Tracking TCP connections with netstat

I’ve been troubleshooting some possible problems on a mail server recently, and have been digging into TCP connections some. The ‘netstat’ command has a ‘-o’ option that displays some timers that are useful:

[mail]# netstat -on |grep
tcp        0      8           ESTABLISHED on (17.00/4/0)
tcp        0    452           ESTABLISHED on (36.09/6/0)

This displays countdown timers for each TCP State. For example, if a connection is in FIN_WAIT and you run the command over and over with “watch”, you can watch the time count down to 0 and then go away. The man pages and documentation I could find didn’t explain the timers very well, so this is what I have learned by watching it. (read: this is not official).

When a connection is in the ESTABLISHED state, the timer can be either on or off. From what I can tell, the counter turns to ON when there is some kind of trouble with the connection. It looks like when a retransmission occurs, the timer is flipped ON, and then the countdown timer starts. The countdown timer has 3 numbers. The first is a countdown in seconds, the second is incremented for each retransmission, and the third one is always 0, so I’m not sure what it does

Now that I have a basic understanding of the output, I still have to figure out why these connections just hang. My guess at this point is that it is poorly written spamming software and maxed out bandwidth on all a whole much of compromised machines throughout the world that are hitting my mail servers.

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