As already answered, the commandline sent from the ssh client is put into the SSH_ORIGINAL_COMMAND environment variable, only the ForcedCommand is executed.
If you use the information in SSH_ORIGINAL_COMMAND in your ForcedCommand you must take care of security implications. An attacker can augment your command with arbitrary additional commands by sending e.g. ; rm -rf / at the end of the commandline.
This article shows a generic script which can be used to lock down allowed parameters. It also contains links to relevant information.
The described method (the 'only' script) works as follows:
Make the 'only' script the ForcedCommand, and give it the allowed
command as its parameter. Note that more then one allowed command may be used.
Put a .onlyrules files into the home directory of user_1 and fill it with rules (regular expressions) which are matched against the
commandline sent by the ssh client.
Your example would look like:
Match User user_1
ForceCommand /usr/local/bin/only /home/user_1/user_1_shell
and if, for example, you want to allow as parameters only 'set_new_mode' with exactly two alphanumeric arbitrary parameters the .onlyrules file would look like this:
\:^/home/user_1/user_1_shell set_new_mode [[:alnum:]]\{1,\} [[:alnum:]]\{1,\}$:{p;q}
Note that for sending the command to the server you must use the whole commandline:
/home/user_1/user_1_shell set_new_mode param1 param2
'only' looks up the command on the server and uses its name for matching the rules. If any of these checks fail, the command is not run.
