我已经尝试了几乎所有 iptables 规则来阻止 openvpn 服务器上的 smtp，但客户端仍然可以在端口 25 上访问远程 smtp 服务器。
我正在使用 openvpn 访问服务器。它创建两个接口 as0t0 和 as0t1。
所有用户分配的 ip 均来自 172.16.0.0/12。
请帮助我编写什么规则来解决这个问题。
openvpn接入服务器设置的iptables规则：
#Generated by iptables-save v1.4.7 on Sun Apr 10 13:03:56 2016
*nat
:PREROUTING ACCEPT [566:72410]
:POSTROUTING ACCEPT [36:2340]
:OUTPUT ACCEPT [36:2340]
:AS0_NAT - [0:0]
:AS0_NAT_POST_REL_EST - [0:0]
:AS0_NAT_PRE - [0:0]
:AS0_NAT_PRE_REL_EST - [0:0]
:AS0_NAT_TEST - [0:0]
-A PREROUTING -m state --state RELATED,ESTABLISHED -j AS0_NAT_PRE_REL_EST
-A POSTROUTING -m state --state RELATED,ESTABLISHED -j AS0_NAT_POST_REL_EST
-A POSTROUTING -m mark --mark 0x2000000/0x2000000 -j AS0_NAT_PRE
-A AS0_NAT -o eth0 -j SNAT --to-source 91.13.18.170
-A AS0_NAT -j ACCEPT
-A AS0_NAT_POST_REL_EST -j ACCEPT
-A AS0_NAT_PRE -m mark --mark 0x8000000/0x8000000 -j AS0_NAT
-A AS0_NAT_PRE -d 192.168.0.0/16 -j AS0_NAT_TEST
-A AS0_NAT_PRE -d 172.16.0.0/12 -j AS0_NAT_TEST
-A AS0_NAT_PRE -d 10.0.0.0/8 -j AS0_NAT_TEST
-A AS0_NAT_PRE -j AS0_NAT
-A AS0_NAT_PRE_REL_EST -j ACCEPT
-A AS0_NAT_TEST -o as0t+ -j ACCEPT
-A AS0_NAT_TEST -m mark --mark 0x4000000/0x4000000 -j ACCEPT
-A AS0_NAT_TEST -d 172.27.224.0/20 -j ACCEPT
-A AS0_NAT_TEST -j AS0_NAT
COMMIT
# Completed on Sun Apr 10 13:03:56 2016
# Generated by iptables-save v1.4.7 on Sun Apr 10 13:03:56 2016
*mangle
:PREROUTING ACCEPT [146:10130]
:INPUT ACCEPT [6422:1226373]
:FORWARD ACCEPT [8289:2947415]
:OUTPUT ACCEPT [5446:2764996]
:POSTROUTING ACCEPT [13735:5712411]
:AS0_MANGLE_PRE_REL_EST - [0:0]
:AS0_MANGLE_TUN - [0:0]
-A PREROUTING -m state --state RELATED,ESTABLISHED -j AS0_MANGLE_PRE_REL_EST
-A PREROUTING -i as0t+ -j AS0_MANGLE_TUN
-A AS0_MANGLE_PRE_REL_EST -j ACCEPT
-A AS0_MANGLE_TUN -j MARK --set-xmark 0x2000000/0xffffffff
-A AS0_MANGLE_TUN -j ACCEPT
COMMIT
# Completed on Sun Apr 10 13:03:56 2016
# Generated by iptables-save v1.4.7 on Sun Apr 10 13:03:56 2016
*filter
:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [3970:2307554]
:AS0_ACCEPT - [0:0]
:AS0_IN - [0:0]
:AS0_IN_NAT - [0:0]
:AS0_IN_POST - [0:0]
:AS0_IN_PRE - [0:0]
:AS0_IN_ROUTE - [0:0]
:AS0_OUT - [0:0]
:AS0_OUT_LOCAL - [0:0]
:AS0_OUT_POST - [0:0]
:AS0_OUT_S2C - [0:0]
:AS0_WEBACCEPT - [0:0]
-A INPUT -m state --state RELATED,ESTABLISHED -j AS0_ACCEPT
-A INPUT -i lo -j AS0_ACCEPT
-A INPUT -m mark --mark 0x2000000/0x2000000 -j AS0_IN_PRE
-A INPUT -d 91.13.18.170/32 -p udp -m state --state NEW -m udp --dport 1194 -j AS0_ACCEPT
-A INPUT -d 91.13.18.170/32 -p tcp -m state --state NEW -m tcp --dport 443 -j AS0_ACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j AS0_WEBACCEPT
-A INPUT -d 91.13.18.170/32 -p tcp -m state --state NEW -m tcp --dport 943 -j AS0_WEBACCEPT
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -p icmp -j ACCEPT
-A INPUT -i lo -j ACCEPT
-A INPUT -p tcp -m state --state NEW -m tcp --dport 22 -j ACCEPT
-A INPUT -p tcp -m tcp --dport 25 -j DROP
-A INPUT -p udp -m udp --dport 25 -j DROP
-A INPUT -j REJECT --reject-with icmp-host-prohibited
-A INPUT -i eth0 -p tcp -m tcp --dport 25 -j DROP
-A INPUT -i as0t0 -p tcp -m tcp --dport 25 -j DROP
-A INPUT -i as0t1 -p tcp -m tcp --dport 25 -j DROP
-A INPUT -p tcp -m tcp --dport 25 -j REJECT --reject-with icmp-port-unreachable
-A FORWARD -m state --state RELATED,ESTABLISHED -j AS0_ACCEPT
-A FORWARD -m mark --mark 0x2000000/0x2000000 -j AS0_IN_PRE
-A FORWARD -o as0t+ -j AS0_OUT_S2C
-A FORWARD -j REJECT --reject-with icmp-host-prohibited
-A FORWARD -i eth0 -p tcp -m tcp --dport 25 -j DROP
-A FORWARD -i as0t0 -p tcp -m tcp --dport 25 -j DROP
-A FORWARD -i as0t1 -p tcp -m tcp --dport 25 -j DROP
-A FORWARD -i lo -p tcp -m tcp --dport 25 -j DROP
-A FORWARD -i as0t+ -p tcp -m tcp --dport 25 -j DROP
-A FORWARD -i as0t0 -p tcp -m tcp --dport 25 -j DROP
-A FORWARD -i as0t1 -p tcp -m tcp --dport 25 -j DROP
-A OUTPUT -o as0t+ -j AS0_OUT_LOCAL
-A OUTPUT -p tcp -m tcp --dport 25 -j DROP
-A OUTPUT -p tcp -m tcp --dport 25 -j DROP
-A AS0_ACCEPT -j ACCEPT
-A AS0_IN -d 172.27.224.1/32 -j ACCEPT
-A AS0_IN -j AS0_IN_POST
-A AS0_IN_NAT -j MARK --set-xmark 0x8000000/0x8000000
-A AS0_IN_NAT -j ACCEPT
-A AS0_IN_POST -o as0t+ -j AS0_OUT
-A AS0_IN_POST -j DROP
-A AS0_IN_PRE -d 192.168.0.0/16 -j AS0_IN
-A AS0_IN_PRE -d 172.16.0.0/12 -j AS0_IN
-A AS0_IN_PRE -d 10.0.0.0/8 -j AS0_IN
-A AS0_IN_PRE -j ACCEPT
-A AS0_IN_ROUTE -j MARK --set-xmark 0x4000000/0x4000000
-A AS0_IN_ROUTE -j ACCEPT
-A AS0_OUT -j AS0_OUT_POST
-A AS0_OUT_LOCAL -p icmp -m icmp --icmp-type 5 -j DROP
-A AS0_OUT_LOCAL -j ACCEPT
-A AS0_OUT_POST -j DROP
-A AS0_OUT_S2C -j AS0_OUT
-A AS0_WEBACCEPT -j ACCEPT
COMMIT
# Completed on Sun Apr 10 13:03:56 2016