differences between win and unix tcp clients.

["Fabiano Reis" <silos.reis@ig.com.br>]

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Hi list,

I was doing tests with iptables on redhat 7.3 , iptables version 1.2.5 and I observed that windows tcp client (telnet) doesn´t answer in the same time that unix tcp client (telnet) is doing.

(This test is so simple to do)

For example: I have a telnet daemon listening on port 23 of a Redhat 7.3 (192.168.0.1) and two more machines that will work as client of this daemon on this example. One of them Windows 2000 (192.168.0.2) and the another Solaris 2.6 (192.168.0.3). I have my lan connection OK, so the clients can achieve the server using a telnet client normally.

To start the test I added a rule to block the connections that came from the two clients I have.

iptables -A INPUT -p tcp --source 192.168.0.2 --dport 23 -j REJECT
iptables -A INPUT -p tcp --source 192.168.0.3 --dport 23 -j REJECT

On the client side I do a "telnet 192.168.0.1" and what I got:

On the Solaris machine: a very fast response that the connection was refused.

On the Windows 2000 machine: after for about 20 seconds I got the message that the connect failed (I thing this is the same as connection refused).

Doing another test on the Windows2000 machine. That time I used telnet to connect on a port that doesn´t exists on my RedHat server. 

c:> telnet 192.168.0.1 30
Connecting To 192.168.0.1...Could not open a connection to host on port 30: Connect failed

This is the same error I got when I use the port that is blocked with "reject" by iptables on server side, BUT the difference is that I get the error after 1 or 2 seconds after executing the command.

Conclusion: I think this was because the Windows implementation of TCP detects something different on this connection, maybe it detects in some way that there is a filter on the server side and this is why the time for the error message take so long to appear.

Am I right? Is there a workaround for this problem? I need to make windows think that the server is really "down" and that is why i´m writting to you people.

I will appreciate any help.

Thanks in advance,

Fabiano

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