Porting Bad Apple to Telnet and Successfully Screening It

This post is automatically translated with LLM. The translation content has NOT been reviewed and may contain errors.

Today, after tinkering for half a day, I successfully ported Bad Apple to my VPS's Telnet service using the command-line animation version I published earlier.

To view it yourself:
telnet xuyh0120.tk or telnet 5.175.156.249

Round 1. Porting to SSH

Since I originally promised a classmate I'd implement this via SSH, I started there. First, I uploaded Bad Apple to my VPS at /opt/badapple, which contains two files: badapple and badapple.in (the resource file with a modified extension).

useradd badapple
passwd badapple
# 密码我设的badapple
su badapple
# 切换过去
chsh
# 输入/opt/badapple/badapple
exit

When testing via SSH, I encountered Error 2 (resource file not found). I recompiled locally (the VPS lacks FPC), changed paths from relative to absolute, re-uploaded, and successfully displayed the animation upon reconnecting.

Next, I needed to automatically disconnect users after playback. A classmate suggested pkill -9 sshd—effective but disruptive to concurrent viewers. However, I discovered that OpenSSH automatically terminates the connection when my Pascal program exits.

SSH's powerful features posed security risks. For example, its built-in SFTP bypasses the shell. I disabled it by commenting out the relevant line in /etc/ssh/sshd_config:

Subsystem sftp /usr/libexec/openssh/sftp-server```

I also disabled TCP/IP port forwarding by adding:

AllowTcpForwarding no```

Later, I found SCP file transfer—deeply integrated with SSH and impossible to disable—forcing me to abandon SSH and delete the user.

Round 2. Porting to Telnet

Try telnet towel.blinkenlights.nl on your machine (safe, but Vista+ users may not see effects). This inspired me to replicate the ASCII-art experience via Telnet.

apt-get install telnet telnetd xinetd
cd /etc/xinetd.d
nano telnet
#输入：
#service telnet
#{
#        disable=no
#        flags=REUSE
#        socket_type=stream
#        wait=no
#        user=root
#        server=/usr/sbin/in.telnetd
#        log_in_failure+=USERID
#}
service xinetd restart

Initially, Telnet required authentication via the badapple user. To achieve password-free access like the Star Wars demo, I studied telnetd and discovered its -L parameter for custom login programs. After unsuccessful attempts via xinetd, I leveraged the default login path:

cd /usr/lib
mv telnetlogin telnetlogin_backup
ln -sf /opt/badapple/badapple telnetlogin
service xinetd restart

This replaces the login shell with Bad Apple, starting playback immediately upon connection. Like SSH, Telnet disconnects automatically after playback.

The result is impressive—despite a brief "Debian GNU Linux 6.0" header, the core objective is achieved. Mission complete.

Demo: telnet xuyh0120.tk or telnet 5.175.156.249