iptable에서 TCP SYN Flooding 차단 설정

방화벽에서 비정상 접속 패킷 차단. 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
# 룰 적용 후 iptables-save 명령어로 config 를 확인하여,
/etc/sysconfig/iptables 파일을 수정한다.

# 커널파라메터 수정
/etc/rc.local 에 등록하거나,
sysctl -w net.ipv4.tcp_syncookies=1
sysctl -w net.ipv4.tcp_fin_timeout=30
sysctl -w net.ipv4.tcp_window_scaling=0
sysctl -w net.ipv4.tcp_sack=0
sysctl -w net.ipv4.tcp_max_syn_backlog=1024

/etc/sysctl.conf 설정 수정.
net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_fin_timeout = 30
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_sack = 0
net.ipv4.tcp_max_syn_backlog = 1024
sysctl -p 명령어로 적용.
확인방법
sysctl -a | grep net.ipv4.tcp_max_syn_backlog

# ddos 서버 IP 확인.
netstat -anp |grep 'tcp|udp' | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n


Check whether it shows lot of SYN_WAIT / TIME_WAIT / FIN_WAIT. If yes its due to the high number of connections. You can reduce these by adding some rules to the Iptables.
# 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
# service iptables save
# service iptables restart
Adding rules to sysctl.conf
Also you can reduce these by adding some rules to sysctl.conf, the details given below.
# echo 1 > /proc/sys/net/ipv4/tcp_syncookies
Put following in /etc/sysctl.conf
# 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 = 30
# Turn off the tcp_window_scaling
net.ipv4.tcp_window_scaling = 0
# Turn off the tcp_sack
net.ipv4.tcp_sack = 0


Then execute the command :-
# /sbin/sysctl -p

A quick and usefull command for checking if a server is under ddos is:
# netstat -anp |grep 'tcp|udp' | awk '{print $5}' | cut -d: -f1 | sort | uniq -c | sort -n
Reference : http://linuxhow2.net/?p=9

# Set default policies
/sbin/iptables -P INPUT ACCEPT
/sbin/iptables -P OUTPUT ACCEPT
/sbin/iptables -P FORWARD DROP
/sbin/iptables -F
/sbin/iptables -F INPUT
/sbin/iptables -F OUTPUT
/sbin/iptables -F FORWARD
/sbin/iptables -F -t mangle
/sbin/iptables -X
/sbin/iptables -A INPUT -i lo -j ACCEPT
/sbin/iptables -A INPUT -d 127.0.0.0/8 -j REJECT
/sbin/iptables -A INPUT -i eth0 -j ACCEPT
/sbin/iptables -A INPUT -m state --state INVALID -j DROP
### chains to DROP too many SYN-s ######
/sbin/iptables -N syn-flood
/sbin/iptables -A syn-flood -m limit --limit 100/second --limit-burst 150 -j RETURN
/sbin/iptables -A syn-flood -j LOG --log-prefix "SYN flood: "
/sbin/iptables -A syn-flood -j DROP
Reference : http://www.webhostingtalk.com/archive/index.php/t-355411.html