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标题: TCP三次握手过程及利用iptables防syn攻击 [打印本页]

作者: 思考的牛 时间: 2011-12-27 10:27
标题: TCP三次握手过程及利用iptables防syn攻击
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

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