From 5ed357bca135aa0d5b659b934fc2729c5e32c961 Mon Sep 17 00:00:00 2001
From: Peter Solnica <peter@solnica.online>
Date: Thu, 24 Sep 2026 11:17:49 +0000
Subject: [PATCH] feat(metrics): attach sentry.is_localhost to metrics

---
 lib/sentry/metric.ex                     | 39 ++++++++++++++++++++-
 test/sentry/metrics_integration_test.exs | 43 ++++++++++++++++++++++++
 2 files changed, 81 insertions(+), 1 deletion(-)

diff --git a/lib/sentry/metric.ex b/lib/sentry/metric.ex
index f826a54f..74f82b68 100644
--- a/lib/sentry/metric.ex
+++ b/lib/sentry/metric.ex
@@ -49,12 +49,22 @@ defmodule Sentry.Metric do
   This adds Sentry-specific attributes like environment, release, SDK info, and server name.
   Per the Sentry Metrics spec, default attributes should be attached before the
   `before_send_metric` callback is applied (step 5 before step 6).
+
+  It also adds `"sentry.is_localhost"`: `true` when the calling process is serving a
+  request whose host is `localhost`, a `*.localhost` name, or a loopback IP address
+  (`127.0.0.0/8` or `::1`), and `false` otherwise, including when the process is not
+  serving a request. The request is the one recorded in the process's
+  `Sentry.Context` request context, as set by `Sentry.PlugContext`. A value of the
+  same name in the metric's attributes takes precedence.
+
+  *Changed in 14.0.0:* adds `"sentry.is_localhost"`.
   """
   @spec attach_default_attributes(t()) :: t()
   def attach_default_attributes(%__MODULE__{} = metric) do
     default_attrs = %{
       "sentry.sdk.name" => "sentry.elixir",
-      "sentry.sdk.version" => @sdk_version
+      "sentry.sdk.version" => @sdk_version,
+      "sentry.is_localhost" => serving_localhost_request?()
     }
 
     # Add optional attributes if configured
@@ -83,6 +93,33 @@ defmodule Sentry.Metric do
     :atomics.add_get(:persistent_term.get(@sequence_key), 1, 1) - 1
   end
 
+  defp serving_localhost_request? do
+    case request_host(Sentry.Context.get_all().request) do
+      host when is_binary(host) -> localhost?(String.downcase(host))
+      nil -> false
+    end
+  end
+
+  defp request_host(%{env: %{"SERVER_NAME" => host}}) when is_binary(host), do: host
+  defp request_host(%{url: url}) when is_binary(url), do: URI.parse(url).host
+  defp request_host(_request), do: nil
+
+  defp localhost?("localhost"), do: true
+
+  defp localhost?(host) do
+    String.ends_with?(host, ".localhost") or loopback_address?(host)
+  end
+
+  defp loopback_address?(host) do
+    unbracketed_host = host |> String.trim_leading("[") |> String.trim_trailing("]")
+
+    case :inet.parse_address(to_charlist(unbracketed_host)) do
+      {:ok, {127, _, _, _}} -> true
+      {:ok, {0, 0, 0, 0, 0, 0, 0, 1}} -> true
+      _ -> false
+    end
+  end
+
   defp maybe_put_attr(attrs, _key, nil), do: attrs
   defp maybe_put_attr(attrs, key, value), do: Map.put(attrs, key, value)
 
diff --git a/test/sentry/metrics_integration_test.exs b/test/sentry/metrics_integration_test.exs
index 6e75109e..dfe143d5 100644
--- a/test/sentry/metrics_integration_test.exs
+++ b/test/sentry/metrics_integration_test.exs
@@ -175,6 +175,38 @@ defmodule Sentry.MetricsIntegrationTest do
     end
   end
 
+  describe "sentry.is_localhost attribute" do
+    test "is true for a metric recorded while serving a localhost request", ctx do
+      serve_request("http://localhost:4000/checkout")
+
+      Metrics.count("orders.placed", 1)
+
+      assert is_localhost_attribute(ctx.ref) == %{"type" => "boolean", "value" => true}
+    end
+
+    test "is true for a metric recorded while serving a request to a loopback address", ctx do
+      serve_request("http://[::1]:4000/checkout")
+
+      Metrics.count("orders.placed", 1)
+
+      assert is_localhost_attribute(ctx.ref) == %{"type" => "boolean", "value" => true}
+    end
+
+    test "is false for a metric recorded while serving a request to a public host", ctx do
+      serve_request("https://shop.example.com/checkout")
+
+      Metrics.count("orders.placed", 1)
+
+      assert is_localhost_attribute(ctx.ref) == %{"type" => "boolean", "value" => false}
+    end
+
+    test "is false for a metric recorded outside of a request", ctx do
+      Metrics.count("orders.placed", 1)
+
+      assert is_localhost_attribute(ctx.ref) == %{"type" => "boolean", "value" => false}
+    end
+  end
+
   describe "trace context on recorded metrics" do
     test "records the metric with the trace of the surrounding span", ctx do
       put_test_config(traces_sample_rate: 1.0)
@@ -193,6 +225,17 @@ defmodule Sentry.MetricsIntegrationTest do
     end
   end
 
+  defp serve_request(url) do
+    :get
+    |> Plug.Test.conn(url)
+    |> Sentry.PlugContext.call(Sentry.PlugContext.init([]))
+  end
+
+  defp is_localhost_attribute(ref) do
+    [[{_header, %{"items" => [metric]}}]] = collect_envelopes(ref, 1)
+    metric["attributes"]["sentry.is_localhost"]
+  end
+
   defp assert_metric_dropped(ctx, crashing_callback) do
     put_test_config(before_send_metric: crashing_callback)