lightning-dev-sendcustommsg – Low-level interface to send protocol messages to peers


dev-sendcustommsg node_id msg


The dev-sendcustommsg RPC method allows the user to inject a custom message into the communication with the peer with the given node_id. This is intended as a low-level interface to implement custom protocol extensions on top, not for direct use by end-users.

The message must be a hex encoded well-formed message, including the 2-byte type prefix, but excluding the length prefix which will be added by the RPC method. The messages must not use even-numbered types, since these may require synchronous handling on the receiving side, and can cause the connection to be dropped. The message types may also not use one of the internally handled types, since that may cause issues with the internal state tracking of c-lightning.

The node specified by node_id must be a peer, i.e., it must have a direct connection with the node receiving the RPC call, and the connection must be established. For a method to send arbitrary messages over multiple hops, including hops that do not understand the custom message, see the createonion and sendonion RPC methods. Messages can only be injected if the connection is handled by openingd or channeld. Messages cannot be injected when the peer is handled by onchaind or closingd since these do not have a connection, or are synchronous daemons that do not handle spontaneous messages.

On the reveiving end a plugin may implement the custommsg plugin hook and get notified about incoming messages.


The method will validate the arguments and queue the message for delivery through the daemon that is currently handling the connection. Queuing provides best effort guarantees and the message may not be delivered if the connection is terminated while the message is queued. The RPC method will return as soon as the message is queued.

If any of the above limitations is not respected the method returns an explicit error message stating the issue.


Christian Decker <> is mainly responsible.


lightning-createonion(7), lightning-sendonion(7)