rsync Daemon

📈 Architecture Overview

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🛠 Server Configuration

/etc/rsyncd.conf:

[usbbackup]  # Module name
  path = /tmp/rsync-test     # Destination directory on the server
  read only = false
  auth users = backupuser    # User defined for rsync access
  secrets file = /etc/rsyncd.secrets

🔐 Authentication Explained

• auth users defines which usernames can access this module.
• secrets file stores the username-password mapping in the format:

/etc/rsyncd.secrets:

backupuser:supersecret123

🔸 The backupuser does not need to exist on the OS. 🔸 This authentication is entirely internal to the rsync daemon. 🔸 /etc/passwd and OS accounts are not involved. ✅ This creates a safer, isolated login system for file sync.

Make sure to restrict the file permission:

sudo chmod 600 /etc/rsyncd.secrets

💡 If auth users is omitted, the module becomes publicly writable (not recommended).

To start the daemon:

sudo systemctl daemon-reload
sudo systemctl restart rsync

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💻 Client Usage

1. Test the Connection

# List available modules
rsync --list-only rsync://backupuser@192.168.68.51/

You will be prompted to enter the password.

2. Sync Files

# Upload: Sync local → server
rsync -av /some/local/folder/ rsync://backupuser@192.168.68.51/usbbackup/

# Download: Sync server → local
rsync -av rsync://backupuser@192.168.68.51/usbbackup/ /some/local/folder/

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⚠️ Common Error & Solutions

❗ Problem

rsync: [receiver] mkstemp "/.hello-rsync.txt.CH2LTc" (in usbbackup) failed: Permission denied (13)

Even though the rsync daemon runs as root, it doesn't write as root by default during file transfers.

By default, if uid and gid are not specified in rsyncd.conf, the daemon uses nobody:nogroup, which lacks permissions to write to /tmp/rsync-test.

✅ Solution 1: Change Directory Ownership to nobody

sudo chown -R nobody:nogroup /tmp/rsync-test
sudo chmod -R 755 /tmp/rsync-test

This allows the daemon (running as nobody) to write data successfully.

✅ Solution 2: Use a Specific User (e.g., eric)

Update rsyncd.conf:

check the uid and gid settings:

id
# uid=1000(eric) gid=1000(eric) groups=1000(eric) ...

Then modify /etc/rsyncd.conf:

[usbbackup]
  path = /tmp/rsync-test
  read only = false
  uid = 1000
  gid = 1000
  auth users = backupuser
  secrets file = /etc/rsyncd.secrets

Then adjust directory permissions:

sudo chown -R eric:eric /tmp/rsync-test

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🧠 Optional Enhancements

1. Enable Module Listing

This helps tools like Hyper Backup or CLI debugging:

list = yes
comment = USB Backup POC

rsync rsync://backupuser@192.168.68.51/
# Output:
# usbbackup       USB Backup POC

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2. Restrict IP Access (Firewall)

Allow access only from LAN or specific clients:

hosts allow = 192.168.68.0/24
hosts deny = *

Or even stricter:

hosts allow = 192.168.68.50

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3. Enable Persistent Logging (via systemd)

sudo systemctl edit rsync

Add the following to the override:

[Service]
ExecStart=
ExecStart=/usr/bin/rsync --daemon --no-detach --config=/etc/rsyncd.conf
StandardOutput=journal
StandardError=journal

Then reload:

sudo systemctl daemon-reload
sudo systemctl restart rsync

Watch logs in real-time:

journalctl -u rsync -f

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🔄 Reverting to Default

1. Stop & Disable the Daemon

sudo systemctl stop rsync
sudo systemctl disable rsync

2. Remove Config Files

sudo rm /etc/rsyncd.conf
sudo rm /etc/rsyncd.secrets

Optionally, you may only remove rsyncd.conf and keep the secrets file.

3. Remove systemd Overrides (If Applied)

sudo systemctl revert rsync

4. Confirm Status

sudo systemctl status rsync

Expected output:

● rsync.service - fast remote file copy program daemon
   Loaded: loaded (/lib/systemd/system/rsync.service; disabled; vendor preset: enabled)
   Active: inactive (dead)

Summary

Action	Command
Stop & disable daemon	sudo systemctl stop/disable rsync
Remove config file	sudo rm /etc/rsyncd.conf
Remove secrets file (optional)	sudo rm /etc/rsyncd.secrets
Remove systemd override	sudo systemctl revert rsync
Check service status	sudo systemctl status rsync

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🧹 Synchronization Notes: --delete

By default, rsync does not delete files on the destination that are missing from the source.

sudo rsync -av ./ rsync://backupuser@192.168.68.51/usbbackup/

• ✅ New and updated files are synced.
• ❌ Deleted source files remain on the server.

To keep the destination fully in sync (mirroring):

sudo rsync -av --delete ./ rsync://backupuser@192.168.68.51/usbbackup/

🚨 Caution: This will remove files on the server that are not present on the client.

🧪 Always test first:

sudo rsync -av --delete --dry-run ./ rsync://backupuser@192.168.68.51/usbbackup/

Common --delete Options

Option	Description
--delete	Delete files on the target that no longer exist in the source.
--delete-before	Perform deletions before transfer.
--delete-during	Delete files during the transfer (more efficient).
--delete-excluded	Also delete files that were excluded with --exclude.