Bash Shell Scripting

These tutorials cover various aspects of bash shell scripting, starting with basic script creation and execution, then progressing to more advanced topics such as variables, loops, conditional statements, quoting, arithmetic operations, and file redirections. Each section provides practical examples and explanations to help beginners understand and apply bash scripting concepts effectively.

1. Hello bourne - Bash Shell Scripting
First you need to find out where is your bash interpreter located. Enter the following into your command line:

$ which bash

bash interpreter location:
/bin/bash

Open up you favorite text editor and a create file called hello_BOURNE.sh. Insert the following lines to a file:
NOTE:Every bash shell script in this tutorial starts with shebang:"#!" which is not read as a comment. First line is also a place where you put your interpreter which is in this case: /bin/bash.
Here is our first bash shell script example:

#!/bin/bash
# declare STRING variable
STRING="Hello bourne"
#print variable on a screen
echo $STRING

Navigate to a directory where your hello_bourne.sh is located and make the file executable:

$ chmod +x hello_bourne.sh

Make bash shell script executable
Now you are ready to execute your first bash script:

./hello_bourne.sh

Example of simple bash shell script


2. Simple Backup bash shell script

#!/bin/bash
tar -czf myhome_directory.tar.gz /home/linuxconfig

Simple Backup bash script

3. Variables
In this example we declare simple bash variable and print it on the screen ( stdout ) with echo command.

#!/bin/bash
STRING="HELLO BOURNE!!!"
echo $STRING

Bash string Variables in bash script

Your backup script and variables:

#!/bin/bash
OF=myhome_directory_$(date +%Y%m%d).tar.gz
tar -czf $OF /home/linuxconfig

Bash backup Script with bash Variables

3.1. Global vs. Local variables

#!/bin/bash
#Define bash global variable
#This variable is global and can be used anywhere in this bash script
VAR="global variable"
function bash {
#Define bash local variable
#This variable is local to bash function only
local VAR="local variable"
echo $VAR
}
echo $VAR
bash
# Note the bash global variable did not change
# "local" is bash reserved word
echo $VAR

Global vs. Local Bash variables in bash script

4. Passing arguments to the bash script

#!/bin/bash
# use predefined variables to access passed arguments
#echo arguments to the shell
echo $1 $2 $3 ' -> echo $1 $2 $3'

# We can also store arguments from bash command line in special array
args=("$@")
#echo arguments to the shell
echo ${args[0]} ${args[1]} ${args[2]} ' -> args=("$@"); echo ${args[0]} ${args[1]} ${args[2]}'

#use $@ to print out all arguments at once
echo $@ ' -> echo $@'

# use $# variable to print out
# number of arguments passed to the bash script
echo Number of arguments passed: $# ' -> echo Number of arguments passed: $#'

/arguments.sh Bash Scripting Tutorial

Passing arguments to the bash script

5. Executing shell commands with bash

#!/bin/bash
# use backticks " ` ` " to execute shell command
echo `uname -o`
# executing bash command without backticks
echo uname -o

Executing shell commands with bash

6. Reading User Input

#!/bin/bash
echo -e "Hi, please type the word: \c "
read word
echo "The word you entered is: $word"
echo -e "Can you please enter two words? "
read word1 word2
echo "Here is your input: \"$word1\" \"$word2\""
echo -e "How do you feel about bash scripting? "
# read command now stores a reply into the default build-in variable $REPLY
read
echo "You said $REPLY, I'm glad to hear that! "
echo -e "What are your favorite colours ? "
# -a makes read command to read into an array
read -a colours
echo "My favorite colours are also ${colours[0]}, ${colours[1]} and ${colours[2]}:-)"

Reading User Input with bash

7. Bash Trap Command

#!/bin/bash
# bash trap command
trap bashtrap INT
# bash clear screen command
clear;
# bash trap function is executed when CTRL-C is pressed:
# bash prints message => Executing bash trap subrutine !
bashtrap()
{
echo "CTRL+C Detected !...executing bash trap !"
}
# for loop from 1/10 to 10/10
for a in `seq 1 10`; do
echo "$a/10 to Exit."
sleep 1;
done
echo "Exit Bash Trap Example!!!"

8. Arrays


8.1. Declare simple bash array

#!/bin/bash
#Declare array with 4 elements
ARRAY=( 'Debian Linux' 'Redhat Linux' Ubuntu Linux )
# get number of elements in the array
ELEMENTS=${#ARRAY[@]}

# echo each element in array
# for loop
for (( i=0;i<$ELEMENTS;i++)); do
echo ${ARRAY[${i}]}
done

Declare simple bash array

8.2. Read file into bash array

#!/bin/bash
# Declare array
declare -a ARRAY
# Link filedescriptor 10 with stdin
exec 10<&0
# stdin replaced with a file supplied as a first argument
exec < $1
let count=0

while read LINE; do

ARRAY[$count]=$LINE
((count++))
done

echo Number of elements: ${#ARRAY[@]}
# echo array's content
echo ${ARRAY[@]}
# restore stdin from filedescriptor 10
# and close filedescriptor 10
exec 0<&10 10<&-

Bash script execution with an output:

linuxconfig.org $ cat bash.txt
Bash
Scripting
Tutorial
Guide

linuxconfig.org $ ./bash-script.sh bash.txt
Number of elements: 4
Bash Scripting Tutorial Guide
linuxconfig.org $

9. Bash if / else / fi statements

9.1. Simple Bash if/else statement

Please note the spacing inside the [ and ] brackets! Without the spaces, it won't work!

#!/bin/bash
directory="./BashScripting"

# bash check if directory exists
if [ -d $directory ]; then
echo "Directory exists"
else
echo "Directory does not exists" 

fi

Bash if else fi statement

9.2. Nested if/else

#!/bin/bash
# Declare variable choice and assign value 4
choice=4
# Print to stdout
echo "1. Bash"
echo "2. Scripting"
echo "3. Tutorial"
echo -n "Please choose a word [1,2 or 3]? "
# Loop while the variable choice is equal 4
# bash while loop
while [ $choice -eq 4 ]; do

# read user input
read choice
# bash nested if/else
if [ $choice -eq 1 ] ; then

echo "You have chosen word: Bash"

else

if [ $choice -eq 2 ] ; then
echo "You have chosen word: Scripting"
else

if [ $choice -eq 3 ] ; then
echo "You have chosen word: Tutorial"
else
echo "Please make a choice between 1-3 !"
echo "1. Bash"
echo "2. Scripting"
echo "3. Tutorial"
echo -n "Please choose a word [1,2 or 3]? "
choice=4
fi
fi
fi
done

10. Bash Comparisons

10.1. Arithmetic Comparisons

-lt <
-gt >
-le <=
-ge >=
-eq ==
-ne !=

#!/bin/bash
# declare integers
NUM1=2
NUM2=2
if [ $NUM1 -eq $NUM2 ]; then
echo "Both Values are equal"
else
echo "Values are NOT equal"
fi

Bash Arithmetic Comparisons - values are NOT equal

#!/bin/bash
# declare integers
NUM1=2
NUM2=1
if [ $NUM1 -eq $NUM2 ]; then
echo "Both Values are equal"
elif [ $NUM1 -gt $NUM2 ]; then
echo "NUM1 is greater then NUM2"
else
echo "NUM2 is greater then NUM1"
fi

Bash Arithmetic Comparisons - greater then

10.2. String Comparisons

= equal
!= not equal
< less then
> greater then
-n s1 string s1 is not empty
-z s1 string s1 is empty

#!/bin/bash
#Declare string S1
S1="Bash"
#Declare string S2
S2="Scripting"
if [ $S1 = $S2 ]; then
echo "Both Strings are equal"
else
echo "Strings are NOT equal"
fi

Bash String Comparisons - values are NOT equal

#!/bin/bash
#Declare string S1
S1="Bash"
#Declare string S2
S2="Bash"
if [ $S1 = $S2 ]; then
echo "Both Strings are equal"
else
echo "Strings are NOT equal"
fi

bash interpreter location: /bin/bash

11. Bash File Testing

-b filename Block special file
-c filename Special character file
-d directoryname Check for directory existence
-e filename Check for file existence
-f filename Check for regular file existence not a directory
-G filename Check if file exists and is owned by effective group ID.
-g filename true if file exists and is set-group-id.
-k filename Sticky bit
-L filename Symbolic link
-O filename True if file exists and is owned by the effective user id.
-r filename Check if file is a readable
-S filename Check if file is socket
-s filename Check if file is nonzero size
-u filename Check if file set-ser-id bit is set
-w filename Check if file is writable
-x filename Check if file is executable

#!/bin/bash
file="./file"
if [ -e $file ]; then
echo "File exists"
else
echo "File does not exists"
fi

Bash File Testing - File does not exist Bash File Testing - File exists

Similarly for example we can use while loop to check if file does not exists. This script will sleep until file does exists. Note bash negator "!" which negates the -e option.

#!/bin/bash

while [ ! -e myfile ]; do
# Sleep until file does exists/is created
sleep 1
done

12. Loops

12.1. Bash for loop

#!/bin/bash

# bash for loop
for f in $( ls /var/ ); do
echo $f
done

Running for loop from bash shell command line:
$ for f in $( ls /var/ ); do echo $f; done

Bash for loop

12.2. Bash while loop

#!/bin/bash
COUNT=6
# bash while loop
while [ $COUNT -gt 0 ]; do
echo Value of count is: $COUNT
let COUNT=COUNT-1
done




Bash while loop

12.3. Bash until loop

#!/bin/bash
COUNT=0
# bash until loop
until [ $COUNT -gt 5 ]; do
echo Value of count is: $COUNT
let COUNT=COUNT+1
done




Bash until loop

12.4. Control bash loop with
Here is a example of while loop controlled by standard input. Until the redirection chain from STDOUT to STDIN to the read command exists the while loop continues.
#!/bin/bash
# This bash script will locate and replace spaces
# in the filenames
DIR="."
# Controlling a loop with bash read command by redirecting STDOUT as
# a STDIN to while loop
# find will not truncate filenames containing spaces
find $DIR -type f | while read file; do
# using POSIX class [:space:] to find space in the filename
if [[ "$file" = *[[:space:]]* ]]; then
# substitute space with "_" character and consequently rename the file
mv "$file" `echo $file | tr ' ' '_'`
fi;
# end of while loop
done




Bash script to replace spaces in the filenames with _

13. Bash Functions

!/bin/bash
# BASH FUNCTIONS CAN BE DECLARED IN ANY ORDER
function function_B {
echo Function B.
}
function function_A {
echo $1
}
function function_D {
echo Function D.
}
function function_C {
echo $1
}
# FUNCTION CALLS
# Pass parameter to function A
function_A "Function A."
function_B
# Pass parameter to function C
function_C "Function C."
function_D




Bash Functions

14. Bash Select

#!/bin/bash

PS3='Choose one word: '

# bash select
select word in "linux" "bash" "scripting" "tutorial"
do
echo "The word you have selected is: $word"
# Break, otherwise endless loop
break
done

exit 0




Bash Select

15. Case statement conditional

#!/bin/bash
echo "What is your preferred programming / scripting language"
echo "1) bash"
echo "2) perl"
echo "3) phyton"
echo "4) c++"
echo "5) I do not know !"
read case;
#simple case bash structure
# note in this case $case is variable and does not have to
# be named case this is just an example
case $case in
1) echo "You selected bash";;
2) echo "You selected perl";;
3) echo "You selected phyton";;
4) echo "You selected c++";;
5) exit
esac




bash case statement conditiona

16. Bash quotes and quotations
Quotations and quotes are important part of bash and bash scripting. Here are some bash quotes and quotations basics.

16.1. Escaping Meta characters
Before we start with quotes and quotations we should know something about escaping meta characters. Escaping will suppress a special meaning of meta characters and therefore meta characters will be read by bash literally. To do this we need to use backslash "\" character. Example:
#!/bin/bash

#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

#when meta character such us "$" is escaped with "\" it will be read literally
echo \$BASH_VAR

# backslash has also special meaning and it can be suppressed with yet another "\"
echo "\\"



escaping meta characters in bash

16.2. Single quotes
Single quotes in bash will suppress special meaning of every meta characters. Therefore meta characters will be read literally. It is not possible to use another single quote within two single quotes not even if the single quote is escaped by backslash.
#!/bin/bash

#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

# meta characters special meaning in bash is suppressed when using single quotes
echo '$BASH_VAR "$BASH_VAR"'




Using single quotes in bash

16.3. Double Quotes
Double quotes in bash will suppress special meaning of every meta characters except "$", "\" and "`". Any other meta characters will be read literally. It is also possible to use single quote within double quotes. If we need to use double quotes within double quotes bash can read them literally when escaping them with "\". Example:
#!/bin/bash

#Declare bash string variable
BASH_VAR="Bash Script"

# echo variable BASH_VAR
echo $BASH_VAR

# meta characters and its special meaning in bash is
# suppressed when using double quotes except "$", "\" and "`"

echo "It's $BASH_VAR and \"$BASH_VAR\" using backticks: `date`"



Using double quotes in bash

16.4. Bash quoting with ANSI-C style
There is also another type of quoting and that is ANSI-C. In this type of quoting characters escaped with "\" will gain special meaning according to the ANSI-C standard.








































\a alert (bell) \b backspace
\e an escape character \f form feed
\n newline \r carriage return
\t horizontal tab \v vertical tab
\\ backslash \` single quote
\nnn octal value of characters ( see [http://www.asciitable.com/ ASCII table] ) \xnn hexadecimal value of characters ( see [http://www.asciitable.com/ ASCII table] )

The syntax fo ansi-c bash quoting is: $'' . Here is an example:
#!/bin/bash

# as a example we have used \n as a new line, \x40 is hex value for @
# and \56 is octal value for .
echo $'web: www.linuxconfig.org\nemail: web\x40linuxconfig\56org'




quoting in bash with ansi-c stype

17. Arithmetic Operations


17.1. Bash Addition Calculator Example

#!/bin/bash

let RESULT1=$1+$2
echo $1+$2=$RESULT1 ' -> # let RESULT1=$1+$2'
declare -i RESULT2
RESULT2=$1+$2
echo $1+$2=$RESULT2 ' -> # declare -i RESULT2; RESULT2=$1+$2'
echo $1+$2=$(($1 + $2)) ' -> # $(($1 + $2))'




Bash Addition Calculator

17.2. Bash Arithmetics

#!/bin/bash

echo '### let ###'
# bash addition
let ADDITION=3+5
echo "3 + 5 =" $ADDITION

# bash subtraction
let SUBTRACTION=7-8
echo "7 - 8 =" $SUBTRACTION

# bash multiplication
let MULTIPLICATION=5*8
echo "5 * 8 =" $MULTIPLICATION

# bash division
let DIVISION=4/2
echo "4 / 2 =" $DIVISION

# bash modulus
let MODULUS=9%4
echo "9 % 4 =" $MODULUS

# bash power of two
let POWEROFTWO=2**2
echo "2 ^ 2 =" $POWEROFTWO

echo '### Bash Arithmetic Expansion ###'
# There are two formats for arithmetic expansion: $[ expression ]
# and $(( expression #)) its your choice which you use

echo 4 + 5 = $((4 + 5))
echo 7 - 7 = $[ 7 - 7 ]
echo 4 x 6 = $((3 * 2))
echo 6 / 3 = $((6 / 3))
echo 8 % 7 = $((8 % 7))
echo 2 ^ 8 = $[ 2 ** 8 ]

echo '### Declare ###'

echo -e "Please enter two numbers \c"
# read user input
read num1 num2
declare -i result
result=$num1+$num2
echo "Result is:$result "

# bash convert binary number 10001
result=2#10001
echo $result

# bash convert octal number 16
result=8#16
echo $result

# bash convert hex number 0xE6A
result=16#E6A
echo $result




Bash Arithmetic Operations

17.3. Round floating point number

#!/bin/bash
# get floating point number
floating_point_number=3.3446
echo $floating_point_number
# round floating point number with bash
for bash_rounded_number in $(printf %.0f $floating_point_number); do
echo "Rounded number with bash:" $bash_rounded_number
done




Round floating point number with bash

17.4. Bash floating point calculations

#!/bin/bash
# Simple linux bash calculator
echo "Enter input:"
read userinput
echo "Result with 2 digits after decimal point:"
echo "scale=2; ${userinput}" | bc
echo "Result with 10 digits after decimal point:"
echo "scale=10; ${userinput}" | bc
echo "Result as rounded integer:"
echo $userinput | bc


Bash floating point calculations

18. Redirections


18.1. STDOUT from bash script to STDERR

#!/bin/bash

echo "Redirect this STDOUT to STDERR" 1>&2

To prove that STDOUT is redirected to STDERR we can redirect script's output to file:


STDOUT from bash script to STDERR

18.2. STDERR from bash script to STDOUT

#!/bin/bash

cat $1 2>&1

To prove that STDERR is redirected to STDOUT we can redirect script's output to file:


STDERR from bash script to STDOUT

18.3. stdout to screen
The simple way to redirect a standard output ( stdout ) is to simply use any command, because by default stdout is automatically redirected to screen. First create a file "file1":
$ touch file1
$ ls file1
file1


As you can see from the example above execution of ls command produces STDOUT which by default is redirected to screen.

18.4. stdout to file
The override the default behavior of STDOUT we can use ">" to redirect this output to file:
$ ls file1 > STDOUT
$ cat STDOUT
file1




18.5. stderr to file
By default STDERR is displayed on the screen:
$ ls
file1 STDOUT
$ ls file2
ls: cannot access file2: No such file or directory


In the following example we will redirect the standard error ( stderr ) to a file and stdout to a screen as default. Please note that STDOUT is displayed on the screen, however STDERR is redirected to a file called STDERR:

$ ls
file1 STDOUT
$ ls file1 file2 2> STDERR
file1
$ cat STDERR
ls: cannot access file2: No such file or directory




18.6. stdout to stderr
It is also possible to redirect STDOUT and STDERR to the same file. In the next example we will redirect STDOUT to the same descriptor as STDERR. Both STDOUT and STDERR will be redirected to file "STDERR_STDOUT".

$ ls
file1 STDERR STDOUT
$ ls file1 file2 2> STDERR_STDOUT 1>&2
$ cat STDERR_STDOUT
ls: cannot access file2: No such file or directory
file1



File STDERR_STDOUT now contains STDOUT and STDERR.

18.7. stderr to stdout
The above example can be reversed by redirecting STDERR to the same descriptor as SDTOUT:

$ ls
file1 STDERR STDOUT
$ ls file1 file2 > STDERR_STDOUT 2>&1
$ cat STDERR_STDOUT
ls: cannot access file2: No such file or directory
file1




18.8. stderr and stdout to file
Previous two examples redirected both STDOUT and STDERR to a file. Another way to achieve the same effect is illustrated below:

$ ls
file1 STDERR STDOUT
$ ls file1 file2 &> STDERR_STDOUT
$ cat STDERR_STDOUT
ls: cannot access file2: No such file or directory
file1



or

$ ls file1 file2 >& STDERR_STDOUT
$ cat STDERR_STDOUT
ls: cannot access file2: No such file or directory
file1

Mastering Packet Analysis with Tcpdump - Packet Analysing.

Tcpdump is a powerful command-line packet analyzer tool used for network troubleshooting and analysis. It allows the user to intercept and display the packets transmitted or received over a network to which the computer is attached. In this guide, we'll explore how to use tcpdump for various purposes, including capturing packets, filtering traffic, and analyzing packet content.

1. Display Available Interfaces:

To see a list of available network interfaces on your system:

tcpdump -D

2. Capture Packets from a Specific Interface:

To start capturing packets from a specific interface (e.g., venet0):

tcpdump -i venet0

3. Capture Only N Number of Packets:

To limit the capture to a specific number of packets (e.g., 2 packets):

tcpdump -c 2 -i venet0

4. Print Captured Packets in ASCII:

To view the captured packets in ASCII format:

tcpdump -c 2 -A -i venet0

5. Display Captured Packets in HEX and ASCII:

To view the packet's contents in both HEX and ASCII formats:

tcpdump -c 2 -XX -i venet0

Advanced Packet Capturing

6. Capture and Save Packets in a File:

To capture packets and save them to a file for later analysis:

tcpdump -w capture.pcap -i venet0 -c 2

7. Read Captured Packets from a File:

To read packets from a previously saved file:

tcpdump -r capture.pcap

8. Capture Packets from a Specific IP Address:

To capture packets involving a particular IP address:

tcpdump -n -i venet0 -c 2 src 117.229.105.142

9. Capture Only TCP Packets:

To capture only TCP packets:

tcpdump tcp -n -i venet0 -c 2

10. Capture Packets from a Specific Port:

To capture packets from a particular port (e.g., SSH port 22):

tcpdump -i venet0 -c 2 port 22

Filtering and Analyzing Traffic

11. Capture Packets with a Readable Timestamp:

To capture packets with a more readable timestamp:

tcpdump -i venet0 -c 2 -tttt

12. Read Packets Longer than N Bytes:

To capture and read packets longer than a certain size (e.g., 10 bytes):

tcpdump -i venet0 greater 10 -c 2

13. Filter Packets – Exclude ARP and RARP:

To capture all packets other than ARP and RARP:

tcpdump -i venet0 not arp and not rarp -c 2

Conclusion

Tcpdump is an incredibly versatile tool that can be used for a wide range of network analysis tasks. By understanding how to use its various options and filters, you can diagnose network issues, monitor traffic in real-time, and perform in-depth protocol analysis. Remember, while tcpdump can capture sensitive data, it should be used responsibly and ethically. Happy analyzing!

Navigating Server Security: A Comprehensive Guide to Mitigating DDoS Attacks

Distributed Denial of Service (DDoS) attacks can cripple your server, disrupting operations and services. Understanding how to detect and mitigate these attacks is crucial for maintaining uptime and security. This blog provides a structured approach to identifying, combating, and preventing DDoS attacks.

Confirming a Potential Attack

Understanding Connection States: Use the netstat command to analyze the state of network connections:

netstat -an|awk '/tcp/ {print $6}'|sort|uniq -c

This command will list the number of connections in various states:

1. ESTABLISHED: Valid connections to the server.
2. SYN_SENT: Active attempts at establishing connections.
3. SYN_RECV: Received connection requests.
4. FIN_WAIT: Closing sockets.
5. TIME_WAIT: Sockets waiting post-closure to handle remaining packets.
6. LISTEN: Sockets listening for incoming connections.
7. LAST_ACK: Awaiting acknowledgment after remote shutdown.

A high count in SYN_SENT, SYN_RECV, TIME_WAIT, or FIN_WAIT indicates a likely attack.

Initial Defensive Tweaks

Adjusting System Configuration: Tweak the /etc/sysctl.conf settings to reduce vulnerability:

# Enable TCP SYN cookie protection
net.ipv4.tcp_syncookies = 1
# Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 3
# Turn off the tcp_window_scaling and tcp_sack
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_sack = 0

Apply changes with sysctl -p.

Identifying the Attack Vector

Determining the Source: If the attack comes from a single or few IPs, block them using the firewall. If the attack is distributed, deeper investigation is necessary.

Pinpointing the Targeted Port: When SYN_RECV connections are high, identify the targeted port with:

netstat -lpan | grep SYN_RECV | awk '{print $4}' | cut -d: -f2 | sort | uniq -c | sort -nk 1

Use tcpdump to further analyze traffic to a specific port:

tcpdump -nn -tttt -i any port 80

Mitigating the Attack

Adjusting Apache Settings: If the attack targets a web service, modify Apache configurations to handle increased load:

MaxClients 500 
KeepAlive On 
KeepAliveTimeout 3 
/etc/init.d/httpd restart

Isolating the Affected Domain/IP: Use netstat to determine if a specific IP is targeted. Check Apache logs or use the top command to identify if a particular domain is under attack.

Advanced Defense Strategies

Employing ModSecurity and Firewall Rules:

1. Disable ModSecurity’s automatic IP blocking.
2. Add specific rules to block malicious traffic to targeted domains.
3. Use iptables to block access to the domain on port 80:

iptables -I INPUT -p tcp --dport 80 -m string --string "domain.com" --algo bm -j DROP

Implementing Bandwidth Throttling: Limit connections to the targeted domain to reduce the load.

Nullrouting as a Last Resort: If the situation doesn't improve, nullroute the IP causing issues:

iptables -I INPUT -d XX.XX.XX.XX -j DROP

For dedicated IPs, remove them from /etc/ips and restart ipaliases. For shared IPs, consider reducing the TTL and reassigning domains.

Preventative Measures

Fortifying the Server with iptables: Add rules to mitigate future attacks:

iptables -A INPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
iptables -A INPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
iptables -A INPUT -p tcp --tcp-flags FIN,RST FIN,RST -j DROP
iptables -A INPUT -p tcp --tcp-flags ACK,FIN FIN -j DROP

Conclusion: DDoS attacks are a formidable threat, but with the right knowledge and tools, you can protect your server and ensure it remains secure and operational. Regularly update your configurations, monitor your traffic, and stay informed about new attack methods to maintain robust security.