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== Linux tcpdrop kernel module == (c) 2012 Roman Arutyunyan <email@example.com> Module: ====== This module lets you drop TCP connections from working Linux system. It also supports unscheduling TIME_WAIT sockets. The module creates pseudo-file /proc/net/tcp_drop which expects input of the following format: saddr:sport daddr:dport Note: Spaces between the two can be of any type & size. Requirements: ============ Linux kernel 3.0.0 Build & install: =============== Untar/ungzip & cd to module directory & just run make. Load module: sudo insmod ./tcp_drop.ko Unload module: sudo rmmod tcp_drop Note: remember you need root privileges to write to /proc/net/tcp_drop. Example1: ======== We have 4 connected sockets by IMAP client (port 143). Let's drop the last one: netstat -n|grep ESTABLISHED|grep 143 tcp 0 0 10.31.1.141:51292 192.168.0.1:143 ESTABLISHED tcp 0 0 10.31.1.141:51293 192.168.0.1:143 ESTABLISHED tcp 0 0 10.31.1.141:51436 192.168.0.1:143 ESTABLISHED ^.............copy this...............^ Just copy the middle part of string (incl. tabs/spaces) to /proc/net/tcp_drop: echo "10.31.1.141:51436 192.168.0.1:143" > /proc/net/tcp_drop Now it'dead: netstat -n|grep ESTABLISHED|grep 143 tcp 0 0 10.31.1.141:51292 192.168.0.1:143 ESTABLISHED tcp 0 0 10.31.1.141:51293 192.168.0.1:143 ESTABLISHED IMAP client has received a socket error & will reconnect when needed. Example2: ======== Let's kill a TIME_WAIT socket. I've just created a TIME_WAIT socket with a netcat connection to localhost:8080: netstat -n|grep TIME_WAIT tcp 0 0 127.0.0.1:34790 127.0.0.1:8080 TIME_WAIT ^....................................^ Here's how to kill it (it's better to say 'unschedule'): echo "127.0.0.1:34790 127.0.0.1:8080" > /proc/net/tcp_drop Let's see: netstat -n|grep TIME_WAIT <nothing here> IPv6 support: ============ If built for non-ancient (>2.6.19) kernel IPv6 is fully supported. Dropping IPv6 connections is done the same way as IPv4: echo "::1:34717 ::1:8080" > /proc/net/tcp_drop Standard representation of IPv6 address with port ([ipv6addr]:port) is also supported: echo "[::1]:34717 [::1]:8080" > /proc/net/tcp_drop What server/client code receives: ================================ All code which has been using the socket killed receives network error (it's like receiving TCP RESET): telnet localhost 8080 Trying ::1... Trying 127.0.0.1... Connected to localhost. Escape character is '^]'. Connection closed by foreign host.