Private Networks

⚠️ This article is outdated. By now geth is not able to seal Ethash or Clique blocks. It only works in PoS mode and in concert with a consensus client. It is recommended to read Private network via Kurtosis instead.

This guide explains how to set up a private network of multiple Geth nodes. An Ethereum network is private if the nodes are not connected to the main network. In this context private only means reserved or isolated, rather than protected or secure. A fully controlled, private Ethereum network is useful as a backend for core developers working on issues relating to networking/blockchain syncing etc. Private networks are also useful for Dapp developers testing multi-block and multi-user scenarios.

Prerequisites

To follow the tutorial on this page it is necessary to have a working Geth installation (instructions here). It is also helpful to understand Geth fundamentals (see Getting Started).

Private Networks

A private network is composed of multiple Ethereum nodes that can only connect to each other. In order to run multiple nodes locally, each one requires a separate data directory (--datadir). The nodes must also know about each other and be able to exchange information, share an initial state and a common consensus algorithm. The remainder of this page will explain how to configure Geth so that these basic requirements are met, enabling a private network to be started.

Choosing A Network ID

Ethereum Mainnet has Network ID = 1. There are also many other networks that Geth can connect to by providing alternative Chain IDs, some are testnets and others are alternative networks built from forks of the Geth source code. Providing a network ID that is not already being used by an existing network or testnet means the nodes using that network ID can only connect to each other, creating a private network. A list of current network IDs is available at Chainlist.org. The network ID is controlled using the networkid flag, e.g.

geth --networkid 12345

Choosing A Consensus Algorithm

While the main network uses proof-of-stake (PoS) to secure the blockchain, Geth also supports the 'Clique' proof-of-authority (PoA) consensus algorithm and the Ethash proof-of-work algorithm as alternatives for private networks. Clique is strongly recommended for private testnets because PoA is far less resource-intensive than PoW. The key differences between the consensus algorithms available in Geth are:

Ethash

Geth's PoW algorithm, Ethash, is a system that allows open participation by anyone willing to dedicate resources to mining. While this is a critical property for a public network, the overall security of the blockchain strictly depends on the total amount of resources used to secure it. As such, PoW is a poor choice for private networks with few miners. The Ethash mining 'difficulty' is adjusted automatically so that new blocks are created approximately 12 seconds apart. As more mining resources are deployed on the network, creating a new block becomes harder so that the average block time matches the target block time.

Clique

⚠️ Since geth v1.14 clique has been deprecated

Clique consensus is a PoA system where new blocks can be created by authorized 'signers' only. The clique consensus protocol is specified in EIP-225. The initial set of authorized signers is configured in the genesis block. Signers can be authorized and de-authorized using a voting mechanism, thus allowing the set of signers to change while the blockchain operates. Clique can be configured to target any block time (within reasonable limits) since it isn't tied to the difficulty adjustment.

Creating The Genesis Block

Every blockchain starts with a genesis block. When Geth is run with default settings for the first time, it commits the Mainnet genesis to the database. For a private network, it is generally preferable to use a different genesis block. The genesis block is configured using a genesis.json file whose path must be provided to Geth on start-up. When creating a genesis block, a few initial parameters for the private blockchain must be defined:

Clique Example

⚠️ Since geth v1.14 clique has been deprecated

Below is an example of a genesis.json file for a PoA network. The config section ensures that all known protocol changes are available and configures the 'clique' engine to be used for consensus. Note that the initial signer set must be configured through the extradata field. This field is required for Clique to work.

The signer account keys can be generated using the geth account command (this command can be run multiple times to create more than one signer key).

geth account new --datadir data

The Ethereum address printed by this command should be recorded. To encode the signer addresses in extradata, concatenate 32 zero bytes, all signer addresses and 65 further zero bytes. The result of this concatenation is then used as the value accompanying the extradata key in genesis.json. In the example below, extradata contains a single initial signer address, 0x7df9a875a174b3bc565e6424a0050ebc1b2d1d82.

The period configuration option sets the target block time of the chain.

{
  "config": {
    "chainId": 12345,
    "homesteadBlock": 0,
    "eip150Block": 0,
    "eip155Block": 0,
    "eip158Block": 0,
    "byzantiumBlock": 0,
    "constantinopleBlock": 0,
    "petersburgBlock": 0,
    "istanbulBlock": 0,
    "berlinBlock": 0,
    "clique": {
      "period": 5,
      "epoch": 30000
    }
  },
  "difficulty": "1",
  "gasLimit": "8000000",
  "extradata": "0x00000000000000000000000000000000000000000000000000000000000000007df9a875a174b3bc565e6424a0050ebc1b2d1d820000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
  "alloc": {
    "7df9a875a174b3bc565e6424a0050ebc1b2d1d82": { "balance": "300000" },
    "f41c74c9ae680c1aa78f42e5647a62f353b7bdde": { "balance": "400000" }
  }
}

Ethash Example

Since Ethash is the default consensus algorithm, no additional parameters need to be configured in order to use it. The initial mining difficulty is influenced using the difficulty parameter, but note that the difficulty adjustment algorithm will quickly adapt to the amount of mining resources deployed on the chain.

{
  "config": {
    "chainId": 12345,
    "homesteadBlock": 0,
    "eip150Block": 0,
    "eip155Block": 0,
    "eip158Block": 0,
    "byzantiumBlock": 0,
    "constantinopleBlock": 0,
    "petersburgBlock": 0,
    "istanbulBlock": 0,
    "berlinBlock": 0,
    "ethash": {}
  },
  "difficulty": "1",
  "gasLimit": "8000000",
  "alloc": {
    "7df9a875a174b3bc565e6424a0050ebc1b2d1d82": { "balance": "300000" },
    "f41c74c9ae680c1aa78f42e5647a62f353b7bdde": { "balance": "400000" }
  }
}

Initializing the Geth Database

To create a blockchain node that uses this genesis block, first use geth init to import and sets the canonical genesis block for the new chain. This requires the path to genesis.json to be passed as an argument.

geth init --datadir data genesis.json

When Geth is started using --datadir data the genesis block defined in genesis.json will be used. For example:

geth --datadir data --networkid 12345

The default value for the storage scheme is hash. In case the plan is to use the path based storage scheme, the --state.scheme=path needs to be passed during the init step. This will ensure that the database is initialized with the correct storage scheme for the network.

geth --state.scheme=path init --datadir data genesis.json

Scheduling Hard Forks

As Ethereum protocol development progresses, new features become available. To enable these features on an existing private network, a hard fork must be scheduled. To do this, a future block number must be chosen which determines precisely when the hard fork will activate. Continuing the genesis.json example above and assuming the current block number is 35421, a hard fork might be scheduled for block 40000. This hard fork might upgrade the network to conform to the 'London' specs. First, all the Geth instances on the private network must be recent enough to support the specific hard fork. If so, genesis.json can be updated so that the londonBlock key gets the value 40000. The Geth instances are then shut down and geth init is run to update their configuration. When the nodes are restarted they will pick up where they left off and run normally until block 40000, at which point they will automatically upgrade.

The modification to genesis.json is as follows:

{
  "config": {
    "londonBlock": 40000
  }
}

The upgrade command is:

geth init --datadir data genesis.json

Setting Up Networking

With the node configured and initialized, the next step is to set up a peer-to-peer network. This requires a bootstrap node. The bootstrap node is a normal node that is designated to be the entry point that other nodes use to join the network. Any node can be chosen to be the bootstrap node.

To configure a bootstrap node, the IP address of the machine the bootstrap node will run on must be known. The bootstrap node needs to know its own IP address so that it can broadcast it to other nodes. On a local machine this can be found using tools such as ifconfig and on cloud instances such as Amazon EC2 the IP address of the virtual machine can be found in the management console. Any firewalls must allow UDP and TCP traffic on port 30303.

The bootstrap node IP is set using the --nat flag (the command below contains an example address - replace it with the correct one).

geth --datadir data --networkid 15 --nat extip:172.16.254.4

The 'node record' of the bootnode can be extracted using the JS console:

geth attach --exec admin.nodeInfo.enr data/geth.ipc

This command should print a base64 string such as the following example. Other nodes will use the information contained in the bootstrap node record to connect to the peer-to-peer network.

"enr:-Je4QEiMeOxy_h0aweL2DtZmxnUMy-XPQcZllrMt_2V1lzynOwSx7GnjCf1k8BAsZD5dvHOBLuldzLYxpoD5UcqISiwDg2V0aMfGhGlQhqmAgmlkgnY0gmlwhKwQ_gSJc2VjcDI1NmsxoQKX_WLWgDKONsGvxtp9OeSIv2fRoGwu5vMtxfNGdut4cIN0Y3CCdl-DdWRwgnZf"

If the nodes are intended to connect across the Internet, the bootnode and all other nodes must have public IP addresses assigned, and both TCP and UDP traffic can pass their firewalls. If Internet connectivity is not required or all member nodes connect using well-known IPs, Geth should be set up to restrict peer-to-peer connectivity to an IP subnet. Doing so will further isolate the network and prevents cross-connecting with other blockchain networks in case the nodes are reachable from the Internet. Use the --netrestrict flag to configure a whitelist of IP networks:

geth <other-flags> --netrestrict 172.16.254.0/24

With the above setting, Geth will only allow connections from the 172.16.254.0/24 subnet, and will not attempt to connect to other nodes outside of the set IP range.

Running Member Nodes

Before running a member node, it must be initialized with the same genesis file as used for the bootstrap node. With the bootnode operational and externally reachable (telnet <ip> <port> will confirm that it is indeed reachable), more Geth nodes can be started and connected to them via the bootstrap node using the --bootnodes flag. The process is to start Geth on the same machine as the bootnode, with a separate data directory and listening port and the bootnode node record provided as an argument:

For example, using data directory (example: data2) and listening port (example: 30305):

geth --datadir data2 --networkid 12345 --port 30305 --bootnodes <bootstrap-node-record>

With the member node running, it is possible to check that it is connected to the bootstrap node or any other node in the network by attaching a console and running admin.peers. It may take up to a few seconds for the nodes to get connected.

geth attach data2/geth.ipc --exec admin.peers

Running A Signer (Clique)

:> [!WARNING] Since geth v1.14 clique is not an option anymore

To set up Geth for signing blocks in Clique, a signer account must be available. The account must already be available as a keyfile in the keystore. To use it for signing blocks, it must be unlocked. The following command, for address 0x7df9a875a174b3bc565e6424a0050ebc1b2d1d82 will prompt for the account password, then start signing blocks:

geth <other-flags> --unlock 0x7df9a875a174b3bc565e6424a0050ebc1b2d1d82 --mine

Mining can be further configured by changing the default gas limit blocks converge to (with --miner.gastarget) and the price transactions are accepted at (with --miner.gasprice).

Running A Miner (Ethash)

For PoW in a simple private network, a single CPU miner instance is enough to create a stable stream of blocks at regular intervals. To start a Geth instance for mining, it can be run with all the usual flags plus the following to configure mining:

geth <other-flags> --mine --miner.threads=1 --miner.etherbase=0xf41c74c9ae680c1aa78f42e5647a62f353b7bdde

This will start mining bocks and transactions on a single CPU thread, crediting all block rewards to the account specified by --miner.etherbase.

End-to-end example

This section will run through the commands for setting up a simple private network of two nodes. Both nodes will run on the local machine using the same genesis block and network ID. The data directories for each node will be named node1 and node2.

`mkdir node1 node2`

Each node will have an associated account that will receive some ether at launch. The following command creates an account for Node 1:

geth --datadir node1 account new

This command returns a request for a password. Once a password has been provided the following information is returned to the terminal:

The keyfile and account password should be backed up securely. These steps can then be repeated for Node 2. These commands create keyfiles that are stored in the keystore directory in node1 and node2 data directories. In order to unlock the accounts later the passwords for each account should be saved to a text file in each node's data directory.

In each data directory save a copy of the following genesis.json to the top level project directory. The account addresses in the alloc field should be replaced with those created for each node in the previous step (without the leading 0x). Additionally, replace the initial signer address in extradata with the account address of Node 1.

{
  "config": {
    "chainId": 123454321,
    "homesteadBlock": 0,
    "eip150Block": 0,
    "eip155Block": 0,
    "eip158Block": 0,
    "byzantiumBlock": 0,
    "constantinopleBlock": 0,
    "petersburgBlock": 0,
    "istanbulBlock": 0,
    "muirGlacierBlock": 0,
    "berlinBlock": 0,
    "londonBlock": 0,
    "arrowGlacierBlock": 0,
    "grayGlacierBlock": 0,
    "clique": {
      "period": 5,
      "epoch": 30000
    }
  },
  "difficulty": "1",
  "gasLimit": "800000000",
  "extradata": "0x0000000000000000000000000000000000000000000000000000000000000000C1B2c0dFD381e6aC08f34816172d6343Decbb12b0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
  "alloc": {
    "C1B2c0dFD381e6aC08f34816172d6343Decbb12b": { "balance": "1000000000000000000" },
    "c94d95a5106270775351eecfe43f97e8e75e59e8": { "balance": "1000000000000000000" }
  }
}

The nodes can now be set up using geth init as follows:

geth init --datadir node1 genesis.json

This should be repeated for both nodes. The following will be returned to the terminal:

The next step is to configure a bootnode. This can be any node, but for this tutorial the developer tool bootnode will be used to quickly and easily configure a dedicated bootnode. First the bootnode requires a key, which can be created with the following command, which will save a key to boot.key:

bootnode -genkey boot.key

This key can then be used to generate a bootnode as follows:

bootnode -nodekey boot.key -addr :30305

The choice of port passed to -addr is arbitrary, but public Ethereum networks use 30303, so this is best avoided. The bootnode command returns the following logs to the terminal, confirming that it is running:

The two nodes can now be started. Open separate terminals for each node, leaving the bootnode running in the original terminal. In the terminal for Node 1, run the following command (replacing bootnodes, account address and etherbase for Node 1. The password file for Node 1 must also be provided):

geth --datadir node1 --port 30306 --bootnodes enode://f7aba85ba369923bffd3438b4c8fde6b1f02b1c23ea0aac825ed7eac38e6230e5cadcf868e73b0e28710f4c9f685ca71a86a4911461637ae9ab2bd852939b77f@127.0.0.1:0?discport=30305  --networkid 123454321 --unlock 0xC1B2c0dFD381e6aC08f34816172d6343Decbb12b --password node1/password.txt --authrpc.port 8551 --mine --miner.etherbase 0xC1B2c0dFD381e6aC08f34816172d6343Decbb12b

In the terminal for Node 2, run the following command (replacing bootnodes and account address for Node 2. The password file for Node 2 must also be provided):

geth --datadir node2 --port 30307 --bootnodes enode://f7aba85ba369923bffd3438b4c8fde6b1f02b1c23ea0aac825ed7eac38e6230e5cadcf868e73b0e28710f4c9f685ca71a86a4911461637ae9ab2bd852939b77f@127.0.0.1:0?discport=30305  --networkid 123454321 --unlock 0xc94d95a5106270775351eecfe43f97e8e75e59e8 --password node2/password.txt --authrpc.port 8552

These will start the nodes using the bootnode as an entry point and run Node 1 as a signer. In each terminal, the following logs indicate success:

In the first terminal that is currently running the logs resembling the following will be displayed, showing the discovery process in action:

It is now possible to attach a Javascript console to either node to query the network properties:

geth attach node1/geth.ipc

Once the Javascript console is running, check that the node is connected to one other peer (Node 2):

net.peerCount

The details of this peer can also be queried and used to check that the peer really is Node 2:

admin.peers

This should return the following:

The account associated with Node 1 was supposed to be funded with some ether at the chain genesis. This can be checked easily using eth.getBalance():

eth.getBalance(eth.accounts[0])

This account can then be unlocked and some ether sent to Node 2, using the following commands:

// send some Wei
eth.sendTransaction({
  to: '0xc94d95a5106270775351eecfe43f97e8e75e59e8',
  from: eth.accounts[0],
  value: 25000
});

//check the transaction was successful by querying Node 2's account balance
eth.getBalance('0xc94d95a5106270775351eecfe43f97e8e75e59e8');

The same steps can then be repeated to attach a console to Node 2.

Summary

This page explored the various options for configuring a local private network. A step by step guide showed how to set up and launch a private network, unlock the associated accounts, attach a console to check the network status and make some basic interactions.