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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.

Milestone blocksMilestone blocks for the network.
chainIDChain ID for the network.
ethashSpecifies network uses Ethash and contains fixeddifficulty.
cliqueSpecifies network uses Clique and contains Clique configuration items.
ibft2Specifies network uses IBFT 2.0 and contains IBFT 2.0 configuration items.
qbftSpecifies network uses QBFT and contains QBFT configuration items.
transitionsSpecifies block at which to change IBFT 2.0 or QBFT validators.
contractSizeLimitMaximum contract size in bytes. Specify in free gas networks. The default is 24576 and the maximum size is 2147483647.
evmStackSizeMaximum stack size. Specify to increase the maximum stack size in private networks with complex smart contracts. The default is 1024.
ecCurveSpecifies the elliptic curve to use. Default is secp256k1.
discoverySpecifies discovery configuration items. The discovery object can be left empty.

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.

coinbaseAddress to pay mining rewards to. Can be any value in the genesis block (commonly set to 0x0000000000000000000000000000000000000000).
gasLimitBlock gas limit. Total gas limit for all transactions in a block.
nonceUsed in block computation. Can be any value in the genesis block (commonly set to 0x0).
timestampCreation date and time of the block. Must be before the next block so we recommend specifying 0x0 in the genesis file.
allocDefines accounts with balances or contracts.

If a Supplied genesis block does not match stored chain data error occurs, use the genesis file matching the genesis block of the data directory, or use the --data-path option to specify a different data directory.

Milestone blocks

In public networks, the milestone blocks specify the blocks at which the network changed protocol. See a full list of Ethereum protocol releases and their corresponding milestone blocks.

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,
"muirGlacierBlock": 9200000,
"berlinBlock": 12244000,
"londonBlock": 12965000,
"arrowGlacierBlock": 13773000,
"grayGlacierBlock": 15050000,

Ensure you include a milestone far enough in advance in the genesis file. Not doing so can lead to unexpected and inconsistent behavior without specific errors.

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

Private network milestone block
"config": {
"berlinBlock": 0,

In private networks, we recommend specifying the latest milestone block. It's implied this includes the preceding milestones. This ensures you use the most up-to-date protocol and have access to the most recent opcodes.

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.

Discovery configuration items

Use the discovery configuration items to specify the bootnodes and discovery-dns-url in the genesis file, in place of using CLI options or listing them in the configuration file. If either CLI option is used, it takes precedence over the genesis file. Anything listed in the configuration file also takes precedence.

"config": {
"discovery": {
"bootnodes": [
"dns": "enrtree://"