Canon EOS-1D C Manual (en)

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Changing Settings
Configured in [FTP server]  [Passive mode].
Enable this setting in network environments protected by a firewall. If an 
Error 41 occurs (“Cannot connect to FTP server”), setting passive mode 
to [Enable] may enable access to the FTP server.
To prevent accidental changes to LAN 
settings, set [Protect settings] to [On].
Passive Mode
Preventing Changes to Settings