# 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` run your plugins from the `--lightning-dir`, 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 without any params. It MUST return a JSON object similar to this example: ```json { "options": [ { "name": "greeting", "type": "string", "default": "World", "description": "What name should I call you?" } ], "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." } ], "subscriptions": [ "connect", "disconnect" ], "hooks": [ "openchannel", "htlc_accepted" ], "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, (unsigned) integers, and bool 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 `[]`. The `dynamic` indicates if the plugin can be managed after `lightningd` has been started. Critical plugins that should not be stop should set it to false. 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. ### 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: ```json { "options": { "greeting": "World" }, "configuration": { "lightning-dir": "/home/user/.lightning", "rpc-file": "lightning-rpc", "startup": true } } ``` 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`: ```json ... "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][jsonrpc-spec] 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. ### Notification Types #### `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 } } ``` #### `connect` A notification for topic `connect` is sent every time a new connection to a peer is established. ```json { "id": "02f6725f9c1c40333b67faea92fd211c183050f28df32cac3f9d69685fe9665432", "address": "1.2.3.4" } ``` #### `disconnect` A notification for topic `disconnect` is sent every time a connection to a peer was lost. ```json { "id": "02f6725f9c1c40333b67faea92fd211c183050f28df32cac3f9d69685fe9665432" } ``` #### `invoice_payment` A notification for topic `invoice_payment` is sent every time an invoie is paid. ```json { "invoice_payment": { "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... ```json { "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: ` chan #:`,`lightningd():`, `plugin-:`, `():`, `jsonrpc:`, `jcon fd :`, `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`. ```json { "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 ```json { "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]. ## 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 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. Hooks and notifications sounds 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, however only one plugin may register for any hook topic at any point in time (we cannot disambiguate between multiple plugins returning contradictory results from a hook callback). 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. ### Hook Types #### `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: ```json { "peer": { "id": "03864ef025fde8fb587d989186ce6a4a186895ee44a926bfc370e2c366597a3f8f", "addr": "34.239.230.56:9735", "globalfeatures": "", "localfeatures": "" } } ``` 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. #### `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! ```json { "writes": [ "PRAGMA foreign_keys = ON" ] } ``` Any response but "true" will cause lightningd to error without committing to the database! #### `invoice_payment` This hook is called whenever a valid payment for an unpaid invoice has arrived. ```json { "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 non-zero `failure_code` field as defined for final nodes in [BOLT 4][bolt4-failure-codes], or otherwise an empty object 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: ```json { "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][bolt2-open-channel]. 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. #### `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: ```json { "onion": { "payload": "", "per_hop_v0": { "realm": "00", "short_channel_id": "1x2x3", "forward_amount": "42msat", "outgoing_cltv_value": 500014 } }, "next_onion": "[1365bytes of serialized onion]", "shared_secret": "0000000000000000000000000000000000000000000000000000000000000000", "htlc": { "amount": "43msat", "cltv_expiry": 500028, "cltv_expiry_relative": 10, "payment_hash": "0000000000000000000000000000000000000000000000000000000000000000" } } ``` The `per_hop_v0` will only be present if the per hop payload has format `0x00` as defined by the specification. If not present an object representing the type-length-vale (TLV) payload will be added (pending specification). For detailed information about each field please refer to [BOLT 04 of the specification][bolt4], the following is just a brief summary: - `onion.payload` contains the unparsed payload that was sent to us from the sender of the payment. - `onion.per_hop_v0`: - `realm` will always be `00` since that value determines that we are using the `per_hop_v0` format. - `short_channel_id` determines the channel that the sender is hinting should be used next (set to `0x0x0` if we are the recipient of the payment). - `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. - `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: ```json { "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. ```json { "result": "fail", "failure_code": 4301 } ``` `fail` will tell `lightningd` to fail the HTLC with a given numeric `failure_code` (please refer to the [spec][bolt4-failure-codes] for details). ```json { "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. [jsonrpc-spec]: https://www.jsonrpc.org/specification [jsonrpc-notification-spec]: https://www.jsonrpc.org/specification#notification [bolt4]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md [bolt4-failure-codes]: https://github.com/lightningnetwork/lightning-rfc/blob/master/04-onion-routing.md#failure-messages [bolt2-open-channel]: https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#the-open_channel-message