Sockets API for legacy bundled services

Caution: This solution is no longer recommended: Apps that use this API can only run in the Go 1.11 runtime and will need to upgrade to a recommended solution before migrating to the Go 1.12+ runtime.

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
  Note: Google Compute Engine IP addresses are not considered to be in Google IP ranges. You can use sockets to connect App Engine apps to Compute Engine instances.

• 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.

  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.