From 2de79abc00a2bf3f244bc5b60827c8405bc12ce0 Mon Sep 17 00:00:00 2001
From: Jared Kirschner <jkirschner@hashicorp.com>
Date: Mon, 13 Dec 2021 08:26:35 -0800
Subject: [PATCH] http: improve 404 Not Found response message

When a URL path is not found, return a non-empty message with the 404 status
code to help the user understand what went wrong. If the URL path was not
prefixed with '/v1/', suggest that may be the cause of the problem (which is a
common mistake).
---
 .changelog/11818.txt | 3 +++
 agent/http.go        | 6 ++++++
 2 files changed, 9 insertions(+)
 create mode 100644 .changelog/11818.txt

diff --git a/.changelog/11818.txt b/.changelog/11818.txt
new file mode 100644
index 0000000000..728e7360f1
--- /dev/null
+++ b/.changelog/11818.txt
@@ -0,0 +1,3 @@
+```release-note:improvement
+http: when a URL path is not found, include a message with the 404 status code to help the user understand why (e.g., HTTP API endpoint path not prefixed with /v1/)
+```
\ No newline at end of file
diff --git a/agent/http.go b/agent/http.go
index a1d8461d0d..e1f6e475bc 100644
--- a/agent/http.go
+++ b/agent/http.go
@@ -587,6 +587,12 @@ func (s *HTTPHandlers) Index(resp http.ResponseWriter, req *http.Request) {
 	// Check if this is a non-index path
 	if req.URL.Path != "/" {
 		resp.WriteHeader(http.StatusNotFound)
+
+		if strings.Contains(req.URL.Path, "/v1/") {
+			fmt.Fprintln(resp, "Invalid URL path: not a recognized HTTP API endpoint")
+		} else {
+			fmt.Fprintln(resp, "Invalid URL path: if attempting to use the HTTP API, ensure the path starts with '/v1/'")
+		}
 		return
 	}