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How To: Create Your Own Private Ethereum Blockchain
image-courtsey-mercuryprotocol
“Hello, Testnet”
This is a guide for starting your own custom Ethereum blockchain on Mac, not to be confused with starting a node on the main Ethereum blockchain. Here we are starting an entirely new and separate blockchain that cannot interact with Ethereum mainnet.
Starting your own Ethereum blockchain is useful, educational, and safer than the public testnet. Learning to set up a private testnet provides tangibility to otherwise abstract concepts such as mining, network peers, and even the geth datadir.
Prerequisites
You need to have Geth installed. The easiest way to do this is through homebrew.
1.Open Terminal and install homebrew​
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ruby -e “$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
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2.Now install geth​
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 brew tap ethereum/ethereum
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 brew install ethereum
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Create Genesis File
The Genesis block is the first block in the chain, the Genesis file is a JSON file that defines the characteristics of that initial block and subsequently the rest of the blockchain.
1.Create a directory to hold your network files
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mkdir my-eth-chain
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cd my-eth-chain
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2.Create your genesis file
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 touch myGenesis.json
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3.Open your genesis file and paste the following
{ "config": { "chainId": 1994, "homesteadBlock": 0, "eip155Block": 0, "eip158Block": 0, "byzantiumBlock": 0 }, "difficulty": "400", "gasLimit": "2000000", "alloc": { "7b684d27167d208c66584ece7f09d8bc8f86ffff": { "balance": "100000000000000000000000" }, "ae13d41d66af28380c7af6d825ab557eb271ffff": { "balance": "120000000000000000000000" } } }
config
chainId — this is your chain’s identifier, and is used in replay protection.
homesteadBlock, eip155Block, eip158Block, byzantiumBlock — these relate to chain forking and versioning, so in our case
lets leave them 0 since we’re starting a new blockchain.
difficulty
This dictates how difficult it is to mine a block. Setting this value low (~10–10000) is helpful in a private blockchain as it lets you mine blocks quickly, which equals fast transactions, and plenty of ETH to test with. For comparison, the Ethereum mainnet Genesis file defines a difficulty of 17179869184.
gasLimit
This is the the total amount of gas that can be used in each block. With such a low mining difficulty, blocks will be moving pretty quick, but you should still set this value pretty high to avoid hitting the limit and slowing down your network.
alloc
Here you can allocate ETH to specific addresses. This won’t create the account for you, so make sure its an account you already have control of. You will need to add the account to your private chain in order to use it, and to do that you need access to the keystore/utc file. For example, Geth and MyEtherWallet give you access to this file when you create an account, but Metamask and Coinbase do not. The addresses provided are not real addresses, they are just examples. Here we allocate 100,000 and 120,000 ETH respectively.
Start your Node!
Now the real fun begins, we’re all set to fire up our new Ethereum blockchain.
1. Instantiate your data directory
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geth --datadir ./myDataDir init ./myGenesis.json
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2. Start your Ethereum peer node.
Networkid helps ensure the privacy of your network. You can use any number here (where we used “1114”), but other peers joining your network must use the same one.
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geth --datadir ./myDataDir --networkid 1114 console 2>> myEth.log
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Output should look like this:
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Welcome to the Geth JavaScript console!
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​
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instance: Geth/v1.7.3-stable-4bb3c89d/darwin-amd64/go1.8.3
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coinbase: 0xae13d41d66af28380c7af6d825ab557eb271ffff
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at block: 5 (Thu, 07 Dec 2017 17:08:48 PST)
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datadir: /Users/test/my-eth-chain/myDataDir
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modules: admin:1.0 clique:1.0 debug:1.0 eth:1.0 miner:1.0 net:1.0 personal:1.0 rpc:1.0 txpool:1.0 web3:1.0
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​
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>
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This is the geth JavaScript console. Any command with the symbol > should be typed here.
3. Display your Ethereum logs
Open another terminal window
cd to my-eth-chain
Type tail -f myEth.log
4. Import/Create an Account
If you allocated ETH in the Genesis file, import the corresponding account by dragging the UTC file​
into the myDataDir/keystoredirectory and skip to step 5.
In the geth JavaScript console, create an account:
 `>
personal.newAccount("")`
Do not forget this passphrase! You will be typing this a lot, so for this test
network you can keep it simple.
5. Set Default Account
Check your default account, type
 > eth.coinbase
If this address is the same as the one from step 4, skip the rest of step 5.
To set your default account, type 
 `>
miner.setEtherbase(web3.eth.accounts[0])`
6. Start mining
Check your balance with 
 > eth.getBalance(eth.coinbase)
Run 
 > miner.start()
Look at your other terminal window, you should see some mining action in the logs. Check your balance again and it should
be higher.
To end mining, type 
 > miner.stop()
Optional: Add Other Peers
Add more nodes to your private Ethereum network.
1. Start another peer
On your same machine instantiate a new datadir:
geth --datadir ./peer2DataDir init ./myGenesis.json
Launch the 2nd peer on a different port:
geth --datadir ./peer2DataDir --networkid 1114 --port 30304 console 2>> myEth2.log
2. Display your Ethereum logs
Open another terminal window
cd to my-eth-chain
Type tail -f myEth2.log
3. Join the 1st peer
In the geth JavaScript console of your 1st peer, type:
admin.nodeInfo.enode
Output will look like this: “enode://dcff6d9dcb14eeb1d1b7575b0653fa1025ad1b7722c6d652d0449f0966e97931b[email protected][::]:30303”
In the geth JavaScript console of your new 2nd peer, type:
admin.addPeer( “enode:// ” )
Make sure you replace “enode://…@” above with the output from admin.nodeInfo.enode which is specific to you. As shown above, the “[::]” is replaced with “127.0.0.1:30303” which is the IP:Port of the 1st peer.
4. Verify your nodes are now communicating
In the geth JavaScript console of your new 2nd peer, type:
admin.peers
Output should show that peer 2 is connected to127.0.0.1:30303
Helpful: geth console commands
admin.nodeInfo.enode
 net.listening
 net.peerCount
 admin.peers
 eth.coinbase
 eth.getBalance(eth.coinbase)
 personal
 eth.accounts
 miner.setEtherbase(web3.eth.accounts[0])
 miner.setEtherbase(“0xae13d41d66af28380c7af6d825ab557eb271ffff”)
 miner.start()
 miner.stop()
 miner.hashrate
 eth.getBlock(0)
 eth.getBlock(“latest”)
 eth.blockNumber 
 web3.eth.getBlock(BLOCK_NUMBER).hash
 eth.syncing
 debug.verbosity(6) //highest logging level, 3 is default
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How To: Create Your Own Private Ethereum Blockchain