TCP三次握手过程及利用iptables防syn攻击

思考的牛

SYN_RECV状态，顾名思义，是收到SYN包后应该置的状态。关 于SYN_RECV状态,受某些教科书的误导,我以前一直理解为服务器收到 SYN包后应该置此状态。也没细想到底是置那个socket的状态，最近在看三次握手协议在linux内核中的实现时,才仔细思考这个问题应该是置连接套 接字的状态而非监听套接字的状态。

通常,SYN包只用于TCP三次握手协议中。常见的tcp三次握手协议过程(当然还有同时连接、半连接等其它一些情况)，如下：
1、client SYN包---> server
2、client <---SYN包/ACK包 server
3、client ACK包---> server
根据tcp状态图,对应下述4个状态的变化
a、client发送完毕，状态变成SYN_SEND;
b、server收到SYN报并发送ack确认包和SYN包，状态变为SYN_RECV
c、client发送ack包完毕，状态变成ESTABLISHED
d、server发送ack包完毕，状态变成ESTABLISHED 在linux内核中,上述几个状态对应为TCP_SYN_SEND、TCP_SYN_RECV、TCP_ESTABLISHED.
RFC793中关于SYN_RECV状态的描述如下：
SYN-RECEIVED - represents waiting for a confirming connection
request acknowledgment after having both received and sent a
connection request.


利用iptables防tcp_syn攻击

修改iptables
iptables -A INPUT -p tcp --syn -m limit --limit 1/s -j ACCEPT
iptables -A INPUT -p tcp --tcp-flags SYN,ACK,FIN,RST RST -m limit --limit 1/s -j ACCEPT
iptables -A INPUT -p icmp --icmp-type echo-request -m limit --limit 1/s -j ACCEPT
echo 2048 > /proc/sys/net/ipv4/tcp_max_syn_backlog
echo 1 > /proc/sys/net/ipv4/tcp_synack_retries
echo 1 > /proc/sys/net/ipv4/tcp_syn_retries
echo 1 > /proc/sys/net/ipv4/tcp_syncookies

查询某ip并发连接数
netstat -na|grep SYN|awk '{print $5}'|awk -F: '{print $1}'|sort|uniq -c|sort -r

统计
netstat -na |grep SYN_RECV |grep 80 |wc -l

查看各个状态总数
netstat -n | awk '/^tcp/ {++S[$NF]} END {for(a in S) print a, S[a]}'

sysctl内核
net.ipv4.ip_forward = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 0
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 68719476736
kernel.shmall = 4294967296
net.ipv4.tcp_max_syn_backlog = 65536
net.core.netdev_max_backlog = 32768
net.core.somaxconn = 32768
net.ipv4.tcp_max_tw_buckets = 5000
net.core.wmem_default = 8388608
net.core.rmem_default = 8388608
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 65536 16777216
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_synack_retries = 2
net.ipv4.tcp_syn_retries = 2
net.ipv4.tcp_tw_recycle = 1
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_mem = 196608 262144 393216
net.ipv4.tcp_max_orphans = 3276800
net.ipv4.tcp_fin_timeout = 30
net.ipv4.tcp_keepalive_time = 120
net.ipv4.ip_local_port_range = 1024  65535