The channel opening on lightning network involves broadcasting an on-chain bitcoin transaction assigning a transaction fee per byte of transaction size to the miners. It’s possible to batch the transactions together to save space on the blockchain which also implies lower transaction fee. One of the ways to batch the channel opening is to use PSBT in which the inputs of transaction can be used for a large number of channels.

I have already written an article about PSBT channel opening from bitcoin-cli which can be used when you have never used lightning network yet or wish to use the funds that you have within bitcoin-cli. Recently, that two of the channels I had on the lighting network got closed by a peer and I had two UTXO available in the lncli. This article is an extension of the PSBT channel opening from bitcoin-cli to describe how lncli psbt can be made.

The workflow goes through the points of the following list:

  1. connect all relevant nodes
  2. start funding using openchannel –psbt to all nodes (10 minutes counter starts), all but the last with --no_publish option
  3. create the funding PSBT transaction from lncli (explanation below)
    • you can check it in the bitcoin-cli decodepsbt <psbt> (but perhaps later, now the time counter is on)
  4. enter the PSBT transaction to each and every openchannel instance
  5. finalize the transaction in lncli (explanation below)
  6. enter the finalized transatction to each and every openchannel instance (make sure to enter it to the one without --no_publish option as the last one)
  7. the channels are batched for the opening

To remind you the point 2. is done using the command:

lncli openchannel --psbt <node_ID> --local_amt <amount> --no_publish

which gives you the address where the funds should be send within the bitcoin-cli example

...

Example with bitcoind:
	bitcoin-cli walletcreatefundedpsbt [] '[{"bcrt1qqzgeg5ty6hz3v2gckwkgyg8s0596zrvrjulgvly8efaeqe3uu60snfrfd5":0.01000000}]'

...
Paste the funded PSBT here to continue the funding flow.
Base64 encoded PSBT:

The point 3. and 5. need further explanation as those are the ones, which I haven’t use in the bitcoin-cli setup. In my case I have used the command:

lncli wallet psbt fund --conf_target 420 --inputs='[<utxo_1>, ... , <utxo_N>]' --outputs='{<addr_1>:<amount_1>,...,<addr_M>:<amount_M>}'

The --inputs options are used to select the inputs numbers that we would like to spend. We can find out which UTXOs we have available using lncli listunspend command. The <utxo_x> are exactly those which were specified in the lncli openchannel example and for you need to make sure, that you insert the amounts in satoshi rather than bitcoins.

As I wanted to use all the funds in the inputs to be transferred to the channels I selected the output amount plus desired fee (fairly low as I don’t care if the channels open in a few days) to be just a bit lower than the input amounts. Then I had a bit of trouble telling lncli that it should not create yet another output for the change, as I wanted all the funds to be used for the channel. I finally I overcame the issue by setting the confirmation target to a large value using --conf_target 420 which means that I target for channel to be opened in 420 blocks.

Once the command is finalized you obtain the <PSBT> string that can be entered to all the terminal windows with the prompt for the channel opening.

Finally, use the

lncli wallet psbt finalize <PSBT>

That gives you a final_tx hexadecimal string that is again entered to the channel opening prompts. Just make sure, that the channel opening without --no_publish option goes as the last one.

If everything goes through smoothly, you are returned the TXID of the transaction opening your channels. However my experience was not that straight forward. As one of the peers I have tried to open the channel with had a large minimal channel amount that I was planing to submit, my transaction failed before the final transaction could be broadcast. The channels with the other peers stayed pending. I had to abandon the pending channels with lncli abandonchannel that is only available in dev build of lncli (so I had to recompile from the source using make tags=dev && make install tags=dev). Before I could re-initiate the channel opening as specified in the lnd git issue.

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