# Lightning Loop
 
Lightning Loop is a non-custodial service offered by
[Lightning Labs](https://lightning.engineering/) to bridge on-chain and
off-chain Bitcoin using submarine swaps. This repository is home to the Loop
client and depends on the Lightning Network daemon
[lnd](https://github.com/lightningnetwork/lnd). All of lnd’s supported chain
backends are fully supported when using the Loop client: Neutrino, Bitcoin
Core, and btcd.

In the current iteration of the Loop software, only off-chain to on-chain
swaps are supported, where the Loop client sends funds off-chain in
exchange for the funds back on-chain. This is called a **Loop Out**.

The service can be used in various situations:

- Acquiring inbound channel liquidity from arbitrary nodes on the Lightning
    network
- Depositing funds to a Bitcoin on-chain address without closing active
    channels
- Paying to on-chain fallback addresses in the case of insufficient route
    liquidity

Loop also allow offers an experimental testnet version of on-chain to off-chain
swaps, called **Loop In**. This allows you to use on-chain funds to increase
the local balance of a channel.

Potential uses for Loop In:

- Refilling depleted channels with funds from cold-wallets or exchange
    withdrawals
- Servicing off-chain Lightning withdrawals using on-chain payments, with no
    funds in channels required
- As a failsafe payment method that can be used when channel liquidity along a
    route is insufficient

## Development and Support

The Loop client is current in an early beta state, and offers a simple command
line application. Future APIs will be added to support implementation or use of
the Loop service.

The [GitHub issue tracker](https://github.com/lightninglabs/loop/issues) can be
used to request specific improvements or register and get help with any
problems. Community support is also available in the
[LND Slack](https://join.slack.com/t/lightningcommunity/shared_invite/enQtMzQ0OTQyNjE5NjU1LWRiMGNmOTZiNzU0MTVmYzc1ZGFkZTUyNzUwOGJjMjYwNWRkNWQzZWE3MTkwZjdjZGE5ZGNiNGVkMzI2MDU4ZTE)
.

## Setup and Install

LND and the loop client are using Go modules. Make sure that the `GO111MODULE`
env variable is set to `on`.

In order to execute a swap, **You need to run lnd 0.6.0+, or master built with
sub-servers enabled.**

### LND

If you are using a binary release of lnd that is 0.6 or higher, it is
compatible with Lightning Loop. Otherwise, you will need to build from source.

If you are building from source make sure that your lnd is built with build
tags `chainrpc invoicesrpc signrpc walletrpc` that enable additional lnd APIs.

To build lnd using these tags:

```
cd lnd
make install tags="signrpc walletrpc chainrpc invoicesrpc"
```

### Loopd

After lnd is installed, you will need to either download a Loop binary release
or clone the Lightning Loop repo and install the command line interface and
swap client service.

To install from source:

```
git clone https://github.com/lightninglabs/loop.git
cd loop/cmd
go install ./...
```

## Execute a Swap

After you have lnd and the Loop client installed, you can execute a Loop swap.

The Loop client needs its own short-lived daemon which will deal with the swaps
in progress.

Command to start `loopd`::

```
loopd

// Or if you want to do everything in the same terminal and background loopd
loopd &

// For testnet mode, you'll need to specify the network as mainnet is the
default:
loopd --network=testnet
```

By default `loopd` attempts to connect to the lnd instance running on
`localhost:10009` and reads the macaroon and tls certificate from `~/.lnd`.
This can be altered using command line flags. See `loopd --help`.

`loopd` only listens on localhost and uses an unencrypted and unauthenticated
connection.

Now that loopd is running, you can initiate a simple Loop Out. This will pay
out Lightning off-chain funds and you will receive Bitcoin on-chain funds in
return. There will be some chain and routing fees associated with this swap.

```
loop out <amt_in_satoshis>
```

This will take some time, as it requires an on-chain confirmation. When the
swap is initiated successfully, `loopd` will see the process through.

To query in-flight swap statuses, run `loop monitor`.

## Resume

When `loopd` is terminated (or killed) for whatever reason, it will pickup
pending swaps after a restart. 

Information about pending swaps is stored persistently in the swap database.
Its location is `~/.loopd/<network>/loop.db`.

## Multiple Simultaneous Swaps

It is possible to execute multiple swaps simultaneously. Just keep loopd 
running.