openssh bugs - Apr 2008 - [Bug 1459] New: Request for better documentation of shell used to run commands

https://bugzilla.mindrot.org/show_bug.cgi?id=1459

           Summary: Request for better documentation of shell used to run  
                    commands                                               
    Classification: Unclassified                                           
           Product: Portable OpenSSH                                       
           Version: 5.0p1                                                  
          Platform: Other                                                  
        OS/Version: All                                                    
            Status: NEW                                                    
          Severity: minor                                                  
          Priority: P2                                                     
         Component: sshd                                                   
        AssignedTo: bitbucket at mindrot.org                                  
        ReportedBy: Joseph.Krahn at gmail.com                                 


It would be helpful if the documentation gave a more complete
description of how the shell is selected to run commands. Recently, the
sshd man page added documentation that sshrc commands are run by sh,
and not the user shell. There are still some other things that could
use more detail. For example, sshd never uses the SHELL environment
variable, but ssh-agent does. I think it should be stated that commands
are always run under the shell defined by getpw(), including commands
for ssh_config keys with the "command=" option.

This came up when trying to set up an ssh key to allow remote execution
of a pre-defined task using the "command=" feature. I also wanted to
disable logins for that account, so it could only be used to invoke
that specific task. Obviously, setting the shell to /sbin/nologin
didn't work. However, searching for ways to achieve this, I found that
it is not uncommon for people to assume that commands are run as if by
system(). In most cases, perhaps it should be obvious that a "remote
shell" always runs commands under the account's shell, but I think this
should be clarified at least for the "command=" feature. Consider a
situation where an ssh-key is defined to invoke a specific task, but a
user figures out a way to change the account shell in a way that
affects the access allowed by that ssh key.

It's not a major issue, but I think its worth including a few extra
sentences so that you don't have to read the source code just to figure
out the details of how commands are run.

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