Explicitly match on-the-fly HTLCs after a restart - #3357
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When we restart our node, we look at HTLCs that have been received but not relayed yet: they must be failed back, otherwise we would let them in the incoming channel until their timeout is reached, which would force our peer to force-close. There is an exception for HTLCs that are paying for an on-the-fly funded channel: we will explicitly retry relaying those HTLCs, even after a restart, because that's how we get paid for the funding fees. We were only matching them by `payment_hash`, which wasn't precise enough: an attacker could initiate an on-the-fly channel and then send unrelated HTLCs that use the same `payment_hash`, just to mess up with us and get some of our channels force-closed. We already have the detailed information about on-the-fly HTLCs in our DB, so we now use that to match HTLCs on the corresponding `channel_id` and `htlc_id`, which guarantees uniqueness.
pm47
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Aug 19, 2026
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LGTM. We could probably type the pair (case class HtlcCoordinates(...) ?)
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I hesitated but it was hard to find a good name, and it required touching more files, so I deferred it for now. |
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When we restart our node, we look at HTLCs that have been received but not relayed yet: they must be failed back, otherwise we would let them in the incoming channel until their timeout is reached, which would force our peer to force-close.
There is an exception for HTLCs that are paying for an on-the-fly funded channel: we will explicitly retry relaying those HTLCs, even after a restart, because that's how we get paid for the funding fees. We were only matching them by
payment_hash, which wasn't precise enough: an attacker could initiate an on-the-fly channel and then send unrelated HTLCs that use the samepayment_hash, just to mess up with us and get some of our channels force-closed.We already have the detailed information about on-the-fly HTLCs in our DB, so we now use that to match HTLCs on the corresponding
channel_idandhtlc_id, which guarantees uniqueness.This was reported by Loupe (https://github.com/project-loupe).