Last update: December 14, 2020

Genesis file

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

Configuration items

The following table lists the network configuration items in the config object specified in the genesis file.

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. The default is `24576` and the maximum size is `2147483647`.
`evmStackSize`	Maximum stack size. Specify to increase the maximum stack size in private networks with complex smart contracts. The 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.

The following table describes the genesis block parameters with the same purpose across all consensus protocols.

Item	Description
`coinbase`	Address to pay mining rewards to. 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. It is implied this includes the preceding milestones.

Fixed difficulty

Use fixeddifficulty to specify a fixed difficulty in private networks using Ethash. This will keep the network’s difficulty constant and override the difficulty parameter from the genesis file.

Example

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

   },
  ...
}

Tip

Using fixeddifficulty is not recommended for use with Ethash outside of test environments. For production networks using Ethash, we recommend setting a low difficulty value in the genesis file instead. Ethash will adjust the difficulty of the network based on hashrate to produce blocks at the targeted frequency.