Plugins

Plugins are a simple yet powerful way to extend the functionality provided by c-lightning. They are subprocesses that are started by the main lightningd daemon and can interact with lightningd in a variety of ways:

  • Command line option passthrough allows plugins to register their own command line options that are exposed through lightningd so that only the main process needs to be configured.
  • JSON-RPC command passthrough adds a way for plugins to add their own commands to the JSON-RPC interface.
  • Event stream subscriptions provide plugins with a push-based notification mechanism about events from the lightningd.
  • Hooks are a primitive that allows plugins to be notified about internal events in lightningd and alter its behavior or inject custom behaviors.

A plugin may be written in any language, and communicates with lightningd through the plugin’s stdin and stdout. JSON-RPCv2 is used as protocol on top of the two streams, with the plugin acting as server and lightningd acting as client. The plugin file needs to be executable (e.g. use chmod a+x plugin_name)

A day in the life of a plugin

During startup of lightningd you can use the --plugin= option to register one or more plugins that should be started. In case you wish to start several plugins you have to use the --plugin= argument once for each plugin (or --plugin-dir or place them in the default plugin dirs, usually /usr/local/libexec/c-lightning/plugins and ~/.lightningd/plugins). An example call might look like:

lightningd --plugin=/path/to/plugin1 --plugin=path/to/plugin2

lightningd will run your plugins from the --lightning-dir/networkname, then will write JSON-RPC requests to the plugin’s stdin and will read replies from its stdout. To initialize the plugin two RPC methods are required:

  • getmanifest asks the plugin for command line options and JSON-RPC commands that should be passed through. This can be run before lightningd checks that it is the sole user of the lightning-dir directory (for --help) so your plugin should not touch files at this point.
  • init is called after the command line options have been parsed and passes them through with the real values (if specified). This is also the signal that lightningd’s JSON-RPC over Unix Socket is now up and ready to receive incoming requests from the plugin.

Once those two methods were called lightningd will start passing through incoming JSON-RPC commands that were registered and the plugin may interact with lightningd using the JSON-RPC over Unix-Socket interface.

The getmanifest method

The getmanifest method is required for all plugins and will be called on startup with optional parameters (in particular, it may have allow-deprecated-apis: false, but you should accept, and ignore, other parameters). It MUST return a JSON object similar to this example:

{
  "options": [
    {
      "name": "greeting",
      "type": "string",
      "default": "World",
      "description": "What name should I call you?",
      "deprecated": false
    }
  ],
  "rpcmethods": [
    {
      "name": "hello",
      "usage": "[name]",
      "description": "Returns a personalized greeting for {greeting} (set via options)."
    },
    {
      "name": "gettime",
      "usage": "",
      "description": "Returns the current time in {timezone}",
      "long_description": "Returns the current time in the timezone that is given as the only parameter.\nThis description may be quite long and is allowed to span multiple lines.",
      "deprecated": false
    }
  ],
  "subscriptions": [
    "connect",
    "disconnect"
  ],
  "hooks": [
    { "name": "openchannel", "before": ["another_plugin"] },
    { "name": "htlc_accepted" }
  ],
  "featurebits": {
    "node": "D0000000",
    "channel": "D0000000",
    "init": "0E000000",
    "invoice": "00AD0000"
  },
  "dynamic": true
}

The options will be added to the list of command line options that lightningd accepts. The above will add a --greeting option with a default value of World and the specified description. Notice that currently string, integers, bool, and flag options are supported.

The rpcmethods are methods that will be exposed via lightningd’s JSON-RPC over Unix-Socket interface, just like the builtin commands. Any parameters given to the JSON-RPC calls will be passed through verbatim. Notice that the name, description and usage fields are mandatory, while the long_description can be omitted (it’ll be set to description if it was not provided). usage should surround optional parameter names in [].

options and rpcmethods can mark themselves deprecated: true if you plan on removing them: this will disable them if the user sets allow-deprecated-apis to false (which every developer should do, right?).

The dynamic indicates if the plugin can be managed after lightningd has been started. Critical plugins that should not be stopped should set it to false.

The featurebits object allows the plugin to register featurebits that should be announced in a number of places in the protocol. They can be used to signal support for custom protocol extensions to direct peers, remote nodes and in invoices. Custom protocol extensions can be implemented for example using the sendcustommsg method and the custommsg hook, or the sendonion method and the htlc_accepted hook. The keys in the featurebits object are node for features that should be announced via the node_announcement to all nodes in the network, init for features that should be announced to direct peers during the connection setup, channel for features which should apply to channel_announcement, and invoice for features that should be announced to a potential sender of a payment in the invoice. The low range of featurebits is reserved for standardize features, so please pick random, high position bits for experiments. If you’d like to standardize your extension please reach out to the specification repository to get a featurebit assigned.

Plugins are free to register any name for their rpcmethod as long as the name was not previously registered. This includes both built-in methods, such as help and getinfo, as well as methods registered by other plugins. If there is a conflict then lightningd will report an error and exit.

Types of Options

There are currently four supported option ‘types’:

  • string: a string
  • bool: a boolean
  • int: parsed as a signed integer (64-bit)
  • flag: no-arg flag option. Is boolean under the hood. Defaults to false.

Nota bene: if a flag type option is not set, it will not appear in the options set that is passed to the plugin.

Here’s an example option set, as sent in response to getmanifest

  "options": [
    {
      "name": "greeting",
      "type": "string",
      "default": "World",
      "description": "What name should I call you?"
    },
    {
      "name": "run-hot",
      "type": "flag",
      "default": None,  // defaults to false
      "description": "If set, overclocks plugin"
    },
    {
      "name": "is_online",
      "type": "bool",
      "default": false,
      "description": "Set to true if plugin can use network"
    },
    {
      "name": "service-port",
      "type": "int",
      "default": 6666,
      "description": "Port to use to connect to 3rd-party service"
    }
  ],

The init method

The init method is required so that lightningd can pass back the filled command line options and notify the plugin that lightningd is now ready to receive JSON-RPC commands. The params of the call are a simple JSON object containing the options:

{
  "options": {
    "greeting": "World"
  },
  "configuration": {
    "lightning-dir": "/home/user/.lightning/testnet",
    "rpc-file": "lightning-rpc",
    "startup": true,
    "network": "testnet",
    "feature_set": {
        "init": "02aaa2",
        "node": "8000000002aaa2",
        "channel": "",
        "invoice": "028200"
    },
    "proxy": {
        "type": "ipv4",
        "address": "127.0.0.1",
        "port": 9050
    },
    "torv3-enabled": true,
    "use_proxy_always": false
  }
}

The plugin must respond to init calls, however the response can be arbitrary and will currently be discarded by lightningd. JSON-RPC commands were chosen over notifications in order not to force plugins to implement notifications which are not that well supported.

The startup field allows a plugin to detect if it was started at lightningd startup (true), or at runtime (false).

JSON-RPC passthrough

Plugins may register their own JSON-RPC methods that are exposed through the JSON-RPC provided by lightningd. This provides users with a single interface to interact with, while allowing the addition of custom methods without having to modify the daemon itself.

JSON-RPC methods are registered as part of the getmanifest result. Each registered method must provide a name and a description. An optional long_description may also be provided. This information is then added to the internal dispatch table, and used to return the help text when using lightning-cli help, and the methods can be called using the name.

For example the above getmanifest result will register two methods, called hello and gettime:

  ...
  "rpcmethods": [
    {
      "name": "hello",
      "usage": "[name]",
      "description": "Returns a personalized greeting for {greeting} (set via options)."
    },
    {
      "name": "gettime",
      "description": "Returns the current time in {timezone}",
      "usage": "",
      "long_description": "Returns the current time in the timezone that is given as the only parameter.\nThis description may be quite long and is allowed to span multiple lines."
    }
  ],
  ...

The RPC call will be passed through unmodified, with the exception of the JSON-RPC call id, which is internally remapped to a unique integer instead, in order to avoid collisions. When passing the result back the id field is restored to its original value.

Note that if your result for an RPC call includes "format-hint": "simple", then lightning-cli will default to printing your output in “human-readable” flat form.

Event notifications

Event notifications allow a plugin to subscribe to events in lightningd. lightningd will then send a push notification if an event matching the subscription occurred. A notification is defined in the JSON-RPC specification as an RPC call that does not include an id parameter:

A Notification is a Request object without an “id” member. A Request object that is a Notification signifies the Client’s lack of interest in the corresponding Response object, and as such no Response object needs to be returned to the client. The Server MUST NOT reply to a Notification, including those that are within a batch request.

Notifications are not confirmable by definition, since they do not have a Response object to be returned. As such, the Client would not be aware of any errors (like e.g. “Invalid params”,”Internal error”).

Plugins subscribe by returning an array of subscriptions as part of the getmanifest response. The result for the getmanifest call above for example subscribes to the two topics connect and disconnect. The topics that are currently defined and the corresponding payloads are listed below.

channel_opened

A notification for topic channel_opened is sent if a peer successfully funded a channel with us. It contains the peer id, the funding amount (in millisatoshis), the funding transaction id, and a boolean indicating if the funding transaction has been included into a block.

{
  "channel_opened": {
    "id": "03864ef025fde8fb587d989186ce6a4a186895ee44a926bfc370e2c366597a3f8f",
    "funding_satoshis": "100000000msat",
    "funding_txid": "4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b",
    "funding_locked": false
  }
}

channel_state_changed

A notification for topic channel_state_changed is sent every time a channel changes its state. The notification includes the peer_id and channel_id, the old and new channel states, the type of cause and a message.

{
    "channel_state_changed": {
        "peer_id": "03bc9337c7a28bb784d67742ebedd30a93bacdf7e4ca16436ef3798000242b2251",
        "channel_id": "a2d0851832f0e30a0cf778a826d72f077ca86b69f72677e0267f23f63a0599b4",
        "short_channel_id" : "561820x1020x1",
        "old_state": "CHANNELD_NORMAL",
        "new_state": "CHANNELD_SHUTTING_DOWN",
        "cause" : "remote",
        "message" : "Peer closes channel"
    }
}

A cause can have the following values:

  • “unknown” Anything other than the reasons below. Should not happen.
  • “local” Unconscious internal reasons, e.g. dev fail of a channel.
  • “user” The operator or a plugin opened or closed a channel by intention.
  • “remote” The remote closed or funded a channel with us by intention.
  • “protocol” We need to close a channel because of bad signatures and such.
  • “onchain” A channel was closed onchain, while we were offline.

Most state changes are caused subsequentially for a prior state change, e.g. “CLOSINGD_COMPLETE” is followed by “FUNDING_SPEND_SEEN”. Because of this, the cause reflects the last known reason in terms of local or remote user interaction, protocol reasons, etc. More specifically, a new_state “FUNDING_SPEND_SEEN” will likely not have “onchain” as a cause but some value such as “REMOTE” or “LOCAL” depending on who initiated the closing of a channel.

Note: If the channel is not closed or being closed yet, the cause will reflect which side “remote” or “local” opened the channel.

Note: If the cause is “onchain” this was very likely a conscious decision of the remote peer, but we have been offline.

connect

A notification for topic connect is sent every time a new connection to a peer is established.

{
  "id": "02f6725f9c1c40333b67faea92fd211c183050f28df32cac3f9d69685fe9665432",
  "address": "1.2.3.4"
}

disconnect

A notification for topic disconnect is sent every time a connection to a peer was lost.

{
  "id": "02f6725f9c1c40333b67faea92fd211c183050f28df32cac3f9d69685fe9665432"
}

invoice_payment

A notification for topic invoice_payment is sent every time an invoice is paid.

{
  "invoice_payment": {
    "label": "unique-label-for-invoice",
    "preimage": "0000000000000000000000000000000000000000000000000000000000000000",
    "msat": "10000msat"
  }
}

invoice_creation

A notification for topic invoice_creation is sent every time an invoice is created.

{
  "invoice_creation": {
    "label": "unique-label-for-invoice",
    "preimage": "0000000000000000000000000000000000000000000000000000000000000000",
    "msat": "10000msat"
  }
}

warning

A notification for topic warning is sent every time a new BROKEN /UNUSUAL level(in plugins, we use error/warn) log generated, which means an unusual/borken thing happens, such as channel failed, message resolving failed…

{
  "warning": {
    "level": "warn",
    "time": "1559743608.565342521",
    "source": "lightningd(17652): 0821f80652fb840239df8dc99205792bba2e559a05469915804c08420230e23c7c chan #7854:",
    "log": "Peer permanent failure in CHANNELD_NORMAL: lightning_channeld: sent ERROR bad reestablish dataloss msg"
  }
}
  1. level is warn or error: warn means something seems bad happened and it’s under control, but we’d better check it; error means something extremely bad is out of control, and it may lead to crash;
  2. time is the second since epoch;
  3. source means where the event happened, it may have the following forms: <node_id> chan #<db_id_of_channel>:,lightningd(<lightningd_pid>):, plugin-<plugin_name>:, <daemon_name>(<daemon_pid>):, jsonrpc:, jcon fd <error_fd_to_jsonrpc>:, plugin-manager;
  4. log is the context of the original log entry.

forward_event

A notification for topic forward_event is sent every time the status of a forward payment is set. The json format is same as the API listforwards.

{
  "forward_event": {
    "payment_hash": "f5a6a059a25d1e329d9b094aeeec8c2191ca037d3f5b0662e21ae850debe8ea2",
    "in_channel": "103x2x1",
    "out_channel": "103x1x1",
    "in_msatoshi": 100001001,
    "in_msat": "100001001msat",
    "out_msatoshi": 100000000,
    "out_msat": "100000000msat",
    "fee": 1001,
    "fee_msat": "1001msat",
    "status": "settled",
    "received_time": 1560696342.368,
    "resolved_time": 1560696342.556
  }
}

or

{
  "forward_event": {
    "payment_hash": "ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff",
    "in_channel": "103x2x1",
    "out_channel": "110x1x0",
    "in_msatoshi": 100001001,
    "in_msat": "100001001msat",
    "out_msatoshi": 100000000,
    "out_msat": "100000000msat",
    "fee": 1001,
    "fee_msat": "1001msat",
    "status": "local_failed",
    "failcode": 16392,
    "failreason": "WIRE_PERMANENT_CHANNEL_FAILURE",
    "received_time": 1560696343.052
  }
}
  • The status includes offered, settled, failed and local_failed, and they are all string type in json.
    • When the forward payment is valid for us, we’ll set offered and send the forward payment to next hop to resolve;
    • When the payment forwarded by us gets paid eventually, the forward payment will change the status from offered to settled;
    • If payment fails locally(like failing to resolve locally) or the corresponding htlc with next hop fails(like htlc timeout), we will set the status as local_failed. local_failed may be set before setting offered or after setting offered. In fact, from the time we receive the htlc of the previous hop, all we can know the cause of the failure is treated as local_failed. local_failed only occuors locally or happens in the htlc between us and next hop;
      • If local_failed is set before offered, this means we just received htlc from the previous hop and haven’t generate htlc for next hop. In this case, the json of forward_event sets the fields of out_msatoshi, out_msat,fee and out_channel as 0;
        • Note: In fact, for this case we may be not sure if this incoming htlc represents a pay to us or a payment we need to forward. We just simply treat all incoming failed to resolve as local_failed.
      • Only in local_failed case, json includes failcode and failreason fields;
    • failed means the payment forwarded by us fails in the latter hops, and the failure isn’t related to us, so we aren’t accessed to the fail reason. failed must be set after offered.
      • failed case doesn’t include failcode and failreason fields;
  • received_time means when we received the htlc of this payment from the previous peer. It will be contained into all status case;
  • resolved_time means when the htlc of this payment between us and the next peer was resolved. The resolved result may success or fail, so only settled and failed case contain resolved_time;
  • The failcode and failreason are defined in [BOLT 4][bolt4-failure-codes].

sendpay_success

A notification for topic sendpay_success is sent every time a sendpay succeeds (with complete status). The json is the same as the return value of the commands sendpay/waitsendpay when these commands succeed.

{
  "sendpay_success": {
    "id": 1,
    "payment_hash": "5c85bf402b87d4860f4a728e2e58a2418bda92cd7aea0ce494f11670cfbfb206",
    "destination": "035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d",
    "msatoshi": 100000000,
    "amount_msat": "100000000msat",
    "msatoshi_sent": 100001001,
    "amount_sent_msat": "100001001msat",
    "created_at": 1561390572,
    "status": "complete",
    "payment_preimage": "9540d98095fd7f37687ebb7759e733934234d4f934e34433d4998a37de3733ee"
  }
}

sendpay doesn’t wait for the result of sendpay and waitsendpay returns the result of sendpay in specified time or timeout, but sendpay_success will always return the result anytime when sendpay successes if is was subscribed.

sendpay_failure

A notification for topic sendpay_failure is sent every time a sendpay completes with failed status. The JSON is same as the return value of the commands sendpay/waitsendpay when these commands fail.

{
  "sendpay_failure": {
    "code": 204,
    "message": "failed: WIRE_UNKNOWN_NEXT_PEER (reply from remote)",
    "data": {
      "id": 2,
      "payment_hash": "9036e3bdbd2515f1e653cb9f22f8e4c49b73aa2c36e937c926f43e33b8db8851",
      "destination": "035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d",
      "msatoshi": 100000000,
      "amount_msat": "100000000msat",
      "msatoshi_sent": 100001001,
      "amount_sent_msat": "100001001msat",
      "created_at": 1561395134,
      "status": "failed",
      "erring_index": 1,
      "failcode": 16394,
      "failcodename": "WIRE_UNKNOWN_NEXT_PEER",
      "erring_node": "022d223620a359a47ff7f7ac447c85c46c923da53389221a0054c11c1e3ca31d59",
      "erring_channel": "103x2x1",
      "erring_direction": 0
    }
  }
}

sendpay doesn’t wait for the result of sendpay and waitsendpay returns the result of sendpay in specified time or timeout, but sendpay_failure will always return the result anytime when sendpay fails if is was subscribed.

coin_movement

A notification for topic coin_movement is sent to record the movement of coins. It is only triggered by finalized ledger updates, i.e. only definitively resolved HTLCs or confirmed bitcoin transactions.

{
	"coin_movement": {
		"version":1,
		"node_id":"03a7103a2322b811f7369cbb27fb213d30bbc0b012082fed3cad7e4498da2dc56b",
		"movement_idx":0,
		"type":"chain_mvt",
		"account_id":"wallet",
		"txid":"0159693d8f3876b4def468b208712c630309381e9d106a9836fa0a9571a28722", // (`chain_mvt` type only, mandatory)
		"utxo_txid":"0159693d8f3876b4def468b208712c630309381e9d106a9836fa0a9571a28722", // (`chain_mvt` type only, optional)
		"vout":1, // (`chain_mvt` type only, optional)
		"payment_hash": "xxx", // (either type, optional on `chain_mvt`)
		"part_id": 0, // (`channel_mvt` type only, mandatory)
		"credit":"2000000000msat",
		"debit":"0msat",
		"tag":"deposit",
		"blockheight":102, // (`channel_mvt` type only. may be null)
		"timestamp":1585948198,
		"coin_type":"bc"
	}
}

version indicates which version of the coin movement data struct this notification adheres to.

node_id specifies the node issuing the coin movement.

movement_idx is an increment-only counter for coin moves emitted by this node.

type marks the underlying mechanism which moved these coins. There are two ‘types’ of coin_movements:

  • channel_mvts, which occur as a result of htlcs being resolved and,
  • chain_mvts, which occur as a result of bitcoin txs being mined.

account_id is the name of this account. The node’s wallet is named ‘wallet’, all channel funds’ account are the channel id.

txid is the transaction id of the bitcoin transaction that triggered this ledger event. utxo_txid and vout identify the bitcoin output which triggered this notification. (chain_mvt only) In most cases, the utxo_txid will be the same as the txid, except for spend_track notficiations. Notifications tagged chain_fees and journal_entry do not have a utxo_txid as they’re not represented in the utxo set.

payment_hash is the hash of the preimage used to move this payment. Only present for HTLC mediated moves (both chain_mvt and channel_mvt) A chain_mvt will have a payment_hash iff it’s recording an htlc that was fulfilled onchain.

part_id is an identifier for parts of a multi-part payment. useful for aggregating payments for an invoice or to indicate why a payment hash appears multiple times. channel_mvt only

credit and debit are millisatoshi denominated amounts of the fund movement. A ‘credit’ is funds deposited into an account; a debit is funds withdrawn.

tag is a movement descriptor. Current tags are as follows:

  • deposit: funds deposited
  • withdrawal: funds withdrawn
  • chain_fees: funds paid for onchain fees. chain_mvt only
  • penalty: funds paid or gained from a penalty tx. chain_mvt only
  • invoice: funds paid to or recieved from an invoice. channel_mvt only
  • routed: funds routed through this node. channel_mvt only
  • journal_entry: a balance reconciliation event, typically triggered by a penalty tx onchain. chain_mvt only
  • onchain_htlc: funds moved via an htlc onchain. chain_mvt only
  • pushed: funds pushed to peer. channel_mvt only.
  • spend_track: informational notification about a wallet utxo spend. chain_mvt only.

blockheight is the block the txid is included in. chain_mvt only. In the case that an output is considered dust, c-lightning does not track its return to our wallet. In those cases, the blockheight will be null, as they’re recorded before confirmation.

The timestamp is seconds since Unix epoch of the node’s machine time at the time lightningd broadcasts the notification.

coin_type is the BIP173 name for the coin which moved.

openchannel_peer_sigs

When opening a channel with a peer using the collaborative transaction protocol (opt_dual_fund), this notification is fired when the peer sends us their funding transaction signatures, tx_signatures. We update the in-progress PSBT and return it here, with the peer’s signatures attached.

{
	"openchannel_peer_sigs": {
		"channel_id": "<hex of a channel id (note, v2 format)>",
		"signed_psbt": "<Base64 serialized PSBT of funding transaction,
				with peer's sigs>"
	}
}

Hooks

Hooks allow a plugin to define custom behavior for lightningd without having to modify the c-lightning source code itself. A plugin declares that it’d like to be consulted on what to do next for certain events in the daemon. A hook can then decide how lightningd should react to the given event.

When hooks are registered, they can optionally specify “before” and “after” arrays of plugin names, which control what order they will be called in. If a plugin name is unknown, it is ignored, otherwise if the hook calls cannot be ordered to satisfy the specifications of all plugin hooks, the plugin registration will fail.

The call semantics of the hooks, i.e., when and how hooks are called, depend on the hook type. Most hooks are currently set to single-mode. In this mode only a single plugin can register the hook, and that plugin will get called for each event of that type. If a second plugin attempts to register the hook it gets killed and a corresponding log entry will be added to the logs.

In chain-mode multiple plugins can register for the hook type and they are called in any order they are loaded (i.e. cmdline order first, configuration order file second: though note that the order of plugin directories is implementation-dependent), overriden only by before and after requirements the plugin’s hook registrations specify. Each plugin can then handle the event or defer by returning a continue result like the following:

{
  "result": "continue"
}

The remainder of the response is ignored and if there are any more plugins that have registered the hook the next one gets called. If there are no more plugins then the internal handling is resumed as if no hook had been called. Any other result returned by a plugin is considered an exit from the chain. Upon exit no more plugin hooks are called for the current event, and the result is executed. Unless otherwise stated all hooks are single-mode.

Hooks and notifications are very similar, however there are a few key differences:

  • Notifications are asynchronous, i.e., lightningd will send the notifications but not wait for the plugin to process them. Hooks on the other hand are synchronous, lightningd cannot finish processing the event until the plugin has returned.
  • Any number of plugins can subscribe to a notification topic and get notified in parallel, however only one plugin may register for single-mode hook types, and in all cases only one plugin may return a non-continue response. This avoids having multiple contradictory responses.

Hooks are considered to be an advanced feature due to the fact that lightningd relies on the plugin to tell it what to do next. Use them carefully, and make sure your plugins always return a valid response to any hook invocation.

As a convention, for all hooks, returning the object { "result" : "continue" } results in lightningd behaving exactly as if no plugin is registered on the hook.

peer_connected

This hook is called whenever a peer has connected and successfully completed the cryptographic handshake. The parameters have the following structure if there is a channel with the peer:

{
  "peer": {
    "id": "03864ef025fde8fb587d989186ce6a4a186895ee44a926bfc370e2c366597a3f8f",
    "addr": "34.239.230.56:9735",
    "features": ""
  }
}

The hook is sparse on purpose, since the plugin can use the JSON-RPC listpeers command to get additional details should they be required. The addr field shows the address that we are connected to ourselves, not the gossiped list of known addresses. In particular this means that the port for incoming connections is an ephemeral port, that may not be available for reconnections.

The returned result must contain a result member which is either the string disconnect or continue. If disconnect and there’s a member error_message, that member is sent to the peer before disconnection.

commitment_revocation

This hook is called whenever a channel state is updated, and the old state was revoked. State updates in Lightning consist of the following steps:

  1. Proposal of a new state commitment in the form of a commitment transaction
  2. Exchange of signatures for the agreed upon commitment transaction
  3. Verification that the signatures match the commitment transaction
  4. Exchange of revocation secrets that could be used to penalize an eventual misbehaving party

The commitment_revocation hook is used to inform the plugin about the state transition being completed, and deliver the penalty transaction. The penalty transaction could then be sent to a watchtower that automaticaly reacts in case one party attempts to settle using a revoked commitment.

The payload consists of the following information:

{
	"commitment_txid": "58eea2cf538cfed79f4d6b809b920b40bb6b35962c4bb4cc81f5550a7728ab05",
	"penalty_tx": "02000000000101...ac00000000"
}

Notice that the commitment_txid could also be extracted from the sole input of the penalty_tx, however it is enclosed so plugins don’t have to include the logic to parse transactions.

Not included are the htlc_success and htlc_failure transactions that may also be spending commitment_tx outputs. This is because these transactions are much more dynamic and have a predictable timeout, allowing wallets to ensure a quick checkin when the CLTV of the HTLC is about to expire.

The commitment_revocation hook is a chained hook, i.e., multiple plugins can register it, and they will be called in the order they were registered in. Plugins should always return {"result": "continue"}, otherwise subsequent hook subscribers would not get called.

db_write

This hook is called whenever a change is about to be committed to the database. It is currently extremely restricted:

  1. a plugin registering for this hook should not perform anything that may cause a db operation in response (pretty much, anything but logging).
  2. a plugin registering for this hook should not register for other hooks or commands, as these may become intermingled and break rule #1.
  3. the hook will be called before your plugin is initialized!

This hook, unlike all the other hooks, is also strongly synchronous: lightningd will stop almost all the other processing until this hook responds.

{
  "data_version": 42,
  "writes": [
    "PRAGMA foreign_keys = ON"
  ]
}

This hook is intended for creating continuous backups. The intent is that your backup plugin maintains three pieces of information (possibly in separate files): (1) a snapshot of the database, (2) a log of database queries that will bring that snapshot up-to-date, and (3) the previous data_version.

data_version is an unsigned 32-bit number that will always increment by 1 each time db_write is called. Note that this will wrap around on the limit of 32-bit numbers.

writes is an array of strings, each string being a database query that modifies the database. If the data_version above is validated correctly, then you can simply append this to the log of database queries.

Your plugin MUST validate the data_version. It MUST keep track of the previous data_version it got, and:

  1. If the new data_version is exactly one higher than the previous, then this is the ideal case and nothing bad happened and we should save this and continue.
  2. If the new data_version is exactly the same value as the previous, then the previous set of queries was not committed. Your plugin MAY overwrite the previous set of queries with the current set, or it MAY overwrite its entire backup with a new snapshot of the database and the current writes array (treating this case as if data_version were two or more higher than the previous).
  3. If the new data_version is less than the previous, your plugin MUST halt and catch fire, and have the operator inspect what exactly happend here.
  4. Otherwise, some queries were lost and your plugin SHOULD recover by creating a new snapshot of the database: copy the database file, back up the given writes array, then delete (or atomically rename if in a POSIX filesystem) the previous backups of the database and SQL statements, or you MAY fail the hook to abort lightningd.

The “rolling up” of the database could be done periodically as well if the log of SQL statements has grown large.

Any response other than {"result": "continue"} will cause lightningd to error without committing to the database! This is the expected way to halt and catch fire.

invoice_payment

This hook is called whenever a valid payment for an unpaid invoice has arrived.

{
  "payment": {
    "label": "unique-label-for-invoice",
    "preimage": "0000000000000000000000000000000000000000000000000000000000000000",
    "msat": "10000msat"
  }
}

The hook is sparse on purpose, since the plugin can use the JSON-RPC listinvoices command to get additional details about this invoice. It can return a failure_message field as defined for final nodes in BOLT 4, a result field with the string reject to fail it with incorrect_or_unknown_payment_details, or a result field with the string continue to accept the payment.

openchannel

This hook is called whenever a remote peer tries to fund a channel to us, and it has passed basic sanity checks:

{
  "openchannel": {
    "id": "03864ef025fde8fb587d989186ce6a4a186895ee44a926bfc370e2c366597a3f8f",
    "funding_satoshis": "100000000msat",
    "push_msat": "0msat",
    "dust_limit_satoshis": "546000msat",
    "max_htlc_value_in_flight_msat": "18446744073709551615msat",
    "channel_reserve_satoshis": "1000000msat",
    "htlc_minimum_msat": "0msat",
    "feerate_per_kw": 7500,
    "to_self_delay": 5,
    "max_accepted_htlcs": 483,
    "channel_flags": 1
  }
}

There may be additional fields, including shutdown_scriptpubkey and a hex-string. You can see the definitions of these fields in BOLT 2’s description of the open_channel message.

The returned result must contain a result member which is either the string reject or continue. If reject and there’s a member error_message, that member is sent to the peer before disconnection.

For a ‘continue’d result, you can also include a close_to address, which will be used as the output address for a mutual close transaction.

e.g.

{
    "result": "continue",
    "close_to": "bc1qlq8srqnz64wgklmqvurv7qnr4rvtq2u96hhfg2"
}

Note that close_to must be a valid address for the current chain; an invalid address will cause the node to exit with an error.

htlc_accepted

The htlc_accepted hook is called whenever an incoming HTLC is accepted, and its result determines how lightningd should treat that HTLC.

The payload of the hook call has the following format:

{
  "onion": {
    "payload": "",
    "type": "legacy",
    "short_channel_id": "1x2x3",
    "forward_amount": "42msat",
    "outgoing_cltv_value": 500014,
    "shared_secret": "0000000000000000000000000000000000000000000000000000000000000000",
    "next_onion": "[1365bytes of serialized onion]"
  },
  "htlc": {
    "amount": "43msat",
    "cltv_expiry": 500028,
    "cltv_expiry_relative": 10,
    "payment_hash": "0000000000000000000000000000000000000000000000000000000000000000"
  }
}

For detailed information about each field please refer to BOLT 04 of the specification, the following is just a brief summary:

  • onion:
    • payload contains the unparsed payload that was sent to us from the sender of the payment.
    • type is legacy for realm 0 payments, tlv for realm > 1.
    • short_channel_id determines the channel that the sender is hinting should be used next. Not present if we’re the final destination.
    • forward_amount is the amount we should be forwarding to the next hop, and should match the incoming funds in case we are the recipient.
    • outgoing_cltv_value determines what the CLTV value for the HTLC that we forward to the next hop should be.
    • total_msat specifies the total amount to pay, if present.
    • payment_secret specifies the payment secret (which the payer should have obtained from the invoice), if present.
    • next_onion is the fully processed onion that we should be sending to the next hop as part of the outgoing HTLC. Processed in this case means that we took the incoming onion, decrypted it, extracted the payload destined for us, and serialized the resulting onion again.
    • shared_secret is the shared secret we used to decrypt the incoming onion. It is shared with the sender that constructed the onion.
  • htlc:
    • amount is the amount that we received with the HTLC. This amount minus the forward_amount is the fee that will stay with us.
    • cltv_expiry determines when the HTLC reverts back to the sender. cltv_expiry minus outgoing_cltv_expiry should be equal or larger than our cltv_delta setting.
    • cltv_expiry_relative hints how much time we still have to claim the HTLC. It is the cltv_expiry minus the current blockheight and is passed along mainly to avoid the plugin having to look up the current blockheight.
    • payment_hash is the hash whose payment_preimage will unlock the funds and allow us to claim the HTLC.

The hook response must have one of the following formats:

{
  "result": "continue"
}

This means that the plugin does not want to do anything special and lightningd should continue processing it normally, i.e., resolve the payment if we’re the recipient, or attempt to forward it otherwise. Notice that the usual checks such as sufficient fees and CLTV deltas are still enforced.

It can also replace the onion.payload by specifying a payload in the response. Note that this is always a TLV-style payload, so unlike onion.payload there is no length prefix (and it must be at least 4 hex digits long). This will be re-parsed; it’s useful for removing onion fields which a plugin doesn’t want lightningd to consider.

{
  "result": "fail",
  "failure_message": "2002"
}

fail will tell lightningd to fail the HTLC with a given hex-encoded failure_message (please refer to the spec for details: incorrect_or_unknown_payment_details is the most common).

{
  "result": "fail",
  "failure_onion": "[serialized error packet]"
}

Instead of failure_message the response can contain a hex-encoded failure_onion that will be used instead (please refer to the spec for details). This can be used, for example, if you’re writing a bridge between two Lightning Networks. Note that lightningd will apply the obfuscation step to the value returned here with its own shared secret (and key type ammag) before returning it to the previous hop.

{
  "result": "resolve",
  "payment_key": "0000000000000000000000000000000000000000000000000000000000000000"
}

resolve instructs lightningd to claim the HTLC by providing the preimage matching the payment_hash presented in the call. Notice that the plugin must ensure that the payment_key really matches the payment_hash since lightningd will not check and the wrong value could result in the channel being closed.

Warning: lightningd will replay the HTLCs for which it doesn’t have a final verdict during startup. This means that, if the plugin response wasn’t processed before the HTLC was forwarded, failed, or resolved, then the plugin may see the same HTLC again during startup. It is therefore paramount that the plugin is idempotent if it talks to an external system.

The htlc_accepted hook is a chained hook, i.e., multiple plugins can register it, and they will be called in the order they were registered in until the first plugin return a result that is not {"result": "continue"}, after which the event is considered to be handled. After the event has been handled the remaining plugins will be skipped.

rpc_command

The rpc_command hook allows a plugin to take over any RPC command. It sends the received JSON-RPC request to the registered plugin,

{
    "rpc_command": {
        "id": 3,
        "method": "method_name",
        "params": {
            "param_1": [],
            "param_2": {},
            "param_n": "",
        }
    }
}

which can in turn:

Let lightningd execute the command with

{
    "result" : "continue"
}

Replace the request made to lightningd:

{
    "replace": {
        "id": 3,
        "method": "method_name",
        "params": {
            "param_1": [],
            "param_2": {},
            "param_n": "",
        }
    }
}

Return a custom response to the request sender:

{
    "return": {
        "result": {
        }
    }
}

Return a custom error to the request sender:

{
    "return": {
        "error": {
        }
    }
}

custommsg

The custommsg plugin hook is the receiving counterpart to the dev-sendcustommsg RPC method and allows plugins to handle messages that are not handled internally. The goal of these two components is to allow the implementation of custom protocols or prototypes on top of a c-lightning node, without having to change the node’s implementation itself.

The payload for a call follows this format:

{
	"peer_id": "02df5ffe895c778e10f7742a6c5b8a0cefbe9465df58b92fadeb883752c8107c8f",
	"message": "1337ffffffff"
}

This payload would have been sent by the peer with the node_id matching peer_id, and the message has type 0x1337 and contents ffffffff. Notice that the messages are currently limited to odd-numbered types and must not match a type that is handled internally by c-lightning. These limitations are in place in order to avoid conflicts with the internal state tracking, and avoiding disconnections or channel closures, since odd-numbered message can be ignored by nodes (see “it’s ok to be odd” in the specification for details). The plugin must implement the parsing of the message, including the type prefix, since c-lightning does not know how to parse the message.

The result for this hook is currently being discarded. For future uses of the result we suggest just returning {'result': 'continue'}. This will ensure backward compatibility should the semantics be changed in future.

Bitcoin backend

C-lightning communicates with the Bitcoin network through a plugin. It uses the bcli plugin by default but you can use a custom one, multiple custom ones for different operations, or write your own for your favourite Bitcoin data source!

Communication with the plugin is done through 5 JSONRPC commands, lightningd can use from 1 to 5 plugin(s) registering these 5 commands for gathering Bitcoin data. Each plugin must follow the below specification for lightningd to operate.

getchaininfo

Called at startup, it’s used to check the network lightningd is operating on and to get the sync status of the backend.

The plugin must respond to getchaininfo with the following fields: - chain (string), the network name as introduced in bip70 - headercount (number), the number of fetched block headers - blockcount (number), the number of fetched block body - ibd (bool), whether the backend is performing initial block download

estimatefees

Polled by lightningd to get the current feerate, all values must be passed in sat/kVB.

If fee estimation fails, the plugin must set all the fields to null.

The plugin, if fee estimation succeeds, must respond with the following fields: - opening (number), used for funding and also misc transactions - mutual_close (number), used for the mutual close transaction - unilateral_close (number), used for unilateral close (/commitment) transactions - delayed_to_us (number), used for resolving our output from our unilateral close - htlc_resolution (number), used for resolving HTLCs after an unilateral close - penalty (number), used for resolving revoked transactions - min_acceptable (number), used as the minimum acceptable feerate - max_acceptable (number), used as the maximum acceptable feerate

getrawblockbyheight

This call takes one parameter, height, which determines the block height of the block to fetch.

The plugin must set all fields to null if no block was found at the specified height.

The plugin must respond to getrawblockbyheight with the following fields: - blockhash (string), the block hash as a hexadecimal string - block (string), the block content as a hexadecimal string

getutxout

This call takes two parameter, the txid (string) and the vout (number) identifying the UTXO we’re interested in.

The plugin must set both fields to null if the specified TXO was spent.

The plugin must respond to gettxout with the following fields: - amount (number), the output value in sats - script (string), the output scriptPubKey

sendrawtransaction

This call takes two parameters, a string tx representing a hex-encoded Bitcoin transaction, and a boolean allowhighfees, which if set means suppress any high-fees check implemented in the backend, since the given transaction may have fees that are very high.

The plugin must broadcast it and respond with the following fields: - success (boolean), which is true if the broadcast succeeded - errmsg (string), if success is false, the reason why it failed