SMTP port open - but not open

Posted by Frederik Nielsen on Server Fault See other posts from Server Fault or by Frederik Nielsen
Published on 2012-07-04T08:49:25Z Indexed on 2012/07/04 9:17 UTC
Read the original article Hit count: 478

Filed under:

firewall

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smtp

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exchange-2010

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port-forwarding

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packet

As some of you might know, I am setting up an exchange server.

Now I ran into another problem:
I cannot connect to the SMTP service from outside the server!

The ports are opened in the gateway device (a ZyXEL USG50), Windows firewall is off.

I see the packets travekl through the ZyXEL firewall, and I can also see the packets with wireshark on the server, so I know they are getting all the way in to the server.

I also know it receives them, and sends out the reply - and this is where things go bad!

Analyzing with wireshark, I get these errors in the return packets:

Header checksum: 0x0000 [incorrect, should be 0x0779 (may be caused by "IP checksum offload"?)]
And:
Acknowledgment Number: 0x8e3337d1 [should be 0x00000000 because ACK flag is not set]

What the (sorry my French) hell is going on?
I really cant figure it out..

Thanks in advance.

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