VPN Setupを参考にして、Kali Linuxで試してみた。がDocker上のKali Linuxでは難しいことがわかった。
そして仮想マシンで試したところ、(おそらくうまく接続できた結果)グローバルIPアドレスでSSH接続できなくなったorz

設定とエラーメッセージ

# curl https://ifconfig.me
109.x.x.x
# apt install openvpn
# mkdir openvpn
# cd openvpn/
# wget -d https://www.vpnbook.com/free-openvpn-account/VPNBook.com-OpenVPN-PL226.zip
...
# unzip VPNBook.com-OpenVPN-PL226.zip
...
# rm VPNBook.com-OpenVPN-PL226.zip
# ls
vpnbook-pl226-tcp443.ovpn  vpnbook-pl226-tcp80.ovpn  vpnbook-pl226-udp25000.ovpn  vpnbook-pl226-udp53.ovpn
# openvpn --config vpnbook-pl226-udp53.ovpn
Sat Mar  7 13:25:47 2020 OpenVPN 2.4.7 x86_64-pc-linux-gnu [SSL (OpenSSL)] [LZO] [LZ4] [EPOLL] [PKCS11] [MH/PKTINFO] [AEAD] built on Feb 20 2019
Sat Mar  7 13:25:47 2020 library versions: OpenSSL 1.1.1d  10 Sep 2019, LZO 2.10
Enter Auth Username:vpnbook
Enter Auth Password:
...
Sat Mar  7 13:26:25 2020 Initialization Sequence Completed
Sat Mar  7 13:26:28 2020 AEAD Decrypt error: bad packet ID (may be a replay): [ #1 ] -- see the man page entry for --no-replay and --replay-window for more info or silence this warning with --mute-replay-warnings
Sat Mar  7 13:26:32 2020 AEAD Decrypt error: bad packet ID (may be a replay): [ #2 ] -- see the man page entry for --no-replay and --replay-window for more info or silence this warning with --mute-replay-warnings