Drop TCP connections

On FreeBSD you can drop existing TCP connection using the tcpdrop command. For instance you can drop all ESTABLISHED connections using tcpdrop -s ESTABLISHED. Or you can even list them all with:

$ tcpdrop -la
tcpdrop ::1 59298 ::1 1180
tcpdrop 59299 80
tcpdrop 59300 22
tcpdrop 59301 443

Notice the fun thing here. Those are actual commands that you can use to drop the connections. In fact you can use this to filter which connection you want to drop. For example:

# Drop all but SSH connections
tcpdrop -la | grep -vw 22 | sh

# Drop all incoming HTTP connections
tcpdrop -la | grep -v " 80 " | sh

# Drop all connections to a specific IP
tcpdrop -la | grep -vw | sh

This can be useful for instance on a desktop when you just switched interface, or say just started a VPN daemon, and want all prior TCP connections not originating from your new addresses to be killed. Then you would just add those IP you would like to keep and filter them out:

# List of IPs you want to keep
echo > keep-ip.txt
echo >> keep-ip.txt

tcpdrop -la | grep -Ev "(::1|" | grep -vwf keep-ip.txt | sh

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