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.

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, 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 c-lightning@lists.ozlabs.org, or on the Lightning Network-wide mailing list lightning-dev@lists.linuxfoundation.org.

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 (suggested mainly for developers or if you need one of the still unreleased features)

Please refer to the PPA release page and the installation documentation for detailed instructions.

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

In order to start lightningd you will need to have a local bitcoind node running (in this case we start testnet):

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 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=testnet --log-level=debug

Please refer to lightningd --help for all other command line options.

JSON-RPC Interface

c-lightning exposes a JSON-RPC 2.0 interface over a Unix Domain socket located in its home directory (default: $HOME/.lightning). The Unix Domain Socket has the advantage of not being exposed over the network by default, allowing users to add their own authentication and authorization mechanism, while still providing a fully functional RPC interface out of the box.

You can use lightning-cli help to print a table of the available RPC methods that can be called. The JSON-RPC interface is also documented in the following manual pages:

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

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

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>
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.

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. By default this will be $HOME/.lightning/config.

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

alias=SLEEPYDRAGON
rgb=008000
network=testnet

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

lightningd --help

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.