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networking - How to diagnose large number of TIME_WAIT connections

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We have a production issue with only
one of our servers and have correlated slow performance to an abundance of sockets in
the TIME_WAIT state. Without drawing this question into a huge
backstory, we basically know that every time the server is slow, about 80% of the
server's sockets are in this TIME_WAIT state, which of course
we see by running a netstat). Specifically, because
TIME_WAIT times out and go away, when our server is slow we see
these TIME_WAITs crop up very frequently (about ever 5 - 10
minutes).



I did a little digging and see that
TIME_WAITs occur when the server closes an active connection
but keeps it around in case any delayed packets come through. Eventually
TIME_WAIT times
out.



Anyway to see exactly why an individual
socket went into the TIME_WAIT state to begin with? This is
CentOS 5 - does Linux log this info in var/logs anywhere, or is
there any way to do a tcpdump and look for a specific pattern that leads to a
TIME_WAIT? Thanks in advance.



Answer




Short answer - it is due to an app. The app creates sockets for a short time ,
closes them, then it immediately needs to open another socket. The sluggishness is
related to the process(es) running out of sockets to
use.




When creating a socket there are
options - SO_REUSEADDR abnd SO_REUSEPORT. They have somewhat similar functions, but I
suspect in Centos 5 SO_REUSEPORT is not available. Anyway, the optional setting on a
socket call allows the port to be immediately reused.



So, a commonly used fix is to recode. It is
probably a net app that connects for a few seconds then ends the
session.


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