scponly(8) System Manager's Manual scponly(8)

scponly

scponly

— limited shell for secure file transfers

scponly is an alternative "shell" (of sorts) for system administrators who would like to provide access to remote users to both read and write local files without providing any remote execution privileges. Functionally, it is best described as a wrapper to the trusted suite of ssh applications.

A typical usage of scponly is in creating a semi-public account not unlike the concept of anonymous login for ftp. This allows an administrator to share files in the same way an anonymous ftp setup would, only employing all the protection that ssh provides. This is especially significant if you consider that ftp authentications traverse public networks in a plaintext format.

Instead of just a single anonymous user, scponly supports configuring potentially many users, each of which could be set up to provide access to distinct directory trees. Aside from the installation details, each of these users would have their default shell in /etc/passwd set to "/usr/local/bin/scponly" (or wherever you choose to install it). This would mean users with this shell can neither login interactively or execute commands remotely. They can however, scp files in and out, governed by the usual Unixish file permissions.

  • Logging: scponly logs time, client IP address, username, and the actual request to syslog.
  • chroot: scponly can chroot to the user's home directory (or any other directory the user has permissions for), disallowing access to the rest of the filesystem.
  • sftp compatibility: My testing of sftp against an scponly user worked great. This is probably the cleanest and most usable way for an scponly user to access files.
  • Security checks: root login is disallowed (though root should never be configured to be using scponly as the default shell.)
  • WinSCP 2.0 compatibility: scponly can be compiled in WinSCP compatibility mode that will permit a "semi-interactive" shell that WinSCP can use.
  • gftp compatibility: scponly is compatible with gftp if you set "use ssh2 sftp subsys" in your gftp options.

scponly doesn't do anything to manage read/write permissions. The ssh applications already do that just fine. If you use scponly, be aware that good old Unix-style file permissions are still doing the work of protecting your files.

  • As recent as June 26, 2002, vulnerabilities have been discovered in OpenSSH. There is also a SSH1 protocol vulnerability. If you're going to use scponly , be aware it is no more secure than the ssh installation it runs on.
  • I've since discovered that ssh.com's commercial ssh offering supports BOTH "dummy users" as well as functionality paralleling scponly. I have not been able to find any notes on these features, but I did read that they exist. I will make a point to include more information later. It appears that OpenSSH does not yet support these features. At this time, I have no plans to end-of-life scponly, though ultimately, I recognize that scponly should eventually become just a feature of whichever sshd you may run.

/usr/local/bin/scponly
The default location for the shell itself.
/usr/local/sbin/scponlyc
The default location for the chrooted version of scponly
/etc/shells
To be a proper shell, it has to be included here.

chroot(2), ssh(1), scp(1), sftp(1), shells(5), sshd(8).

Joe Boyle ⟨joe@sublimation.org⟩.

Copyright (C) 2001, 2002, 2003
Joe Boyle ⟨joe@sublimation.org⟩.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Versions prior to 2.4 have a vulnerability wherein the .ssh/environment file can be used to override $PATH and $LD_LIBRARY_PATH vars, compromising the shell.

Sept 03, 2002