Reject invalid HTTP methods and resources (#1019)

This change addresses the issue that currently, any HTTP method is handled
by returning success and metrics data, which causes network scanners to
report issues.

Details:

* This change rejects any HTTP methods and resources other than the following:

    OPTIONS (any) - returns 200 and an 'Allow' header indicating allowed methods
    GET (any) - returns 200 and metrics
    GET /favicon.ico - returns 200 and no body (this is no change)

  Other HTTP methods than these are rejected with 405 "Method Not Allowed"
  and an 'Allow' header indicating the allowed HTTP methods.

  Any returned HTTP errors are also displayed in the response body after a
  hash sign and with a brief hint,
  e.g. "# HTTP 405 Method Not Allowed: XXX; use OPTIONS or GET".

Signed-off-by: Andreas Maier <[email protected]>

docs/content/exporting/http/_index.md

@@ -52,4 +52,23 @@ chain is used (see Python [ssl.SSLContext.load_default_certs()](https://docs.pyt
 from prometheus_client import start_http_server
 
 start_http_server(8000, certfile="server.crt", keyfile="server.key")
-```
+```
+
+# Supported HTTP methods
+
+The prometheus client will handle the following HTTP methods and resources:
+
+* `OPTIONS (any)` - returns HTTP status 200 and an 'Allow' header indicating the
+  allowed methods (OPTIONS, GET)
+* `GET (any)` - returns HTTP status 200 and the metrics data
+* `GET /favicon.ico` - returns HTTP status 200 and an empty response body. Some
+  browsers support this to display the returned icon in the browser tab.
+
+Other HTTP methods than these are rejected with HTTP status 405 "Method Not Allowed"
+and an 'Allow' header indicating the allowed methods (OPTIONS, GET).
+
+Any returned HTTP errors are also displayed in the response body after a hash
+sign and with a brief hint. Example:
+```
+# HTTP 405 Method Not Allowed: XXX; use OPTIONS or GET
+```

prometheus_client/exposition.py

@@ -118,12 +118,24 @@ def prometheus_app(environ, start_response):
         accept_header = environ.get('HTTP_ACCEPT')
         accept_encoding_header = environ.get('HTTP_ACCEPT_ENCODING')
         params = parse_qs(environ.get('QUERY_STRING', ''))
-        if environ['PATH_INFO'] == '/favicon.ico':
+        method = environ['REQUEST_METHOD']
+
+        if method == 'OPTIONS':
+            status = '200 OK'
+            headers = [('Allow', 'OPTIONS,GET')]
+            output = b''
+        elif method != 'GET':
+            status = '405 Method Not Allowed'
+            headers = [('Allow', 'OPTIONS,GET')]
+            output = '# HTTP {}: {}; use OPTIONS or GET\n'.format(status, method).encode()
+        elif environ['PATH_INFO'] == '/favicon.ico':
             # Serve empty response for browsers
             status = '200 OK'
             headers = [('', '')]
             output = b''
         else:
+            # Note: For backwards compatibility, the URI path for GET is not
+            # constrained to the documented /metrics, but any path is allowed.
             # Bake output
             status, headers, output = _bake_output(registry, accept_header, accept_encoding_header, params, disable_compression)
         # Return output