To bypass firewall we need to use valid packets for authorized protocols and authorized protocols are not the same for all servers. There are many commons protocols (used on servers in the production environment) that can be useful to bypass firewall:
NTP (take examples on HTTP, DNS and IRC implementations and use the RFC to define how you can sent commands and responses and how you should implements the protocol)
NRPE (take examples on HTTP, DNS and IRC implementations and use documentations to define how you can sent commands and responses and how you should implements the protocol)
SNMP (take examples on HTTP, DNS and IRC implementations and use the RFC version 1, version 2, version 3 to define how you can sent commands and responses and how you should implements the protocol)
LDAP (take examples on HTTP, DNS and IRC implementations and use the RFC to define how you can sent commands and responses and how you should implements the protocol)
FTP (take examples on HTTP, DNS and IRC implementations and use the RFC to define how you can sent commands and responses and how you should implements the protocol)
SSH (take examples on HTTP, DNS and IRC implementations and use the RFC to define how you can sent commands and responses and how you should implements the protocol)
SMB (take examples on HTTP, DNS and IRC implementations and use the documentation to define how you can sent commands and responses and how you should implements the protocol)
SMTP (take examples on HTTP, DNS and IRC implementations and use the RFC to define how you can sent commands and responses and how you should implements the protocol)
DNS over TCP (take examples on HTTP, DNS and IRC implementations and use the RFC to define how you can sent commands and responses and how you should implements the protocol)
DNS over TLS (take examples on HTTP, DNS and IRC implementations and use the RFC
DNS over HTTPS (take examples on HTTP, DNS and IRC implementations and use the RFC
To bypass firewall we need to use valid packets for authorized protocols and authorized protocols are not the same for all servers. There are many commons protocols (used on servers in the production environment) that can be useful to bypass firewall: