OpenWRT Firewarll INPUT Chain 允许一切？

Question

我没有对默认防火墙规则做任何事情，只是从 wan 打开一个 ssh 端口，并添加本教程中定义的 OpenVPN 规则：http://wiki.openwrt.org/doc/howto/vpn.openvpn，但我担心由于输出 iptables -L，我留下了比预期更多的开放端口。我已将完整的输出放在下方，但特别是：

Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            ctstate RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere
syn_flood  tcp  --  anywhere             anywhere            tcp flags:FIN,SYN,RST,ACK/SYN
input_rule  all  --  anywhere             anywhere
input      all  --  anywhere             anywhere
from the:
ACCEPT     all  --  anywhere             anywhere

但这是否意味着任何地方的一切都可以被接受？

完整的 IP 表输出以供参考：

# iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            ctstate RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere
syn_flood  tcp  --  anywhere             anywhere            tcp flags:FIN,SYN,RST,ACK/SYN
input_rule  all  --  anywhere             anywhere
input      all  --  anywhere             anywhere
Chain FORWARD (policy DROP)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            ctstate RELATED,ESTABLISHED
forwarding_rule  all  --  anywhere             anywhere
forward    all  --  anywhere             anywhere
reject     all  --  anywhere             anywhere
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere            ctstate RELATED,ESTABLISHED
ACCEPT     all  --  anywhere             anywhere
output_rule  all  --  anywhere             anywhere
output     all  --  anywhere             anywhere
Chain forward (1 references)
target     prot opt source               destination
zone_lan_forward  all  --  anywhere             anywhere
zone_wan_forward  all  --  anywhere             anywhere
zone_vpn_forward  all  --  anywhere             anywhere
Chain forwarding_lan (1 references)
target     prot opt source               destination
Chain forwarding_rule (1 references)
target     prot opt source               destination
Chain forwarding_vpn (1 references)
target     prot opt source               destination
Chain forwarding_wan (1 references)
target     prot opt source               destination
Chain input (1 references)
target     prot opt source               destination
ACCEPT     udp  --  anywhere             anywhere            udp dpt:openvpn
zone_lan   all  --  anywhere             anywhere
zone_wan   all  --  anywhere             anywhere
zone_vpn   all  --  anywhere             anywhere
Chain input_lan (1 references)
target     prot opt source               destination
Chain input_rule (1 references)
target     prot opt source               destination
Chain input_vpn (1 references)
target     prot opt source               destination
Chain input_wan (1 references)
target     prot opt source               destination
Chain output (1 references)
target     prot opt source               destination
zone_lan_ACCEPT  all  --  anywhere             anywhere
zone_wan_ACCEPT  all  --  anywhere             anywhere
zone_vpn_ACCEPT  all  --  anywhere             anywhere
Chain output_rule (1 references)
target     prot opt source               destination
Chain reject (7 references)
target     prot opt source               destination
REJECT     tcp  --  anywhere             anywhere            reject-with tcp-reset
REJECT     all  --  anywhere             anywhere            reject-with icmp-port-unreachable
Chain syn_flood (1 references)
target     prot opt source               destination
RETURN     tcp  --  anywhere             anywhere            tcp flags:FIN,SYN,RST,ACK/SYN limit: avg 25/sec burst 50
DROP       all  --  anywhere             anywhere
Chain zone_lan (1 references)
target     prot opt source               destination
input_lan  all  --  anywhere             anywhere
zone_lan_ACCEPT  all  --  anywhere             anywhere
Chain zone_lan_ACCEPT (3 references)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
Chain zone_lan_DROP (0 references)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere
DROP       all  --  anywhere             anywhere
Chain zone_lan_REJECT (1 references)
target     prot opt source               destination
reject     all  --  anywhere             anywhere
reject     all  --  anywhere             anywhere
Chain zone_lan_forward (1 references)
target     prot opt source               destination
zone_wan_ACCEPT  all  --  anywhere             anywhere
forwarding_lan  all  --  anywhere             anywhere
zone_lan_REJECT  all  --  anywhere             anywhere
Chain zone_vpn (1 references)
target     prot opt source               destination
input_vpn  all  --  anywhere             anywhere
zone_vpn_ACCEPT  all  --  anywhere             anywhere
Chain zone_vpn_ACCEPT (3 references)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
Chain zone_vpn_DROP (0 references)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere
DROP       all  --  anywhere             anywhere
Chain zone_vpn_REJECT (0 references)
target     prot opt source               destination
reject     all  --  anywhere             anywhere
reject     all  --  anywhere             anywhere
Chain zone_vpn_forward (1 references)
target     prot opt source               destination
zone_wan_ACCEPT  all  --  anywhere             anywhere
zone_lan_ACCEPT  all  --  anywhere             anywhere
forwarding_vpn  all  --  anywhere             anywhere
zone_vpn_ACCEPT  all  --  anywhere             anywhere
Chain zone_wan (1 references)
target     prot opt source               destination
ACCEPT     udp  --  anywhere             anywhere            udp dpt:bootpc
ACCEPT     icmp --  anywhere             anywhere            icmp echo-request
ACCEPT     tcp  --  anywhere             anywhere            tcp dpt:23232
input_wan  all  --  anywhere             anywhere
zone_wan_REJECT  all  --  anywhere             anywhere
Chain zone_wan_ACCEPT (3 references)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere
ACCEPT     all  --  anywhere             anywhere
Chain zone_wan_DROP (0 references)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere
DROP       all  --  anywhere             anywhere
Chain zone_wan_REJECT (2 references)
target     prot opt source               destination
reject     all  --  anywhere             anywhere
reject     all  --  anywhere             anywhere
Chain zone_wan_forward (1 references)
target     prot opt source               destination
forwarding_wan  all  --  anywhere             anywhere
zone_wan_REJECT  all  --  anywhere             anywhere

Answer 1

抱歉，我看不出问题出在哪里。是的，你的防火墙允许任何数据包让 INPUT 和 FORWARD 都通过。这是新安装的 OpenWRT 防火墙的标准配置，我的配置也是一样。

基本上，iptables规则应按顺序读取：从第一个读取到最后一个，第一个符合的规则将被应用，其余规则甚至无需测试。如果测试了所有规则，但没有一个符合，则应用默认策略（例如，对于 INPUT 链，默认策略为 ACCEPT）。

以您的 INPUT 链为例，测试传入的数据包以确定它是否与现有连接相关；如果相关，则应用规则，因此数据包被接受。如果不是（因此数据包属于尚未建立的连接），我们转到第二条规则。第二条规则没有要满足的标准，因此所有数据包都符合该规则；因此应用该规则，其应用意味着：接受。

其余规则永远不会成为问题。这就是为什么您应该在永久保存 iptables 规则之前仔细检查它们：您提供规则的顺序可能与应用规则的顺序不同，从而导致不愉快的后果。

这个总体策略（应用第一个适合的规则，忽略后面的规则）不同于内核应用路由规则的策略：在这种情况下，最合适的限制性规则都会被应用，无论路由规则的存储顺序如何。