Backing Up Your C-Lightning Node

Lightning Network channels get their scalability and privacy benefits from the very simple technique of not telling anyone else about your in-channel activity. This is in contrast to onchain payments, where you have to tell everyone about each and every payment and have it recorded on the blockchain, leading to scaling problems (you have to push data to everyone, everyone needs to validate every transaction) and privacy problems (everyone knows every payment you were ever involved in).

Unfortunately, this removes a property that onchain users are so used to, they react in surprise when learning about this removal. Your onchain activity is recorded in all archival fullnodes, so if you forget all your onchain activity because your storage got fried, you just go redownload the activity from the nearest archival fullnode.

But in Lightning, since you are the only one storing all your financial information, you cannot recover this financial information from anywhere else.

This means that on Lightning, you have to responsibly back up your financial information yourself, using various processes and automation.

The discussion below assumes that you know where you put your $LIGHTNINGDIR, and you know the directory structure within. By default your $LIGHTNINGDIR will be in ~/.lightning/${COIN}. For example, if you are running --mainnet, it will be ~/.lightning/bitcoin.

hsm_secret

/!\ WHO SHOULD DO THIS: Everyone.

You need a copy of the hsm_secret file regardless of whatever backup strategy you use.

The hsm_secret is created when you first create the node, and does not change. Thus, a one-time backup of hsm_secret is sufficient.

This is just 32 bytes, and you can do something like the below and write the hexadecimal digits a few times on a piece of paper:

cd $LIGHTNINGDIR
xxd hsm_secret

You can re-enter the hexdump into a text file later and use xxd to convert it back to a binary hsm_secret:

cat > hsm_secret_hex.txt <<HEX
00: 30cc f221 94e1 7f01 cd54 d68c a1ba f124
10: e1f3 1d45 d904 823c 77b7 1e18 fd93 1676
HEX
xxd -r hsm_secret_hex.txt > hsm_secret
chmod 0400 hsm_secret

Notice that you need to ensure that the hsm_secret is only readable by the user, and is not writable, as otherwise lightningd will refuse to start. Hence the chmod 0400 hsm_secret command.

Alternately, if you are deploying a new node that has no funds and channels yet, you can generate BIP39 words using any process, and create the hsm_secret using the hsmtool generatehsm command. If you did make install then hsmtool is installed as lightning-hsmtool, else you can find it in the tools/ directory of the build directory.

lightning-hsmtool generatehsm hsm_secret

Then enter the BIP39 words, plus an optional passphrase.

You can regenerate the same hsm_secret file using the same BIP39 words, which again, you can back up on paper.

Recovery of the hsm_secret is sufficient to recover any onchain funds. Recovery of the hsm_secret is necessary, but insufficient, to recover any in-channel funds. To recover in-channel funds, you need to use one or more of the other backup strategies below.

backup Plugin And Remote NFS Mount

/!\ WHO SHOULD DO THIS: Casual users.

You can get the backup plugin here: https://github.com/lightningd/plugins/tree/master/backup

The backup plugin requires Python 3.

  • cd into its directory and install requirements.

    • pip3 install -r requirements.txt

  • Figure out where you will put the backup files.

    • Ideally you have an NFS or other network-based mount on your system, into which you will put the backup.

  • Stop your Lightning node.

  • /path/to/backup-cli init ${LIGHTNINGDIR} file:///path/to/nfs/mount. This creates an initial copy of the database at the NFS mount.

  • Add these settings to your lightningd configuration:

    • important-plugin=/path/to/backup.py

  • Restart your Lightning node.

It is recommended that you use a network-mounted filesystem for the backup destination. For example, if you have a NAS you can access remotely.

Do note that files are not stored encrypted, so you should really not do this with rented space (”cloud storage”).

Alternately, you could put it in another storage device (e.g. USB flash disk) in the same physical location.

To recover:

  • Re-download the backup plugin and install Python 3 and the requirements of backup.

  • /path/to/backup-cli restore file:///path/to/nfs/mount ${LIGHTNINGDIR}

If your backup destination is a network-mounted filesystem that is in a remote location, then even loss of all hardware in one location will allow you to still recover your Lightning funds.

However, if instead you are just replicating the database on another storage device in a single location, you remain vulnerable to disasters like fire or computer confiscation.

Filesystem Redundancy

/!\ WHO SHOULD DO THIS: Filesystem nerds, data hoarders, home labs, enterprise users.

You can set up a RAID-1 with multiple storage devices, and point the $LIGHTNINGDIR to the RAID-1 setup. That way, failure of one storage device will still let you recover funds.

You can use a hardware RAID-1 setup, or just buy multiple commodity storage media you can add to your machine and use a software RAID, such as (not an exhaustive list!):

  • mdadm to create a virtual volume which is the RAID combination of multiple physical media.

  • BTRFS RAID-1 or RAID-10, a filesystem built into Linux.

  • ZFS RAID-Z, a filesystem that cannot be legally distributed with the Linux kernel, but can be distributed in a BSD system, and can be installed on Linux with some extra effort, see ZFSonLinux.

RAID-1 (whether by hardware, or software) like the above protects against failure of a single storage device, but does not protect you in case of certain disasters, such as fire or computer confiscation.

You can “just” use a pair of high-quality metal-casing USB flash devices (you need metal-casing since the devices will have a lot of small writes, which will cause a lot of heating, which needs to dissipate very fast, otherwise the flash device firmware will internally disconnect the flash device from your computer, reducing your reliability) in RAID-1, if you have enough USB ports.

Example: BTRFS on Linux

On a Linux system, one of the simpler things you can do would be to use BTRFS RAID-1 setup between a partition on your primary storage and a USB flash disk. The below “should” work, but assumes you are comfortable with low-level Linux administration. If you are on a system that would make you cry if you break it, you MUST stop your Lightning node and back up all files before doing the below.

  • Install btrfs-progs or btrfs-tools or equivalent.

  • Get a 32Gb USB flash disk.

  • Stop your Lightning node and back up everything, do not be stupid.

  • Repartition your hard disk to have a 30Gb partition.

    • This is risky and may lose your data, so this is best done with a brand-new hard disk that contains no data.

  • Connect the USB flash disk.

  • Find the /dev/sdXX devices for the HDD 30Gb partition and the flash disk.

    • lsblk -o NAME,TYPE,SIZE,MODEL should help.

  • Create a RAID-1 btrfs filesystem.

    • mkfs.btrfs -m raid1 -d raid1 /dev/${HDD30GB} /dev/${USB32GB}

    • You may need to add -f if the USB flash disk is already formatted.

  • Create a mountpoint for the btrfs filesystem.

  • Create a /etc/fstab entry.

    • Use the UUID option instad of /dev/sdXX since the exact device letter can change across boots.

    • You can get the UUID by lsblk -o NAME,UUID. Specifying either of the devices is sufficient.

    • Add autodefrag option, which tends to work better with SQLITE3 databases.

    • e.g. UUID=${UUID} ${BTRFSMOUNTPOINT} btrfs defaults,autodefrag 0 0

  • mount -a then df to confirm it got mounted.

  • Copy the contents of the $LIGHTNINGDIR to the BTRFS mount point.

    • Copy the entire directory, then chown -R the copy to the user who will run the lightningd.

    • If you are paranoid, run diff -R on both copies to check.

  • Remove the existing $LIGHTNINGDIR.

  • ln -s ${BTRFSMOUNTPOINT}/lightningdirname ${LIGHTNINGDIR}.

    • Make sure the $LIGHTNINGDIR has the same structure as what you originally had.

  • Add crontab entries for root that perform regular btrfs maintenance tasks.

    • 0 0 * * * /usr/bin/btrfs balance start -dusage=50 -dlimit=2 -musage=50 -mlimit=4 ${BTRFSMOUNTPOINT} This prevents BTRFS from running out of blocks even if it has unused space within blocks, and is run at midnight everyday. You may need to change the path to the btrfs binary.

    • 0 0 * * 0 /usr/bin/btrfs scrub start -B -c 2 -n 4 ${BTRFSMOUNTPOINT} This detects bit rot (i.e. bad sectors) and auto-heals the filesystem, and is run on Sundays at midnight.

  • Restart your Lightning node.

If one or the other device fails completely, shut down your computer, boot on a recovery disk or similar, then:

  • Connect the surviving device.

  • Mount the partition/USB flash disk in degraded mode:

    • mount -o degraded /dev/sdXX /mnt/point

  • Copy the lightningd.sqlite3 and hsm_secret to new media.

    • Do not write to the degraded btrfs mount!

  • Start up a lightningd using the hsm_secret and lightningd.sqlite3 and close all channels and move all funds to onchain cold storage you control, then set up a new Lightning node.

If the device that fails is the USB flash disk, you can replace it using BTRFS commands. You should probably stop your Lightning node while doing this.

  • btrfs replace start /dev/sdOLD /dev/sdNEW ${BTRFSMOUNTPOINT}.

    • If /dev/sdOLD no longer even exists because the device is really really broken, use btrfs filesystem show to see the number after devid of the broken device, and use that number instead of /dev/sdOLD.

  • Monitor status with btrfs replace status ${BTRFSMOUNTPOINT}.

More sophisticated setups with more than two devices are possible. Take note that “RAID 1” in btrfs means “data is copied on up to two devices”, meaning only up to one device can fail. You may be interested in raid1c3 and raid1c4 modes if you have three or four storage devices. BTRFS would probably work better if you were purchasing an entire set of new storage devices to set up a new node.

PostgreSQL Cluster

/!\ WHO SHOULD DO THIS: Enterprise users, whales.

lightningd may also be compiled with PostgreSQL support. PostgreSQL is generally faster than SQLITE3, and also supports running a PostgreSQL cluster to be used by lightningd, with automatic replication and failover in case an entire node of the PostgreSQL cluster fails.

Setting this up, however, is more involved.

By default, lightningd compiles with PostgreSQL support only if it finds libpq installed when you ./configure. To enable it, you have to install a developer version of libpq. On most Debian-derived systems that would be libpq-dev. To verify you have it properly installed on your system, check if the following command gives you a path:

pg_config --includedir

Versioning may also matter to you. For example, Debian Stable (”buster”) as of late 2020 provides PostgreSQL 11.9 for the libpq-dev package, but Ubuntu LTS (”focal”) of 2020 provides PostgreSQL 12.5. Debian Testing (”bullseye”) uses PostgreSQL 13.0 as of this writing. PostgreSQL 12 had a non-trivial change in the way the restore operation is done for replication. You should use the same PostgreSQL version of libpq-dev as what you run on your cluster, which probably means running the same distribution on your cluster.

Once you have decided on a specific version you will use throughout, refer as well to the “synchronous replication” document of PostgreSQL for the specific version you are using:

You then have to compile lightningd with PostgreSQL support.

  • Clone or untar a new source tree for lightning and cd into it.

    • You could just use make clean on an existing one, but for the avoidance of doubt (and potential bugs in our Makefile cleanup rules), just create a fresh source tree.

  • ./configure

    • Add any options to configure that you normally use as well.

  • Double-check the config.vars file contains HAVE_POSTGRES=1.

    • grep 'HAVE_POSTGRES' config.vars

  • make

  • If you install lightningd, sudo make install.

If you were not using PostgreSQL before but have compiled and used lightningd on your system, the resulting lightningd will still continue supporting and using your current SQLITE3 database; it just gains the option to use a PostgreSQL database as well.

If you just want to use PostgreSQL without using a cluster (for example, as an initial test without risking any significant funds), then after setting up a PostgreSQL database, you just need to add --wallet=postgresql://${USER}:${PASSWORD}@${HOST}:${PORT}/${DB} to your lightningd config or invocation.

To set up a cluster for a brand new node, follow this (external) guide by @gabridome.

The above guide assumes you are setting up a new node from scratch. It is also specific to PostgreSQL 12, and setting up for other versions will have differences; read the PostgreSQL manuals linked above.

If you want to continue a node that started using an SQLITE3 database, note that we do not support this. You should set up a new PostgreSQL node, move funds from the SQLITE3 node to the PostgreSQL node, then shut down the SQLITE3 node permanently.

There are also more ways to set up PostgreSQL replication. In general, you should use synchronous replication (13), since lightningd assumes that once a transaction is committed, it is saved in all permanent storage. This can be difficult to create remote replicas due to the latency.

Database File Backups

/!\ WHO SHOULD DO THIS: Those who already have at least one of the other backup methods, those who are #reckless.

This is the least desirable backup strategy, as it can lead to loss of all in-channel funds if you use it. However, having no backup strategy at all will lead to loss of all in-channel funds, so this is still better than nothing.

This backup method is undesirable, since it cannot recover the following channels:

  • Channels with peers that do not support option_dataloss_protect.

    • Most nodes on the network already support option_dataloss_protect as of November 2020.

    • If the peer does not support option_dataloss_protect, then the entire channel funds will be revoked by the peer.

    • Peers can claim to honestly support this, but later steal funds from you by giving obsolete state when you recover.

  • Channels created after the copy was made are not recoverable.

    • Data for those channels does not exist in the backup, so your node cannot recover them.

Because of the above, this strategy is discouraged: you can potentially lose all funds in open channels.

However, again, note that a “no backups #reckless” strategy leads to definite loss of funds, so you should still prefer this strategy rather than having no backups at all.

Even if you have one of the better options above, you might still want to do this as a worst-case fallback, as long as you:

  • Attempt to recover using the other backup options above first. Any one of them will be better than this backup option.

  • Recover by this method ONLY as a last resort.

  • Recover using the most recent backup you can find. Take time to look for the most recent available backup.

Again, this strategy can lead to only partial recovery of funds, or even to complete failure to recover, so use the other methods first to recover!

Offline Backup

While lightningd is not running, just copy the lightningd.sqlite3 file in the $LIGHTNINGDIR on backup media somewhere.

To recover, just copy the backed up lightningd.sqlite3 into your new $LIGHTNINGDIR together with the hsm_secret.

You can also use any automated backup system as long as it includes the lightningd.sqlite3 file (and optionally hsm_secret, but note that as a secret key, thieves getting a copy of your backups may allow them to steal your funds, even in-channel funds) and as long as it copies the file while lightningd is not running.

Backing Up While lightningd Is Running

Since sqlite3 will be writing to the file while lightningd is running, cping the lightningd.sqlite3 file while lightningd is running may result in the file not being copied properly if sqlite3 happens to be committing database transactions at that time, potentially leading to a corrupted backup file that cannot be recovered from.

You have to stop lightningd before copying the database to backup in order to ensure that backup files are not corrupted, and in particular, wait for the lightningd process to exit. Obviously, this is disruptive to node operations, so you might prefer to just perform the cp even if the backup potentially is corrupted. As long as you maintain multiple backups sampled at different times, this may be more acceptable than stopping and restarting lightningd; the corruption only exists in the backup, not in the original file.

If the filesystem or volume manager containing $LIGHTNINGDIR has a snapshot facility, you can take a snapshot of the filesystem, then mount the snapshot, copy lightningd.sqlite3, unmount the snapshot, and then delete the snapshot. Similarly, if the filesystem supports a “reflink” feature, such as cp -c on an APFS on MacOS, or cp --reflink=always on an XFS or BTRFS on Linux, you can also use that, then copy the reflinked copy to a different storage medium; this is equivalent to a snapshot of a single file. This reduces but does not eliminate this race condition, so you should still maintain multiple backups.

You can additionally perform a check of the backup by this command:

echo 'PRAGMA integrity_check;' | sqlite3 ${BACKUPFILE}

This will result in the string ok being printed if the backup is likely not corrupted. If the result is anything else than ok, the backup is definitely corrupted and you should make another copy.

In order to make a proper uncorrupted backup of the SQLITE3 file while lightningd is running, we would need to have lightningd perform the backup itself, which, as of the version at the time of this writing, is not yet implemented.

Even if the backup is not corrupted, take note that this backup strategy should still be a last resort; recovery of all funds is still not assured with this backup strategy.

You might be tempted to use sqlite3 .dump or VACUUM INTO. Unfortunately, these commands exclusive-lock the database. A race condition between your .dump or VACUUM INTO and lightningd accessing the database can cause lightningd to crash, so you might as well just cleanly shut down lightningd and copy the file at rest.