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](/docs/install-and-build/installing-geth)). It is also helpful to understand Geth fundamentals (see [Getting Started](/docs/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.
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](https://chainlist.org/). The network ID is controlled using the `networkid` flag, e.g.
While the main network uses proof-of-work (PoW) to secure the blockchain, Geth also supports the the 'Clique' proof-of-authority (PoA) consensus algorithm as an alternative for private networks. Clique is strongly recommended for private testnets because PoA is far less resource-intensive than PoW. Clique is currently used as the consensus algorithm in public testnets such as [Rinkeby](https://www.rinkeby.io) and [Görli](https://goerli.net). The key differences between the consensus algorithms available in Geth are:
#### Ethash
Geth's PoW algorithm, [Ethhash](https://ethereum.org/en/developers/docs/consensus-mechanisms/pow/mining-algorithms/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
Clique consensus is a PoA system where new blocks can be created by authorized 'signers' only. The clique consenus protocol is specified in [EIP-225][clique-eip]. 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.
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:
- Ethereum platform features enabled at launch (`config`). Enabling and disabling features once the blockchain is running requires scheduling a [hard fork](https://ethereum.org/en/glossary/#hard-fork).
- Initial block gas limit (`gasLimit`). This impacts how much EVM computation can happen within a single block. Mirroring the main Ethereum network is generally a [good choice][gaslimit-chart]. The block gas limit can be adjusted after launch using the `--miner.gastarget` command-line flag.
- Initial allocation of ether (`alloc`). This determines how much ether is available to the addresses listed in the genesis block. Additional ether can be created through mining as the chain progresses.
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](./managing-your-accounts) command (this command can be run multiple times to create more than one signer key).
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`.
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.
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.
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.
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 bootsrap 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.
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.
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
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.
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:
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.
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:
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:
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`.
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`).
The nodes can now be set up using `geth init` as follows:
```shell
geth init --datadir node1 genesis.json
```
This should be repeated for both nodes. The following will be returned to the terminal:
```terminal
INFO [05-13|15:41:47.520] Maximum peer count ETH=50 LES=0 total=50
INFO [05-13|15:41:47.520] Smartcard socket not found, disabling err="stat /run/pcscd/pcscd.comm: no such file or directory"
INFO [05-13|15:41:47.520] Set global gas cap cap=50,000,000
INFO [05-13|15:41:47.520] Allocated cache and file handles database=/home/go-ethereum/node2/geth/chaindata cache=16.00MiB handles=16
INFO [05-13|15:41:47.542] Writing custom genesis block
INFO [05-13|15:41:47.542] Persisted trie from memory database nodes=3 size=397.00B time="41.246µs" gcnodes=0 gcsize=0.00B gctime=0s livenodes=1 livesize=0.00B
INFO [05-13|15:41:47.543] Successfully wrote genesis state database=chaindata hash=c9a158..d415a0
INFO [05-13|15:41:47.543] Allocated cache and file handles database=/home/go-ethereum/node2/geth/chaindata cache=16.00MiB handles=16
INFO [05-13|15:41:47.556] Writing custom genesis block
INFO [05-13|15:41:47.557] Persisted trie from memory database nodes=3 size=397.00B time="81.801µs" gcnodes=0 gcsize=0.00B gctime=0s livenodes=1 livesize=0.00B
INFO [05-13|15:41:47.558] Successfully wrote genesis state database=chaindata hash=c9a158..d415a0
```
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`:
```shell
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:
Note: you're using cmd/bootnode, a developer tool.
We recommend using a regular node as bootstrap node for production deployments.
INFO [05-13|15:50:03.645] New local node record seq=1,652,453,403,645 id=a2d37f4a7d515b3a ip=nil udp=0 tcp=0
```
The two nodes can now be started. Open separate terminals for each node, leaving the bootnode running in the original terminal. In each terminal, run the following command (replacing `node1` with `node2` where appropriate, and giving each node a different port ID. The account address and password file for node 1 must also be provided:
This will start the node using the bootnode as an entry point. Repeat the same command with the information appropriate to node 2. In each terminal, the following logs indicate success:
```terminal
INFO [05-13|16:17:40.061] Maximum peer count ETH=50 LES=0 total=50
INFO [05-13|16:17:40.061] Smartcard socket not found, disabling err="stat /run/pcscd/pcscd.comm: no such file or directory"
INFO [05-13|16:17:40.061] Set global gas cap cap=50,000,000
INFO [05-13|16:17:40.061] Allocated trie memory caches clean=154.00MiB dirty=256.00MiB
INFO [05-13|16:17:40.061] Allocated cache and file handles database=/home/go-ethereum/node1/geth/chaindata cache=512.00MiB handles=524,288
INFO [05-13|16:17:40.094] Opened ancient database database=/home/go-ethereum/node1/geth/chaindata/ancient readonly=false
INFO [05-13|16:17:40.100] Starting peer-to-peer node instance=Geth/v1.10.18-unstable-8d84a701-20220503/linux-amd64/go1.18.1
INFO [05-13|16:17:40.130] New local node record seq=1,652,454,949,228 id=f1364e6d060c4625 ip=127.0.0.1 udp=30306 tcp=30306
INFO [05-13|16:17:40.130] Started P2P networking self=enode://87606cd0b27c9c47ca33541d4b68cf553ae6765e22800f0df340e9788912b1e3d2759b3d1933b6f739c720701a56ce26f672823084420746d04c25fc7b8c6824@127.0.0.1:30306
INFO [05-13|16:17:40.133] IPC endpoint opened url=/home/go-ethereum/node1/geth.ipc
INFO [05-13|16:17:40.785] Unlocked account address=0xC1B2c0dFD381e6aC08f34816172d6343Decbb12b
INFO [05-13|16:17:42.636] New local node record seq=1,652,454,949,229 id=f1364e6d060c4625 ip=82.11.59.221 udp=30306 tcp=30306
INFO [05-13|16:17:43.309] Mapped network port proto=tcp extport=30306 intport=30306 interface="UPNP IGDv1-IP1"
INFO [05-13|16:17:43.822] Mapped network port proto=udp extport=30306 intport=30306 interface="UPNP IGDv1-IP1"
[05-13|16:17:50.150] Looking for peers peercount=0 tried=0 static=0
INFO [05-13|16:18:00.164] Looking for peers peercount=0 tried=0 static=0
```
In the first terminal that is currently running the logs resembling the following will be displayed, showing the discovery process in action:
```terminal
INFO [05-13|15:50:03.645] New local node record seq=1,652,453,403,645 id=a2d37f4a7d515b3a ip=nil udp=0 tcp=0
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()`:
```shell
eth.getBalance(eth.accounts[0])
```
This account can then be unlocked and some ether sent to Node 2, using the following commands:
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.
