scponly.8

.\" Copyright 2001, 2002, 2003 joe@sublimation.org.
.\" All rights reserved.
.Dd Sept 03, 2002
.Dt scponly 8
.Sh NAME
.Nm scponly
.Sh SYNOPSIS
.Nm
.Nd limited shell for secure file transfers
.Sh DESCRIPTION
.Nm
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.
.Pp
A typical usage of
.Nm
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.
.Pp
Instead of just a single anonymous user,
.Nm
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.
.Sh FEATURES
.Bl -bullet
.It
Logging:
.Nm
logs time, client IP address, username, and the actual request to syslog.
.It
chroot:
.Nm
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.
.It
sftp compatibility:  My testing of sftp against an
.Nm
user worked great.  This is probably the cleanest and most usable way for an
.Nm
user to access files.
.It
Security checks:  root login is disallowed (though root should never be
configured to be using
.Nm
as the default shell.)
.It
WinSCP 2.0 compatibility:
.Nm
can be compiled in WinSCP compatibility mode that will permit a
"semi-interactive" shell that WinSCP can use.
.It
gftp compatibility:
.Nm
is compatible with gftp if you set "use ssh2 sftp subsys" in your gftp options.
.El
.Pp
.Nm
doesn't do anything to manage read/write permissions.  The ssh applications
already do that just fine.  If you use
.Nm ,
be aware that good old Unix-style file permissions are still doing the work of
protecting your files.
.Sh NOTES
.Bl -bullet
.It
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
.Nm
, be aware it is no more secure than the ssh installation it runs on.
.It
I've since discovered that ssh.com's commercial ssh offering supports BOTH
"dummy users" as well as functionality paralleling
.Nm .
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
.Nm ,
though ultimately, I recognize that
.Nm
should eventually become just a feature of whichever sshd you may run.
.El
.Sh FILES
.Bl -tag -width base/sup/collection/checkouts*xx -compact
.It Pa /usr/local/bin/scponly
The default location for the shell itself.
.It Pa /usr/local/sbin/scponlyc
The default location for the chrooted version of 
.Nm
.It Pa /etc/shells
To be a proper shell, it has to be included here.
.El
.Sh SEE ALSO
.Xr chroot 2 ,
.Xr ssh 1 ,
.Xr scp 1 ,
.Xr sftp 1 ,
.Xr shells 5 ,
.Xr sshd 8 .
.Sh AUTHORS
.An Joe Boyle Aq joe@sublimation.org .
.Sh LEGALITIES
Copyright (C) 2001, 2002, 2003
.An Joe Boyle Aq joe@sublimation.org .
.Pp
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:
.Pp
1. Redistributions of source code must retain the above copyright notice, this
list of conditions and the following disclaimer.
.Pp
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.
.Pp
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.
.Sh BUGS
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.