Essential Linux Remote Data Sync with Rsync: A Complete Guide

Introduction

Rsync (remote synchronize) is a data‑synchronization tool that quickly syncs files between multiple hosts over LAN or WAN. It uses the rsync algorithm to transfer only the differing parts of files, which makes subsequent synchronizations fast.

Rsync runs on most Unix‑like systems (Linux, Solaris, BSD) and has Windows ports such as cwRsync and Sync2NAS.

Principle

Originally created to replace rcp, rsync is maintained at rsync.samba.org and its configuration file format resembles Samba’s main config.

Rsync can operate via rsh or ssh, or run as a daemon listening on port 873. After the first full transfer, later runs perform incremental backups.

Key Features

Can mirror entire directory trees and file systems.

Preserves original permissions, timestamps, and symbolic/hard links.

Installs without special privileges.

Optimized workflow yields high transfer efficiency.

Supports rsh, ssh, or direct socket connections.

Allows anonymous transfers.

SSH Mode Examples

Local‑to‑Local Sync

# mkdir src</code>
<code># touch src/{1,2,3,4}</code>
<code># mkdir dest</code>
<code># rsync -av src/ dest/    # sync contents of src into dest (exclude src itself)</code>
<code># rsync -av src dest/    # sync src directory itself into dest</code>
<code># rsync -avR src/ dest/   # same effect even with trailing slash

LAN Sync

# mkdir src</code>
<code># touch src/{a,b,c,d}</code>
<code># mkdir dest</code>
<code># rsync -av 172.16.22.12:/data/test/src/ dest/    # remote → local (enter root password)</code>
<code># rsync -av src/ 172.16.22.12:/data/test/dest/    # local → remote</code>
<code># rsync -av src 172.16.22.12:/data/test/dest/    # sync whole directory</code>
<code># rm -rf src/d    # delete a file</code>
<code># rsync -av --delete src/ 172.16.22.12:/data/test/dest/    # delete extraneous files on target

LAN Sync with Specified User

# useradd george</code>
<code># passwd george</code>
<code># mkdir /home/george/test</code>
<code># touch /home/george/test/g{1,2,3,4}</code>
<code># rsync -av src '-e ssh -l george' 172.16.22.12:/home/george    # local → remote as george</code>
<code># rsync -av 172.16.22.12:/home/george/test/g* '-e ssh -l george -p 22' dest/

Daemon Mode

Service Startup

Heavy‑load server (stand‑alone daemon)

# yum install rsync xinetd</code>
<code># /usr/bin/rsync --daemon

Light‑load server (xinetd)

# yum install rsync xinetd</code>
<code># vim /etc/xinetd.d/rsync   # set "disable = no"</code>
<code># /etc/init.d/xinetd start</code>
<code># chkconfig rsync on</code>
<code># netstat -ntpl | grep 873   # verify service

Configuration Details (rsyncd.conf)

Global parameters (apply to all modules unless overridden):

address – IP address for stand‑alone mode; ignored by xinetd.

port – listening port (default 873); ignored by xinetd.

motd file – message displayed to clients on connection.

pid file – file where the daemon writes its PID.

log file – path for the daemon’s log.

syslog facility – syslog priority for messages.

socket options – custom TCP options.

lockfile – location of lock file.

timeout = 600 – inactivity timeout (seconds).

Module parameters (define which directories are exposed):

path – required, the directory to sync.

comment – description shown to clients.

Module control parameters (examples):

use chroot = true – chroot to path before transfer (requires root).

uid / gid – run as specified user/group (default nobody).

max connections – maximum concurrent connections (0 = unlimited).

read only / write only – control upload/download permissions.

ignore errors – skip I/O errors.

hosts allow / hosts deny – whitelist/blacklist client IPs.

auth users – list of allowed usernames (stored in secrets file).

Server Configuration Example

# vim /etc/rsyncd.conf

uid = root

gid = root

use chroot = no

max connections = 5

timeout = 600

pid file = /var/run/rsyncd.pid

lockfile = /var/run/rsyncd.lock

log file = /var/log/rsyncd.log

[web1] # module name

# mkdir /data/test/src</code>
<code># mkdir /data/test/src/george</code>
<code># touch /data/test/src/{1,2,3}</code>
<code># echo "web:123" > /etc/web.passwd</code>
<code># chmod 600 /etc/web.passwd</code>
<code># service xinetd restart

Testing

Client Setup

# yum -y install rsync</code>
<code># mkdir /data/test

Basic Sync Command

# rsync -avzP [email protected]::web1 /data/test/

Option explanations:

-a – archive mode (equivalent to -rlptgoD).

-r – recursive.

-l – copy symlinks as symlinks.

-i – list files on the server.

-p – preserve permissions.

-t – preserve timestamps.

-g – preserve group.

-o – preserve owner.

-D – preserve device files.

-z – compress during transfer.

-P – show progress.

-v – verbose output.

# rsync -avzP --delete [email protected]::web1 /data/test/</code>
<code># rsync -avzP --delete /data/test/ [email protected]::web1</code>
<code># rsync -avzP --delete /data/test/ [email protected]::web1/george</code>
<code># rsync -ir --password-file=/tmp/rsync.password [email protected]::web1</code>
<code># rsync -avzP --exclude="*3*" --password-file=/tmp/rsync.password [email protected]::web1 /data/test/

Password‑File Sync

# echo "123" > /tmp/rsync.password</code>
<code># chmod 600 /tmp/rsync.password</code>
<code># rsync -avzP --delete --password-file=/tmp/rsync.password [email protected]::web1 /data/test/

Automated Client Sync (cron)

# crontab -e</code>
<code>10 0 * * * rsync -avzP --delete --password-file=/tmp/rsync.password [email protected]::web1 /data/test/</code>
<code># crontab -l

Real‑Time Sync with Inotify‑tools

Overview

Inotify‑tools provide C libraries and command‑line utilities ( inotifywait, inotifywatch) to monitor filesystem events on Linux.

Installation

# yum install -y gcc</code>
<code># mkdir /usr/local/inotify</code>
<code># tar -xf inotify-tools-3.14.tar.gz</code>
<code># cd inotify-tools-3.14</code>
<code># ./configure --prefix=/usr/local/inotify/</code>
<code># make && make install

Environment Setup

# vim /root/.bash_profile</code>
<code>export PATH=/usr/local/inotify/bin/:$PATH</code>
<code># source /root/.bash_profile</code>
<code># echo '/usr/local/inotify/lib' >> /etc/ld.so.conf</code>
<code># ldconfig</code>
<code># ln -s /usr/local/inotify/include /usr/include/inotify

Common Inotify Options

-m – keep listening (default exits after an event).

-r – recursive monitoring.

-q – quiet output.

-e – specify events (access, modify, attrib, open, delete, create, move, etc.).

--exclude, --excludei – pattern‑based exclusion.

--timefmt – format timestamps.

--format – custom output format.

One‑Way Real‑Time Sync Test

Monitor /data/test/src on the source machine and push changes to the destination via SSH.

# mkdir /data/test/dest/   # on destination</code>
<code># mkdir /data/test/src/   # on source</code>
<code># rsync -av --delete /data/test/src/ 192.168.22.12:/data/test/dest   # test command</code>
<code># vim /data/test/test.sh</code>
<code>#!/bin/bash</code>
<code>/usr/local/inotify/bin/inotifywait -mrq -e modify,create,move,delete,attrib /data/test/src \</code>
<code>  | while read events; do</code>
<code>    rsync -a --delete /data/test/src/ 192.168.22.12:/data/test/dest</code>
<code>    echo "$(date +'%F %T') event: $events" >> /tmp/rsync.log 2>&1</code>
<code>done</code>
<code># chmod 755 /data/test/test.sh</code>
<code># /data/test/test.sh &</code>
<code># echo '/data/test/test.sh &' >> /etc/rc.local   # start on boot

For bidirectional sync, a similar script can be placed on the target machine swapping source and destination paths.