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Comment by amelius | Hacker News Reader

I always wondered why there is no standard unix tool to accomplish this.

Also useful would be a tool to close a TCP connection, regardless of the processes that have handles to it.

BSD's have tcpdrop(8):

     If	a connection to	httpd(8) is causing congestion on a network link, one
     can drop the TCP session in charge:

	   # sockstat -c | grep	httpd
	   www	    httpd      16525 3	tcp4 \
		   192.168.5.41:80	192.168.5.1:26747

     The following command will	drop the connection:

	   # tcpdrop 192.168.5.41 80 192.168.5.1 26747

You can achieve this for TCP by faking an RST (reset) packet that purports to be from the other end of the connection.

[EDIT]

These tools are a reasonable starting point.

http://linuxpoison.blogspot.co.uk/2008/10/tools-for-creating...

This sort of does what you want. But there must be some traffic even if it just keep alive.

  sudo tcpkill -9 -i eth0 host 1.2.3.4 and port 80

It is part of dsniff.

I've had much better luck with killcx:

http://killcx.sourceforge.net/

In particular, it sends a spoofed SYN packet to induce an ACK packet containing the sequence numbers necessary to send the RST. That means it doesn't rely on there being traffic already.

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