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# Lightning Loop
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Lightning Loop is a non-custodial service offered by
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[Lightning Labs](https://lightning.engineering/) to bridge on-chain and
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off-chain Bitcoin using submarine swaps. This repository is home to the Loop
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client and depends on the Lightning Network daemon
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[lnd](https://github.com/lightningnetwork/lnd). All of lnd’s supported chain
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backends are fully supported when using the Loop client: Neutrino, Bitcoin
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Core, and btcd.
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In the current iteration of the Loop software, only off-chain to on-chain
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swaps are supported, where the Loop client sends funds off-chain in
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exchange for the funds back on-chain. This is called a **Loop Out**.
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The service can be used in various situations:
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- Acquiring inbound channel liquidity from arbitrary nodes on the Lightning
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network
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- Depositing funds to a Bitcoin on-chain address without closing active
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channels
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- Paying to on-chain fallback addresses in the case of insufficient route
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liquidity
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Loop also allow offers an experimental testnet version of on-chain to off-chain
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swaps, called **Loop In**. This allows you to use on-chain funds to increase
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the local balance of a channel.
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Potential uses for Loop In:
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- Refilling depleted channels with funds from cold-wallets or exchange
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withdrawals
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- Servicing off-chain Lightning withdrawals using on-chain payments, with no
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funds in channels required
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- As a failsafe payment method that can be used when channel liquidity along a
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route is insufficient
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## Development and Support
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The Loop client is current in an early beta state, and offers a simple command
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line application. Future APIs will be added to support implementation or use of
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the Loop service.
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The [GitHub issue tracker](https://github.com/lightninglabs/loop/issues) can be
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used to request specific improvements or register and get help with any
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problems. Community support is also available in the
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[LND Slack](https://join.slack.com/t/lightningcommunity/shared_invite/enQtMzQ0OTQyNjE5NjU1LWRiMGNmOTZiNzU0MTVmYzc1ZGFkZTUyNzUwOGJjMjYwNWRkNWQzZWE3MTkwZjdjZGE5ZGNiNGVkMzI2MDU4ZTE)
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.
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## Setup and Install
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LND and the loop client are using Go modules. Make sure that the `GO111MODULE`
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env variable is set to `on`.
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In order to execute a swap, **You need to run lnd 0.6.0+, or master built with
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sub-servers enabled.**
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### LND
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If you are building from source, and not using a 0.6.0 or higher release of
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lnd, make sure that you are using the `master` branch of lnd. You can get this
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by git cloning the repository
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```
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git clone https://github.com/lightningnetwork/lnd.git
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```
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Once the lnd repository is cloned, it will need to be built with special build
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tags that enable the swap. This enables the required lnd rpc services.
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```
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cd lnd
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make install tags="signrpc walletrpc chainrpc invoicesrpc"
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```
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Check to see if you have already installed lnd. If you have, you will need to
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delete the `.macaroon` files from your lnd directory and restart lnd.
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**Do not delete any other files other than the `.macaroon` files**
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```
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// Example on Linux to see macaroons in the default directory:
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ls ~/.lnd/data/chain/bitcoin/mainnet
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```
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This should show no `.macaroon` files. If it does? Stop lnd, delete macaroons,
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restart lnd.
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```
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lncli stop
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```
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Now delete the .macaroon files and restart lnd. (don't delete any other files)
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### Loopd
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After lnd is installed, you will need to clone the Lightning Loop repo and
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install the command line interface and swap client service.
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```
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git clone https://github.com/lightninglabs/loop.git
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cd loop/cmd
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go install ./...
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```
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## Execute a Swap
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After you have lnd and the Loop client installed, you can execute a Loop swap.
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The Loop client needs its own short-lived daemon which will deal with the swaps
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in progress.
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Command to start `loopd`::
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```
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loopd
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// Or if you want to do everything in the same terminal and background loopd
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loopd &
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// For testnet mode, you'll need to specify the network as mainnet is the
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default:
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loopd --network=testnet
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```
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By default `loopd` attempts to connect to the lnd instance running on
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`localhost:10009` and reads the macaroon and tls certificate from `~/.lnd`.
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This can be altered using command line flags. See `loopd --help`.
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`loopd` only listens on localhost and uses an unencrypted and unauthenticated
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connection.
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Now that loopd is running, you can initiate a simple Loop Out. This will pay
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out Lightning off-chain funds and you will receive Bitcoin on-chain funds in
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return. There will be some chain and routing fees associated with this swap.
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```
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loop out <amt_in_satoshis>
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```
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This will take some time, as it requires an on-chain confirmation. When the
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swap is initiated successfully, `loopd` will see the process through.
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To query in-flight swap statuses, run `loop monitor`.
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## Resume
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When `loopd` is terminated (or killed) for whatever reason, it will pickup
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pending swaps after a restart.
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Information about pending swaps is stored persistently in the swap database.
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Its location is `~/.loopd/<network>/loop.db`.
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## Multiple Simultaneous Swaps
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It is possible to execute multiple swaps simultaneously. Just keep loopd
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running.
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