109 lines
4.3 KiB
Markdown
109 lines
4.3 KiB
Markdown
---
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title: Private network
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---
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An Ethereum network is a private network if the nodes are not connected to the main
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network nodes. In this context private only means reserved or isolated, rather than
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protected or secure.
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## Choosing A Network ID
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Since connections between nodes are valid only if peers have identical protocol version
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and network ID, you can effectively isolate your network by setting either of these to a
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non default value. We recommend using the `--networkid` command line option for this. Its
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argument is an integer, the main network has id 1 (the default). So if you supply your own
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custom network ID which is different than the main network your nodes will not connect to
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other nodes and form a private network.
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## Creating The Genesis Block
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Every blockchain starts with the genesis block. When you run geth with default settings
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for the first time, the main net genesis block is committed to the database. For a private
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network, you usually want a different genesis block.
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Here's an example of a custom genesis.json file. The `config` section ensures that certain
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protocol upgrades are immediately available. The `alloc` section pre-funds accounts.
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```json
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{
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"config": {
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"chainId": 15,
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"homesteadBlock": 0,
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"eip155Block": 0,
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"eip158Block": 0
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},
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"difficulty": "200000000",
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"gasLimit": "2100000",
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"alloc": {
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"7df9a875a174b3bc565e6424a0050ebc1b2d1d82": { "balance": "300000" },
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"f41c74c9ae680c1aa78f42e5647a62f353b7bdde": { "balance": "400000" }
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}
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}
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```
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To create a database that uses this genesis block, run the following command. This will
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import and set the canonical genesis block for your chain.
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```text
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geth --datadir path/to/custom/data/folder init genesis.json
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```
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Future runs of geth on this data directory will use the genesis block you have defined.
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```text
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geth --datadir path/to/custom/data/folder --networkid 15
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```
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## Network Connectivity
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With all nodes that you want to run initialized to the desired genesis state, you'll need
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to start a bootstrap node that others can use to find each other in your network and/or
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over the internet. The clean way is to configure and run a dedicated bootnode:
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```text
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bootnode --genkey=boot.key
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bootnode --nodekey=boot.key
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```
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With the bootnode online, it will display an enode URL that other nodes can use to connect
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to it and exchange peer information. Make sure to replace the displayed IP address
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information (most probably [::]) with your externally accessible IP to get the actual
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enode URL.
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Note: You can also use a full fledged Geth node as a bootstrap node.
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### Starting Up Your Member Nodes
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With the bootnode operational and externally reachable (you can try `telnet <ip> <port>`
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to ensure it's indeed reachable), start every subsequent Geth node pointed to the bootnode
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for peer discovery via the --bootnodes flag. It will probably also be desirable to keep
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the data directory of your private network separated, so do also specify a custom
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`--datadir` flag.
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```text
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geth --datadir path/to/custom/data/folder --networkid 15 --bootnodes <bootnode-enode-url-from-above>
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```
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Since your network will be completely cut off from the main and test networks, you'll also
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need to configure a miner to process transactions and create new blocks for you.
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## Running A Private Miner
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Mining on the public Ethereum network is a complex task as it's only feasible using GPUs,
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requiring an OpenCL or CUDA enabled ethminer instance. For information on such a setup,
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please consult the EtherMining subreddit and the Genoil miner repository.
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In a private network setting however, a single CPU miner instance is more than enough for
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practical purposes as it can produce a stable stream of blocks at the correct intervals
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without needing heavy resources (consider running on a single thread, no need for multiple
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ones either). To start a Geth instance for mining, run it with all your usual flags,
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extended by:
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```text
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$ geth <usual-flags> --mine --minerthreads=1 --etherbase=0x0000000000000000000000000000000000000000
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```
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Which will start mining bocks and transactions on a single CPU thread, crediting all
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proceedings to the account specified by --etherbase. You can further tune the mining by
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changing the default gas limit blocks converge to (`--targetgaslimit`) and the price
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transactions are accepted at (`--gasprice`).
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