Category: Networking


How to use sendmail as a backup MX/backup mail server

This is a quick guide on how to use sendmail as a backup MX server. If you want to use qmail as a backup MX/backup mail server, please see our Qmail as backup MX guide here.


1. Installing sendmail

If you do not have sendmail installed, you can install it using your OS package manager. In this example, we use yum on CentOS:

yum install sendmail


2. Configuring the sendmail service

First off, we want to make sure it auto-starts upon reboot:

chkconfig sendmail on

By default, sendmail will only listen on localhost; for this example we will set it to listen on all interfaces instead:

1. Open /etc/mail/

2. Find the line starting with DaemonPortOptions and then modify it so it looks like this:

DaemonPortOptions=Port=smtp,Addr=, Name=MTA


3. Configuring sendmail to run as backup MX/backup mail server

First, we will need to modify the sendmail access file.

1. Open the access file:

vim /etc/mail/access

2. For every domain that you want to allow this sendmail instance to serve as a backup mail server for, add the following line (replace with your real domain name):                            RELAY

3. Next, you will need to specify which mail route this sendmail instance will attempt to deliver mail to (replace with your real domain name):

4. Finally, we will recreate the database files:

makemap hash /etc/mail/mailertable.db < /etc/mail/mailertable
makemap hash /etc/mail/access.db < /etc/mail/access

5. Make sure to restart sendmail so that our new configuration is taken in to effect:

/etc/init.d/sendmail restart


4. Update DNS

Now that you have configured your sendmail service to act as a backup mail server, you will need to update your DNS so that other mail servers know that you have a backup MX.

If you have not already, create a DNS A record for the backup mail server, eg:

Next, create a new MX record that points to your DNS name for the backup mail server with a higher priority number than the primary mail server. Remember, the higher the priority number, the less precedence it has, eg in order of preference: MX 10 MX 50


Now that you have configured sendmail and your DNS, you should now have a backup MX configured ! If you have any issues, questions or concerns, please comment and I will assist where I can.



Featured Application #2: Mouse Without Borders – share keyboard and mouse over network

In 2010, I wrote an article about Input Director, since then a new application has emerged for sharing mouse and keyboard over network for Windows: Mouse without Borders

It’s a simplified version of Input Director, with a handy feature – shared clipboard and the ability to copy and paste files between workstations.

A screenshot of the setup between my PC and laptop:

machine setup 300x193 Featured Application #2: Mouse Without Borders – share keyboard and mouse over network


Mouse without Borders can be downloaded here and configured within a few seconds :)


Kloxo now supports CentOS 5 64-bit (x86_64)

As of Kloxo 6.1.7, it now supported on 64-bit CentOS systems !

For all of who wanted Kloxo on CentOS 5 64-bit can now do so. I have installed Kloxo on CentOS 5.7 64-bit and everything appears to working 100% !



How to stop and reset/clear MySQL replication

This is a quick article that describes how to reset/delete/disable/clear MySQL replication in master-master or master-slave replication.

For each server that you wish to reset/clear MySQL replication on, do the following:

1. Enter MySQL prompt and run the following commands:


2. Remove replication user(s). In this example, we remove replication user “replicator”:

 mysql> use mysql;
 mysql> delete from user where User="replicator";
 mysql> flush privileges;

3. Exit MySQL prompt and open up /etc/my.cnf – remove any reference to replication (eg: replicate-* master-* etc)

# vim /etc/my.cnf

4. Lastly, restart your MySQL instance:

# /etc/init.d/mysqld restart





Featured Application: Input Director – share keyboard and mouse over network

Input Director is a completely free Windows application that allows you to share a keyboard and mouse across multiple physical systems over a network connection.

This is extremely useful if you have a few separate PC’s and laptops in the same workspace, and don’t want the hassle of using multiple keyboards and mice, or a physical KVM switch to manage each system.

The configuration is extremely simple: install the application on each system that you would like keyboard and mouse shared over, set your main system as the Master, and then configure each secondary system as a Slave. As a plus, you can enable encryption between the Master and Slave systems for added privacy over the network.

This is my config at home, a PC with two monitors, as well as a laptop with another screen attached. Input Director virtually allows me to treat this setup as a single device with four screens attached:

inputdirector m1 Featured Application: Input Director   share keyboard and mouse over network


Input Director also shares content on the clipboard between Master and Slave(s). Go ahead and download Input Director here:

For cross-platform keyboard and mouse over network sharing, see the Synergy+ project. Synergy+ allows you to share keyboard and mouse over network across Windows, Mac and Linux clients. I will do a write-up on Synergy+ soon.

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    Muchas gracias por tu aporte :)...

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    The listed solution worked fine for me under Win7 64bit Though I had to run the copy as Asministr...

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    The files are already in the C:\Windows\SysWOW64\wbem\en-US and they need to be in the system32 fold...

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    @Jordan, I had the same problem also on Windows 7 64 bits. It worked for my user account and Admini...

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