Iptables ACCEPT and DROP

Iptables ACCEPT and DROP

[Ravi Verma]

Dear Friends:

I have observe a behavior of iptables which I need to understand.

216.155.193.168 is the IP address of Yahoo's messenger site and it
listens on port 5050.

The following command will allows the machine to connect to
216.155.193.168.

iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
ACCEPT

After that, I see output like following:

#telnet 216.155.193.168 5050
Trying 216.155.193.168...
Connected to 216.155.193.168.
Escape character is '^]'.

Now when I issue
iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
DROP
And
iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
REJECT

Still, it allows connection to 216.155.193.168 on port 5050.

How does this work? It seems -j DROP is not opposite of -j ACCEPT. How
can stop this?

Kind regards.

Ravi Verma

Iptables ACCEPT and DROP

[Ravi Verma]

Dear Friends:

I have observe a behavior of iptables which I need to understand.

216.155.193.168 is the IP address of Yahoo's messenger site and it
listens on port 5050.

The following command will allows the machine to connect to
216.155.193.168.

iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
ACCEPT

After that, I see output like following:

#telnet 216.155.193.168 5050
Trying 216.155.193.168...
Connected to 216.155.193.168.
Escape character is '^]'.

Now when I issue
iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
DROP And iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport
5050 -j REJECT

Still, it allows connection to 216.155.193.168 on port 5050.

How does this work? It seems -j DROP is not opposite of -j ACCEPT. How
can stop this?

Kind regards.

Ravi Verma

Re: Iptables ACCEPT and DROP

[Antony Stone]

On Friday 16 April 2004 6:56 pm, Ravi Verma wrote:

> The following command will allows the machine to connect to
> 216.155.193.168.
>
> iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
> ACCEPT
>
> Now when I issue
> iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport 5050 -j
> DROP
>
> And
>
> iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168 --dport
> 5050 -j REJECT
>
> Still, it allows connection to 216.155.193.168 on port 5050.
>
> How does this work? It seems -j DROP is not opposite of -j ACCEPT. How
> can stop this?

"-A" means append - in other words, "add on to the end of my ruleset".

You have not said that you have flushed the OUTPUT chain (with "iptables -F 
OUTPUT") between adding the ACCEPT rule and applying more rules after it, so 
I think you still have the ACCEPT rule in your chain, and that is the first 
one the packets see.

Try "iptables -L OUTPUT -nvx" and see what rules you have, and in what order.

Regards,

Antony.

-- 
"Black holes are where God divided by zero."

 - Steven Wright

                                                     Please reply to the list;
                                                           please don't CC me.

Re: Iptables ACCEPT and DROP

[IT Clown]

will the following help:


iptables -A INPUT -i eth0 -p tcp -s 216.155.193.168 --sport
5050 -j DROP
iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168
--dport 5050 -j DROP

service iptables stop
/etc/init.d/iptables stop
iptables -F
service iptables start
/etc/init.d/iptables start

or am i missing it completely?

Regards

On Fri, 16 Apr 2004 10:43:07 -0700
 "Ravi Verma" <ravi.verma@telecommand.com> wrote:
> Dear Friends:
> 
> I have observe a behavior of iptables which I need to
> understand.
> 
> 216.155.193.168 is the IP address of Yahoo's messenger
> site and it
> listens on port 5050.
> 
> The following command will allows the machine to connect
> to
> 216.155.193.168.
> 
> iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168
> --dport 5050 -j
> ACCEPT
> 
> After that, I see output like following:
> 
> #telnet 216.155.193.168 5050
> Trying 216.155.193.168...
> Connected to 216.155.193.168.
> Escape character is '^]'.
> 
> Now when I issue
> iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168
> --dport 5050 -j
> DROP
> And
> iptables -A OUTPUT -o eth0 -p tcp -d 216.155.193.168
> --dport 5050 -j
> REJECT
> 
> Still, it allows connection to 216.155.193.168 on port
> 5050.
> 
> How does this work? It seems -j DROP is not opposite of
> -j ACCEPT. How
> can stop this?
> 
> Kind regards.
> 
> Ravi Verma
> 
> 

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