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Old 06-08-2004, 09:03 AM   #1
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drop incoming/outgoing packets using iptables


I am running a java application from my linux machine, which connects over internet to a remote computer on a given port. Now, during the run of the application, if the internet link goes down, then the application behaviour is unpredictable, so I want to reproduce the timeout problem by dropping the incoming and outgoing packets from the given IP using iptables. Following are the rules I am using for dropping the incoming/outgoing packets :

iptables -t mangle -I OUTPUT -p tcp --syn -d -j DROP
iptables -t mangle -I PREROUTING -p tcp --syn -s -j DROP

Now, if I apply these rules before starting my application, then it properly drops the packets and application gives timeout. But let say I haven't applied these rules at the start of the application, (means the IP is not blocked when the application starts), so application connects to the remote IP on the specified port, opens input/output streams on it. If I apply the above rules now to disable the incoming/outgoing traffics from the given IP, then it doesn't work, means it will transfer data on both the input and output streams properly and no timeout comes.

What is the problem here and what would be the work around?

Old 06-08-2004, 11:38 AM   #2
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Registered: Mar 2003
Distribution: Fedora
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Your rules are only going to drop the initial syn packets that establish the connection. It's likely that once the connection has been initiated and the stream established, you won't see any further syn packets. I'd guess that you have a default policy of accept or some other rule that allows the rest of the traffic through (something like a rule allowing established/related traffic or some rule that allows non-syn traffic through). It's hard to say specifically without see you entire firewall ruleset, so post them if you want specifics.

If you wanted to mimic the effects of the link going down, you could probably do something a little more drastic like:

iptables -I INPUT -j DROP
iptables -I OUTPUT -j DROP
iptables -I FORWARD -j DROP


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