ssh_config(5) says: ProxyCommand Specifies the command to use to connect to the server. The com- mand string extends to the end of the line, and is executed using the user's shell `exec' directive to avoid a lingering shell process. Personally, I would find this explanation much more transparent if it mentioned up front that the ssh command line arguments "destination" and "-p port" are ignored when ProxyCommand is specified. Perhaps something like: ProxyCommand Specifies the command to use to connect to the server. If this option is provided then ssh will ignore command line arguments such as "destination" and "-p port". Instead, Ssh will run the given command, and will attempt to communicate with the remote server via the standard input and output of the command's process. Specifying no proxy command is equivalent to "ProxyCommand nc host port" (where "nc" is the "netcat" utility). The command string extends to the end of the line. The command is executed using the user's shell `exec' directive to avoid a lingering shell process. [...]
Those options aren't ignored though, they are made available to the proxy command via token command line arguments. E.g. ProxyCommand="nc %h %p"
Thank you. Second attempt ProxyCommand Specifies the command to use to connect to the server. If this option is provided then ssh(1) will not use the command line arguments such as "destination" and "-p port" for connecting. Instead, ssh(1) will run the given command, and will attempt to communicate with the remote server via the input and output of the command's process. Thus, specifying no proxy command (the default) would be semantically equivalent to `ProxyCommand="nc %h %p"`. ("nc" is the "netcat" utility, and %h and %p are expanded to host and port as defined under TOKENS below) The command string extends to the end of the line. The command is executed using the user's shell `exec' directive to avoid a lingering shell process. [...] There is a bit of repetition in the next paragraph ("Arguments to ProxyCommand accept the tokens [...]") but I think this is OK. (?)