Updated 2019-12-19

Genesis File

The genesis file contains network configuration items and genesis block parameters.

Configuration Items

Network configuration items are specified in the genesis file in the config object.

Item	Description
Chain ID	Chain ID for the network
Milestone blocks	Milestone blocks for the network
`ethash`	Specifies network uses Ethash and contains `fixeddifficulty`
`clique`	Specifies network uses Clique and contains Clique configuration items
`ibft2`	Specifies network uses IBFT 2.0 and contains IBFT 2.0 configuration items
`transitions`	Specifies block at which to change IBFT 2.0 validators
`contractSizeLimit`	Maximum contract size in bytes. Specify in free gas networks. Default is `24576` and the maximum size is `2147483647`.
`evmStackSize`	Maximum stack size. Specify to increase the maximum stack size in private networks with very complex smart contracts. Default is `1024`.

Genesis Block Parameters

The purpose of some genesis block parameters varies depending on the consensus protocol (Ethash, Clique, or IBFT 2.0). These parameters include:

difficulty
extraData
mixHash

Genesis block parameters with the same purpose across all consensus protocols are described below.

Item	Description
`coinbase`	Address to which mining rewards are paid. Can be any value in the genesis block (commonly set to `0x0000000000000000000000000000000000000000`).
`gasLimit`	Block gas limit. Total gas limit for all transactions in a block.
`nonce`	Used in block computation. Can be any value in the genesis block (commonly set to `0x0`).
`timestamp`	Creation date and time of the block. Must be before the next block so we recommend specifying `0x0` in the genesis file.
`alloc`	Defines accounts with balances or contracts.

Milestone Blocks

In public networks, the milestone blocks specify the blocks at which the network changed protocol.

Ethereum Mainnet Milestone Blocks

{
  "config": {
    ...
    "homesteadBlock": 1150000,
    "daoForkBlock": 1920000,
    "daoForkSupport": true,
    "eip150Block": 2463000,
    "eip150Hash": "0x2086799aeebeae135c246c65021c82b4e15a2c451340993aacfd2751886514f0",
    "eip155Block": 2675000,
    "eip158Block": 2675000,
    "byzantiumBlock": 4370000,
    "constantinopleBlock": 7280000,
    "constantinopleFixBlock": 7280000,
    ...
  },
}

In private networks, the milestone block defines the protocol version for the network.

Private Network Milestone Block

{
  "config": {
    ...
    "constantinopleFixBlock": 0,
    ...
  },
}

Tip

When specifying the milestone block for private networks, you only need to specify the latest milestone because the milestones that precede it are implied.

Fixed Difficulty

Use fixeddifficulty to specify a fixed difficulty in private networks using Ethash.

Example

{
  "config": {
      ...
      "ethash": {
        "fixeddifficulty": 1000
      },

   },
  ...
}