** icmpecho
reply every ping command.

to try it:
1. make
2. sudo ./icmpecho
3. in another window, set ip of tun0 device.
   sudo ifconfig tun0 192.168.0.1/24
   or
   sudo ifconfig tun0 10.0.0.1/24
4. ping 192.168.0.2
   or
   ping 192.168.0.3
   or
   ping 192.168.0.4
   or
   ping 192.168.0.254

** rejectall
reject all tcp connections.

to try it:
1. make
2. sudo ./rejectall
3. in another window, set ip of tun0 device.
   sudo ifconfig tun0 192.168.0.1/24
   or
   sudo ifconfig tun0 10.0.0.1/24
4. nc 192.168.0.2 1000
   or
   nc 192.168.0.3 1000
   or
   nc 192.168.0.3 4000

** acceptall
accept all tcp connections.

to try it:
1. make
2. sudo ./acceptall
3. in another window, set ip of tun0 device.
   sudo ifconfig tun0 192.168.0.1/24
   or
   sudo ifconfig tun0 10.0.0.1/24
4. nc 192.168.0.2 1000
   or
   nc 192.168.0.3 1000
   or
   nc 192.168.0.3 4000

** discardall
accept all tcp connections, and discard all incoming data.

to try it:
1. make
2. sudo ./discardall
3. in another window, set ip of tun0 device.
   sudo ifconfig tun0 192.168.0.1/24
   or
   sudo ifconfig tun0 10.0.0.1/24
4. nc 192.168.0.2 1000
   or
   nc 192.168.0.3 1000
   or
   nc 192.168.0.3 4000

** connectmany
connect to ip:port for many times concurrently.

to try it:
1. make
2. sudo ./connectmany 192.168.0.1 2007 1000
3. in another window, set ip of tun0 device.
   sudo ifconfig tun0 192.168.0.1/24
4. start a concurrent tcp server which listens port 2007
   eg. muduo/examples/simple/echo
5. hit ENTER in 1st window, start connecting