How to set up a multi-node private Ethereum blockchain on your Mac

8 min readAug 17, 2018

In this tutorial we will:

Setup a blockchain with multiple nodes
Setup mining nodes
Connect the multiple nodes and setup the blockchain network
Test the blockchain network by mining blocks and verifying that blocks are propagated to all the nodes
Verify that the local copy of blockchain on all the nodes is updated

Setup and Prerequisite Software

Go Etheruem (Geth) is a command line client interface tool that allows you to interact with your private Ethereum blockchain.

If you need to install Homebrew on your Mac, place the following line in your terminal command prompt:

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Once Homebrew is installed, install Geth:

$ brew tap ethereum/ethereum
$ brew install ethereum

Configure the Genesis Block

Create a new project directory. Within it create the genesis.json file using any editor of your choice. I use Visual Studio code on Mac, which is a free editor:

$ mkdir project3
$ cd project3

Copy the following JSON into your genesis.json file, and save the file in your project directory.

\\genesis.json file{"config": {"chainId": 4321,"homesteadBlock": 0,"eip155Block": 0,"eip158Block": 0},"alloc": {},"difficulty" : "0x20000","gasLimit"   : "0x8880000"}

Set up the first node (Node 1)

Once the genesis.json file is saved, you are ready to create your first node. To create your first node, open a new terminal window and navigate to your project folder, and type in the following command:

$ geth --datadir blkchain1 init genesis.json

This command initializes a new blockchain node using the configuration specified in the genesis.json file. By using — -datadir we specify the name of the directory where the local copy of the blockchain will be stored on the node.

This command will create a directory called blkchain1 in your project directory. This directory contains the geth and keystore directories. The blockchain data will be stored in the local database in the geth directory.

You can verify this by navigating into the blkchain1 directory in your command terminal and typing in ls -l. Also navigate to the geth sub-directory and type in ls -l.

Now that your Node 1 is initialized, let’s start Node 1 using Geth. Within your terminal window, navigate to your project folder (where you saved your genesis.json file) and type in the following:

$ geth --datadir blkchain1 --nodiscover --networkid 1234 console

This command will start your first node and bring up the Geth console, where you can type in commands to interact with the Node1 blockchain.

You will see the following:

Let’s break down this “geth console” command and understand it:

— -datadir blkchain1:
specifies the data directory of the blockchain. If you do not specify this, it will default to the main Ethereum blockchain.

--nodiscover:
disables the peer discovery mechanism and enables manual peer addition.

--networkid 1234:
identity of your Ethereum network, other peer nodes will also have the same network identifier. The network id can be any random integer value.

Now that you are in the Geth Console, to get more information about the node we will use the admin command. Type in the following command in the Geth console:

> admin.nodeInfo

This will give you more information about your node:

Notice the following:

by default the listener port is 30303
the enode id is your node id
the discovery port is ‘0’, since we set it to --nodiscover.

To get a list of all admin commands, type in “admin.” and press <tab>

Now, let’s setup an account where the mined ethers will be collected. Type in the following in the Geth console:

> personal.newAccount()

You can put in any passphrase — this is your password. The command returns the account id.

To get a list of all the accounts:

> personal.listAccounts

Node 1 is now ready!

Keep the terminal window (with the Geth Console for Node1) open and running. We will open a new terminal window to setup Node 2.

Set up the second node (Node 2)

Now let’s setup a second node in the blockchain network. The process will be similar to setting up Node1.

Open a new terminal window and navigate to the project folder that contains the genesis.json file.

Initialize the new node with the following command:

$ geth --datadir blkchain2 init genesis.json

Note: Since we want this node to be part of the same blockchain, we use the same genesis block.

This will create a new node whose data will be stored in a new directory called blkchain2 (this will contain the local copy of the blockchain database for the node 2).

To get the node 2 up and running with a Geth console, type in the following:

$ geth --datadir blkchain2 --nodiscover --networkid 1234 --port 30304 console

This will start up Node 2 and bring up the Geth console connected to Node 2.

Note the following differences for Node 2:

we specify a port number 30304
The default port 30303 is already being used by the first node, so if we do not specify a separate port it will throw an error.
we give the same networkid as we did for node 1. This is important, since we want both the nodes to be part of the network
we specify that the — -datadir for node2 will be in blkchain2

Run the admin.nodeInfo command in Geth Console for Node2:

> admin.nodeInfo

Notice that this provides the node information for Node 2. You can compare the enode id by running the same admin.nodeInfo command in the Terminal window running the Geth console for Node1.

Similar to Node 1, we can setup an account on Node 2 by typing in personal.newAccount() in the Geth console for Node2.

You can use any passphrase for the account.

> personal.newAccount()

Node 2 is now ready!

Keep open both the terminal windows, one running the Geth Console (Node1) and the second running the Geth Console (Node2), side by side.

In the next step we will connect Node1 and Node2 and create the blockchain network!

Connect the Nodes

Congratulations! You have 2 blockchain nodes running.

The next step is to connect them to each other and create a network. We will do this by adding one of the nodes to the other as a “peer”.

Run the following command on the Geth Consoles of both Node1 and Node2:

> admin.peers

Notice that both return an empty array, since the nodes are not connected to any other nodes.

Let’s add Node1 as a peer to Node2.

Run the admin.nodeInfo command on Node1

2. Copy the enode id for Node 1:

"enode://549468e6d00e135128af33e03a6d27b0ee5fda7fbd0154b2e83fe68afdfda869eb6ace6ccaefe84ed7a5b804529dcef49f0b5d64be97da87b1e28ddecfca227a@[::]:30303?discport=0"

3. In the Geth Console for Node2, add the Node1 as a peer to Node2 using the command admin.addPeer(“//enode id”):

> admin.addPeer("enode://549468e6d00e135128af33e03a6d27b0ee5fda7fbd0154b2e83fe68afdfda869eb6ace6ccaefe84ed7a5b804529dcef49f0b5d64be97da87b1e28ddecfca227a@[::]:30303?discport=0")

Now when you run the admin.peers command in either of the Geth Console you will see the other Node listed as a peer!

Hurray! You now have a connect blockchain network!

But how do we know for sure?

Let’s put it to the test, shall we? Let’s update something in Node1 and see if that change is propagated through the blockchain network to Node2.

Verify that you have a connected blockchain and a distributed database

Ok, thus far you should have:

Two terminal windows open
One running Geth Console for Node1, and the second running Get Console for Node2
Node 1 saves its local copy of the blockchain in the blkchain1 folder
Node 2 saves its local copy of the blockchain in the blkchain2 folder
Also, in the previous step you connected the two nodes using admin.addPeer()

Thus, if the new blocks are mined and added in Node 1, this new block should propagate over the blockchain network, and the local blockchain copy of node2 should update automatically.

Let us verify if this happens.

In the Geth Console(Node1) let’s begin mining:

> miner.start(1)

To check how many blocks have been mined at Node1 at any point, run the following in the Geth Console (Node1):

> eth.blockNumber

This will return the number of the current block mined (or the blockchain height).

After a few blocks have been mined, go to the Geth Console (Node 2) and run the eth.blockNumber command.

You will notice that the block height in Node 2 matches with the block height in Node 1! This verifies that the Blocks that were mined in Node 1 were propagated over the Blockchain to Node 2.

To kick it up a notch, you can verify the data in the actual blocks on Node 1 and Node 2.

Wait until about 10 blocks have been mined and then run the following command in Geth Console(Node1) and Geth Console(Node2):

> eth.getBlock(3)

This command displays the contents of Block 3 from the blockchain. Notice that the data displayed in Geth Console (Node1) and Geth Console(Node2) is identical, indicating that the Block3 that was mined by Node1 was propagated over the blockchain network and is also part of the local blockchain data in Node 2.