Eclair (ACINQ)

Eclair is ACINQ's Lightning implementation. Scala-based, runs on JVM, designed primarily as a routing/payment infrastructure node rather than personal wallet.

Versions and upgrade path

Release line as of September 2026 (dates from the ACINQ/eclair GitHub releases page):

• Bitcoin Core floor: v0.14.0 raised the dependency to Core 30.x to get v3/TRUC transactions and ephemeral dust; v0.14.1 raised it again to 31.x. The v0.14.3 README states "Eclair requires Bitcoin Core 31 or higher". The node must still be synchronized, wallet-enabled, non-pruning, tx-indexing and ZMQ-enabled.
• Java: Eclair targets Java 21 (Adoptium OpenJDK 21) as of v0.14.3.
• Non-anchor channels are gone in v0.14.0. Legacy channels must be closed before upgrading — v0.13.1 is the last release that can operate them. List them while still on v0.13.1 with eclair-cli channels and select commitments whose commitmentFormat is legacy, then close (or forceclose if the peer is offline).
• Channel splicing, taproot channels (unannounced) and zero-fee commitments shipped their final form in v0.14.0; zero-fee commitments remain flagged experimental there.
• Experimental peer scoring was added in v0.14.0 to optimize routing. Off by default (eclair.peer-scoring.enabled = false); it first only recommends liquidity and relay-fee actions, and must be explicitly configured before it acts on its own.
• Upgrades within the v0.14.x line need no channel closes: stop, upgrade, restart.

Components

• eclair-node — main daemon.
• eclair-cli — CLI tool wrapping HTTP API.
• HTTP/JSON-RPC API (no gRPC by default).
• Phoenix (mobile wallet) talks to ACINQ's Eclair backend via custom protocol.

Configuration

eclair.conf:

eclair {
  chain = "mainnet"
  bitcoind {
    host = "127.0.0.1"
    rpcport = 8332
    rpcuser = "..."
    rpcpassword = "..."
    zmqblock = "tcp://127.0.0.1:28332"
    zmqtx = "tcp://127.0.0.1:28333"
  }
  api {
    enabled = true
    port = 8080
    binding-ip = "127.0.0.1"
    password = "..."
  }
  node-alias = "eclair-node"
  channel {
    min-public-funding-satoshis = 100000
    min-private-funding-satoshis = 100000
    // 50 BTC; added in v0.14.2, channels above this are rejected
    max-funding-satoshis = 5000000000
  }
  on-chain-fees {
    feerate-tolerance.ratio-low = 0.5
    feerate-tolerance.ratio-high = 10.0
    // sat/byte cap on funding and splice feerates; added in v0.14.3
    max-funding-feerate = 50
  }
  router.sync {
    // per-peer gossip-query limits added in v0.14.2
    max-queries-per-second = 5
    max-queries-per-sync = 2000
  }
}

HOCON syntax (superset of JSON).

API

HTTP basic auth with single password (set in conf). Calls:

curl -u :password -X POST http://localhost:8080/getinfo
curl -u :password -d "uri=03abc...@host:9735" \
  http://localhost:8080/connect
curl -u :password -d "nodeId=03abc..." -d "fundingSatoshis=1000000" \
  http://localhost:8080/open
curl -u :password -d "invoice=lnbc..." \
  http://localhost:8080/payinvoice

Plugin architecture

Eclair plugins are JVM JAR files dropped into the plugins/ directory. Implement Plugin interface in Scala/Java. Hooks into:

• Channel events.
• Payment events.
• Routing decisions.

Watchtower

Eclair has no watchtower, client or server. As of September 2026 there is no watchtower code in ACINQ/eclair (checked at tag v0.14.3) and no eclair.conf setting for one — breach protection depends on the node itself being online to see a revoked commitment and publish the penalty transaction.

• Penalty material lives in the node's own database. Eclair keeps per-HTLC revocation info for the lifetime of each channel and prunes it in batches once a close or splice confirms (eclair.db.revoked-htlc-info-cleaner).
• eclair.blockchain-watchdog is not a watchtower. It cross-checks the local chain tip against external header sources (bitcoinheaders.net, blockcypher.com, blockstream.info, mempool.space by default) and warns when the node falls behind (missing-blocks-threshold = 7). It detects eclipse/desync, not channel breaches.
• Budget for downtime accordingly: eclair.channel.unhandled-exception-strategy defaults to local-close, and reference.conf explicitly warns that an unmonitored node gives peers a window to publish a revoked commitment.

See watchtowers/SKILL.md for the third-party options.

Implementation strengths

• Liquidity advertisement: Eclair pioneered "liquidity ads" for selling channel inbound to peers.
• PathFinding: variant of Dijkstra with sophisticated probability tracking.
• Backend / server-grade: designed for high-throughput routing nodes.
• Phoenix backend: ACINQ's hosted infrastructure for Phoenix users.

Phoenix integration

Phoenix mobile wallet uses a hybrid model:

• Mobile holds keys (self-custodial).
• ACINQ-hosted Eclair server is "trusted intermediary" for channel-on-pay and routing.
• "Splice-on-demand" — Phoenix can request capacity changes.

Memory / resource

JVM-based: ~500 MB - 1 GB RAM typical. Heavier than LND/CLN/LDK but not unreasonable for a backend.

Common issues

• JVM tuning: heap too small → OOM under load.
• HTTP API password leak via process listing — use env var or file reference instead.
• Backup: SQLite database in ~/.eclair/eclair.sqlite.db. Standard warnings about not restoring old DB.
• Plugin compatibility breaks across Eclair versions; verify before upgrading.
• Remote bitcoind: since v0.14.2 ACINQ documents that bitcoind must run on the same machine as Eclair, or behind a tunnel providing both encryption and authentication. A plain RPC connection over the network is treated as a broken setup.
• Tor: eclair.tor.auth defaults to safecookie from v0.14.2 onward, and password auth is rejected when eclair.tor.host is not a local address (v0.14.3 re-allows it on private networks). The onion private key is sent to the control port at every startup, so a remote control port leaks it regardless of auth method.
• Upgrading past v0.13.1 with non-anchor channels still open leaves no code path to operate them — close them first (see Versions and upgrade path).

See also

• bolts/SKILL.md
• channels/SKILL.md
• consumer-wallets/SKILL.md