Alright, so I'm convinced replace-by-fee is a good thing. I've read it's opt-in, so how can I enable it in my bitcoin-core wallet? I don't find any flag to pass it when launching it. I want all my outgoing transaction have replace-by-fee enabled.
Bitcoin Core v0.14.0 introduced the startup option
walletrbf=1 which causes any transactions sent from the wallet to opt into replaceability. Since Bitcoin Core v0.16.0 transactions created from the GUI would signal replaceability by default.
Bitcoin Core v24.0.1 changed the default value to
walletrbf=1 which extended this behavior also to all transactions created via RPC.
According to bitcoin core version 0.12.0 release notes:
It is now possible to replace transactions in the transaction memory pool of Bitcoin Core 0.12 nodes.
Transaction replacement can be disabled with a new command line option, -mempoolreplacement=0.
Note that the wallet in Bitcoin Core 0.12 does not yet have support for creating transactions that would be replaceable under BIP 125.
So the mempool can be configured to accept/reject replacement transactions, but there is not support yet for creating replaceable transactions.
Bitcoin v0.12.1 release notes do not include any modification regarding RBF.
The functionality for the wallets is split in two: