Notes about SSH

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How to restrict ssh port forwarding, without denying it?

Source: https://superuser.com/questions/516417/how-to-restrict-ssh-port-forwarding-without-denying-it

OpenSSH has a feature for restricting -L style opens on the server side:

  • In the server configuration file there is a PermitOpen option. This option can be used to specify hosts and ports for which forwards can be established. This option can be used inside a Match block, so it can be restricted by user, group, or hostname or IP address pattern.
  • In an authorized_keys file, options can be associated with a particular key. There is a permitopen option which works similarly to the server-config one.

Notes/Limitations:

  • The option AllowTcpForwarding disables all forwarding in both directions, preventing listening ports from being set up on the server, as well as active forwards.
  • There is no PermitOpen access control for -R style connections. This is probably okay. What it means is that the users can use ssh to open various non-privileged ports for listening on the server. Where these connect to on the other side of the SSH connection is the client's problem. If we restrict forwarding in the -L direction, the user has no way of using those -R ports (at least not through ssh, if that user is not able to create an arbitrary interactive session).
  • There doesn't seem to be a way to create an empty list of permitted opens, to prevent users from making any -L style connections whatsoever. However, a workaround is to use a harmless, nonexistent or impossible host name, such as the empty string. Concretely, permitopen=":1234" does the trick.

Secure Secure Shell - stribika guide

link: https://stribika.github.io/2015/01/04/secure-secure-shell.html