Fedora Administration and Troubleshooting

Limit number of telnet connection from a client
You can specify how many telnet connections are allowed to a telnet server from a host.
This can be done using the Firewall tool "iptables" and its module "connlimit".

The configuration for doing this is:

iptables -A INPUT -p tcp --syn --dport 23 -m connlimit --connlimit-above 3 -j REJECT

The above command will allow only 3 concurrent telnet connection from a host.

Reverse command of cat is tac, head is tail, cut is paste...
cat filename shows the content of the file.
But "tac"? What it does?
It displayes the file in reverse order.
So funny!! Why they had implemented so??

Try "head & tail" command and "cut & paste" command.

Adding an admin account in Linux/Fedora
Multiple "Admin" account is possible in Linux.

First create an account & set the password.
Commnad: useradd -g users admin1 [Create a user with name admin1 in users group].

Now edit the corresponding line in /etc/passwd file.
Put zero in third field(user id) and zero in forth field(group id).
Save the file

Now log on to the system with this new account.
You will get "#" prompt!!

Increase or decrease number of telnet session on Linux/Fedora
Edit the file /etc/xinetd.conf.
Change the parameter "instances" to a suitable value you need.
Save the file.
Restart xinetd service. [Command: service xinetd restart]

Install the Flash Plugin with Firefox on IBM XSeries X86_64
Follow the instruction from http://www.linuxheadquarters.com/howto/64-bit/flash64.shtml to download, install & enable flash plugin.

Prevent ping/icmp attack using kernel parameter
Prevent ping/icmp attacks using kernel parameter

Write the following line in the file "/etc/sysctl.conf"
net.ipv4.icmp_echo_ignore_all = 1
Save the file & restart network service [command: service network restart]
OR
Save the file & run "sysctl -p"

We can also change the value of this runtime kernel parameter to 0 (zero) using the command:
echo "0" > /proc/sys/net/ipv4/icmp_echo_ignore_all

If the value is "1" -->> The system will ignore all icmp/ping requests.
If the value is "0" -->> The system will accept all icmp/ping requests.

Ext4 FS and File System Encryption in Fedora 9
New Ext4 File System support in Fedora 9

New features in Ext4 File System:

>> Delayed Allocation
>> Uninitialized groups
>> Online Defragmentation

More details at: http://fedoraproject.org/wiki/Features/Ext4

Encrypted File Systems

Now users don't have to bother to use a third party software to encrypt their files. This new feature will take care of data security.
Its a good and importance news for laptop users.

More details at: http://fedoraproject.org/wiki/Releases/FeatureEncryptedFilesystems

Installing Skype on Fedora 7
Installing Skype on Fedora 7
Instating Skype on Fedora is easy. The dependencies are only qt-4.4To install Skype using yum,

I have created a file called “skype.repo” in “/etc/yum.repos.d/” directory with the following content.

[skype] name=Skype Repository baseurl=http://download.skype.com/linux/repos/fedora/updates/i586/ gpgkey=http://www.skype.com/products/skype/linux/rpm-public-key.asc

Then, I have used the command “yum install skype” to download & install Skype and its dependencies (qt4 & qt4-x11).

Note: It may fail to install Skype with the error “GPG key parsing failed: No key found in given key data”.
If it is so, download the Skype rpm file from http://www.skype.com/download/skype/linux/choose/
Then install using “rpm –i skype-2.2.0xxxxxxxx.rpm”

Fedora hangs to start X on IBM xSeries
Fedora hangs to start X on IBM xSeries

I have found some problem with Fedora & IBM xSeries 225, 226.
After Fedora installation (I have tried Fedora 6, 7, 9) when the server starts in GUI mode (runlavel 5), the machine hangs forever.

I have tried several tips & tricks. But failed. One day I found Fedora supports color Depth 15.
So I applied that.
And I got the success.
I have followed the following to do that.

I had to start the server in runlevel one (1). [Note: you can start in runlevel 3 also]
Then, change the line following line in /etc/inittab file
id:5:initdefault:
as
id:3:initdefault:
so that, the server can start in CUI mode every time it boots.

Then I changed the display configuration file "/etc/X11/xorg.conf ".
Then, changed the color Depth to 15 in each "Display" SubSection.

Then I had used "reboot". Now I am ready in runlavel 3.
Then, after login, I used "startx" to enable GUI mode.

Create a Local yum repository
Create a Local yum repository
Current Fedora distributions are installed with default yum configuration. Default yum configuration will search for packages from fedora or fedora mirror sites. So it will be slow.
But I can customize yum configuration to search from Local repository.
To do that, I have followed the steps from: http://tqmcube.com/repo.php
You can try this page also: http://dotancohen.com/howto/yum_repo.html

Playing mp3 and wma on Fedora
Playing mp3 and wma on Fedora
The default distribution of Fedora can’t supply the required library & player to play mp3 & wma files due their copyright & licensing issues.
But I can play it easily by installing three packages.
I have used the command “yum install xmms xmms-mp3 xmms-wma” to install the player and library.
Now I can play those files.
Run “xmms” from any terminal & open the file to be played.
I have created a link to xmms on my Desktop.
Note: If you fail to install it using “yum”, you can download the latest packages from http://freshrpms.net/packages/ , and then you can install them manually.

Optimizing MySQL Performance With MySQLTuner
MySQLTuner is a high-performance perl tuning script that will provide you with a report of a MySQL server’s health. Based on the statistics gathered, MySQLTuner will provide specific recommendations that will increase a MySQL server’s efficiency and performance. The script gives you automated MySQL tuning that is expected on the level of what you would receive from a MySQL DBA.

Installation:
Download perl script --->> wget http://mysqltuner.com/mysqltuner.pl
Change file permission --->> chmod +x mysqltuner.pl
Run --->> ./mysqltuner.pl

Increase or decrease number of telnet session on Linux/Fedora
Edit the file /etc/xinetd.conf.
Change the parameter "instances" to a suitable value you need.
Save the file.
Restart xinetd service. [Command: service xinetd restart]