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

c-lightning is a lighweight, highly customizable and standard compliant implementation of the Lightning Network protocol.

• Getting Started
  • Installation
  • Starting lightningd
  • Using the JSON-RPC Interface
  • Care And Feeding Of Your New Lightning Node
  • Opening A Channel
  • Sending and Receiving Payments
  • Configuration File
• Further Information
  • Pruning
  • Developers

Project Status

This implementation has been in production use on the Bitcoin mainnet since early 2018, with the launch of the Blockstream Store. We recommend getting started by experimenting on testnet (or regtest), but the implementation is considered stable and can be safely used on mainnet.

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 [email protected], or on the Lightning Network-wide mailing list [email protected].

Getting Started

c-lightning only works on Linux and Mac OS, and requires a locally (or remotely) running bitcoind (version 0.16 or above) that is fully caught up with the network you're testing on. Pruning (prune=n option in bitcoin.conf) is partially supported, see here for more details.

Installation

There are 4 supported installation options:

• Installation from the Ubuntu PPA
• Installation of a pre-compiled binary from the release page on Github
• Using one of the provided docker images on the Docker Hub
• Compiling the source code yourself as described in the installation documentation.

For the impatient here's the gist of it for Ubuntu:

sudo apt-get install -y software-properties-common
sudo add-apt-repository -u ppa:bitcoin/bitcoin
sudo add-apt-repository -u ppa:lightningnetwork/ppa
sudo apt-get install bitcoind lightningd

Starting lightningd

If you want to experiment with lightningd, there's a script to set up a bitcoind regtest test network of two local lightning nodes, which provides a convenient start_ln helper:

. contrib/startup_regtest.sh

To test with real bitcoin, you will need to have a local bitcoind node running:

bitcoind -daemon

Wait until bitcoind has synchronized with the network.

Make sure that you do not have walletbroadcast=0 in your ~/.bitcoin/bitcoin.conf, or you may run into trouble. Notice that running lightningd against a pruned node may cause some issues if not managed carefully, see below for more information.

You can start lightningd with the following command:

lightningd --network=bitcoin --log-level=debug

This creates a .lightning/ subdirectory in your home directory: see man -l doc/lightningd.8 (or https://lightning.readthedocs.io/) for more runtime options.

Using The JSON-RPC Interface

c-lightning exposes a JSON-RPC 2.0 interface over a Unix Domain socket; the lightning-cli tool can be used to access it, or there is a python client library.

You can use lightning-cli help to print a table of RPC methods; lightning-cli help <command> will offer specific information on that command.

Useful commands:

• newaddr: get a bitcoin address to deposit funds into your lightning node.
• listfunds: see where your funds are.
• connect: connect to another lightning node.
• fundchannel: create a channel to another connected node.
• invoice: create an invoice to get paid by another node.
• pay: pay someone else's invoice.
• plugin: commands to control extensions.

Care And Feeding Of Your New Lightning Node

Once you've started for the first time, there's a script called contrib/bootstrap-node.sh which will connect you to other nodes on the lightning network.

You can encrypt the BIP32 root seed (what is stored in hsm_secret) by passing the --encrypted-hsm startup argument. You can start lightningd with --encrypted-hsm on an already existing lightning-dir (with a not encrypted hsm_secret). If you pass that option, you will not be able to start lightningd (with the same wallet) again without the password, so please beware with your password management. Also beware of not feeling too safe with an encrypted hsm_secret: unlike for bitcoind where the wallet encryption can restrict the usage of some RPC command, lightningd always need to access keys from the wallet which is thus not locked (yet), even with an encrypted BIP32 master seed.

There are also numerous plugins available for c-lightning which add capabilities: in particular there's a collection at:

https://github.com/lightningd/plugins

Including helpme which guides you through setting up your first channels and customizing your node.

You can also chat to other users at #c-lightning @ freenode.net; we are always happy to help you get started!

Opening A Channel

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

# Returns an address <address>
lightning-cli newaddr

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

lightningd will register the funds once the transaction is confirmed.

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

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

Confirm lightningd got funds by:

# Returns an array of on-chain funds.
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>:

lightning-cli connect <node_id> <ip> [<port>]
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 3 confirmation in order for the channel to be usable, and 6 to be announced for others to use. You can check the status of the channel using lightning-cli listpeers, which after 3 confirmations (1 on testnet) should say that state is CHANNELD_NORMAL; after 6 confirmations you can use lightning-cli listchannels to verify that the public field is now true.

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:

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:

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 (eg. ~/.lightning/config). See man -l doc/lightningd-config.5.

Further information

Pruning

c-lightning requires JSON-RPC access to a fully synchronized bitcoind in order to synchronize with the Bitcoin network. Access to ZeroMQ is not required and bitcoind does not need to be run with txindex like other implementations. The lightning daemon will poll bitcoind for new blocks that it hasn't processed yet, thus synchronizing itself with bitcoind. If bitcoind prunes a block that c-lightning has not processed yet, e.g., c-lightning was not running for a prolonged period, then bitcoind will not be able to serve the missing blocks, hence c-lightning will not be able to synchronize anymore and will be stuck. In order to avoid this situation you should be monitoring the gap between c-lightning's blockheight using lightning-cli getinfo and bitcoind's blockheight using bitcoin-cli getblockchaininfo. If the two blockheights drift apart it might be necessary to intervene.

Developers

Developers wishing to contribute should start with the developer guide here. You should also configure with --enable-developer to get additional checks and options.