How It Works:
inotify is a Linux subsystem available from the 2.6.13 release of the Linux kernel that monitors changes made to a file system and reports these changes to interested applications. lsyncd is an application written in lua that uses the inotify service to inform it of changes made to monitored directories and then when notified of a change uses rsync to replicate the changes on a remote service.

Getting lsyncd
The lsyncd sourcecode can be downloaded from Google Code and compiled but depending on the distribution your server is running, the install process be even easier as lsyncd is included in the repositories of many popular Linux distros.

Installing On Debian / Ubuntu
While lsyncd is included in both the repositories of Debian and Ubuntu but unfortunately its an outdated version so check what version you are going to get beforehand with the command:

apt-cache show lsyncd

If apt-cache tells you that only one of the 1.x versions are available, I would recommend grabbing one of the 2.x versions from the Debian testing repository instead, the amd64 package can be found here  and one for the i386 architecture here. Then install using the dpkg command i.e

cd /root
wget wget http://ftp.au.debian.org/debian/pool/main/l/lsyncd/lsyncd_2.0.4-1_amd64.deb
apt-get install lua5.1 rsync
dpkg -i lsyncd_2.0.4-1_amd64.deb

CentOS 5 Installation

yum install lsyncd.x86_64

Preparation
First step for getting started with lsync over two hosts is to create a shared SSH key to allow authentication without a password.

ssh-keygen

Now transfer the servers newly generated public key to the second server you are going to mirror content to.

scp ~/.ssh/id_rsa.pub root@my2ndserver:/tmp

After copying the file the second server add the public key to the ssh authorized_keys file.

cat /tmp/id_rsa.pub >> ~/.ssh/authorized_keys

While you are logged into the second machine also double check that rsync is installed and install it if it isn’t. Now decide the directory you would like to mirror across your machines. In this example I am just going to keep it simple by starting fresh and creating a new directory in the same location on both of my servers:

mkdir /home/mirror

Quick Test Run
cd /home/mirror
touch test_file
lsyncd -log all -nodaemon -rsyncssh /home/mirror sync@my2ndserver /home/mirror

(Note: Unlike the scp command the target host and directory are given as two separate arguments to lsyncd at the command line)

Which should produce output that looks something similar to:

kernels clocks_per_sec=100
Call: configure()
Inotify: inotify fd = 3
Call: initialize()
10:08:17 Function: Inotify.addWatch(/home/mirror/, (true), (nil), (nil))
10:08:17 Inotify: addwatch(/home/mirror/)->1
10:08:17 Normal: recursive startup rsync: /home/mirror/ -> sync@192.168.20.127:/home/mirror/
10:08:17 Exec: /usr/bin/rsync [--delete] [-r] [-lts] [/home/mirror/] [sync@192.168.20.127:/home/mirror/]
10:08:17 Call: getAlarm()
10:08:17 Debug: getAlarm returns: (false)
10:08:17 Masterloop: going into select (no timeout).
10:08:17 Call: collectProcess()
10:08:17 Delay: collected an event
10:08:17 Normal: Startup of ‘/home/mirror/’ finished.
10:08:17 Normal: Finished Blanket on /home/mirror/ = 0
10:08:17 Delay: Finish of Blanket on /home/mirror/ = 0
10:08:17 Call: cycle()
10:08:17 Function: invokeActions(‘Sync1′,(Timestamp: 17184171.29))
10:08:17 Call: getAlarm()
10:08:17 Debug: getAlarm returns: (false)
10:08:17 Masterloop: going into select (no timeout).

Look in the mirror directory on the second server and with any luck you should see your test file has migrated across from the first server.

Potential Issues
When performing the test run you may get some output that appears similar to the text below:

kernels clocks_per_sec=100
Call: configure()
Inotify: inotify fd = 3
Call: initialize()
10:14:39 Function: Inotify.addWatch(/home/mirror/, (true), (nil), (nil))
10:14:39 Inotify: addwatch(/home/mirror/)->1
10:14:39 Normal: recursive startup rsync: /home/mirror/ -> root@192.168.20.131:/home/mirror/
10:14:39 Exec: /usr/bin/rsync [--delete] [-r] [-lts] [/home/mirror/] [root@192.168.20.131:/home/mirror/]
10:14:39 Call: getAlarm()
10:14:39 Debug: getAlarm returns: (false)
10:14:39 Masterloop: going into select (no timeout).
rsync: on remote machine: -sltre.iLsf: unknown option
rsync error: syntax or usage error (code 1) at main.c(1231) [server=2.6.8]
rsync: connection unexpectedly closed (0 bytes received so far) [sender]
rsync error: error in rsync protocol data stream (code 12) at io.c(601) [sender=3.0.7]
10:14:39 Call: collectProcess()
10:14:39 Delay: collected an event
10:14:39 Error: Failure on startup of ‘/home/mirror/’.

This tends to happen if you are using an older version of rsync on the 2nd server, no need to stress though I found when creating a config file for lysncd removing the -s rsync option seemed to solve this issue.

Creating A Config File
Fire up your preferred editor and create a new file to store your configuration:

nano /etc/lsyncd.conf

With the contents:

settings = {
   logfile    = “/var/log/lsyncd.log”,
}

sync{default.rsync, source=”/home/mirror”, target=”192.168.20.131:/home/mirror”, rsyncOps=”-rltvu”}

Then start the deamon with the comand:

lsyncd /etc/lsyncd.conf

This will start the daemon in the background, go create a new file in your mirror directory then watch the log with the tail command:

tail -f /var/log/lsyncd.log

With any luck you should be able to see your newly created file getting picked up by lsyncd and mirrored on your second server.

Sun Jul 17 11:03:09 2011 Normal: Calling rsync with filter-list of new/modified files/dirs
/myfile.txt
/
building file list … done
./
myfile.txt

sent 139 bytes  received 48 bytes  374.00 bytes/sec
total size is 0  speedup is 0.00
Sun Jul 17 11:03:09 2011 Normal: Finished a list = 0

As you can see the process is not instantaneous with around a 5 – 15 second delay before initiating a transfer and obviously the delay while the file itself transfers across but it is handy for backups and other situations where a small delay is not too much of a concern.

Further Reading:

The lsyncd manual

This article will walk you through the process of setting up a PowerDNS server with a replicated MySQL back end on two Ubuntu 9.10 systems.

Step 1 – Configuring the Master Nameserver

This nameserver is going to be your primary server, any changes that need to be made to the DNS records are made on this server and they will be replicated on the the slave. To get started log into your primary name server via SSH and execute:

apt-get update
apt-get install apt-get install pdns-server pdns-backend-mysql mysql-server

When prompted by the installer create a password for the MySQL root user and then configure the MySQL service itself:

pico /etc/mysql/my.cnf

Go down to line 53, it should look like the line below comment it out.

bind-address = 127.0.0.1 <- becomes -> # bind-address = 127.0.0.1

This is needed so the slave server can talk to the master and be aware of any changes made to its records. Save the file, then restart the MySQL service to make your changes active.

/etc/init.d/mysql restart

Okay now its time to sort out the database side of things by creating a MySQL user for PowerDNS to use and the database for the storage of its DNS records:

mysql -u root -p

Whilst in the MySQL client execute the following commands:

CREATE DATABASE dns_server;

GRANT ALL ON dns_server.* TO ‘pdns’@'localhost’ IDENTIFIED BY ‘pdns_pass’;

NOTE: Be sure to change the password for the pdns user to something a tad stronger.

GRANT ALL ON dns_server.* TO ‘pdns’@'localhost.localdomain’ IDENTIFIED BY ‘pdns_pass’;

FLUSH PRIVILEGES;

USE dns_server;

CREATE TABLE domains (
id INT auto_increment,
name VARCHAR(255) NOT NULL,
master VARCHAR(128) DEFAULT NULL,
last_check INT DEFAULT NULL,
type VARCHAR(6) NOT NULL,
notified_serial INT DEFAULT NULL,
account VARCHAR(40) DEFAULT NULL,
primary key (id)
);

CREATE UNIQUE INDEX name_index ON domains(name);

CREATE TABLE records (
id INT auto_increment,
domain_id INT DEFAULT NULL,
name VARCHAR(255) DEFAULT NULL,
type VARCHAR(6) DEFAULT NULL,
content VARCHAR(255) DEFAULT NULL,
ttl INT DEFAULT NULL,
prio INT DEFAULT NULL,
change_date INT DEFAULT NULL,
primary key(id)
);

CREATE INDEX rec_name_index ON records(name);
CREATE INDEX nametype_index ON records(name,type);
CREATE INDEX domain_id ON records(domain_id);

CREATE TABLE supermasters (
ip VARCHAR(25) NOT NULL,
nameserver VARCHAR(255) NOT NULL,
account VARCHAR(40) DEFAULT NULL
);

Then exit the MySQL client with the command:

exit;

Alternatively if you don’t feel that comfortable using the command line MySQL client you could install phpmyadmin and use that to create the database and user within web based environment.

Now that the database has been setup we need to configure PowerDNS to use our newly created MySQL database and user for any lookups it needs to make.

pico /etc/powerdns/pdns.conf

Go down to line 82 or thereabouts and find the line:

# launch=

Uncomment this line and modify so that it reads:

launch=gmysql

Edit the file /etc/powerdns/pdns.d/pdns.local

:

pico  /etc/powerdns/pdns.d/pdns.local

Add the following lines so that PowerDNS can connect with the MySQL service.

gmysql-host=127.0.0.1
gmysql-user=pdns
gmysql-password=pdns_pass
gmysql-dbname=dns_server

Save the file and its show time, just restart the PowerDNS service for your changes to take effect and the DNS side of your server should be good to go!

/etc/init.d/pdns restart

Step 2 – Setting up PowerAdmin

Okay its working but not very useful at the present, as it has no DNS records to serve. So lets install PowerAdmin so we can manage our DNS records using a web based interface. I am assuming the server you are working on already has Apache2, PHP and its modules installed already. If this is not the case you will need to install these before continuing as they are required for the operation PowerAdmin.

PowerAdmin also requires the mdb2 module to work properly, which i doubt many people will have installed already. So:

apt-get install php-mdb2-driver-mysql

Or on some older versions of Ubuntu it wont be in the repository so make sure you have PEAR installed and

apt-get install php-pear
pear install MDB2-2.5.0b2
pear install MDB2_Driver_mysql-1.5.0b2

Then continue the poweradmin install with:

cd /tmp
wget https://www.poweradmin.org/download/poweradmin-2.1.3.tgz
tar zxvf poweradmin-2.1.3.tgz
mv poweradmin-2.1.3 /var/www/poweradmin
cd /var/www/poweradmin

Now the code base is in place open your browser and point it at the Poweradmin location to continue the install process e.g http://ns1.mydomain.com/poweradmin/install

Follow the install instructions and when you reach step 6 of the install process copy the PHP code displayed on screen, go back to your terminal window create the file:

pico /var/www/poweradmin/inc/config.inc.php

Then paste the code from the browser window into the new file and save.

NOTE: The browser gives you the code with white space between some of the lines make sure these have been removed before saving the file. Otherwise after logging in you will be greeted by a “Warning: Cannot modify header information” PHP error.

Step 7 is the last step of the install process and simply asks you to delete the install directory. If you fail to complete this step, the program will not let you login!

rm -rf /var/www/poweradmin/install

The operation of PowerAdmin is pretty straight forward although if you have only used Bind in the past you may find some of the terms a little confusing. A quick look at the PowerDNS documentation should point you in the right direction though.

Step 3 – Replicating Changes on the Slave Server

Log in to the server you will be using as your MySQL slave. Update the repositories and then install the MySQL server application:

apt-get update
apt-get install mysql-server

On your master server edit the /etc/mysql/my.cnf file go down to around line 92. Uncomment and modify the lines that regard replication so they resemble:

server-id = 1
log_bin = /var/log/mysql/mysql-bin.log
expire_logs_days = 10
max_binlog_size = 100M
binlog_do_db = dns_server

Save your changes to the file and then open up the MySQL client to create a new user for the replication process:

CREATE USER ‘replication_user’@’192.168.0.2′ IDENTIFIED BY ‘mypassword’;

GRANT REPLICATION SLAVE ON * . * TO ‘replication_user’@’192.168.0.2′ IDENTIFIED BY ‘mypassword’;

NOTE: replace the IP address ‘192′.168.0.2‘ with the IP of your slave server, and change the password to something a bit stronger than ‘mypassword‘

Now edit the /etc/mysql/my.cnf file of your slave server and append the following lines after the [mysqld] declaration so that the slave can connect to its master:

server-id=2
master-host = 192.168.0.1
master-user = replication_user
master-password = mypassword
master-port = 3306

Be sure to change the ip address of the master server and the users password to match your setup. Save the file and restart the MySQL service on both the Master and Slave nodes so that your changes will take effect.

On the slave now use the mysql command line tool and execute the following commands:

start slave;
show slave status\G;

You you should see something like:

*************************** 1. row ***************************
Slave_IO_State: Waiting for master to send event
Master_Host: 192.168.0.1
Master_User: replication_user
Master_Port: 3306
Connect_Retry: 60
Master_Log_File: mysqld-bin.000001
Read_Master_Log_Pos: 98
Relay_Log_File: mysqld-relay-bin.000002
Relay_Log_Pos: 236
Relay_Master_Log_File: mysqld-bin.000001
Slave_IO_Running: Yes
Slave_SQL_Running: Yes
Replicate_Do_DB:
Replicate_Ignore_DB:
Replicate_Do_Table:
Replicate_Ignore_Table:
Replicate_Wild_Do_Table:
Replicate_Wild_Ignore_Table:
Last_Errno: 0
Last_Error:
Skip_Counter: 0
Exec_Master_Log_Pos: 98
Relay_Log_Space: 236
Until_Condition: None
Until_Log_File:
Until_Log_Pos: 0
Master_SSL_Allowed: No
Master_SSL_CA_File:
Master_SSL_CA_Path:
Master_SSL_Cert:
Master_SSL_Cipher:
Master_SSL_Key:
Seconds_Behind_Master: 0
1 row in set (0.00 sec)

Now test it by creating a record on the master and with any luck you will also be able to see it on the slave. That said i stuffed around a bit when setting mine up and the slave service wouldn’t start as it was looking for the wrong bin file on the master. I was able to follow these instructions though to point the slave in the right direction to get it working again.