IP tables basic overview and usage

I have been working with iptables for quiet some time. iptables rules are stored iin the file /etc/sysconfig/iptables

There are mainly 3 tables under iptables filter,nat and mangle
filter : used for packet filtering
nat    : used to provide packet modification capabilities; NAT/PAT and IP masquerading
mangle : used for setting packet options and marking packets for further filtering or routing

We deal with filter table here,Some of the basic commands which we need to understand when manipulating the default table filter of iptables are as follows

iptables -L INPUT %To list the currently loaded iptable rules in INPUT chain %
iptabes-save % command to save the currently configured iptable rules%
iptables -C %command to check whether the reported command is already there in theiptables%
iptables -N % creates a new chain with a user specified name
iptables -t filter -A INPUT -j <customchain> %Adding custom chain to iptables%
iptables -g %Jump to chain with no return%
iptables -t %Table to manupulate default is filter%
iptables -t filter -A INPUT -j <custom-chain> % Adding custom chain to the INPUT table%
iptables -X <chain-name> % If you want to delete the iptables chain you have created%
iptables -A INPUT 4 -s 192.168.1.0/24 -j ACCEPT % Inputs the rule in a specific location %
iptables -A INPUT -m limit --limit % using module limit state etc%


Kernal files which could be used to prevent and during attack

# PREVENT YOU SYSTEM FROM ANSWERING ICMP ECHO REQUESTS
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_all

# DROP ICMP ECHO-REQUEST MESSAGES SENT TO BROADCAST OR MULTICAST ADDRESSES
echo 1 > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts

# DONT ACCEPT ICMP REDIRECT MESSAGES
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects

# DONT SEND ICMP REDIRECT MESSAGES
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects

# DROP SOURCE ROUTED PACKETS
echo 0 > /proc/sys/net/ipv4/conf/all/accept_source_route

# ENABLE TCP SYN COOKIE PROTECTION FROM SYN FLOODS
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

# ENABLE SOURCE ADDRESS SPOOFING PROTECTION
echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter

# LOG PACKETS WITH IMPOSSIBLE ADDRESSES (DUE TO WRONG ROUTES) ON YOUR NETWORK
echo 1 > /proc/sys/net/ipv4/conf/all/log_martians

# DISABLE IPV4 FORWARDING
echo 0 > /proc/sys/net/ipv4/ip_forward

###############
# INPUT

# DROP INVALID
$IPTABLES -A INPUT -m state --state INVALID -j DROP

# ALLOW ONLY ESTABLISHED, RELATED
$IPTABLES -A INPUT -p tcp -i $PUBIF -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A INPUT -p udp -i $PUBIF -m state --state ESTABLISHED,RELATED -j ACCEPT

# DROP INVALID SYN PACKETS
$IPTABLES -A INPUT -p tcp --tcp-flags ALL ACK,RST,SYN,FIN -j DROP
$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

# MAKE SURE NEW INCOMING TCP CONNECTIONS ARE SYN PACKETS; OTHERWISE WE NEED TO DROP THEM
$IPTABLES -A INPUT -p tcp ! --syn -m state --state NEW -j DROP

# DROP PACKETS WITH INCOMING FRAGMENTS. THIS ATTACK RESULT INTO LINUX SERVER PANIC SUCH DATA LOSS
$IPTABLES -A INPUT -f -j DROP

# DROP INCOMING MALFORMED XMAS PACKETS
$IPTABLES -A INPUT -p tcp --tcp-flags ALL ALL -j DROP

# DROP INCOMING MALFORMED NULL PACKETS
$IPTABLES -A INPUT -p tcp --tcp-flags ALL NONE -j DROP

###############
# OUTPUT

# DROP INVALID
$IPTABLES -A OUTPUT -m state --state INVALID -j DROP

# DROP INVALID SYN PACKETS
$IPTABLES -A OUTPUT -p tcp --tcp-flags ALL ACK,RST,SYN,FIN -j DROP
$IPTABLES -A OUTPUT -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
$IPTABLES -A OUTPUT -p tcp --tcp-flags SYN,RST SYN,RST -j DROP

# MAKE SURE NEW OUTGOING TCP CONNECTIONS ARE SYN PACKETS; OTHERWISE WE NEED TO DROP THEM
$IPTABLES -A OUTPUT -p tcp ! --syn -m state --state NEW -j DROP

# DROP PACKETS WITH OUTGOING FRAGMENTS. THIS ATTACK RESULT INTO LINUX SERVER PANIC SUCH DATA LOSS
$IPTABLES -A OUTPUT -f -j DROP

# DROP OUTGOING MALFORMED XMAS PACKETS
$IPTABLES -A OUTPUT -p tcp --tcp-flags ALL ALL -j DROP

# DROP OUTGOING MALFORMED NULL PACKETS
$IPTABLES -A OUTPUT -p tcp --tcp-flags ALL NONE -j DROP