Building a Private Ethereum Blockchain: Creating, Mining, and Transactions

This guide continues our series on setting up a private Ethereum blockchain using CentOS 7.5. Learn how to create a genesis block, initialize your chain, and perform transactions.

Creating the Genesis Block

The genesis block serves as the foundation of your private blockchain, similar to a linked list's head node.

Step-by-Step Setup

1. Create a dedicated directory:

   mkdir private_eth
   cd private_eth
   touch genesis.json

2. Configure genesis.json:

   {
    "config": {
      "chainId": 10001,
      "homesteadBlock": 0,
      "eip150Block": 0,
      "eip155Block": 0,
      "eip158Block": 0,
      "byzantiumBlock": 0,
      "constantinopleBlock": 0,
      "petersburgBlock": 0
    },
    "alloc": {},
    "coinbase": "0x0000000000000000000000000000000000000000",
    "difficulty": "0x20000",
    "extraData": "",
    "gasLimit": "0x2fefd8",
    "nonce": "0x0000000000000042",
    "mixhash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "parentHash": "0x0000000000000000000000000000000000000000000000000000000000000000",
    "timestamp": "0x00"
   }

   (Optional) Pre-fund accounts for testing by replacing "alloc":{} with:

   "alloc": {
     "0x0000000000000000000000000000000000000001": {
       "balance": "111111111"
     },
     "0x0000000000000000000000000000000000000002": {
       "balance": "222222222"
     }
   }

Initializing the Blockchain

Execute the following command to initialize your chain:

geth init genesis.json --datadir test

• test directory: Automatically created to store:

  • geth/: Blockchain data.
  • keystore/: Account files (empty initially).

👉 Pro Tip: Always specify --datadir to avoid genesis block conflicts like:
Fatal: Failed to write genesis block: database contains incompatible genesis...

Launching the Ethereum Client

Start your node with RPC access (for external interactions):

geth --rpc --rpccorsdomain="*" --nodiscover --al

FAQ Section

1. Why is a genesis block necessary?

The genesis block initializes your blockchain’s parameters, ensuring all nodes start with identical configurations.

2. How do I troubleshoot "incompatible genesis" errors?

Use --datadir to isolate your chain’s data. Example:

geth init genesis.json --datadir /custom/path

3. Can I modify the genesis block later?

No—the genesis block is immutable. Changes require creating a new chain.

4. What’s the purpose of alloc in genesis.json?

It pre-funds accounts for testing. Omit it for a fresh start.

5. How do I enable mining?

Add --mine to your startup command and set a miner account with --miner.etherbase.

6. Is --rpccorsdomain="*" safe for production?

No—restrict it to trusted domains (e.g., "https://myapp.com").

Key Takeaways

• Genesis Block: Defines your chain’s rules and initial state.
• Initialization: Use --datadir to avoid conflicts.
• Pre-funding: Simplify testing with alloc in genesis.json.

👉 Explore advanced Ethereum network configurations for scaling options.