get(0).transportUri(), twoThirdsUpgradedTest.nodes.get(0).transportUri()] })
 
     Task upgradedClusterTestRunner = tasks.getByName("${baseName}#upgradedClusterTestRunner")
     upgradedClusterTestRunner.configure {