feat(bigtable): populate reasons on why direct access was not accessible

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/dp/DirectAccessInvestigator.java

@@ -18,6 +18,7 @@
 
 import com.google.api.core.InternalApi;
 import com.google.cloud.bigtable.data.v2.internal.csm.tracers.DirectPathCompatibleTracer;
+import java.net.InetAddress;
 import java.util.logging.Level;
 import java.util.logging.Logger;
 import javax.annotation.Nullable;
@@ -52,8 +53,94 @@ public String getValue() {
   public static void investigateAndReport(
       DirectPathCompatibleTracer tracer, @Nullable Throwable originalError) {
     try {
-      // TODO: Implement checks in a future PR.
-      // For now, default to returning "unknown".
+      // 1. Perform GCE Check
+      if (!GCECheck.isRunningOnGCP()) {
+        recordAndLog(
+            tracer,
+            FailureReason.NOT_IN_GCP,
+            "Direct Access investigation: Not running on GCP.",
+            originalError);
+        return;
+      }
+
+      // 2. Perform MetadataServer Check
+      if (!MetadataServer.isReachable()) {
+        recordAndLog(
+            tracer,
+            FailureReason.METADATA_UNREACHABLE,
+            "Direct Access investigation: Metadata server unreachable.",
+            originalError);
+        return;
+      }
+
+      // 3. Check for Assigned IPs
+      InetAddress ipv4 = MetadataServer.getIPv4();
+      InetAddress ipv6 = MetadataServer.getIPv6();
+      if (ipv4 == null && ipv6 == null) {
+        recordAndLog(
+            tracer,
+            FailureReason.NO_IP_ASSIGNED,
+            "Direct Access investigation: No IP assigned.",
+            originalError);
+        return;
+      }
+
+      // 4. Perform Loopback Check
+      boolean loopbackUp = false;
+      try {
+        loopbackUp = LoopBackInterface.isUp();
+      } catch (Exception e) {
+        LOG.log(Level.FINE, "Exception while checking loopback interfaces", e);
+      }
+
+      if (!loopbackUp) {
+        recordAndLog(
+            tracer,
+            FailureReason.LOOPBACK_DOWN,
+            "Direct Access investigation: Loopback misconfigured.",
+            originalError);
+        return;
+      }
+
+      // 5. IPv4 Loopback Address Configuration Check
+      if (ipv4 != null) {
+        boolean hasIpv4Loopback = false;
+        try {
+          hasIpv4Loopback = LoopBackInterface.hasLocalIpv4Loopback();
+        } catch (Exception e) {
+          LOG.log(Level.FINE, "Failed to check local IPv4 loopback address", e);
+        }
+
+        if (!hasIpv4Loopback) {
+          recordAndLog(
+              tracer,
+              FailureReason.LOOPBACK_V4_MISSING,
+              "Direct Access investigation: IPv4 loopback missing.",
+              originalError);
+          return;
+        }
+      }
+
+      // 6. IPv6 Loopback Address Configuration Check
+      if (ipv6 != null) {
+        boolean hasIpv6Loopback = false;
+        try {
+          hasIpv6Loopback = LoopBackInterface.hasLocalIpv6Loopback();
+        } catch (Exception e) {
+          LOG.log(Level.FINE, "Failed to check local IPv6 loopback address", e);
+        }
+
+        if (!hasIpv6Loopback) {
+          recordAndLog(
+              tracer,
+              FailureReason.LOOPBACK_V6_MISSING,
+              "Direct Access investigation: IPv6 loopback missing.",
+              originalError);
+          return;
+        }
+      }
+
+      // Default fallback if investigation could not determine a specific issue
       recordAndLog(
           tracer,
           FailureReason.UNKNOWN,

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/dp/GCECheck.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.cloud.bigtable.data.v2.internal.dp;
+
+import com.google.api.core.InternalApi;
+import java.io.IOException;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Paths;
+
+@InternalApi
+class GCECheck {
+
+  static boolean isRunningOnGCP() {
+    if (!"Linux".equals(System.getProperty("os.name"))) {
+      return false;
+    }
+    return getSystemProductName().contains("Google");
+  }
+
+  private static String getSystemProductName() {
+    try {
+      return new String(
+          Files.readAllBytes(Paths.get("/sys/class/dmi/id/product_name")), StandardCharsets.UTF_8);
+    } catch (IOException e) {
+      return "";
+    }
+  }
+}

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/dp/LoopBackInterface.java

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.cloud.bigtable.data.v2.internal.dp;
+
+import com.google.api.core.InternalApi;
+import java.net.InetAddress;
+import java.net.NetworkInterface;
+import java.util.Enumeration;
+
+/**
+ * This class verifies two main things: The OS has a functioning loopback interface (lo) with
+ * standard localhost IPs configured.
+ */
+@InternalApi
+class LoopBackInterface {
+
+  static boolean isUp() throws Exception {
+    Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
+    while (interfaces.hasMoreElements()) {
+      NetworkInterface iface = interfaces.nextElement();
+      if (iface.isLoopback() && iface.isUp()) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Verifies that the standard IPv4 localhost address (127.0.0.1) is bound to a loopback interface.
+   */
+  static boolean hasLocalIpv4Loopback() throws Exception {
+    return checkLocalLoopbackAddress("127.0.0.1");
+  }
+
+  /** Verifies that the standard IPv6 localhost address (::1) is bound to a loopback interface. */
+  static boolean hasLocalIpv6Loopback() throws Exception {
+    return checkLocalLoopbackAddress("::1");
+  }
+
+  private static boolean checkLocalLoopbackAddress(String expectedIp) throws Exception {
+    InetAddress expected = InetAddress.getByName(expectedIp);
+    Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
+    while (interfaces.hasMoreElements()) {
+      NetworkInterface iface = interfaces.nextElement();
+      if (iface.isLoopback() && iface.isUp()) {
+        Enumeration<InetAddress> addrs = iface.getInetAddresses();
+        while (addrs.hasMoreElements()) {
+          if (addrs.nextElement().equals(expected)) {
+            return true;
+          }
+        }
+      }
+    }
+    return false;
+  }
+}

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/internal/dp/MetadataServer.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.cloud.bigtable.data.v2.internal.dp;
+
+import com.google.api.core.InternalApi;
+import java.io.BufferedReader;
+import java.io.InputStreamReader;
+import java.net.HttpURLConnection;
+import java.net.InetAddress;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+
+/**
+ * Verifies that the VM can reach the GCP metadata server and checks whether GCP has successfully
+ * assigned DirectPath-eligible IPv4 or IPv6 addresses to the instance's primary network interface
+ * (nic0).
+ */
+@InternalApi
+class MetadataServer {
+  private static final String METADATA_BASE_URL =
+      "http://metadata.google.internal/computeMetadata/v1/";
+  private static final String METADATA_IPV4_URL =
+      "http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ip";
+  private static final String METADATA_IPV6_URL =
+      "http://metadata.google.internal/computeMetadata/v1/instance/network-interfaces/0/ipv6s";
+
+  private static final int REACHABILITY_TIMEOUT_MS = 2000;
+  private static final int FETCH_IP_TIMEOUT_MS = 5000;
+
+  static boolean isReachable() {
+    HttpURLConnection conn = null;
+    try {
+      conn = createConnection(METADATA_BASE_URL, REACHABILITY_TIMEOUT_MS);
+      return conn.getResponseCode() == HttpURLConnection.HTTP_OK;
+    } catch (Exception e) {
+      return false;
+    } finally {
+      if (conn != null) {
+        conn.disconnect();
+      }
+    }
+  }
+
+  static InetAddress getIPv4() {
+    return fetchIP(METADATA_IPV4_URL);
+  }
+
+  static InetAddress getIPv6() {
+    return fetchIP(METADATA_IPV6_URL);
+  }
+
+  private static InetAddress fetchIP(String urlStr) {
+    HttpURLConnection conn = null;
+    try {
+      conn = createConnection(urlStr, FETCH_IP_TIMEOUT_MS);
+      if (conn.getResponseCode() == HttpURLConnection.HTTP_OK) {
+        try (BufferedReader br =
+            new BufferedReader(
+                new InputStreamReader(conn.getInputStream(), StandardCharsets.UTF_8))) {
+          String ipStr = br.readLine();
+          if (ipStr != null && !ipStr.isEmpty()) {
+            return InetAddress.getByName(ipStr.trim());
+          }
+        }
+      }
+    } catch (Exception e) {
+      // investigator handles the exception
+    } finally {
+      if (conn != null) {
+        conn.disconnect();
+      }
+    }
+    return null;
+  }
+
+  /** Helper to consistently configure the HttpURLConnection for the GCE Metadata Server. */
+  private static HttpURLConnection createConnection(String urlStr, int readTimeout)
+      throws Exception {
+    HttpURLConnection conn = (HttpURLConnection) new URL(urlStr).openConnection();
+    conn.setConnectTimeout(MetadataServer.REACHABILITY_TIMEOUT_MS);
+    conn.setReadTimeout(readTimeout);
+    conn.setRequestProperty("Metadata-Flavor", "Google");
+    return conn;
+  }
+}

google-cloud-bigtable/src/test/java/com/google/cloud/bigtable/data/v2/internal/dp/ClassicDirectAccessCheckerTest.java

@@ -149,6 +149,7 @@ public void testInvestigationTriggeredOnFailure() {
 
     // Execute the captured runnable to ensure it safely calls the tracer
     runnableCaptor.getValue().run();
-    verify(mockTracer).recordFailure(DirectAccessInvestigator.FailureReason.UNKNOWN);
+    // just make sure it matches with any FailureReason.
+    verify(mockTracer).recordFailure(any(DirectAccessInvestigator.FailureReason.class));
   }
 }