lightningd: add payment-fronting-node option.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: Config: `payment-fronting-node` option to specify neighbor node(s) to use for all bolt11 invoices, bolt12 offers, invoices and invoice_requests.

Author: rustyrussell

doc/lightningd-config.5.md

@@ -518,6 +518,10 @@ delete the others.
 
 ### Payment and invoice control options:
 
+* **payment-fronting-node**=*nodeid*
+
+  Always use this *nodeid* as the entry point when we generate invoices or offers.  For BOLT11 invoices, this node must be a neighbor: we will use a routehint with the alias for the short channel id to provide limited privacy (we still reveal our node id).  For BOLT12 invoices and offers , we provide a blinded path from the node to us, to provide better privacy.
+
 * **disable-mpp** [plugin `pay`]
 
   Disable the multi-part payment sending support in the `pay` plugin. By default

lightningd/lightningd.c

@@ -384,6 +384,7 @@ static struct lightningd *new_lightningd(const tal_t *ctx)
 	/* The gossip seeker automatically connects to a this many peers */
 	ld->autoconnect_seeker_peers = 10;
 
+	ld->fronting_nodes = tal_arr(ld, struct node_id, 0);
 	return ld;
 }
 

lightningd/lightningd.h

@@ -433,6 +433,9 @@ struct lightningd {
 
 	/* Minimum number of peers seeker should maintain. */
 	u32 autoconnect_seeker_peers;
+
+	/* Nodes to use for invoices / offers */
+	struct node_id *fronting_nodes;
 };
 
 /* Turning this on allows a tal allocation to return NULL, rather than aborting.

lightningd/options.c

@@ -1265,6 +1265,16 @@ static char *opt_add_api_beg(const char *arg, struct lightningd *ld)
 	return NULL;
 }
 
+static char *opt_add_node_id(const char *arg, struct node_id **arr)
+{
+	struct node_id n;
+	if (!node_id_from_hexstr(arg, strlen(arg), &n))
+		return "Unparsable nodeid";
+
+	tal_arr_expand(arr, n);
+	return NULL;
+}
+
 char *hsm_secret_arg(const tal_t *ctx,
 		     const char *arg,
 		     const u8 **hsm_secret)
@@ -1578,6 +1588,10 @@ static void register_opts(struct lightningd *ld)
 		       ld,
 		       "Re-enable a long-deprecated API (which will be removed entirely next version!)");
 	opt_register_logging(ld);
+	clnopt_witharg("--payment-fronting-node",
+		       OPT_MULTI,
+		       opt_add_node_id, NULL,
+		       &ld->fronting_nodes, "Put this node in all invoices and offers, and use blinded path (bolt12) or route hints (bolt11) to route to this node.  Must be a neighboring node.  Can be specified multiple times.");
 
 	/* Old bookkeeper migration flags. */
 	opt_register_early_arg("--bookkeeper-dir",
@@ -1844,6 +1858,7 @@ bool is_known_opt_cb_arg(char *(*cb_arg)(const char *, void *))
 		|| cb_arg == (void *)opt_subd_dev_disconnect
 		|| cb_arg == (void *)opt_set_crash_timeout
 		|| cb_arg == (void *)opt_add_api_beg
+		|| cb_arg == (void *)opt_add_node_id
 		|| cb_arg == (void *)opt_force_featureset
 		|| cb_arg == (void *)opt_force_privkey
 		|| cb_arg == (void *)opt_force_bip32_seed