Configuring lsyncd

At a glance#

• Purpose: Continuously replicate a directory from one Linux server to another as changes happen.
• Applies to: RHEL/CentOS 7+ and Ubuntu 18.04+.
• Risk: Medium — misconfigured paths can overwrite data on the target.
• Time: About 45 minutes.

Overview#

lsyncd (Live Syncing Daemon) watches a directory using the kernel's inotify interface and runs rsync whenever something changes. The result is near-real-time one-way replication without the delay of a cron job.

It is well suited to keeping a warm standby copy of web content or backups. It is not a backup on its own — deletions replicate too, so a file deleted by mistake on the source disappears from the target within seconds.

   SOURCE SERVER                       TARGET SERVER
   /backup  ──inotify──> lsyncd ──rsync over SSH──> /backup

Warning: Replication is one-way and destructive on the target. With delete = true (the default in most examples), anything present on the target but not the source is removed. Confirm the target directory is dedicated to this purpose before starting.

Before you start#

• Root access on both servers.
• Network connectivity from source to target on TCP 22.
• The target directory either empty or holding a known-good copy.
• Sufficient inotify watches for the number of files involved (see Troubleshooting).

The examples use:

Procedure#

1. Set up passwordless SSH from source to target#

lsyncd runs unattended, so it cannot answer a password prompt.

On the source server:

ssh-keygen -t ed25519 -f /root/.ssh/id_ed25519 -N ""

Note: ed25519 is preferred over rsa on any modern system — shorter keys, faster, and stronger. Use -t rsa -b 4096 only if the target is old enough not to support it.

Copy the public key to the target:

ssh-copy-id -i /root/.ssh/id_ed25519.pub root@192.168.213.80

Verify it works without prompting — do this before going further, because every later step depends on it:

ssh -i /root/.ssh/id_ed25519 root@192.168.213.80 "hostname"

If that returns the target's hostname with no password prompt, continue.

2. Install lsyncd#

RHEL / CentOS 7 — lsyncd lives in EPEL, not the base repositories:

sudo yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
sudo yum install -y lsyncd

RHEL 8/9 and Rocky/Alma:

sudo dnf install -y epel-release
sudo dnf install -y lsyncd

Ubuntu / Debian:

sudo apt update && sudo apt install -y lsyncd

Note: On a subscription-managed RHEL system without internet access, EPEL will not be reachable. Either mirror the package internally or install it from a local repository — see Creating Local Repo for Red hat Server.

3. Create the configuration#

lsyncd is configured in Lua. Create the config directory and file:

sudo mkdir -p /etc/lsyncd
sudo nano /etc/lsyncd.conf

settings {
    logfile        = "/var/log/lsyncd/lsyncd.log",
    statusFile     = "/var/log/lsyncd/lsyncd.status",
    statusInterval = 20,
    nodaemon       = false,
}

sync {
    default.rsync,
    source = "/backup",
    target = "192.168.213.80:/backup",
    delay  = 5,
    delete = true,
    rsync  = {
        archive  = true,
        compress = true,
        rsh      = "/usr/bin/ssh -l root -i /root/.ssh/id_ed25519 -o StrictHostKeyChecking=accept-new"
    }
}

Key settings:

4. Create the log directory#

The package does not always create this, and lsyncd fails silently at startup if it is missing:

sudo mkdir -p /var/log/lsyncd
sudo chown root:root /var/log/lsyncd

5. Validate the configuration before starting#

sudo lsyncd -nodaemon -log all /etc/lsyncd.conf

This runs in the foreground with full logging. Watch for a successful initial sync, then stop it with Ctrl+C. Fix any errors before continuing — starting the service with a broken config just moves the failure somewhere less visible.

6. Start and enable the service#

sudo systemctl start lsyncd
sudo systemctl enable lsyncd
sudo systemctl status lsyncd

7. Configure log rotation#

lsyncd logs grow steadily and will eventually fill the partition:

sudo nano /etc/logrotate.d/lsyncd

/var/log/lsyncd/*.log {
    daily
    rotate 7
    compress
    missingok
    notifempty
    copytruncate
}

Verification#

Confirm the service is running:

sudo systemctl status lsyncd

Test replication end to end:

# On the source
echo "sync test $(date)" | sudo tee /backup/synctest.txt

# Wait a few seconds, then on the target
cat /backup/synctest.txt

Test deletion propagation if delete = true:

# Source
sudo rm /backup/synctest.txt

# Target - should be gone within seconds
ls /backup/synctest.txt

Check the status file, which lsyncd rewrites periodically:

sudo cat /var/log/lsyncd/lsyncd.status
sudo tail -f /var/log/lsyncd/lsyncd.log

Troubleshooting#

Related#

• [Windows-to-Linux Rsync Sync for Nextcloud (via SSH)](Windows-to-Linux%20Rsync%20Sync%20for%20Nextcloud%20(via%20SSH%20240bf62b1b5380d3884acbf2c61e2d55.md)
• Creating Local Repo for Red hat Server