Receivers are contracts that implement the IXReceiver interface:

interface IXReceiver {
  function xReceive(
    bytes32 _transferId,
    uint256 _amount,
    address _asset,
    address _originSender,
    uint32 _origin,
    bytes memory _callData
  ) external returns (bytes memory);

These are the main contracts that integrators implement. Connext provides a few abstract receivers that should be inherited in order to gain the built-in security and error handling.


The ForwarderXReceiver is used for unauthenticated calls. The receiver has two virtual functions that the integrator should implement to 1) prepare and 2) forward the call.


function _prepare(
  bytes32 _transferId,
  bytes memory _data,
  uint256 _amount,
  address _asset
) internal virtual returns (bytes memory)

Preparation steps should be performed in _prepare, which can include operations like swapping funds.


function _forwardFunctionCall(
  bytes memory _preparedData,
  bytes32 _transferId,
  uint256 _amount,
  address _asset
) internal virtual returns (bool)

The _forwardFunctionCall should contain the logic that calls the destination target contract.


The AuthForwarderXReceiver is used for authenticated calls. It follows the same two-step pattern as the ForwarderXReceiver (_prepare + _forwardFunctionCall).


In addition, the xReceive method is guarded by an onlyOrigin modifier which ensures that:

  1. The originating call comes from a registered origin domain.

  2. The originating call comes from the expected origin contract of the origin domain.

  3. The call to this contract comes from Connext.

These checks guarantee that any successful call to an AuthForwarderXReceiver contains validated data. For more information on authentication, see the Authentication section.


This contract also holds an originRegistry which maps origin domain IDs to origin sender contracts. This registry is checked to uphold requirement #2 for the modifier above.

The contract is Ownable and only the owner can addOrigin and removeOrigin. All expected origins and senders should be registered in this mapping.