fallback() | Ethernaut #01:

What happens when you call the function that does not exist, or send ether to a contract via send() or transfer() ?

fallback() external [payable] {...}

executes when:

• function does not exist in contract
• ether transfer with non-empty call data
• ether transfer when there is no receive() function

receive() external payable {...}

executes when:

• ether transfer with empty calldata

Code example

contract TestPayable {
    uint x;
    uint y;

    fallback() external payable { x = 1; y = msg.value; }

    receive() external payable { x = 2; y = msg.value; }
}

address(contract).transfer(AMOUNT) => x=2
address(contract).call{value:1}(abi.encodePacked("SOME")) => x=1

Real-world usage: Proxy

Let's look at the code of Proxy.sol

1. proxy.foo(...)
2. foo(...) does not exist in proxy's source code
3. fallback is triggered
4. make delegatecall(...) on implementation
5. return result

Thanks to fallback function arbitrary function can be called on a proxy. Without it we would have to create more complicated mechanism.

Why receive() was introduced?

• To make explicit check if msg.data is empty. Without it, it was up to the programmer to check whether calldata is empty.
• Better UX for plain ETH transfers, so the user who didn't put calldata could be handled independetly. receive() function enforced on programmers thinking about such cases.

Ethernaut 01: Fallback

There was a fallback method which granted you ownership if you contributed before and supplied some ether. This way your contribution could be less than owner but still you can gain ownership.

1. Interact with contract using web3.js

2. Send ether when interacting with an ABI

   • contract.contribute({value: AMOUNT_IN_WEI})

3. Send ether outside of the ABI docs

   • contract.sendTransaction({value: AMOUNT_IN_WEI})

Level completed at tx

sources:
forum answer

medium blog post

solidity docs

stackexchange