Why tcpdump don't work?

EdwardKing zhangsc at neusoft.com
Fri Aug 29 05:03:08 UTC 2008

I have a server and a client,they use TCP to communication.For example,when client send a message to server,the server return the same message to client,then client show it.When client or server shutdown,it will send FIN.So I want to use tcpdump to watch it. The server and client in the same machine,the IP is and the port is 9999

 ether 00:0b:31:76:35:31
 inet6 fe80::20c:29ff:fe76:356a%le0 prefixlen 64 scopeid 0x1 
 inet netmask 0xffffff00 broadcast
 media: Ethernet autoselect
 status: active
plip0: flags=108810<POINTOPOINT,SIMPLEX,MULTICAST,NEEDSGIANT> metric 0 mtu 1500
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
 inet6 ::1 prefixlen 128 
 inet6 fe80::1%lo0 prefixlen 64 scopeid 0x3 
 inet netmask 0xff000000 

#tcpdump tcp port 9999 and host
tcpdump:verbose output suppressed,use -v or -vv for full protocol decode 
listening on le0,link-type EN10MB (Ethernet),capture size 96 bytes

#tcpdump -v tcp port 9999 and host
tcpdump:listening on le0,link-type EN10MB (Ethernet),capture size 96 bytes

when client and server communication or client shutdown,tcpdump don't show any message!
Why? I am very puzzle with it!  How to show server and client tcp communication information by using tcpdump? 


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