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Last update: September 7, 2021

Genesis file items

The Besu 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
Milestone blocks Milestone blocks for the network.
chainID Chain ID 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.
qbft Specifies network uses QBFT and contains QBFT configuration items.
transitions Specifies block at which to change IBFT 2.0 or QBFT 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.
isQuorum Set to true to allow interoperable private transactions between Hyperledger Besu and GoQuorum clients using the Tessera private transaction manager.
ecCurve Specifies the elliptic curve to use. Default is secp256k1.

Genesis block parameters

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


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.


  "config": {
      "ethash": {
        "fixeddifficulty": 1000



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.

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