According to the above page, it means that we can input a IP Address and then ping it.
We gonna try to figure out what the page returns.
It reutns all of the infomation of consequence of command execution.
Then execute other commands like ‘ls‘ to test whether filter someting.
Luckily there is no filter.
In general, the FLAG saved as a file which name contains ‘flag‘.
In fact we find the flag file out finally and use command ‘cat‘ to view it.
We got it.
原文：https://www.cnblogs.com/4fun/p/15139287.html