[CentOS] Not Responding To TCP Connections

Les Mikesell lesmikesell at gmail.com
Sun May 7 04:12:23 UTC 2006

On Sat, 2006-05-06 at 15:13, hkclark at gmail.com wrote:
> I have a CentOS 3 box that appears to be having problems where a TCP
> SYN comes into port 80 for an Apache Tomcat web application, but the
> rest of the TCP "3-way handshake" isn't happening.  When the error
> occurs, I always see this pattern in tcpdump:
>   * A TCP SYN comes in from the client
>   * ~3.25 second later I get another TCP SYN from the client
>   * ~6.5 seconds after that I get a third and final SYN before the
> client gives up
>   * No TCP ACKs are returned to the client to continue the 3-way handshake
> The box is accepting lots of connections on lots of different ports
> and everything else seems to be working, but I'm occassionally getting
> these errors on port 80 for the last few weeks (the box has been up
> for a while).
> Has anyone else run into anything similar to this?
> Am I correct in saying that the Linux kernel, not java and/or Tomcat,
> handles the TCP stack, including the 3-way handshake?  If so, getting
> past the 3-way handshake should have nothing to do with Java or
> Tomcat... IOW, this issue must have something to do with the OS,
> right?

Ultimately the application has to handle connections as fast
as they come in or you are fried one way or the other.  However
the OS should queue pending connections (by completing the
handshake) until the application accepts them according to
the 'backlog' parameter to listen() when the application sets
up the receiving socket.  I don't know enough about tomcat
to know where to look for that value, though.  When the
backlog value is exceeded you should see connection requests
being ignored as you describe.

  Les Mikesell
   lesmikesell at gmail.com

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