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Here port forwarding I means ALL kinds of port forwarding, -D dynamic port forwarding(for socks4 and socks5 now), -L local port forwarding to remote, -R remote port forwarding to local. How to fine grained control them separately? I know there is also permitopen option to permit only specified forwarding to host and port.

And I want to know AllowTcpForwarding in sshd_config will do -L or -R, but not -D?

How to allow -D, but not -L or -R, or this is not possible? I have addedcommand="/bin/echo no-command" to the authorized_keys file.

  1. Allow user to set up an SSH tunnel, but nothing else
  2. How to restrict ssh port forwarding, without denying it?

1 Answer 1

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Allowing -D but not -L is not possible, since they only differ on the client – as far as the protocol is concerned, both are exactly the same thing (they both open direct-tcpip channels over SSH).

The only difference between them is how the ssh client decides which address to connect to (either specified in the command line, or obtained over SOCKS).

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  • Yes, -L will forward local_addr:local_port to remote_host:remote_port, and -D will forward local_addr:local_port to unknown_host:unknown_port. So unknown_host:unknow_port will definite contains remote_host:remote_port?
    – schemacs
    Dec 10, 2015 at 6:32
  • Doesn't matter if it does or not. The server does not care. The server does not receive anything like "prepare a -D tunnel please". The server only receives individual requests like "open a tunnel to host:port right now", and they're identical for both -D and -L.
    – user1686
    Dec 10, 2015 at 6:46
  • OK. Thanks. How about disabling -R using permitopen ?
    – schemacs
    Dec 10, 2015 at 11:39
  • That should be possible, since it's in a different direction, but I can't find an option for it.
    – user1686
    Dec 10, 2015 at 11:50

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