nslookup(on windows) and dig(on linux) are two indepensible tools for system administrator to debug DNS problems. However, the two DNS tools are not easy to use as other commands. First of all, if you issue the following commands to try…

There have been enough articles on how to install vnc server on CentOS6, CentOS7, or ubuntu, debian. But all of those guides tell you to install a desktop such as gnome, kde, before installing vncserver. The first step of their…

Completely disabling postfix to send the  “Undelivered Mail Returned to Sender” emails seems not an easy thing to do. Postfix sends this kind of notification emails in many situations such as the recipients are not found, the destination host is…

You may think google prefers https homepage rather than the http homepage. Nowadays, it seems a common sense that https webpage ranks better than http webpage because it is more “secure”. And google, in its official documents, admits https page…

The authority documents on ip-based virtual host and name-based virtual host are here and here, respectively. But like other official apache documents, the information there is also obscure. You must think hard, guess hard, and do tons of experiments to…

How to configure postfix to hide the client IP address when sending email? By default, postfix will add a “Received:” header to emails it receives. The “Received:” header leaks the privacy such as your home IP address and your computer…

A simple email client does not add too many headers to the email it is sending. Typically only the following headers appear in the message to be sent: “From:”,”To:”, “Subject:”,”MIME-Version:”, “Date:”,”Content-Type:”,”Content-Transfer-Encoding:”. But when you receive the email, you’ll find many…

I occasionally receive some special emails in my email box. Checking the emails I could not find my email  address listed in the “To:” header of the emails. Why can I receive those emails that are not destined to my…

The concept of envelope and email content(formally called “Message”)  is confusing because we see many redundant stuff here. For example, there are an envelope “mail from” and a “from” header in the message itself. There are “rcpt to” in the…

If you have set up an https website, you may want to disable the http version of the website. Or, you just want to keep the http version of website and disable its https version. Disabling the http(https) version of…