Use this reference page for details about what HTTP headers are supported as well as the request and response limits in App Engine. To understand how App Engine receives requests and sends responses, see How Requests Are Handled.
An incoming HTTP request includes the HTTP headers sent by the client. For security purposes, some headers are sanitized or amended by intermediate proxies before they reach the application.
The following headers are removed from the request:
In addition, the header
Strict-Transport-Security is removed from requests
served to any domains other than
These headers relate to the transfer of the HTTP data between the client and
server, and are transparent to the application. For example, the server may
automatically send a gzipped response, depending on the value of the
Encoding request header. The application itself does not need to know which
content encodings the client can accept.
Requests and WSGI
The server determines which Python application object to call by comparing the URL of the request to the URL patterns in the app's configuration file. It then calls the application object using the arguments as defined in the WSGI standard. The application object performs actions appropriate to the request, then prepares a response and returns it as a list of strings.
Most applications use a framework, such as
webapp2, to handle WSGI requests.
webapp2 is included as part of the Python 2.7 runtime.
Requests and CGI
The server determines which Python handler script to run by comparing the URL of the request to the URL patterns in the app's configuration file. It then runs the script in a environment populated with the request data. As described in the CGI standard, the server puts the request data in environment variables and the standard input stream. The script performs actions appropriate to the request, then prepares a response and puts it on the standard output stream.
Most applications use a library to parse CGI requests and return CGI responses, such as the cgi module from the Python standard library, or a web framework that knows the CGI protocol (such as webapp). You can refer to the CGI documentation for details about the environment variables and the format of the input stream data.
App Engine-specific headers
As a service to the app, App Engine adds the following headers to all requests:
- Country from which the request originated, as an ISO 3166-1
alpha-2 country code. App
Engine determines this code from the client's IP address. Note that the country
information is not derived from the WHOIS database; it's possible that an IP
address with country information in the WHOIS database will not have country
information in the
X-AppEngine-Countryheader. Your application should handle the special country code
- Name of region from which the request originated. This value only makes
sense in the context of the country in
X -AppEngine-Country. For example, if the country is "US" and the region is "ca", that "ca" means "California", not Canada. The complete list of valid region values is found in the ISO-3166-2 standard.
- Name of the city from which the request originated. For example, a request
from the city of Mountain View might have the header value
mountain view. There is no canonical list of valid values for this header.
- Latitude and longitude of the city from which the request originated. This string might look like "37.386051,-122.083851" for a request from Mountain View.
X-AppEngine-Https, with example header: "off"
X-AppEngine-User-IP, with example header: "2602:306:3429:520:501f:4a71:9d2c:be5f"
X-Cloud-Trace-Context, with example header: "18ff88cd7f38ff2bf9b79443..."
login:required handlers specified in
App Engine also provides the following set of headers:
X-AppEngine-User-Email, with example header: "email@example.com"
X-AppEngine-Auth-Domain,with example header: "example.com"
X-AppEngine-User-ID, with example header: "100979712376541954724"
X-AppEngine-User-Nickname, with example header: "ange"
X-AppEngine-User-Organization, with example header: "example.com"
X-AppEngine-User-Is-Admin, with example header: "1"
App Engine services may add additional request headers:
- The Task Queue service adds additional headers to requests from that provide details about the task in the request, and the queue it is associated with.
Requests from the Cron Service will also contain a HTTP header:
See Securing URLs for cron for more details.
Requests coming from other App Engine applications will include a header identifying the app making the request, if the requesting app is using the URL Fetch Service:
See the App Identity documentation for more details.
This HTTP header documentation only applies to responses to inbound HTTP requests. The response may be modified before it is returned to the client. For HTTP headers related to outbound requests originated by your App Engine code, see the header documentation for URLFetch.
The following headers are ignored and removed from the response:
* May be re-added if the response is compressed by App Engine.
In addition, the header
Strict-Transport-Security is removed from responses
served from any domains other than
Headers with non-ASCII characters in either the name or value are also removed.
Headers added or replaced
The following headers are added or replaced in the response:
These headers specify caching policy to intermediate web proxies (such as Internet Service Providers) and browsers. If your script sets these headers, they will usually be unmodified, unless the response has a Set-Cookie header, or is generated for a user who is signed in using an administrator account. Static handlers will set these headers as directed by the configuration file. If you do not specify a
Cache-Control, the server may set it to
private, and add a
If you have a Set-Cookie response header, the
Cache-Controlheader will be set to
private(if it is not already more restrictive) and the
Expiresheader will be set to the current date (if it is not already in the past). Generally, this will allow browsers to cache the response, but not intermediate proxy servers. This is for security reasons, since if the response was cached publicly, another user could subsequently request the same resource, and retrieve the first user's cookie.
Depending upon the request headers and response
Content-Type, the server may automatically compress the response body, as described above. In this case, it adds a
Content-Encoding: gzipheader to indicate that the body is compressed. See the section on response compression for more detail.
The server always ignores the
Content-Lengthheader returned by the application. It will either set
Content-Lengthto the length of the body (after compression, if compression is applied), or delete
Content-Length, and use chunked transfer encoding (adding a
If not specified by the application, the server will set a default
Set to the current date and time.
Google Frontend. The development server sets this to
Development/x, where x is the version number.
If you access dynamic pages on your site while signed in using an administrator account, App Engine includes per-request statistics in the response headers:
- An estimate of what 1,000 requests similar to this request would cost in US dollars.
- The resources used by the request, including server-side time as a number of milliseconds.
Responses with resource usage statistics will be made uncacheable.
X-AppEngine-BlobKey header is in the application's response, it and the
X-AppEngine-BlobRange header will be used to replace the body with
all or part of a blobstore blob's content. If
Content-Type is not specified by
the application, it will be set to the blob's MIME type. If a range is
requested, the response status will be changed to
206 Partial Content, and a
Content-Range header will be added. The
-AppEngine-BlobRange headers will be removed from the response. You do not
normally need to set these headers yourself, as the
blobstore_handlers.BlobstoreDownloadHandler class sets them. See Serving a
Response headers set in the application configuration
Custom HTTP Response headers can be set per URL for dynamic and static paths in
your application's configuration file. See the
http_headers sections in the
for more details.