on port 21, metasploitable2 runs vsftpd, a popular ftp server. this particular version contains a backdoor that was slipped into the source code by an unknown intruder. the backdoor was quickly identified and removed, but not before quite a few people downloaded it. if a username is sent that ends in the sequence ":)" [ a happy face ], the backdoored version will open a listening shell on port 6200. here we use metasploit to exploit it.