Why I see many force close on my Lightning node?

If you are a node runner, you have for sure noticed cases in which channels are force closed and you cannot figure out the reason why this happens. Here some info about this.

If your peer (or your own node) becomes unresponsive while an HTLC is in progress, a force close may happen. This means that the channel has to be force closed because the HTLC needs to be 'settled'.

In case the HTLC is too small (e.g. below the dust limit) to get its output,, the HTLC in flight is donated to miners.

Most forced close are due to expired HTLC. A transaction tries to go through your node, ends up in pending status, your node goes offline, after the HTLC expires (please see below the default hours for each implementation) the channel goes into force close. More precisely:

• LND has by default 40

• CLN has by default 34

• Eclair by default 144

A common way to mitigate this is to set CTLV to 144 as many people do. There is a possibility, also, that the HTLC will expire even if your node remains online.

In a tweet of april 24th, Alex Bosworth writes:

LND doubling the default CLTV delta value should mean fewer unwanted force closes of channels. When an HTLC is pending, if the channel peers lose connectivity for some reason and the timeout runs down, LND is forced to auto-close to resolve on-chain. CLTV delta dictates that time