> ## Documentation Index
> Fetch the complete documentation index at: https://docs.optimism.io/llms.txt
> Use this file to discover all available pages before exploring further.

# kona-host

> Canonical hub for kona-host, the fault proof host that serves preimage data to kona-client, covering what it does, who runs it, and where its guides, releases, source, and spec live.

kona-host is the OP Stack's fault proof host program: a native Rust binary,
built as part of the [Kona](/rust/kona/intro/overview) project, that runs the
preimage server supplying chain data to
[kona-client](/op-stack/components/kona-client) while it executes.

## Overview

kona-client runs with no network access, so it cannot fetch chain data
itself: everything it reads arrives as a preimage requested by key over the
preimage oracle ABI. kona-host is the other end of that channel. It fetches
the requested data (L1 headers, transactions, receipts, blobs, L2 state)
from its configured L1, L1 beacon, and L2 endpoints, responds to the
client's hints so the right preimages are ready when asked for, and serves
them back. Without it, the
fault proof program would have no way to learn the chain state it is asked
to verify.

kona-host runs in `single` mode (one pre-interop chain) or `super` mode (an
interop superchain cluster), and its preimage server starts in one of two
ways: `server` mode, where it only serves preimages to a client program run
by an FPVM (when [cannon](/op-stack/components/cannon) executes kona-client,
it launches kona-host as a subprocess, passing the host command after a `--`
separator on `cannon run`), and `native` mode, where kona-host runs
kona-client itself in a native process, useful for witness generation and
testing.

**Who runs it:** dispute game participants, indirectly:
[op-challenger](/op-stack/fault-proofs/challenger)'s `--cannon-kona-server`
flag points at a kona-host binary, which cannon launches when a fault
dispute reaches the execution-trace stage. Developers also run it directly
in native mode to execute kona-client without an FPVM.

## Get started

* [kona-host usage](https://github.com/ethereum-optimism/optimism/tree/develop/rust/kona/bin/host#usage):
  the in-repo README documents the host modes, the preimage server modes,
  and the CLI surface.

## How-tos

* [Run a fault-proof challenger](/use-cases/run-a-fault-proof-challenger):
  stand up an op-challenger deployment, including pointing
  `--cannon-kona-server` at a kona-host build that matches the configured
  prestate.
* [Spin up challenger](/chain-operators/tutorials/create-l2-rollup/op-challenger-setup):
  configure op-challenger for a new chain, including where to get the
  kona-host binary it needs.

## Configuration & flags reference

* kona-host has no configuration reference page on this site yet; it is
  configured entirely through CLI flags, documented by `kona-host --help`
  and the
  [in-repo README](https://github.com/ethereum-optimism/optimism/tree/develop/rust/kona/bin/host#usage).

## Releases

* [kona-host release history](/releases/kona-host)

## Source & spec links

* [Source: `rust/kona/bin/host`](https://github.com/ethereum-optimism/optimism/tree/develop/rust/kona/bin/host)
  in the Optimism monorepo.
* [Pre-image oracle specification](https://specs.optimism.io/fault-proof/index.html#pre-image-oracle):
  the normative definition of the ABI kona-host implements, including the
  preimage key types, hinting, and bootstrapping.