Using Sockets


Traffic from sockets is billed as outgoing bandwidth. App Engine supports outbound sockets through the appengine/socket package.

Libraries that can accept a net.Conn should work without modification.

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.
  • 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: 8.8.8.8, 8.8.4.4, 2001:4860:4860::8888, 2001:4860:4860::8844 port 53
    • Gmail SMTPS: smtp.gmail.com port 465 and 587
    • Gmail POP3S: pop.gmail.com port 995
    • Gmail IMAPS: imap.gmail.com 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 2 minutes of inactivity; any socket operation keeps the socket alive for a further 2 minutes.

    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.