How to drop TCP connection without killing process?

Larry Cook lcook at
Mon May 24 16:43:00 EDT 2004

> What you're asking for is kindof weird

Yes, I guess it is.  Let me explain:

A POP3 server I use doesn't send a response on occasion.  This causes Mozilla 
to stop checking that POP3 account because the connection is still up.  And 
the connection just stays up, with no traffic, as far as I can tell.  I'd like 
to force that connection to drop so that Mozilla will start checking that 
account again.  And I like to do this without having to stop and restart Mozilla.

> but one way of accomplishing
> what you're asking for is to use something like ipchains

I guess this won't be useful since there does not appear to be any traffic on 
that connection.

> This might be useful:
>    tcpdump 'host my-computer tcp port my-local-port'
> If there are any keepalives being sent, tcpdump WILL see them.

That is what I am doing, so I guess Mozilla must not use keepalives.

> Although I need to tell you up front that:
>    1:  by default, TCP doesn't use ANY keepalives.  The application
>        that uses a TCP session needs to enable these manually.
>    2:  If TCP keepalives are enabled, typically these keepalives are
>        sent out every few hours or so.  So, if you're waiting to see
>        an elusive TCP keepalive, you might be waiting for a while.
>        Applications that need some sort of keepalive feature
>        (especially if these are short) have to implement these things
>        themselves and not depend on TCP's keepalive feature.

Thanks for the explaination of keepalives.  I'll leave tcpdump running 
overnight just to confirm there are no keepalives.  I guess Mozilla needs a 
timeout when waiting for POP3 responses.


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