關於iptables的詳細狀態可以查看http://os.51cto.com/art/201108/285209.htm
時常在服務器的防火牆上看到有這些規則，
2 106K 8294K ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED，我覺得有必要搞下這個iptables了
下面就來談談iptables
一.首先iptables有四種狀態
NEW，ESTABLISHED，RELATED，INVALID。
NEW狀態：主機連接目標主機，在目標主機上看到的第一個想要連接的包
ESTABLISHED狀態：主機已與目標主機進行通信，判斷標準只要目標主機迴應了第一個包，就進入該狀態。
RELATED狀態：主機已與目標主機進行通信，目標主機發起新的鏈接方式，例如ftp
INVALID狀態：無效的封包，例如數據破損的封包狀態
二.其次再來談談上述規則的作用
你會發現有這條
2 106K 8294K ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
經常又會看見這條規則
3 0 0 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited #其他主機不滿足RELATED的情況，會給它返回host-prohibited
添加方式：iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -j REJECT --reject-with icmp-host-prohibited
第二條規則的含義其實是：調用狀態模塊，匹配當狀態爲RELATED和ESTABLISHED的所有數據包通過，換句話說就是允許所有已經建立的連接，表現爲本機可以ping其他主機，但是其他主機無法ping本機，只接受自己發出去的響應包，這是萬能的一句話，允許所有自己發出去的包進來。後面跟具體規則
第三條規則的含義其實是：依據第二條來的，所有不滿足第二條規則的，都會被拒絕，而且會給主機返回一個host-prohibited的消息。需要注意的則是，所有位於第三條規則之下的規則都無法生效，位於該規則之上的都會生效
三.口說無憑，下面我們來做個實驗看看
只添加iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT，完全沒有任何作用
[root@iZuf62ds2bbsfbvox5ivxdZ ~]# iptables -t filter -nvL --line-number
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num pkts bytes target prot opt in out source destination
1 14 892 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:1212
2 106K 8294K ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
2.添加兩條規則，防火牆規則如圖，會產生自己能ping其他主機，但是其他主機ping不通自己的情況
[root@iZuf62ds2bbsfbvox5ivxdZ ~]# iptables -t filter -nvL --line-number
Chain INPUT (policy ACCEPT 0 packets, 0 bytes)
num pkts bytes target prot opt in out source destination
1 14 892 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:1212
2 106K 8294K ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
3 0 0 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited #其他主機不滿足RELATED的情況，會給它返回host-prohibited
Chain FORWARD (policy ACCEPT 0 packets, 0 bytes)
num pkts bytes target prot opt in out source destination
Chain OUTPUT (policy ACCEPT 37 packets, 10438 bytes)
num pkts bytes target prot opt in out source destination
效果圖：
[root@iZuf62ds2bbsfbvox5ivxdZ ~]# ping 123.56.16.77
PING 123.56.16.77 (123.56.16.77) 56(84) bytes of data.
64 bytes from 123.56.16.77: icmp_seq=1 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=2 ttl=55 time=24.6 ms
64 bytes from 123.56.16.77: icmp_seq=3 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=4 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=5 ttl=55 time=24.6 ms
64 bytes from 123.56.16.77: icmp_seq=6 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=7 ttl=55 time=24.6 ms
64 bytes from 123.56.16.77: icmp_seq=8 ttl=55 time=24.6 ms
64 bytes from 123.56.16.77: icmp_seq=9 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=10 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=11 ttl=55 time=24.7 ms
64 bytes from 123.56.16.77: icmp_seq=12 ttl=55 time=24.7 ms
^C
--- 123.56.16.77 ping statistics ---
13 packets transmitted, 12 received, 7% packet loss, time 12039ms
rtt min/avg/max/mdev = 24.660/24.720/24.788/0.149 ms
[root@iZuf62ds2bbsfbvox5ivxdZ ~]# exit
logout
Connection to 101.132.109.227 closed.
Welcome to aliyun Elastic Compute Service!
[root@xz-server1 ~]# ping 101.132.109.227
PING 101.132.109.227 (101.132.109.227) 56(84) bytes of data.
From 101.132.109.227 icmp_seq=1 Destination Host Prohibited
From 101.132.109.227 icmp_seq=2 Destination Host Prohibited
From 101.132.109.227 icmp_seq=3 Destination Host Prohibited
From 101.132.109.227 icmp_seq=4 Destination Host Prohibited
From 101.132.109.227 icmp_seq=5 Destination Host Prohibited
From 101.132.109.227 icmp_seq=6 Destination Host Prohibited
From 101.132.109.227 icmp_seq=7 Destination Host Prohibited
From 101.132.109.227 icmp_seq=8 Destination Host Prohibited
From 101.132.109.227 icmp_seq=9 Destination Host Prohibited
From 101.132.109.227 icmp_seq=10 Destination Host Prohibited
From 101.132.109.227 icmp_seq=11 Destination Host Prohibited
From 101.132.109.227 icmp_seq=12 Destination Host Prohibited
From 101.132.109.227 icmp_seq=13 Destination Host Prohibited
From 101.132.109.227 icmp_seq=14 Destination Host Prohibited
^C
--- 101.132.109.227 ping statistics ---
14 packets transmitted, 0 received, +14 errors, 100% packet loss, time 13480ms
結論：下面兩條規則必須連用(filter表INPUT鏈規則是ACCEPT的時候），其他主機ping包過來的時候直接拒絕，並且返回給它host-prohibited信息。
iptables -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -j REJECT --reject-with icmp-host-prohibited
那麼下面還需要說明一點的是，當filter表INPUT鏈默認規則是DROP的時候，命中iptables -A INPUT -j REJECT --reject-with icmp-host-prohibited該規則，依舊會返回host-prohibited信息。沒有命中規則的，不會返回任何信息，直接被drop掉
修改INPUT默認規則：
iptables -P INPUT DROP
[root@iZuf62ds2bbsfbvox5ivxdZ ~]# service iptables status
Table: filter
Chain INPUT (policy DROP)
num target prot opt source destination
1 ACCEPT tcp -- 0.0.0.0/0 0.0.0.0/0 state NEW tcp dpt:1212
2 ACCEPT all -- 0.0.0.0/0 0.0.0.0/0 state RELATED,ESTABLISHED
3 REJECT all -- 0.0.0.0/0 0.0.0.0/0 reject-with icmp-host-prohibited
Chain FORWARD (policy ACCEPT)
num target prot opt source destination
Chain OUTPUT (policy ACCEPT)
num target prot opt source destination
Table: nat
Chain PREROUTING (policy ACCEPT)
num target prot opt source destination
Chain POSTROUTING (policy ACCEPT)
num target prot opt source destination
1 MASQUERADE all -- 192.168.0.0/16 0.0.0.0/0
Chain OUTPUT (policy ACCEPT)
num target prot opt source destination
[root@xz-server1 ~]# ping 101.132.109.227
PING 101.132.109.227 (101.132.109.227) 56(84) bytes of data.
From 101.132.109.227 icmp_seq=1 Destination Host Prohibited
From 101.132.109.227 icmp_seq=2 Destination Host Prohibited
From 101.132.109.227 icmp_seq=3 Destination Host Prohibited
From 101.132.109.227 icmp_seq=4 Destination Host Prohibited
From 101.132.109.227 icmp_seq=5 Destination Host Prohibited
From 101.132.109.227 icmp_seq=6 Destination Host Prohibited
From 101.132.109.227 icmp_seq=7 Destination Host Prohibited
From 101.132.109.227 icmp_seq=8 Destination Host Prohibited
From 101.132.109.227 icmp_seq=9 Destination Host Prohibited
From 101.132.109.227 icmp_seq=10 Destination Host Prohibited
From 101.132.109.227 icmp_seq=11 Destination Host Prohibited
From 101.132.109.227 icmp_seq=12 Destination Host Prohibited
From 101.132.109.227 icmp_seq=13 Destination Host Prohibited
^C
--- 101.132.109.227 ping statistics ---
13 packets transmitted, 0 received, +13 errors, 100% packet loss, time 12441ms