nimbus-eth2/docs/the_nimbus_book/src/trusted-node-sync.md

# Sync from a trusted node

> **Note:** This feature is available from `v1.7.0` onwards

When you [start the beacon node](./quick-start.md) for the first time, it will connect to the beacon chain network and start syncing automatically - a process that can take several days.

Trusted node sync allows you to get started more quickly with Nimbus by fetching a recent checkpoint from a trusted node - you can get started in minutes instead of days.

To use trusted node sync, you must have access to a node that you trust that exposes the [Beacon API](./rest-api.md) (for example a locally running backup node).

Should this node, or your connection to it, be compromised, your node will not be able to detect whether or not it is being served false information.

It is possibly to use trusted node sync with a third-party API provider -- see [here](trusted-node-sync.md#verify-you-synced-the-correct-chain) for how to verify that the chain you are given corresponds to the canonical chain at the time.

## Perform a trusted node sync

> **Tip:** Make sure to replace `http://localhost:5052` in the commands below with the appropriate endpoint for you. `http://localhost:5052` is the endpoint exposed by Nimbus but this is not consistent across all clients. For example, if your trusted node is a [Prysm node](https://docs.prylabs.network/docs/how-prysm-works/ethereum-public-api#performing-requests-against-a-local-prysm-node), it exposes `127.0.0.1:3500` by default. Which means you would run the commands below with
>
> `--trusted-node-url=http://127.0.0.1:3500`

**Mainnet**

To sync Mainnet, from the `nimbus-eth2` directory run:

```sh
build/nimbus_beacon_node trustedNodeSync --network:mainnet \
 --data-dir=build/data/shared_mainnet_0 \
 --trusted-node-url=http://localhost:5052
```

**Prater (testnet)**

To sync Prater, from the `nimbus-eth2` directory run:

```sh
build/nimbus_beacon_node trustedNodeSync --network:prater \
 --data-dir=build/data/shared_prater_0  \
 --trusted-node-url=http://localhost:5052
```

> **Note:**
> Because trusted node sync by default copies all blocks via REST, if you use a third-party service to sync from, you may hit API limits. If this happens to you, you may need to use the `--backfill` option to [delay the backfill of the block history](./trusted-node-sync.md#delay-block-history-backfill).

## Verify you synced the correct chain

When performing a trusted node sync, you can manually verify that the correct chain was synced by comparing the head hash with other sources (e.g. your friends, forums, chats and web sites). If you're syncing using your own backup node you can retrieve the current head from the node using:

```sh
# Make sure to enable the `--rest` option when running your node:

curl http://localhost:5052/eth/v1/beacon/blocks/head/root
```

The `head` root is also printed in the log output at regular intervals.

> **Note:** this same [Beacon API](./rest-api.md) request should work with any third-party provider.
>
> For example, to test it out with our mainnet [testing server](rest-api.md#test-your-tooling-against-our-servers), you could run:
>
> ```
> curl -X GET http://testing.mainnet.beacon-api.nimbus.team/eth/v1/beacon/blocks/head/root
> ```


## Advanced

### Delay block history backfill

By default, both the state and the full block history will be downloaded from the trusted node.

It is possible to get started more quickly by delaying the backfill of the block history using the `--backfill=false` parameter. In this case, the beacon node will first sync to the current head so that it can start performing its duties, then backfill the blocks from the network.

You can also resume the trusted node backfill at any time by simply running the trusted node sync command again.

> **Warning:** While backfilling blocks, your node will not be able to answer historical requests or sync requests. This might lead to you being de-scored, and eventually disconnected, by your peers.

### Modify sync point

By default, the node will sync up to the latest finalized checkpoint of the node that you're syncing with. While you can choose a different sync point using a block hash or a slot number, this block must fall on an epoch boundary:

```
build/nimbus_beacon_node trustedNodeSync --blockId:0x239940f2537f5bbee1a3829f9058f4c04f49897e4d325145153ca89838dfc9e2 ...

```

### Sync from checkpoint files

If you have a state and a block file available, you can start the node using the finalized checkpoint options:

```sh
# Obtain a state and a block from a Beacon API - these must be in SSZ format:

curl -o state.32000.ssz -H 'Accept: application/octet-stream' http://localhost:5052/eth/v2/debug/beacon/states/32000
curl -o block.32000.ssz -H 'Accept: application/octet-stream' http://localhost:5052/eth/v2/beacon/blocks/32000

build/nimbus_beacon_node --data-dir:trusted --finalized-checkpoint-block=block.32000.ssz --finalized-checkpoint-state=state.32000.ssz
```

## Recreate historical state access indices

When performing checkpoint sync, the historical state data from the time before the checkpoint is not available. To recreate the indices and caches necessary for historical state access, run trusted node sync with the `--reindex` flag - this can be done on an already-synced node as well, in which case the process will simply resume where it left off:

```sh
build/nimbus_beacon_node trustedNodeSync --reindex=true
```