c-lightning: A specification compliant Lightning Network implementation in C

c-lightning is a standard compliant implementation of the Lightning Network protocol. The Lightning Network is a scalability solution for Bitcoin, enabling secure and instant transfer of funds between any two parties for any amount.

For more information about the Lightning Network please refer to http://lightning.network.

Project Status

This implementation is still very much a work in progress. It can be used for testing, but it should not be used for real funds. We do our best to identify and fix problems, and implement missing features.

Any help testing the implementation, reporting bugs, or helping with outstanding issues is very welcome. Don't hesitate to reach out to us on IRC at #lightning-dev @ freenode.net, #c-lightning @ freenode.net, or on the implementation-specific mailing list c-lightning@lists.ozlabs.org, or on the Lightning Network-wide mailing list lightning-dev@lists.linuxfoundation.org.

Getting Started

c-lightning currently only works on Linux (and possibly Mac OS with some tweaking), and requires a locally (or remotely) running bitcoind (version 0.15 or above) that is fully caught up with the network you're testing on. Pruning (prune=n option in bitcoin.conf) is not currently supported.

Installation

Please refer to the installation documentation for detailed instructions. For the impatient here's the gist of it for Ubuntu and Debian:

sudo apt-get update
sudo apt-get install -y \
  autoconf automake build-essential git libtool libgmp-dev \
  libsqlite3-dev python python3 net-tools zlib1g-dev
git clone https://github.com/ElementsProject/lightning.git
cd lightning
./configure
make

Or if you like to throw docker into the mix, you can use the official docker image either directly or as a base layer for more complex images. The docker image is elementsproject/lightningd (from this Dockerfile). Image tags with -dev at the end are images built with DEVELOPER=1. If you build the image yourself, you can use the build arg DEVELOPER=1 to build c-lightning in developer mode.

It has the following environment variable:

• EXPOSE_TCP default to false, if true, use expose c-lightning RPC on port 9835. (Use this only for testing)

Here is an example of a docker-compose file with bitcoind and c-lightning on testnet which expose bitcoind's RPC interface on default ports 18332 and c-lightning API on port 9735:

version: "3"
services:
  bitcoind:
    image: nicolasdorier/docker-bitcoin:0.16.3
    container_name: bitcoind
    environment:
      BITCOIN_EXTRA_ARGS: |
        testnet=1
        whitelist=0.0.0.0/0
        server=1
        rpcuser=rpcuser
        rpcpassword=rpcpass
    expose:
      - "18332"
    ports:
      - "0.0.0.0:18333:18333"
    volumes:
      - "bitcoin_datadir:/data"

  clightning_bitcoin:
    image: elementsproject/lightningd
    container_name: lightningd
    command:
      - --bitcoin-rpcconnect=bitcoind
      - --bitcoin-rpcuser=rpcuser
      - --bitcoin-rpcpassword=rpcpass
      - --network=testnet
      - --alias=myawesomenode
      - --log-level=debug
    environment:
      EXPOSE_TCP: "true"
    expose:
      - "9735"
    ports:
      - "0.0.0.0:9735:9735"
    volumes:
      - "clightning_bitcoin_datadir:/root/.lightning"
      - "bitcoin_datadir:/etc/bitcoin"
    links:
      - bitcoind

volumes:
  bitcoin_datadir:
  clightning_bitcoin_datadir:

Starting lightningd

In order to start lightningd you will need to have a local bitcoind node running in either testnet or regtest mode:

bitcoind -daemon -testnet

Wait until bitcoind has synchronized with the testnet network.

Make sure that you do not have walletbroadcast=0 in your ~/.bitcoin/bitcoin.conf, or you may run into trouble. Notice that currently pruned nodes are not supported and may result in lightningd being unable to synchronize with the blockchain.

You can start lightningd with the following command:

lightningd/lightningd --network=testnet --log-level=debug

Listing all commands:

cli/lightning-cli help will print a table of the API and lists the following commands

Opening a channel on the Bitcoin testnet

First you need to transfer some funds to lightningd so that it can open a channel:

# Returns an address <address>
cli/lightning-cli newaddr

# Returns a transaction id <txid>
bitcoin-cli -testnet sendtoaddress <address> <amount_in_bitcoins>

lightningd will register the funds once the transaction is confirmed.

If you don't have any testcoins you can get a few from a faucet such as TPs' testnet faucet or Kiwi's testnet faucet. You can send it directly to the lightningd address.

You may need to generate a p2sh-segwit address if the faucet does not support bech32:

# Return a p2sh-segwit address
cli/lightning-cli newaddr p2sh-segwit

Confirm lightningd got funds by:

# Returns an array of on-chain funds.
cli/lightning-cli listfunds

Once lightningd has funds, we can connect to a node and open a channel. Let's assume the remote node is accepting connections at <ip> (and optional <port>, if not 9735) and has the node ID <node_id>:

cli/lightning-cli connect <node_id> <ip> [<port>]
cli/lightning-cli fundchannel <node_id> <amount_in_satoshis>

This opens a connection and, on top of that connection, then opens a channel. The funding transaction needs 1 confirmation in order for the channel to be usable, and 6 to be broadcast for others to use. You can check the status of the channel using cli/lightning-cli listpeers, which after 3 confirmations (1 on testnet) should say that state is CHANNELD_NORMAL; after 6 confirmations you can use cli/lightning-cli listchannels to verify that the public field is now true.

Different states

• OPENINGD means that lightning_openingd is negotiating channel opening.
• CHANNELD_AWAITING_LOCKIN means that lightning_channeld is waiting until the minimum number of confirmation on the channel funding transaction.
• CHANNELD_NORMAL means your channel is operating normally.
• CHANNELD_SHUTTING_DOWN means one or both sides have asked to shut down the channel, and we're waiting for existing HTLCs to clear.
• CLOSINGD_SIGEXCHANGE means we're trying to negotiate the fee for the mutual close transaction.
• CLOSINGD_COMPLETE means we've broadcast our mutual close transaction (which spends the funding transaction) , but haven't seen it in a block yet.
• FUNDING_SPEND_SEEN means we've seen the funding transaction spent.
• ONCHAIN means that the lightning_onchaind is tracking the onchain closing of the channel.
• AWAITING_UNILATERAL means that we're waiting for a unilateral close to hit the blockchain.

All these states have more information about what's going on in the status field in listpeers.

Sending and receiving payments

Payments in Lightning are invoice based. The recipient creates an invoice with the expected <amount> in millisatoshi (or "any" for a donation), a unique <label> and a <description> the payer will see:

cli/lightning-cli invoice <amount> <label> <description>

This returns some internal details, and a standard invoice string called bolt11 (named after the BOLT #11 lightning spec).

The sender can feed this bolt11 string to the decodepay command to see what it is, and pay it simply using the pay command:

cli/lightning-cli pay <bolt11>

Note that there are lower-level interfaces (and more options to these interfaces) for more sophisticated use.

Configuration File

lightningd can be configured either by passing options via the command line, or via a configuration file. Command line options will always override the values in the configuration file.

To use a configuration file, create a file named "config" within your ".lightning" directory. Usually, this will be ~/.lightning/config

Configuration options are set using a key=value pair on each line of the file, for example:

alias=SLEEPYDRAGON
rgb=008000
port=9735
network=testnet

For a full list of possible lightningd configuration options, run:

lightningd/lightningd --help

Further information

Developers

Developers wishing to contribute should start with the developer guide here.

JSON RPC

JSON-RPC interface is documented in the following manual pages:

• invoice
• listinvoices
• waitinvoice
• waitanyinvoice
• delinvoice
• getroute
• sendpay
• pay
• listpayments
• decodepay

For simple access to the JSON-RPC interface you can use the cli/lightning-cli tool, or the python API client.