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FTP Active vs Passive Mode

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FTP's unusual two-connection design

FTP uses two separate TCP connections: a control channel (port 21) for commands and responses, and a data channel for actual file transfers. This unusual design causes the famous active/passive mode distinction.

When FTP was designed (1971), NAT didn't exist. The challenges that followed when the internet adopted NAT explain why FTP is one of the most troublesome protocols to traverse firewalls.

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