1scponly(8) BSD System Manager's Manual scponly(8)
2
4 scponly
5
7 scponly — limited shell for secure file transfers
8
10 scponly is an alternative "shell" (of sorts) for system administrators
11 who would like to provide access to remote users to both read and write
12 local files without providing any remote execution privileges. Function‐
13 ally, it is best described as a wrapper to the trusted suite of ssh
14 applications.
15 A typical usage of scponly is in creating a semi-public account not
17 unlike the concept of anonymous login for ftp. This allows an adminis‐
18 trator to share files in the same way an anonymous ftp setup would, only
19 employing all the protection that ssh provides. This is especially sig‐
20 nificant if you consider that ftp authentications traverse public net‐
21 works in a plaintext format.
22 Instead of just a single anonymous user, scponly supports configuring
24 potentially many users, each of which could be set up to provide access
25 to distinct directory trees. Aside from the installation details, each
26 of these users would have their default shell in /etc/passwd set to
27 "/usr/bin/scponly" (or wherever you choose to install it). This would
28 mean users with this shell can neither login interactively or execute
29 commands remotely. They can however, scp files in and out, governed by
30 the usual Unixish file permissions.
31
33 · Logging: scponly logs time, client IP address, username, and the
34 actual request to syslog.
35 · chroot: scponly can chroot to the user's home directory (or any other
37 directory the user has permissions for), disallowing access to the
38 rest of the filesystem.
39 · sftp compatibility: My testing of sftp against an scponly user
41 worked great. This is probably the cleanest and most usable way for
42 an scponly user to access files.
43 · Security checks: root login is disallowed (though root should never
45 be configured to be using scponly as the default shell.)
46 · WinSCP 2.0 compatibility: scponly can be compiled in WinSCP compati‐
48 bility mode that will permit a "semi-interactive" shell that WinSCP
49 can use.
50 · gftp compatibility: scponly is compatible with gftp if you set "use
52 ssh2 sftp subsys" in your gftp options.
53 scponly doesn't do anything to manage read/write permissions. The ssh
55 applications already do that just fine. If you use scponly, be aware
56 that good old Unix-style file permissions are still doing the work of
57 protecting your files.
58
60 · As recent as June 26, 2002, vulnerabilities have been discovered in
61 OpenSSH. There is also a SSH1 protocol vulnerability. If you're
62 going to use scponly , be aware it is no more secure than the ssh
63 installation it runs on.
64 · I've since discovered that ssh.com's commercial ssh offering supports
66 BOTH "dummy users" as well as functionality paralleling scponly. I
67 have not been able to find any notes on these features, but I did
68 read that they exist. I will make a point to include more informa‐
69 tion later. It appears that OpenSSH does not yet support these fea‐
70 tures. At this time, I have no plans to end-of-life scponly, though
71 ultimately, I recognize that scponly should eventually become just a
72 feature of whichever sshd you may run.
73
75 /usr/bin/scponly The default location for the shell
76 itself.
77 /usr/sbin/scponlyc The default location for the chrooted
78 version of scponly
79 /etc/shells To be a proper shell, it has to be
80 included here.
81
83 chroot(2), ssh(1), scp(1), sftp(1), shells(5), sshd(8).
84
86 Joe Boyle <joe@sublimation.org>.
87
89 Copyright (C) 2001, 2002, 2003 Joe Boyle <joe@sublimation.org>.
90 Redistribution and use in source and binary forms, with or without modi‐
92 fication, are permitted provided that the following conditions are met:
93 1. Redistributions of source code must retain the above copyright notice,
95 this list of conditions and the following disclaimer.
96 2. Redistributions in binary form must reproduce the above copyright
98 notice, this list of conditions and the following disclaimer in the docu‐
99 mentation and/or other materials provided with the distribution.
100 THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
102 IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
103 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
104 IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDEN‐
105 TAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
106 LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
107 DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
108 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
109 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
110 THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
111
113 Versions prior to 2.4 have a vulnerability wherein the .ssh/environment
114 file can be used to override $PATH and $LD_LIBRARY_PATH vars, compromis‐
115 ing the shell.
116 Sept 03, 2002