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303 - Firewall in CentOS 7

The firewall service in CentOS provided by firewalld is dynamic rather than static because changes to the configuration
can be made at anytime and are immediately implemented, there is no need to save or apply the changes.
No unintended disruption of existing network connections occurs as no part of the firewall has to be reloaded.

To enable firewalld, run the following command as root:

293 - MX records

MX stands for Mail Exchange Records. MX records are used in DNS records(or Zone files) to specify how email should be routed.

You can check the following command to look at MX information.

[root@localhost]# nslookup
> set q=mx
> google.com

1. Multiple mail servers

291 - SELinux

SELinux is a security enhancement to Linux which allows users and administrators more control over access control.

Access can be constrained on such variables as which users and applications can access which resources. These resources may take the form of files. Standard Linux access controls, such as file modes (-rwxr-xr-x) are modifiable by the user and the applications which the user runs. Conversely, SELinux access controls are determined by a policy loaded on the system which may not be changed by careless users or misbehaving applications.

290 - thunderbird 5.7.1 relay access denied

After create a new account in thunderbird, you got the following error when sending an e-mail.

 * thunderbird 5.7.1 relay access denied

1. Check the configuration on thunderbird first

o to Tools > Account Settings > [account name] > Server Settings > Advanced > Smtp and verify the correct SMTP server
is selected there. If you see "Relay access denied" or "Relaying denied" errors, be sure to verify this setting.

283 - IP Masquerade

IP Masquerade is also known as Network Address Translation (NAT) and Network Connection Sharing some other popular operating systems.
It is basically a method for allowing a computer that doesn't have a public Internet wide IP address communicate with other computers on the Internet
with the help of another computer sitting inbetween it and the Internet.

282 - NAT configuration on iptables

Iptables is used to set up, maintain, and inspect the tables of IPv4 packet filter rules in the Linux kernel.
Several different tables may be defined. Each table contains a number of built-in chains and may also contain user-defined chains.

Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches.
This is called a 'target', which may be a jump to a user-defined chain in the same table.

1. Configure NAT PREROUTING by command.

281 - Create Private CA (Certificate Authority)

For a web-server to be able to accept 'https://' requests, a private/public key-pair (for Apache2 w/ mod_ssl) needs to be generated
and stored in the proper location(s). To create a self-signed private/public 1024 bit key-pair that will be valid for 365 days...

1. Create a private CA

278 - DMZs and iptables

iptables rules can be set to route traffic to certain machines, such as a dedicated HTTP or FTP server, in a demilitarized zone (DMZ) - a special local
subnetwork dedicated to providing services on a public carrier such as the Internet.

For example, to set a rule for routing incoming HTTP requests to a dedicated HTTP server at (outside of the range of the LAN),
NAT calls a PREROUTING table to forward the packets to their proper destination :

275 - Apache 301 How to redirect

Search engines consider http://mydomain.com and http://www.mydomain.com different websites.
As a result, if your website has been linked to from other websites using a mix of the two URLs you are effectively
splitting the potential benefit of valuable link popularity.

274 - libXp.so.6

When you are installing openmotif21-2.1.30-11 into the linux server for Oracle EBS  & Database, you encounter the following
error. You need to install another package before openmotif.

1. install openmotif21-2.1.30-11.EL5.i386.rpm

# rpm -ihv openmotif21-2.1.30-11.EL5.i386.rpm

error: Failed dependencies:
libXp.so.6 is needed by openmotif21-2.1.30-11.EL5.i386


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by Dr. Radut