The meaning/specification of "config" in genesis.json

Question

As I tried to search from Google, there are some genesis.json example files to customize the parameter of genesis block in Ethereum private blockchain.

In the file, there is a part called "config"

"config": {
    "chainId": 1,
    "homesteadBlock": 0,
    "eip155Block": 0,
    "eip158Block": 0
}

What is the meaning or specification for those parameter in "config"? May I setup part of parameter(s) only if necessary?

Answer 1

This is the chain configuration field, defined in config.go:

// ChainConfig is the core config which determines the blockchain settings.
//
// ChainConfig is stored in the database on a per block basis. This means
// that any network, identified by its genesis block, can have its own
// set of configuration options.

The available fields and their descriptions are shown below:

type ChainConfig struct {
    ChainId *big.Int `json:"chainId"` // Chain id identifies the current chain and is used for replay protection

    HomesteadBlock *big.Int `json:"homesteadBlock,omitempty"` // Homestead switch block (nil = no fork, 0 = already homestead)
    DAOForkBlock   *big.Int `json:"daoForkBlock,omitempty"`   // TheDAO hard-fork switch block (nil = no fork)
    DAOForkSupport bool     `json:"daoForkSupport,omitempty"` // Whether the nodes supports or opposes the DAO hard-fork

    // EIP150 implements the Gas price changes (https://github.com/ethereum/EIPs/issues/150)
    EIP150Block *big.Int    `json:"eip150Block,omitempty"` // EIP150 HF block (nil = no fork)
    EIP150Hash  common.Hash `json:"eip150Hash,omitempty"`  // EIP150 HF hash (fast sync aid)

    EIP155Block *big.Int `json:"eip155Block,omitempty"` // EIP155 HF block
    EIP158Block *big.Int `json:"eip158Block,omitempty"` // EIP158 HF block

    // Various consensus engines
    Ethash *EthashConfig `json:"ethash,omitempty"`
    Clique *CliqueConfig `json:"clique,omitempty"`
}

The config for the main chain is as follows (again in config.go):

MainnetChainConfig = &ChainConfig{
    ChainId:        MainNetChainID,
    HomesteadBlock: MainNetHomesteadBlock,
    DAOForkBlock:   MainNetDAOForkBlock,
    DAOForkSupport: true,
    EIP150Block:    MainNetHomesteadGasRepriceBlock,
    EIP150Hash:     MainNetHomesteadGasRepriceHash,
    EIP155Block:    MainNetSpuriousDragon,
    EIP158Block:    MainNetSpuriousDragon,
    Ethash:         new(EthashConfig),

With the actual values being defined in util.go.

---

So in answer to your questions...

"config": { "chainId": 1, "homesteadBlock": 0, "eip155Block": 0, "eip158Block": 0 }

What is the meaning or specification for those parameter in "config"?

• Chain id - identifies the current chain and is used for replay protection. You should set it to a unique value for your private chain.
• homesteadBlock - your chain won't be undergoing the switch to Homestead, so leave this as 0.
• eip155Block - your chain won't be hard-forking for these changes, so leave as 0.
• eip158Block - your chain won't be hard-forking for these changes, so leave as 0.

May I setup part of parameter(s) only if necessary?

Yes. Any of the values that set a block or hash number can be ignored for a private chain - they're only really relevant to the public main chain.