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# 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.
[Lightning Labs](https://lightning.engineering/) that makes it easy to move
bitcoin into and out of the Lightning Network.
In the current iteration of the Loop software, two swap types are supported:
* off-chain to on-chain, where the Loop client sends funds off-chain in
* on-chain to off-chain, where the Loop client sends funds to an on-chain
address using an off-chain channel
## Features
- Automated channel balancing
- Privacy-forward non-custodial swaps
- Opportunistic transaction batching to save on fees
- Progress monitoring of in-flight swaps
We call off-chain to on-chain swaps, a **Loop Out**. The service can be used
in various situations:
## Use Cases
- Automate channel balancing with AutoLoop ([Learn more](https://github.com/lightninglabs/loop/blob/master/docs/autoloop.md))
- Pay or deposit to a Bitcoin address without closing channels
- Pay Lightning Invoices with on-chain bitcoin
- Convert outbound liquidity into inbound liquidity
- Refill depleted Lightning channels
- Service Lightning withdrawals with on-chain payments
- 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
## Installation
Download the latest binaries from the [releases](https://github.com/lightninglabs/loop/releases) page.
We call our on-chain to off-chain swaps, a **Loop In**. This allows you to use
on-chain funds to increase the local balance of a channel, effectively
"refilling" an existing 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 currently 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 Loop daemon exposes a [gRPC API](https://lightning.engineering/loopapi/#lightning-loop-grpc-api-reference)
(defaults to port 11010) and a [REST API](https://lightning.engineering/loopapi/index.html#loop-rest-api-reference)
(defaults to port 8081).
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://lightning.engineering/slack.html)
.
## 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 a compatible lnd version built
with the correct sub-servers enabled.**
### LND
To run loop, you need a compatible version of `lnd` running. It is generally
recommended to always keep both `lnd` and `loop` updated to the most recent
released version.
If you are building from source make sure you are using the latest tagged
version of lnd. You can get this by git cloning the repository and checking out
a specific tag:
## Execution
The Loop client needs its own short-lived daemon to facilitate swaps. To start `loopd`:
```
git clone https://github.com/lightningnetwork/lnd.git
cd lnd
git checkout v0.x.x-beta
loopd
```
Once the lnd repository is cloned, it will need to be built with special build
tags that enable the swap. This enables the required lnd rpc services.
To use Loop in testnet, simply pass the network flag:
```
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`.
## LND
Note that Loop requires `lnd` to be built with **all of its subservers**. Download the latest [official release binary](https://github.com/lightningnetwork/lnd/releases/latest) or build `lnd` from source by following the [installation instructions](https://github.com/lightningnetwork/lnd/blob/master/docs/INSTALL.md). If you choose to build `lnd` from source, use the following command to enable all the relevant subservers:
```
make install tags="signrpc walletrpc chainrpc invoicesrpc"
```
Check to see if you have already installed lnd. If you have, you will need to
delete the `.macaroon` files from your lnd directory and restart lnd.
## Usage
**Do not delete any other files other than the `.macaroon` files**
```
// Example on Linux to see macaroons in the default directory:
ls ~/.lnd/data/chain/bitcoin/mainnet
```
This should show no `.macaroon` files. If it does? Stop lnd, delete macaroons,
restart lnd.
```
lncli stop
```
Now delete the .macaroon files and restart lnd. (don't delete any other files)
### Loopd
After lnd is installed, you will need to clone the Lightning Loop repo and
install the command line interface and swap client service.
```
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. If you set `restlisten` variable `loop` will use TLS encryption
and macaroon authentication, see [Authentication and transport security](https://github.com/lightninglabs/loop#authentication-and-transport-security)
for more details.
### Loop Out Swaps
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.
```
NAME:
loop out - perform an off-chain to on-chain swap (looping out)
USAGE:
loop out [command options] amt [addr]
DESCRIPTION:
Attempts to loop out the target amount into either the backing lnd's
wallet, or a targeted address.
The amount is to be specified in satoshis.
Optionally a BASE58/bech32 encoded bitcoin destination address may be
specified. If not specified, a new wallet address will be generated.
OPTIONS:
--channel value the 8-byte compact channel ID of the channel to loop out (default: 0)
--addr value the optional address that the looped out funds should be sent to, if let blank the funds will go to lnd's wallet
--amt value the amount in satoshis to loop out (default: 0)
--conf_target value the number of blocks from the swap initiation height that the on-chain HTLC should be swept within (default: 6)
--max_swap_routing_fee value the max off-chain swap routing fee in satoshis, if not specified, a default max fee will be used (default: 0)
--fast Indicate you want to swap immediately, paying potentially a higher fee. If not set the swap server might choose to wait up to 30 minutes before publishing the swap HTLC on-chain, to save on its chain fees. Not setting this flag therefore might result in a lower swap fee.
```
It's possible to receive more inbound capacity on a particular channel
(`--channel`), and also have the `loop` daemon send the coins to a target
address (`addr`). The latter option allows ones to effectively send on-chain
from their existing channels!
### AutoLoop
AutoLoop makes it easy to keep your channels balanced. Checkout our [autoloop documentation](https://github.com/lightninglabs/loop/blob/master/docs/autoloop.md) for details.
### Loop Out
Use Loop Out to move bitcoins on Lightning into an on-chain Bitcoin address.
To execute a Loop Out:
```
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.
Other notable options:
- Use the `--fast` flag to swap immediately (Note: This opts-out of fee savings made possible by transaction batching)
- Use the `--channel` flag to loop out on specific channels
- Use the `--addr` flag to specify the address the looped out funds should be sent to (Note: By default funds are sent to the lnd wallet)
To query in-flight swap statuses, run `loop monitor`.
Run `loop monitor` to monitor the status of a swap.
The loop client also has the ability to automatically dispatch loop out swaps on your behalf - see our [autoloop documentation](https://github.com/lightninglabs/loop/blob/master/docs/autoloop.md) for details.
### Loop In
Use Loop In to convert on-chain bitcoin into spendable Lightning funds.
### Fees explained
The following is an example output of a 0.01 BTC fast (non-batched) Loop Out
swap from `testnet`:
```bash
$ loop out --amt 1000000 --fast
Max swap fees for 1000000 sat Loop Out: 36046 sat
Fast swap requested.
CONTINUE SWAP? (y/n), expand fee detail (x): x
Estimated on-chain sweep fee: 149 sat
Max on-chain sweep fee: 14900 sat
Max off-chain swap routing fee: 20010 sat
Max off-chain prepay routing fee: 36 sat
Max no show penalty (prepay): 1337 sat
Max swap fee: 1100 sat
CONTINUE SWAP? (y/n):
```
Explanation:
- **Max swap fees for <x> sat Loop Out** (36046 sat): The absolute maximum in
fees that need to be paid. This includes on-chain and off-chain fees. This
represents the ceiling or worst-case scenario. The actual fees will likely be
lower. This is the sum of `14900 + 20010 + 36 + 1100` (see below).
- **Estimated on-chain sweep fee** (149 sat): The estimated cost to sweep the
HTLC in case of success, calculated based on the _current_ on-chain fees.
This value is called `miner_fee` in the gRPC/REST responses.
- **Max on-chain sweep fee** (14900 sat): The maximum on-chain fee the daemon
is going to allow for sweeping the HTLC in case of success. A fee estimation
based on the `--conf_target` flag is always performed before sweeping. The
factor of `100` times the estimated fee is applied in case the fees spike
between the time the swap is initiated and the time the HTLC can be swept. But
that is the absolute worst-case fee that will be paid. If there is no fee
spike, a normal, much lower fee will be used.
- **Max off-chain swap routing fee** (20010 sat): The maximum off-chain
routing fee that the daemon should pay when finding a route to pay the
Lightning invoice. This is a hard limit. If no route with a lower or equal fee
is found, the payment (and the swap) is aborted. This value is calculated
statically based on the swap amount (see `maxRoutingFeeBase` and
`maxRoutingFeeRate` in `cmd/loop/main.go`).
- **Max off-chain prepay routing fee** (36 sat): The maximum off-chain routing
fee that the daemon should pay when finding a route to pay the prepay fee.
This is a hard limit. If no route with a lower or equal fee is found, the
payment (and the swap) is aborted. This value is calculated statically based
on the prepay amount (see `maxRoutingFeeBase` and `maxRoutingFeeRate` in
`cmd/loop/main.go`).
- **Max no show penalty (prepay)** (1337 sat): This is the amount that has to be
pre-paid (off-chain) before the server publishes the HTLC on-chain. This is
necessary to ensure the server's on-chain fees are paid if the client aborts
and never completes the swap _after_ the HTLC has been published on-chain.
If the swap completes normally, this amount is counted towards the full swap
amount and therefore is actually a pre-payment and not a fee. This value is
called `prepay_amt` in the gRPC/REST responses.
- **Max swap fee** (1100 sat): The maximum amount of service fees we allow the
server to charge for executing the swap. The client aborts the swap if the
fee proposed by the server exceeds this maximum. It is therefore recommended
to obtain the maximum by asking the server for a quote first. The actual fees
might be lower than this maximum if user specific discounts are applied. This
value is called `swap_fee` in the gRPC/REST responses.
#### Fast vs. batched swaps
By default, Loop Outs are executed as normal speed swaps. This means the server
will wait up to 30 minutes until it publishes the HTLC on-chain to improve the
chances that it can be batched together with other user's swaps to reduce the
on-chain footprint and fees. The server offers a reduced swap fee for slow swaps
to incentivize users to batch more.
If a swap should be executed immediately, the `--fast` flag can be used. Fast
swaps won't benefit from a reduced swap fee.
### Loop In Swaps
Additionally, Loop In is now also supported for mainnet as well. A Loop In swap
lets one refill their channel (ability to send more coins) by sending to a
special script on-chain.
```
NAME:
loop in - perform an on-chain to off-chain swap (loop in)
USAGE:
loop in [command options] amt
DESCRIPTION:
Send the amount in satoshis specified by the amt argument off-chain.
OPTIONS:
--amt value the amount in satoshis to loop in (default: 0)
--external expect htlc to be published externally
--conf_target value the target number of blocks the on-chain htlc broadcast by the swap client should confirm within (default: 0)
--last_hop value the pubkey of the last hop to use for this swap
```
The `--external` argument allows the on-chain HTLC transacting to be published
_externally_. This allows for a number of use cases like using this address to
withdraw from an exchange _into_ your Lightning channel!
A Loop In swap can be executed a follows:
To execute a Loop In:
```
loop in <amt_in_satoshis>
```
#### Fees explained
The following is an example output of a 0.01 BTC Loop In swap from `testnet`:
```bash
$ loop in --amt 1000000
Max swap fees for 1000000 sat Loop In: 1562 sat
CONTINUE SWAP? (y/n), expand fee detail (x): x
Estimated on-chain HTLC fee: 154 sat
Max swap fee: 1100 sat
CONTINUE SWAP? (y/n):
```
Explanation:
- **Estimated on-chain HTLC fee** (154 sat): The estimated on-chain fee that the
daemon has to pay to publish the HTLC. This is an estimation from `lnd`'s
wallet based on the available UTXOs and current network fees. This value is
called `miner_fee` in the gRPC/REST responses.
- **Max swap fee** (1100 sat): The maximum amount of service fees we allow the
server to charge for executing the swap. The client aborts the swap if the
fee proposed by the server exceeds this maximum. It is therefore recommended
to obtain the maximum by asking the server for a quote first. The actual fees
might be lower than this maximum if user specific discounts are applied. This
value is called `swap_fee` in the gRPC/REST responses.
## 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`.
## Authentication and transport security
## API
The Loop daemon exposes a [gRPC API](https://lightning.engineering/loopapi/#lightning-loop-grpc-api-reference)
(defaults to port 11010) and a [REST API](https://lightning.engineering/loopapi/index.html#loop-rest-api-reference)
(defaults to port 8081).
The gRPC and REST connections of `loopd` are encrypted with TLS and secured with
macaroon authentication the same way `lnd` is.
@ -349,20 +94,16 @@ For more information on macaroons,
**NOTE**: Loop's macaroons are independent from `lnd`'s. The same macaroon
cannot be used for both `loopd` and `lnd`.
## Multiple Simultaneous Swaps
## Build from source
If you’d prefer to build from source:
```
git clone https://github.com/lightninglabs/loop.git
cd loop/cmd
go install ./...
```
It is possible to execute multiple swaps simultaneously. Just keep loopd
running.
## API Support Level
### Server
The Loop server API and on-chain scripts are kept backwards compatible as long
as reasonably possible.
### Client
When breaking changes to the Loop client daemon API are made, old fields will be
marked as deprecated. Deprecated fields will remain supported until the next
minor release.
## Development and Support
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://lightning.engineering/slack.html).