Expose background probing service

Allow operators to enable either built-in probing strategy through the
config file, CLI arguments, or environment variables. Validate strategy
settings and document the risk of probes locking outbound liquidity.

AI assistance: OpenAI Codex was used to implement and verify this
change.

Author: benthecarman

contrib/ldk-server-config.toml

@@ -9,6 +9,17 @@ alias = "ldk_server"                          # Lightning node alias
 #rgs_server_url = "https://rapidsync.lightningdevkit.org/snapshot/v2/"  # Optional: RGS URL for rapid gossip sync
 #async_payments_role = "client"               # Optional async payments role: "client" or "server"
 
+# Background probing service (optional)
+# CAUTION: Probes send real HTLCs and can lock outbound liquidity until they time out.
+#[probing]
+#strategy = "high_degree"                     # "high_degree" or "random_walk"
+#top_node_count = 100                          # Required for "high_degree"
+#max_hops = 5                                  # Required for "random_walk"; values below 2 are clamped to 2
+#interval_secs = 10                            # Time between probe attempts (default: 10 seconds)
+#max_locked_msat = 100000000                   # Maximum total in-flight probe liquidity (default: 100k sats)
+#diversity_penalty_msat = 250                  # Optional; useful with "high_degree"
+#cooldown_secs = 3600                          # Node re-probe cooldown for "high_degree" (default: 1 hour)
+
 # Storage settings
 [storage.disk]
 dir_path = "/tmp/ldk-server/"                 # Path for LDK and BDK data persistence, optional, defaults to ~/Library/Application Support/ldk-server/ on macOS, ~/.ldk-server/ on Linux

docs/configuration.md

@@ -59,6 +59,37 @@ this node to go offline. Set `async_payments_role = "server"` to hold async paym
 and onion messages for peers. The server role requires an announceable node configuration.
 Leave the field unset to disable async payments.
 
+### `[probing]`
+
+Enables LDK Node's background probing service to train the payment scorer with current
+channel-liquidity information. Probing is disabled when this section and the corresponding
+CLI/environment options are absent.
+
+Two strategies are available:
+
+- **`"high_degree"`** - probes toward highly connected public nodes. Set
+  `top_node_count` to control how many top nodes are cycled through.
+- **`"random_walk"`** - constructs random paths through the public graph. Set `max_hops`
+  to limit path length; values below `2` are clamped to `2`.
+
+```toml
+[probing]
+strategy = "high_degree"
+top_node_count = 100
+interval_secs = 30
+max_locked_msat = 500000
+diversity_penalty_msat = 250
+cooldown_secs = 3600
+```
+
+The optional `interval_secs`, `max_locked_msat`, `diversity_penalty_msat`, and
+`cooldown_secs` fields override LDK Node's defaults. `diversity_penalty_msat` and
+`cooldown_secs` are useful with `high_degree`; random walks bypass the scorer and do not
+track destination cooldowns.
+
+> **Warning:** Probes send real HTLCs over real channels. A probe can remain in flight and
+> lock outbound liquidity until its HTLC times out. Set `max_locked_msat` conservatively.
+
 ### `[storage.disk]`
 
 Where persistent data is stored. Defaults to `~/.ldk-server/` on Linux and

ldk-server/src/main.rs

@@ -207,6 +207,10 @@ fn main() {
 		builder.set_gossip_source_rgs(rgs_server_url);
 	}
 
+	if let Some(probing_config) = config_file.probing_config {
+		builder.set_probing_config(probing_config);
+	}
+
 	if let Err(e) = builder.set_async_payments_role(config_file.async_payments_role) {
 		error!("Failed to configure async payments role: {e}");
 		std::process::exit(-1);