How create coinbase transaction with createrawtransaction?

Question

I would like to generate a transaction with createrawtransaction such that it can be passed to submitblock along with the block header.

I tried to use:

[{"txid":"0000000000000000000000000000000000000000000000000000000000000000","vout":0}]{"bc1qd8hza5w3mz7t3tudtlplm9xht4sjwg0e3t9gy2":6.25000000}

but it's not outputting a coinbase transaction:

020000000100000000000000000000000000000000000000000000000000000000000000000000000000ffffffff0140be40250000000016001469ee2ed1d1d8bcb8af8d5fc3fd94d75d612721f900000000

Answer 1

It is not possible to create a coinbase transaction using createrawtransaction. There are numerous rules that coinbase transactions must follow that the createrawtransaction RPC does not have the ability to set the fields to meet those rules. For example, BIP 34 requires the coinbase contain the block's height in the scriptSig. However createrawtransaction does not allow you to set the anything in the scriptSig.

Miners must use software that can independently create the coinbase transaction. Miners actually prefer doing this because they want to be able to modify the coinbase to use as an extra nonce so that when they run out of nonces, they don't necessarily have to re-select the transactions to include in the block. Instead they can change the coinbase to cause the merkle root to change.

Answer 2

According to BIP 34, you must include the block Height to your Coinbase transaction, otherwise, it will be invalid,

The scryptSig of your coinbase transaction, must begin with number of pushed bytes (Actually is 0x03) and then the hex representation of your candidate block, dont forget the little endian conversion

Answer 3

I recommend looking over this code in python which will illustrate how the coinbase is constructed, it is the simplest example I can think of, and is commented well - https://github.com/vsergeev/ntgbtminer/blob/master/ntgbtminer.py#L166