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I have some dead connection in one application which is in hanged state if client machine is dead. 

->192.168.1.214:49029 (ESTABLISHED)

Is there a way to terminate these option from linux command line without restarting the server?

After search I found solution called as tcpkill. But it will not work for me. As it permanently blocks that ip.

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    The answer on using the ss command is much easier and more general than the others. – nealmcb Nov 4 '19 at 20:02
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On linux kernel >= 4.9 you can use the ss command from iproute2 with key -K

ss -K dst 192.168.1.214 dport = 49029

the kernel have to be compiled with CONFIG_INET_DIAG_DESTROY option enabled.

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    For linux, this is really the best way and pretty much the only way if you have idle connections (tcpkill could not work). However, I'll admit that I've not inspected killcx but it feels like a lot of security software would prevent that from working unless you modify your iptables to allow these spoofed packets through. – Seth Robertson May 21 '19 at 17:32
  • Thanks! Worked like a charm with sudo ss -K .... on Ubuntu Bionic 18.04 LTS. I had a tmux process that was stuck at a small screen size because of a remote, but dead, but not timed-out connection. All fixed! – nealmcb Nov 4 '19 at 20:03
  • ss: invalid option -- 'K' ubuntu 18.04.5 LTS – Alexey Sh. Mar 18 at 13:06
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Originally from: http://rtomaszewski.blogspot.sk/2012/11/how-to-forcibly-kill-established-tcp.html

To "kill" a socket, you must send a TCP reset packet. To send it (and be accepted by the other side), you must know the actual TCP sequence number.

1) The already mentioned tcpkill method learns the SEQ number by passively sniffing on the network and waiting for valid packets of this connection to arrive. Then it uses the learned SEQ number to send RSET packets to both sides. However if the connection is idle/hanged and no data flows, it won't do anything and will wait forever.

2) Another method uses perl script called killcx (link to Sourceforge). This actively sends spoofed SYN packets and learns the SEQ number from the answer. It then sends RSET packets the same way as tcpkill.

Alternatively approach (based on what you want to achieve) is to use gdb debugger to attach to a process owning this socket/connection and issue close() syscall on its behalf - as detailed in this answer.

If you want to deal only with hanged connections (the other side is dead), there are various timeouts (TCP keepalive for example), which should automatically close such connections if configured properly on the system.

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  • Would it work if we simply closed the TCP socket by its file descriptor (fd)? exec fd>&- – Alexander Gonchiy Jan 10 '18 at 7:18
  • @AlexanderGonchiy for live connections it would prevent the process to respond to packets, so it would cause the connection to timeout. For idle connections nothing would happen. I'm not sure if kernel would send anything to the network on closing the fd. – Marki555 Jan 11 '18 at 9:41
  • I have sniffed the sequence number. What should i do then? – user3132194 Dec 29 '18 at 11:33
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tcpkill might do it for you. In Ubuntu it is in the dsniff package.

Something like:

$ sudo tcpkill -i wlan0 host 192.168.1.214

(or some other tcpdump like expression for what connection to kill).

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    This works only if the connection is transmitting anything. It will not work for hanged/idle TCP connections (see my answer for details) – Marki555 May 15 '15 at 17:02
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Do - as root netstat -tunp|grep 49029. The last column of the output should show you the PID and program name of the process responsible for that connection.

If you are lucky there is a single process for just that connection.

If you are unlucky it gets more complicated (the PID is responsible for more than just that one connection). What kind of service is this?

Why do you want to terminate that session?

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    I can't kill that process. It is gearman server. I want to just close the connection. – Vivek Goel Apr 10 '13 at 13:34
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tcpkill cannot close a dead (hanged) connection. It is based libpcap, it construct a packet to sent FIN packet. If the connection is already dead, it cannot get the right sequence number.

The only way is to close the process, so makes everywhere is NOT SPOF.

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Adding to Marki555 answer which is a great list of options on how to do this.

I found another option - which is long - but also works for idle connections. It is to use a kernel debugger to get the TCP sequence number and then send FIN packets (I believe) using hping3. From https://blog.habets.se/2017/03/Killing-idle-TCP.html

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You may try to use iptables REJECT with --reject-with tcp-reset, which would send RST to remote when it matches a packet.

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This repo https://github.com/caruccio/tcp_killer/ which is a python3 fix for https://github.com/google/tcp_killer does the job for me. And it works in both Linux and macOS.

It uses the debugger approach mentioned by Marki555 and described in this answer.

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DROP packets for incoming/outgoing IP address using:

sudo iptables -A INPUT -s 50.17.54.5 -j DROP
sudo iptables -A OUTPUT -s 50.17.54.5 -j DROP

Undo the change when finished and delete the rules you added:

sudo iptables -D INPUT -s 50.17.54.5 -j DROP
sudo iptables -D OUTPUT -s 50.17.54.5 -j DROP
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