Unable to `send_probe` to eclair due to lack of payment secret

[AnthonyRonning]

Because of this issue, eclair requires payment secret when sending probes: ACINQ/eclair#1810 (comment)

Right now I'm getting:

INFO [lightning::ln::onion_utils:579] Onion Error[from 03864ef025fde8fb587d989186ce6a4a186895ee44a926bfc370e2c366597a3f8f: invalid_onion_payload(0x4016)] Node indicated that the decrypted onion per-hop payload was not understood by it or is incomplete

And I think it's because when sending probes, it uses sponaneous_empty which does not send payment secret.

rust-lightning/lightning/src/ln/outbound_payment.rs

Lines 984 to 993 in 3dffe54

	match self.pay_route_internal(&route, payment_hash, RecipientOnionFields::spontaneous_empty(),
	None, payment_id, None, onion_session_privs, node_signer, best_block_height, &send_payment_along_path
	) {
	Ok(()) => Ok((payment_hash, payment_id)),
	Err(e) => {
	self.remove_outbound_if_all_failed(payment_id, &e);
	Err(e)
	}
	}
	}

rust-lightning/lightning/src/ln/outbound_payment.rs

Lines 461 to 463 in 3dffe54

	pub fn spontaneous_empty() -> Self {
	Self { payment_secret: None, payment_metadata: None, custom_tlvs: Vec::new() }
	}

Would always sending payment secret be the right approach? Don't all nodes support it at this point? Or would allowing send_probe to pass through RecipientOnionFields be a work around and I can hardcode their node pubkey for it.

[TheBlueMatt]

Hmm, yea, interesting. The error clearly indicates that we got to the recipient, which means for probing purposes we should treat it as a success, but for payment purposes this error implies we should never try to route through this node again so we should blame the inbound channel (which is what we do). The simple fix is, indeed, just add a random payment secret to get them to respond with unknown_payment again, though we could also move to a different NetworkUpdate variant that indicates we got there, but the node did something dumb.

I'm also curious why the node isn't accepting this as a keysend - does eclair not support receiving keysends/spontaneous payments?

[AnthonyRonning]

I'm also curious why the node isn't accepting this as a keysend - does eclair not support receiving keysends/spontaneous payments?

I think they do, this was opened last week: ACINQ/eclair#2724

[TheBlueMatt]

CC @t-bast any chance you can help illuminate here - tony is getting the above error (PERM|invalid_onion_payload) when trying to probe to an eclair node, but it should presumably be treating it as a keysend and simply failing with unknown_payment.

[t-bast]

Hey! Sure, that one is simple enough.

By default, keysend is not activated on eclair nodes (and not activated on ours). That means we'll reject the payment here with an incorrect_or_unknown_payment_details.

But in your case, the keysend preimage field doesn't even seem to be filled, so we don't identify that as a keysend payment at all. Since we have activated payment_secret as mandatory, and we received a payment that is not using keysend and doesn't contain a payment_secret field, we reject it as being an invalid onion.

[TheBlueMatt]

Oh duh lol, y'all identify keysend by the preimage, not the lack of payment secret. Okay, makes sense, we should probably just fill in a random payment secret. Thanks.