Lightning Gossip (BOLT 7)

LN nodes gossip about themselves and channels so the network can build a routing graph. Each node maintains a local copy.

Messages

channel_announcement (256)

Announces a public channel between two nodes:

{
  features,
  chain_hash,
  short_channel_id (block × tx-index × output-index),
  node_id_1, node_id_2,
  bitcoin_key_1, bitcoin_key_2,
  node_signature_1, node_signature_2,        # signed by node keys
  bitcoin_signature_1, bitcoin_signature_2,  # signed by funding keys
}

Bitcoin signatures prove ownership of the funding outpoint. Without those, anyone could forge announcements.

node_announcement (257)

Node metadata:

{
  signature,                  # by node_id
  features, timestamp,
  node_id,
  rgb_color, alias,
  addresses (IPv4, IPv6, Tor v3, etc.),
  TLV (extras),
}

Newer timestamps replace older for same node_id.

channel_update (258)

Per-direction routing policy:

{
  signature,
  chain_hash, short_channel_id,
  timestamp, message_flags, channel_flags,  # incl. enabled bit
  cltv_expiry_delta,
  htlc_minimum_msat,
  fee_base_msat, fee_proportional_millionths,
  htlc_maximum_msat,
}

Two channel_updates per channel (one per direction). Latest timestamp wins; older replaced.

Validation

For channel_announcement:

• Verify both node sigs and both Bitcoin sigs.
• Verify the funding output exists at the specified SCID and matches the declared 2-of-2 multisig of bitcoin_keys.
• For Taproot channels: verify Taproot output matches.

For channel_update:

• Verify signature.
• Verify timestamp > previous for this direction.

For node_announcement:

• Verify signature.
• Verify timestamp > previous.

Failed validation → drop, possibly disconnect peer if many bad messages.

Sync strategies

Initial sync

When connecting to a new peer:

• gossip_queries (feature bit 6/7) enables on-demand sync.
• Without it, peer dumps all gossip (slow, fragile).

query_channel_range

{
  chain_hash,
  first_blocknum,
  number_of_blocks,
  query_options TLV (with checksums for incremental update),
}

Peer responds with reply_channel_range listing SCIDs (and optionally checksums) in that range.

query_short_channel_ids

Request the actual channel_announcement + channel_updates for specific SCIDs.

Incremental updates

Peers gossip in real-time:

• New channel_announcement → flood (one hop, then forward to all).
• channel_update updates → similar.
• node_announcement → similar.

Rate-limited per IP / connection to prevent spam.

Pruning

Channels with no channel_update for ~14 days are considered stale and dropped from local graph.

Closed channels (funding output spent) detected via:

• Watching the chain (each node monitors funding outpoints).
• Receiving an explicit channel_update with disabled flag.

Full prune: when funding output is spent on chain, drop the channel edge.

Large-graph performance

LN graph as of 30 August 2026 (mempool.space, the most recent datapoint published as of September 2026): ~32,700 public channels, ~14,200 announced nodes (~16,200 counting unannounced), ~3,790 BTC public capacity. Storage: ~50-100 MB.

The graph shrank steadily through 2026: ~41,600 channels and ~5,740 BTC capacity on 1 January 2026, ~38,200 channels on 1 August 2026, ~32,700 by 30 August 2026. Crawlers disagree substantially — 1ML reported ~19,900 channels and ~5,900 nodes on 15 September 2026 — because each explorer sees a different slice of the gossip network; treat any single explorer's count as a lower bound.

Periodic full re-gossip on cold start can be 5-30 minutes depending on peer count and bandwidth. Caching the graph locally + diff sync (via query_channel_range checksums) is essential.

Rate limiting

Per BOLT 7 recommendations:

• Max 1 channel_update per direction per channel per second.
• Stale flood detection: drop messages with timestamps too old.
• Per-peer message rate limit (varies by impl).

Privacy

Gossip is fully public:

• Anyone can build the graph.
• Channel capacities revealed via on-chain funding outputs.
• Some data leaks about node identity / location via IP / Tor / etc.

Private channels (announce_channel = false):

• Not gossiped.
• Visible only to channel partners.
• For receive: include route_hints (BOLT 11 'r' tag) in invoices so senders can reach you.

Common bugs

• Failing to verify Bitcoin sigs in channel_announcement → injected fake channels.
• Treating channel_update from disabled channel as routable.
• Race conditions on disconnect: peer sends update for channel that just closed.
• Excessive pruning timeout (e.g., 1 day) → dropping legitimate inactive channels.
• Storing the graph in-memory without periodic disk sync → lose state on crash, full re-sync needed.

See also

• bolts/SKILL.md
• routing/SKILL.md
• channels/SKILL.md