Blocking specified ports with ipchains

In short, ipchains is a tool in Linux that allows you to administer IP packet filters.
# Blocking specified ports
Let’s say you’re running a service, but instead of shutting it down, you just want to filter out the port and block connections to it.  This is made simple with ipchains:

/sbin/ipchains -A input -j REJECT -p tcp -s 0.0.0.0/0 -d 0.0.0.0/0 21

This tells ipchains to REJECT tcp connections from any IP on port 21.  You'll notice that the port number is at the very end of the line.  If you wanted to block out a different service or port, simply replace the number 21 with the new port.