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Consider this basic network setup:

Client -> Router -> "Internet" -> Router -> Server

Problem: the client can open a connection to the server and send data ("request"), but the server data packets ("response") can not be sent to the client.

It looks like a firewall problem. I assume that a packet filter could be the reason, filtering out only the response packets. Is there a technical term for this filter type?

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To clarify - the server is receiving the client's TCP SYN packet, but the SYN ACK response is not getting back to the client? Or are you seeing different symptoms? – Shane Madden Apr 19 '12 at 17:12
the syn/ack response has been sent already, the TCP connnection is established, and the client has sent data packets. ... I am trying to give some troubleshooting hints to a developer, but from his description and code example I figured out it could be port filter problem – mjn Apr 19 '12 at 17:18
up vote 0 down vote accepted

Not exactly, it could be the firewall on the client router not allowing the required ports into the local network, the server's firewall stopping the outgoing packets or the servers router blocking the outgoing packets.

If the routers are blocking it it's still a firewall's packet filter. If the client or server are blocking it they will be doing so with the firewall's packet filtering.

It could also be a routing issue.

I hope that helps.

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My first step in the troubleshooting procedure is to make sure the server is even answering connection requests at all. Both network and host based firewalls both have options to drop connections silently. Secondly I would insure there are no network issues from server to client. this includes routing path issues but also could be something to do with you nat configurations on either side of the connection.


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Are you all part of the same network and do you have VLANS? If so, take a look at your firewall and make sure that the VLAN's can communicate. What type of Firewall do you have in place? ASA, Checkpoint, Sonicwall?

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