這里將載有Openwrt的WR841N的路由表dump出來分析一下。
這個是dump出iptables的命令
root@OpenWrt:/etc/config# iptables-save
這里分為4部分：
1.NAT表
*nat :PREROUTING ACCEPT [37930:3638072] :INPUT ACCEPT [440:34479] :OUTPUT ACCEPT [1004:101848] :POSTROUTING ACCEPT [149:36868] :MINIUPNPD - [0:0] :delegate_postrouting - [0:0] :delegate_prerouting - [0:0] :postrouting_lan_rule - [0:0] :postrouting_rule - [0:0] :postrouting_wan_rule - [0:0] :prerouting_lan_rule - [0:0] :prerouting_rule - [0:0] :prerouting_wan_rule - [0:0] :zone_lan_postrouting - [0:0] :zone_lan_prerouting - [0:0] :zone_wan_postrouting - [0:0] :zone_wan_prerouting - [0:0] -A PREROUTING -j delegate_prerouting -A POSTROUTING -j delegate_postrouting -A delegate_postrouting -m comment --comment "user chain for postrouting" -j postrouting_rule -A delegate_postrouting -o br-lan -j zone_lan_postrouting -A delegate_postrouting -o eth0 -j zone_wan_postrouting -A delegate_postrouting -o pppoe-wan -j zone_wan_postrouting -A delegate_prerouting -m comment --comment "user chain for prerouting" -j prerouting_rule -A delegate_prerouting -i br-lan -j zone_lan_prerouting -A delegate_prerouting -i eth0 -j zone_wan_prerouting -A delegate_prerouting -i pppoe-wan -j zone_wan_prerouting -A zone_lan_postrouting -m comment --comment "user chain for postrouting" -j postrouting_lan_rule -A zone_lan_prerouting -m comment --comment "user chain for prerouting" -j prerouting_lan_rule -A zone_wan_postrouting -m comment --comment "user chain for postrouting" -j postrouting_wan_rule -A zone_wan_postrouting -j MASQUERADE -A zone_wan_prerouting -j MINIUPNPD -A zone_wan_prerouting -m comment --comment "user chain for prerouting" -j prerouting_wan_rule COMMIT
其中方括號里面的數據是當前所使用的流量（數據包數和byte數）。
總結為下圖：
其中從LAN口流入的流量遞交給了miniupnpd處理，這個工具有益於P2P下載。
往WAN口去的流量使用了MASQUERADE，是SNAT的增強型，可以轉換源IP地址，這樣的話就可以發揮路由器的NAT功能了，同時
支持多個客戶端。
2.RAW表
*raw :PREROUTING ACCEPT [3358190:2718603756] :OUTPUT ACCEPT [14202:1858213] :notrack - [0:0] -A PREROUTING -j notrack COMMIT
這里基本上沒有做什么，不進行分析
3.MANGLE表
*mangle :PREROUTING ACCEPT [3358190:2718603756] :INPUT ACCEPT [14538:1853317] :FORWARD ACCEPT [3342456:2716312729] :OUTPUT ACCEPT [14202:1858213] :POSTROUTING ACCEPT [3356900:2718229627] :ASSIGNOUT - [0:0] :NWANOUT - [0:0] :NWANPOS - [0:0] :NWANPRE - [0:0] :fwmark - [0:0] :mssfix - [0:0] -A PREROUTING -j ASSIGNOUT -A PREROUTING -j NWANPRE -A PREROUTING -j fwmark -A FORWARD -j mssfix -A OUTPUT -j NWANOUT -A POSTROUTING -j NWANPOS -A ASSIGNOUT -m state --state RELATED,ESTABLISHED -j RETURN -A NWANOUT -m conntrack --ctstate RELATED,ESTABLISHED -j CONNMARK --restore-mark --nfmask 0xffffffff --ctmask 0xffffffff -A NWANPOS -o pppoe-wan -m state --state NEW -j CONNMARK --set-xmark 0xa/0xffffffff -A NWANPRE -i pppoe-wan -m state --state NEW -j CONNMARK --set-xmark 0xa/0xffffffff -A NWANPRE -i br-lan -m conntrack --ctstate RELATED,ESTABLISHED -j CONNMARK --restore-mark --nfmask 0xffffffff --ctmask 0xffffffff -A mssfix -o eth0 -p tcp -m tcp --tcp-flags SYN,RST SYN -m comment --comment "wan (mtu_fix)" -j TCPMSS --clamp-mss-to-pmtu -A mssfix -o pppoe-wan -p tcp -m tcp --tcp-flags SYN,RST SYN -m comment --comment "wan (mtu_fix)" -j TCPMSS --clamp-mss-to-pmtu COMMIT
這里在prerouting，output和postrouting的時候做了CONNMRK，用於連接管理，比如做QOS管理。在轉發的時候修改了MSS。
4.FILTER表
*filter :INPUT ACCEPT [0:0] :FORWARD DROP [0:0] :OUTPUT ACCEPT [0:0] :MINIUPNPD - [0:0] :delegate_forward - [0:0] :delegate_input - [0:0] :delegate_output - [0:0] :forwarding_lan_rule - [0:0] :forwarding_rule - [0:0] :forwarding_wan_rule - [0:0] :input_lan_rule - [0:0] :input_rule - [0:0] :input_wan_rule - [0:0] :output_lan_rule - [0:0] :output_rule - [0:0] :output_wan_rule - [0:0] :reject - [0:0] :syn_flood - [0:0] :zone_lan_dest_ACCEPT - [0:0] :zone_lan_forward - [0:0] :zone_lan_input - [0:0] :zone_lan_output - [0:0] :zone_lan_src_ACCEPT - [0:0] :zone_lan_src_REJECT - [0:0] :zone_wan_dest_ACCEPT - [0:0] :zone_wan_forward - [0:0] :zone_wan_input - [0:0] :zone_wan_output - [0:0] :zone_wan_src_REJECT - [0:0] -A INPUT -j delegate_input -A FORWARD -j delegate_forward -A OUTPUT -j delegate_output -A delegate_forward -m comment --comment "user chain for forwarding" -j forwarding_rule -A delegate_forward -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A delegate_forward -i br-lan -j zone_lan_forward -A delegate_forward -i eth0 -j zone_wan_forward -A delegate_forward -i pppoe-wan -j zone_wan_forward -A delegate_forward -j reject -A delegate_input -i lo -j ACCEPT -A delegate_input -m comment --comment "user chain for input" -j input_rule -A delegate_input -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A delegate_input -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j syn_flood -A delegate_input -i br-lan -j zone_lan_input -A delegate_input -i eth0 -j zone_wan_input -A delegate_input -i pppoe-wan -j zone_wan_input -A delegate_output -o lo -j ACCEPT -A delegate_output -m comment --comment "user chain for output" -j output_rule -A delegate_output -m conntrack --ctstate RELATED,ESTABLISHED -j ACCEPT -A delegate_output -o br-lan -j zone_lan_output -A delegate_output -o eth0 -j zone_wan_output -A delegate_output -o pppoe-wan -j zone_wan_output -A reject -p tcp -j REJECT --reject-with tcp-reset -A reject -j REJECT --reject-with icmp-port-unreachable -A syn_flood -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -m limit --limit 25/sec --limit-burst 50 -j RETURN -A syn_flood -j DROP -A zone_lan_dest_ACCEPT -o br-lan -j ACCEPT -A zone_lan_forward -m comment --comment "user chain for forwarding" -j forwarding_lan_rule -A zone_lan_forward -m comment --comment "forwarding lan -> wan" -j zone_wan_dest_ACCEPT -A zone_lan_forward -j zone_lan_src_REJECT -A zone_lan_input -m comment --comment "user chain for input" -j input_lan_rule -A zone_lan_input -j zone_lan_src_ACCEPT -A zone_lan_output -m comment --comment "user chain for output" -j output_lan_rule -A zone_lan_output -j zone_lan_dest_ACCEPT -A zone_lan_src_ACCEPT -i br-lan -j ACCEPT -A zone_lan_src_REJECT -i br-lan -j reject -A zone_wan_dest_ACCEPT -o eth0 -j ACCEPT -A zone_wan_dest_ACCEPT -o pppoe-wan -j ACCEPT -A zone_wan_forward -j MINIUPNPD -A zone_wan_forward -m comment --comment "user chain for forwarding" -j forwarding_wan_rule -A zone_wan_forward -j zone_wan_src_REJECT -A zone_wan_input -m comment --comment "user chain for input" -j input_wan_rule -A zone_wan_input -p tcp -m tcp --dport 51413 -m comment --comment "51413" -j ACCEPT -A zone_wan_input -p udp -m udp --dport 51413 -m comment --comment "51413" -j ACCEPT -A zone_wan_input -p tcp -m tcp --dport 9091 -m comment --comment "9091" -j ACCEPT -A zone_wan_input -p udp -m udp --dport 68 -m comment --comment "Allow-DHCP-Renew" -j ACCEPT -A zone_wan_input -p icmp -m icmp --icmp-type 8 -m comment --comment "Allow-Ping" -j ACCEPT -A zone_wan_input -j zone_wan_src_REJECT -A zone_wan_output -m comment --comment "user chain for output" -j output_wan_rule -A zone_wan_output -j zone_wan_dest_ACCEPT -A zone_wan_src_REJECT -i eth0 -j reject -A zone_wan_src_REJECT -i pppoe-wan -j reject COMMIT
這里先不考慮eth0，從WAN口輸入的數據要重點過濾，因為不能讓internet的電腦隨意攻擊路由器，這里允許了ICMP，
51413、9091端口和transmission下載工具有關，68端口和DHCP服務有關。
對於FORWARD的流量，這里基本上已經全部屏蔽了。
對於王WAN口發送的流量全部放行。
5.總結
總的來說，這里做了SNAT實現路由器的基本功能，對連接跟蹤可以管理各個連接。另外的一些策略有助於下載工具的運行。