Recover Bad Superblock in RedHat / CentOS /Fedora Filesystem

If  you get a ¨Damaged Superblock¨ error message at filesystem (fsck) check in Linux Server, Usually fsck will not be able to repair the file system due to bad super block. In these situations, we can recover the damaged super block from the backup. 

Solution:

There are backups of the Superblock located on several positions and we can restore them with a simple command in a Linux server

By default in Linux, the file system creates the backup of  super block in the following locations:

193, 3276, 9804, 16840, 22976 and 29912.


Note: 193 is only on older systems  in many cases. 3276 is the most current position for the first backup

When you get this "damaged superblock or bad superblock error" and if  you get a root-prompt in a recovery console, then issue the following command:

# e2fsck -b 3276 /dev/sda5

Now the System will check the filesystem with the information stored in that backup superblock and if the check was successful it will restore the backup to position 0.

If this is not successful, then try using the other copy of Superblock backup (Refer the backup location of superblock above)

How to make a secure tunnel connection to a Cent OS from a MAC client through ssh

Let's say that you are on a coffe in down-town where you have wi-fi and your laptop with you, and you want to browse secure. You should know that a wireless connection is always unsecure, because anybody can sniff your packets and find out what you're doing on your laptop.

The Cent OS that we are going to use, has a ssh server up and running and connected to internet. So the tunneling that we are going to make can route any applications that supports SOCKS5 proxy.

So let's connect to our ssh server by using terminal:

Now we can setup any applications that supports SOCKS5 proxy to use this secure ssh tunneling.

Example:

Setting up Firefox browser to use our SOCKS5 proxy connection.

We open up firefox and go to preferences: Firefox->preferences, which is different by your version of Firefox. We go to Advanced tab and then Network and we click Settings.

We click manual proxy configuration like in this image, then onsocks input we enter localhost and on the port input we enter 10000 that we used to setup our SOCKS5 proxy tunneling. Then we click OK.


We can check now that we are using ssh tunneling by running a website that shows our ip adress, likehttp://www.ip-adress.com and we should see the Cent OS ip adress.

IPTABLES on CentOS / Fedora / RedHat

Since kernel version 2.4, there is a built in system for package filtering known as Netfilter. To use Netfilter, during kernel compiling CONFIG_NETFILTER must be included. Also ip_forward must be enabled:

echo 1 > /proc/sys/net/ipv4/ip_forward

Package filtering works on Internet layer of TCP/IP protocol. Filtering rules can be defined based on a transport layer header(port number) and connection layer (source IP address). During filtering, package content is not being checked.

Netfilter filtering chains work in kernel mode. In user mode works special tool called – iptables, which requires root user privilegies and it's used to configure:

- filter chains,

- NAT tables,

- mangle tables.

Netfilter uses three filters, INPUT, OUTPUT, FORWARD, realised in form of chains. Each chain contains a set of rules that filters packages. If some package sattisfies a rule, an action gets to be applied, like accepting or rejecting package.

Iptables commands:

-A (Add rule to the end of chain).

-D (Delete rule from chain).

-R (Replace rule in chain).

-I (Add numeric rule in chain).

-L (List rules).

-F (Delete all rules from chain).

Deleting the chains:

# iptables -F INPUT

# iptables -F OUTPUT

# iptables -F FORWARD

Here are some basic examples of iptables usage.

1. Blocking IP with iptables:

# iptables -A INPUT -s xxx.xxx.xxx.xxx -j DROP

# iptables -A OUTPUT -d xxx.xxx.xxx.xxx -j DROP

* instead xxx.xxx.xxx.xxx www.abc.com can be added.

2. Opening ports:

First thing you need to do is check if ports are already opened. It's done using nmap, free program, and it's distributed in most distros.

# nmap -sT xxx.xxx.xxx.xxx

PORT STATE SERVICE

22/tcp open ssh

80/tcp open http

443/tcp open https

Nmap run completed -- 1 IP address scanned in 0.941 seconds.

As we see, on this list port 25 is not opened. Let's open port 25 for SMTP traffic.

# iptables -A INPUT -p tcp --dport 25 -j ACCEPT

3. Enabling other hosts to ping:

# iptables -A INPUT -p icmp -s xxx.xxx.xxx.xxx/xx –icmp-type echo-request -j ACCEPT

# iptables -A INPUT -p icmp -d xxx.xxx.xxx.xxx/xx –icmp-type echo-reply -j ACCEPT

4. Restricting access by time of the day:

# iptables -A INPUT -p tcp -s 0/0 --sport 513:65535 -d xxx.xxx.xxx.xxx --dport 22 -m state
--state NEW,ESTABLISHED -m time --timestart 09:00 --timestop 18:00
--days Mon,Tue,Wed,Thu,Fri -j ACCEPT  

5. Keeping logs about rejected packages:

# iptables -A OUTPUT -j LOG
# iptables -A OUTPUT -j DROP
# iptables -A INPUT -j LOG
# iptables -A INPUT -j DROP
# iptables -A FORWARD -j LOG
# iptables -A FORWARD -j DROP

Crontab for Fedora / RedHat / CentOS / Ubuntu

Crontab can run scripts at regular intervals and perform various tasks. Those intervals can be from 1 minute to 1 year, repeatedly.

To list current crontabs:

# crontab -l

You can create a crontab file by entering the following terminal command:

 # crontab -e

A crontab file has six fields for specifying minute, hour, day of month, month, day of week and the command to be run at that interval:

#################################################################
#minute (0-59),                                                 #
#|      hour (0-23),                                            #
#|      |       day of the month (1-31),                        #
#|      |       |       month of the year (1-12),               #
#|      |       |       |       day of the week (0-6 with 0=Sun)#
#|      |       |       |       |       commands                #
#################################################################

Some examples:

* * * * * #Runs every minute
*/5 * * * * #Runs at every 5 minutes
30 * * * * #Runs at 30 minutes past the hour
45 6 * * * #Runs at 6:45 am every day
45 18 * * * #Runs at 6:45 pm every day
00 1 * * 0 #Runs at 1:00 am every Sunday
00 1 * * 7 #Runs at 1:00 am every Sunday
00 1 * * Sun #Runs at 1:00 am every Sunday
30 8 1 * * #Runs at 8:30 am on the first day of every month
00 0-23/2 02 07 * #Runs every other hour on the 2nd of July

You can also use some special strings:

@reboot #Runs at boot
@yearly #Runs once a year [0 0 1 1 *]
@annually #Runs once a year [0 0 1 1 *]
@monthly #Runs once a month [0 0 1 * *]
@weekly #Runs once a week [0 0 * * 0]
@daily #Runs once a day [0 0 * * *]
@midnight #Runs once a day [0 0 * * *]
@hourly #Runs once an hour [0 * * * *]

You can use multiple commands for the same crontab:

@daily &&

Specifying a crontab file to use

# crontab -u

Example:

# crontab -u tux ~/crontab

-would set Tux's crontab file to that of the file named "crontab" residing in Tux's home directory.

To remove a crontab file for current user:

# crontab -r