# Customize a Chain Spec

After you have a Substrate node crafted, you want to start a network with many peers! This guide shows one method to create chain specification files uniformly and distribute them so other nodes can discover and peer with your network *explicitly*.

This guide illustrates:

* How to generate a `chain-spec.json` and include it for other nodes to join a common network
* How to modify an existing plain chain spec without editing the node source code

### Create a chain specification

**Plain chain specification file**

1. Start in your node's working directory, generate a plain chain spec with this command:

   ```bash
   ./target/release/node-template build-spec > chain-spec-plain.json
   ```

   We have just generated a **plain chain spec** file for the *default* network set in your `chain_spec.rs` file. This file can be passed to other nodes.
2. Modify the plain chain specification (optional):

   This optional step we can leverage an *existing* plain chain specification for a network that otherwise would require modification of the *source* of the node to run on a *new network*. For example, this can be quite useful in the [Prepare a local relay chain](https://docs.infrablockchain.net/infrablockchain-docs/infrablockchain/tutorials/build/build-infra-relay-chain) where we want to create a custom *relay chain* without customizing Polkadot's source.

   Here we use the *same* chain spec, but pass a flag to disable bootnodes, as we want a *new* network where these nodes will be different.

   ```bash
   ./target/release/node-template build-spec --chain chain-spec-plain.json --raw --disable-default-bootnode > no-bootnodes-chain-spec-plain.json
   ```

   This `no-bootnodes-chain-spec-plain.json` can be used to generate a SCALE storage encoded, distributable raw chain spec.

**Raw chain specification file**

1. Generate the raw chain specification.

   With a plain spec available, you can generate a final raw chain spec by running:

   ```bash
   ./target/release/node-template build-spec --chain chain-spec-plain.json --raw > chain-spec.json
   ```

Raw chain specifications should always be used when passed into a node to execute with it.

### Publish the raw chain specification

Because Rust builds that target WebAssembly are optimized, the binaries aren't deterministically reproducible. If each network participant were to generate the plain and/or raw chain specification, the differences in the resulting Wasm blob would break consensus.

It is *conventional* to include the chain specification files for your node *within the source code itself* so that anyone can build your node in the same way, whereby it becomes easy to check for non-determinism by comparing a genesis blob with another. Polkadot, Kusama, Rococo, and more network chain spec files are found [in the source here](https://github.com/paritytech/polkadot-sdk/tree/master/polkadot/node/service/chain-specs) along with a `.gitignore` file to ensure that you don't accidentally change these `!/*.json` files as you build further on your node's software and do [runtime upgrades](https://docs.infrablockchain.net/infrablockchain-docs/infrablockchain/learn/substrate/tutorials/build-a-blockchain/upgrade-a-running-network).

### Start a new node

If you publish a node binary, or have users build their own and then they want to join your network, all then need is the *same* raw chain spec file and to run your binary with:

```bash
# binary named `node-template`
# `chain-spec.json` obtained from canonical common source
node-template --chain chain-spec.json
```

This can also simply be configured to be the *default* network. For reference, you can see how [Polkadot implements](https://github.com/paritytech/polkadot/commits/master/cli/src/command.rs) a default command that uses the chain specs for various networks [in the source here](https://github.com/paritytech/polkadot-sdk/tree/master/polkadot/node/service/chain-specs).

### Examples

* [Add trusted nodes](https://docs.infrablockchain.net/infrablockchain-docs/infrablockchain/learn/substrate/tutorials/build-a-blockchain/add-trusted-nodes)
* [Polkadot-like network chain specs](https://github.com/paritytech/polkadot-sdk/tree/master/polkadot/node/service/chain-specs)
* [Polkadot commands for many networks](https://github.com/paritytech/polkadot/commits/master/cli/src/command.rs)