Following a successful penetration test, you may have large amounts of data to exfiltrate from an environment specifically hardened to make it difficult to exfiltrate data. For example, the network might have a firewall that explicitly blocks common exfiltration methods – such as SSH, HTTPS, HTTP.
It is common that you can still exfiltrate data from these networks by using DNS. For example you could make a request to a domain name that you control where the subdomain contains some information to be exfiltrated. Such as sensitive-data-here.attacker.example.com. DNS is a recursive system, such that if you send this request to a local DNS server, it will forward it on and on until it reaches the authoritative server. If you control the authoritative server, you can simply read the sensitive data from the DNS logs.