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094 - DNS & dig

DNS is an Internet service that translates domain names into IP addresses. Each time you use a domain name,
DNS translates the name into the corresponding IP address. In order to do the translation DNS holds records
for each domain. The most important are the A, CNAME, and MX records. The A record stores the host IP address.
The CNAME is an alias record, which is used to give multiple aliases to a single computer. The MX record is the mail
exchange record, which tells mail servers how to route email for this domain.


091 - reboot, poweroff, halt, shutdown

1. halt, powroff, reboot

Halt notes that the system is being brought down in the file /var/log/wtmp, and then either tells the kernel to halt, reboot or poweroff the system.If halt or reboot is called when the system is not in runlevel 0 or 6, in other words when it's running normally, shutdown will be invoked instead (with the -h or -r flag). The rest of this manpage describes the behaviour in runlevels 0 and 6, that is when the systems shutdown scripts are being run.


086 - Kernel release level

If you install Oracle database, the first system check that you should is to determine the kernel release level of the Linux operating system that you have installed.
Oracle supports Oracle 10g & 11g on any 2.6 kernel with the base release of Red Hat or SUSE.
The following command should tell you the kernel release level.

[MyServer]# uname -r




080 - Use Mail command

The Mail utility allows you to compose, send, receive, forward, and reply to mail. mail has two main modes: compose mode, in which you create
a message, and command mode, in which you manage your mail.

You can also provide much of the information on the command line, as shown in the following example:

[MyServer]# mail james -s "System Log" < /var/log/messages

If you want to send the user outside of the server.


079 - How to SSH without password

Secure Shell is a program to log into another computer over a network, to execute commands in a remote machine, and to move files from one machine
to another. It provides strong authentication and secure communications over insecure channels. It is a replacement for rlogin, rsh, rcp, and rdist.
SSH protects a network from attacks such as IP spoofing, IP source routing, and DNS spoofing. An attacker who has managed to take over a network
can only force ssh to disconnect. He or she cannot play back the traffic or hijack the connection when encryption is enabled.


078 - Using chkconfig to change runlevel

The chkconfig command can also be used to activate and deactivate services. If you use the chkconfig --list  command, you will see a list of system services
and whether they are started (on) or stopped (off) in runlevels 0-6 (at the end of the list, you will see a section for the services managed by xinetd.

If you use chkconfig --list to query a service managed by xinetd, you will see whether the xinetd service is enabled (on) or disabled (off). For example,
the following command shows that finger is enabled as an xinetd service:


074 - Connecting FTP sites with a browser

To connect to a FTP server requiring a username with Firefox,

1. Open Firefox

2. In the locator bar, type ftp://username: password@ftp.xyz.com

For example, if my User name was jess12 and my password was bosox67, the FTP browser syntax would be:

ftp://user01: pass01@ftp.xyz.com

In some cases, the User name includes a domain name such as user01@xyz.com.
In these situations, you would type:


070 - How to know the Host Location

When dealing with remote computers, you might want to know more about the server and domain information. Who owns them? What are the IP address? Where on the network are they located?You can use the following commands to know more detailes about the server.

  • host : Look up hostname, IP address, and DNS info.
  • whois : Look up the registants of internet domains.
  • ping : CHeck if a remote host is reachable.
  • traceroute : View the network path to a remote host.



066 - File Compression and Packaging

Linux can compress file into a variety of formats and uncompress them. The most popular format is GNU Zip (gzip),
whose compressd files are named with .gz suffix.
Other commonly found formats are classic Unix compression (.Z suffix), bzip2 compression (.bz2 suffix) and Zip files
from Windows systems (.zip suffix).


054 - traceroute

If you are having problems connecting to your site or accessing your site's services, "traceroute" is to make sure there isn't a connectivity problem. A traceroute shows the route you take over the internet to reach your site or servers. Information on what routers or back-bone providers you pass through can help you diagnose the problem.


  • In the new terminal window, type the following command: "traceroute -n -w30 yourdomainname.com".
  • You get something similar to the following:



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by Dr. Radut