[Linux] I tried to verify the secure confirmation method of FQDN (CentOS7)

Introduction

I received such a question in a previous article.

** [Previous article] ** [Linux] Why do I, an infrastructure engineer, not use the "hostname" command?

I'm sorry, but I didn't consider the FQDN. (Because there was no situation to check the FQDN in business.)

It's a big deal, so I'll try to verify it.

Information such as OS to be verified this time

OS version

[root@tspdev01 ~]# cat /etc/redhat-release
CentOS Linux release 7.8.2003 (Core)
[root@tspdev01 ~]#

• I made / etc / hostname like this in advance.

hostname

[root@tspdev01 ~]# cat /etc/hostname
tspdev01.example.com
[root@tspdev01 ~]#

However, this time I would like to do it as a root user for verification.

In the first place, what is the output result of "hostname -f"?

Execution command

hostname -f

Execution result

[root@tspdev01 ~]# hostname -f
tspdev01.example.com
[root@tspdev01 ~]#

When you check the execution result, it is displayed with FQDN(tspdev01.example.com).

Try with other commands

I would like to check the FQDN with another command.

Execute with the command that matches the question

Try running the command that matches your question.

Execution command

echo `uname -n`.`dnsdomainname`

The execution result looks like this.

Execution result

[root@tspdev01 ~]# echo `uname -n`.`dnsdomainname`
tspdev01.example.com.example.com
[root@tspdev01 ~]#

As a result, the domain is displayed twice.

Try running with "uname -n"

Execute with this command

Execution command

uname -n

The execution result is here.

Execution result

[root@tspdev01 ~]# uname -n
tspdev01.example.com
[root@tspdev01 ~]#

It was displayed by FQDN!

Try to display the host name as a short name

Short name display of normal host name

Execution command

hostname -s

The execution result is here.

Execution result

[root@tspdev01 ~]# hostname -s
tspdev01
[root@tspdev01 ~]#

Only the abbreviated system of the host name is displayed.

Short host name confirmation method using awk

Execution command

uname -n | awk -F "." '{print $1}'

The execution result is here.

Execution result

[root@tspdev01 ~]# uname -n | awk -F "." '{print $1}'
tspdev01
[root@tspdev01 ~]#

In this way, only the shortened host name can be displayed.

• Separated by . in tspdev01.example.com.

** Reference article ** How to specify the delimiter with awk's -F option

Check only domain name

Normal domain name confirmation method

Execution command

hostname -d

The execution result is here.

Execution result

[root@tspdev01 ~]# hostname -d
example.com
[root@tspdev01 ~]#

How to verify a secure domain name

Execution command

echo `dnsdomainname`

The execution result is here.

Execution result

[root@tspdev01 ~]# echo `dnsdomainname`
example.com
[root@tspdev01 ~]#

Only the domain name is displayed.

Summary

** Display method by FQDN **

uname -n

** Show only abbreviated host name **

uname -n | awk -F "." '{print $1}'

** Show domain name only **

echo `dnsdomainname`

Perhaps there are other ways. I haven't studied enough yet, so I will continue to catch up.