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We seeing this behavior where all of a sudden our TCP response are not getting sent. Even when the connection is up (i.e We are able to receive request )

The TCP client is connected to server over a VPN. It work most of time but some time we don't see any information on the ngrep for response that are suppose to being sent back to server.

ngrep command

ngrep -t -d en01 port 8067

The TCP client is Twisted TCP client.

I tried netstat to check if the Send Queue size.

netstat ouptut

Proto Recv-Q Send-Q Local Address           Foreign Address         State      
tcp        0      0 x.x.x.x.:47617           y.y.y.a:8067    ESTABLISHED
tcp        0      0 x.x.x.x.:50626           y.y.y.b:8067    ESTABLISHED

y.y.y.a - TCP server 1
y.y.y.b - TCP server 2

SEND-Q (0) Look ok to me

even the lsof output mention the QS as 0

lsof -Tqs -i TCP:8067

COMMAND   PID  USER   FD   TYPE    DEVICE SIZE/OFF NODE NAME
twistd  19561 viren    3u  IPv4 183882947      0t0  TCP x-x-x-x:47617->y.y.y.a:8067 (ESTABLISHED QR=0 QS=0)
twistd  19564 viren    3u  IPv4 183879151      0t0  TCP x-x-x-x:50626->y.y.y.b:8067 (ESTABLISHED QR=0 QS=0)

Important note the moment I restart my TCP client I see the message flow.

Not sure what is going wrong and what is happening to the packet.

Any clue??

Operating System

CentOS Linux release 7.2.1511 (Core)

Noobie
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  • Do they failures happen after the connection has been idle? When you say the connection is up, are you referring to from the server perspective? Is there a keep-alive mechanism being used between the client/server? Is there a FW or LB in the connection flow? – Jeff S. Jul 30 '16 at 22:24
  • @JeffS I cannot say that from Server perspective. – Noobie Jul 31 '16 at 01:15
  • @JeffS with regards to Firewall and load balancer even if they are why would this happen all of a sudden.Anyway I will reconfirm this . – Noobie Jul 31 '16 at 01:18
  • The thinking behind look for a lb or fw is to see if there is something in the path with a session table that is timing out after a period of inactivity. – Jeff S. Aug 01 '16 at 11:38

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