A few times, I had this issue with some client connected to one our servers that was filling up the disk space with some garbage.
At that point (customer or not), I needed to block their IP from connecting. There are many ways of doing that (IPTABLES, SELinux, etc.) There is also a way of doing that by rejecting their IP using routing table in Linux.
In this example I will use my two Raspberry PI computers. The first one, 192.168.0.253 (clu) will block the second one 192.168.0.254 (tron).
Method 1
pi@clu $ sudo route add 192.168.0.254 gw 127.0.0.1 pi@clu $
The efect of this command will produce the following output:
pi@clu $ netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0 0 eth0 192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 192.168.0.254 127.0.0.1 255.255.255.255 UGH 0 0 0 lo pi@clu $
Removal is the same command with 'del' instead of 'add'.
pi@clu $ sudo route del 192.168.0.254 gw 127.0.0.1 pi@clu $
Method 2
Another way of accomplishing the same task is the following route table change
pi@clu $ sudo route add -host 192.168.0.254 reject pi@clu $
It creates the following route table entry:
pi@clu $ netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0 0 eth0 192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 192.168.0.254 - 255.255.255.255 !H - - - - pi@clu $
In a similar way, we can use use keyword '-net 192.168.0.0 netmask 255.255.255.0 reject'. This will stop the whole network from entering our server.
Again, in order to remove it, the following needs to be configured:
pi@clu $ sudo route del -host 192.168.0.254 reject pi@clu $
Finally, also a quick method is to use the keyword 'blackhole'.
Method 3
pi@clu $ sudo ip route add blackhole 192.168.0.254/32 pi@clu $
This will create the following entry in the routing table:
pi@clu $ netstat -nr Kernel IP routing table Destination Gateway Genmask Flags MSS Window irtt Iface 0.0.0.0 192.168.0.1 0.0.0.0 UG 0 0 0 eth0 192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 192.168.0.254 0.0.0.0 255.255.255.255 UH 0 0 0 * pi@clu $
In order to remove this entry type in:
pi@clu $ sudo ip route del blackhole 192.168.0.254/32 pi@clu $
