Wireshark · Wireshark-users: [Wireshark-users] help me please

[Miszcsi Miszcsi <miszcsike@xxxxxxxxx>]

Hello!

Please somebody help me with my problem !  I'm new in this and I'm stucked with my project because of this problem and I cannot going further.

Any concrete and real help would be appreciated.

I'm trying to monitor network traffic on windows gateway with Wireshark, specially IM traffic, Yahoo Messenger. I have 2 fastethernet cards in the pc, one for WAN and one for LAN.
If I'm running the sniffer on an internal pc, i have both incoming and outgoing packets from and to yahoo server or in case of peertopeer messaging to and from remote discussion partner.
If I'm running the sniffer on the gateway using WAN interface for capture, I have only incoming packets, and no outgoing. For filtering @capture I'm using the option
"tcp port 5050 and host X.X.X.X" where X.X.X.X is the IP adress of the internal pc.
(Wireshark - Capture Options - and I enter in the Capture Filter field this, after then Start)
I have one staticly assigned real IP on the WAN, and dhcp assigned private IP's for internal pcs (192.168.0.X), they are assigned based on each pc's MAC adress, so they are constant and not interchanging. I'm using source NAT on WAN interface.

What I'm doing wrong or why outgoing packets doesn't appear in Wireshark ?

There is an example in Wireshark User's Guide from where I was inspired :

Example 4.1.  A capture filter for telnet that captures traffic to and from a particular host

tcp port 23 and host 10.0.0.5

Please somebody explain what is the solution or the problem what 
makes that I see only incoming packets and nothing outgoing.

Best Regards 
Miszcsi