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FD Control Protocol

This feature is mainly used for developing Android proxy apps.

The Hysteria client supports sending the file descriptor (fd) of outbound QUIC connections to any process listening on fdControlUnixSocket.

Note: fdControlUnixSocket is currently only effective for outbound QUIC connections. Third-party Android clients using this feature will need to handle additional DNS resolution requests for the Hysteria server domain, or make sure that the server option in the Hysteria client config is an IP address, not a domain.


  • Server: The third-party process listening on fdControlUnixSocket, e.g., the main process of an Android app.
  • Client: The Hysteria client process.

Server Implementation

  1. Listen to a path-based Unix Socket, which must be of type SOCK_STREAM.
  2. Use accept(2) to accept connections initiated by the client.
  3. Receive a single fd sent by the client via recvmsg(2).
  4. Process the fd received in step 3 (e.g., by calling VpnService.protect()) and then close the fd.
  5. Respond to the client with a single byte, notifying the client to continue operations. It is advised to respond with '\x01' under normal conditions; other values are currently undefined.
  6. Close the connection established in step 2 - each connection only processes one fd.