Help for cut the nat connection.

Help for cut the nat connection.

[hanxin]

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Hi,
My prolem is I can't cut a established TCP connection through NAT.

Detail:
I have a Redhat linux 7.2 with default iptables(v1.2.3) installed. The
box has two ethernet cards, eth0(61.139.69.7) direct connect to internet
and eth1(192.168.0.1) is my intranet gateway.
I used this rule for NAT :
iptables -A POSTROUTING -o eth0 -j SNAT --to-source 61.139.69.7
and it worked fine.But for managment, I want to only allow authed client
can use NAT to internet.So I wrote a simple CGI and added some rules for
control NAT client.
The default rules like this:
iptables -t nat -A PREROUTING -d 192.168.0.1 -j ACCEPT
iptables -t nat -A PREROUTING -s 192.168.0.1 -j ACCEPT
iptables -t nat -A PREROUTING -d 61.139.69.7 -j ACCEPT
iptables -t nat -A PREROUTING -s 61.139.69.7 -j ACCEPT
iptables -t nat -P PREROUTING DROP
When a client login correct, my CGI adds a rule:
iptables -t nat -I PREROUTING -s <client_ip> -j ACCEPT
So the client can use NAT now. And if I delete this rule, the client
should not use NAT.
But I found if a client established a TCP connection like telnet, when I
delete the rule, the telnet connection is still active.
eg.:
The client 192.168.0.73 telnet to 61.139.69.11, then I delete its' nat
rule, if 192.168.0.73 has not exit form the telnet, he can still use
this telnet session.But he can't ping 61.139.69.11. Why?
How can I realy cut the client  NAT connection?
Maybe my method to control NAT is stupid. Any advice is welcome.
Thanks.

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-hanxin


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