Sockets API for legacy bundled services

Traffic from sockets is billed as outgoing bandwidth. Async IO (such as Twisted in Python) is not supported. App Engine supports the standard Python socket module API for outbound sockets only. You simply import the standard socket library using the following statement:

import socket

Libraries that import socket, such as poplib or nntplib, and that don't violate the limitations and restrictions listed below, should work without modification.

Although the Python 2 runtime supports sockets, there are certain limitations and behaviors you need to be aware of when using sockets.

You can pickle a socket descriptor and pass it between App Engine instances, such as part of a Task payload. In this scenario, you can open a socket on a frontend instance, and then pass it to a backend instance and use it there.

In SDK versions prior to 1.8.1, you could not call get/set options against sockets. (Doing so raised "Not Implemented" exceptions.) However, the Sockets API now allows this.

For supported options, calls to getsockopt will return a mock value and calls to setsockopt will be silently ignored. Errors will continue to be raised for unsupported options. The supported options are:


Limitations and restrictions

Socket support in App Engine has the following limitations:

  • You cannot create a listen socket; you can only create outbound sockets.
  • FTP is not supported.
  • By default, httplib is configured to use the urlfetch api; if you need to use socket to get around urlfetch limits, you can do so by changing this default so httplib uses sockets instead. For more information, see Making httplib use sockets.
  • You can only use TCP or UDP; arbitrary protocols are not allowed.
  • You cannot bind to specific IP addresses or ports.
  • Port 25 (SMTP) is blocked; you can still use authenticated SMTP on the submission port 587.
  • Private, broadcast, multicast, and Google IP ranges are blocked, except those listed below:

    • Google Public DNS:,, 2001:4860:4860::8888, 2001:4860:4860::8844 port 53
    • Gmail SMTPS: port 465 and 587
    • Gmail POP3S: port 995
    • Gmail IMAPS: port 993
  • Socket descriptors are associated with the App Engine app that created them and are non-transferable (cannot be used by other apps).

  • Sockets may be reclaimed after 10 minutes of inactivity; any socket operation keeps the socket alive for a further 10 minutes.

  • Currently, socket.gethostbyaddr() is not implemented in Python. You can still use the Python SMTP standard library (smtplib) to open a connection:

    # Open a connection to my mail server
    s = smtplib.SMTP('', 587)

Using sockets with the development server

You can run and test code using sockets on the development server, without using any special command line parameters.

Using sockets with OpenSSL

App Engine supports native Python OpenSSL for the Python 2.7 runtime. You must configure your app.yaml file to load the ssl library, as described in OpenSSL Support.

Disabling URL Fetch from handling all outbound requests

If you import httplib, by default it will use the urlfetch api. To change this so that httplib uses sockets instead, add the following environment variable to your app.yaml file:


You can replace anyvalue with any value including an empty string.

App Engine sample using sockets

For a sample using sockets, see the socket demo app in the Google Cloud Platform GitHub.