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5 "scponly" is an alternative 'shell' (of sorts) for system
6 administrators who would like to provide access to remote users to
7 both read and write local files without providing any remote
8 execution privileges. Functionally, it is best described as a
9 wrapper to the mostly trusted suite of ssh applications.
11 A typical usage of scponly is in creating a semi-public account not
12 unlike the concept of anonymous login for ftp. This allows an
13 administrator to share files in the same way an anon ftp setup
14 would, only employing all the protection that ssh provides. This is
15 especially significant if you consider that ftp authentications
16 traverse public networks in a plaintext format.
18 Instead of just a single anon user, scponly supports configuring
19 potentially many users, each of which could be set up to provide
20 access to distinct directory trees. Aside from the installation details
21 (see INSTALL), each of these users would have their default shell in
22 /etc/passwd set to "/usr/local/sbin/scponly" (or wherever you choose
23 to install it). This would mean users with this shell can neither
24 login interactively nor execute commands remotely. They can however,
25 scp files in and out, governed by the usual unix file permissions.
27 Some Features:
30 - logging: scponly logs time, client IP address, username, and the
31 actual request to syslog.
33 - choot: scponly can chroot to the user's home directory (or any
34 other directory the user has permissions for), disallowing access
35 to the rest of the filesystem.
37 - sftp compatibility. my testing of sftp against an scponly user
38 worked great. this is probably the cleanest and most usable way
39 for an scponly user to access files.
41 - sftp logging: if the ./configure directive
42 "--enable-sftp-logging-compat" is used, scponly will support sftp
45 - WinSCP 2.0/3.0 compatibility.
47 - gftp compatibility
49 - rsync compatibility
51 - security checks: root login is disallowed (though root
52 should never be configured to be using scponly as the default shell.)
53 scponly also checks the ownership of directories before chroot-ing
54 into them.
56 How it works:
58 If you were to examine the arguments passed to a shell by sshd upon
59 opening a remote connection, the structure of the argument vector
60 invariably looks like this:
62 <shell name> -c <remote command>
64 scponly validates remote requests by examining the third argument.
65 scponly also verifies the request by disallowing what a normal
66 shell would interpret as "special characters". This prevents
67 someone from piggybacking additional commands onto a valid scp
68 request. It may seem that using scponly would prevent using scp to
69 copy files that really do contain special characters. However,
70 copying files with special characters in their names can be
71 accomplished by using wildcards (which are allowable characters) to
72 match the filenames.
74 scponly doesnt do anything to manage read/write permissions. The
75 ssh applications already do that just fine. If you use scponly, be
76 aware that good old unix file permissions are still doing the work
77 of protecting your files.
80 MAY 2002 ADDENDUM:
81 I've since discovered that ssh.com's commercial ssh offering
82 supports BOTH "dummy users" as well as scponly-ish functionality. I
83 have not been able to find any notes on these features, but I did read
84 that they exist. I will make a point to include more information later.
85 It appears that OpenSSH does not yet support these features. At this time,
86 I have no plans to end-of-life scponly, though ultimately, I recognize
87 that scponly should eventually become just a feature of whichever sshd
88 you may run.
90 NOV 2003 ADDENDUM:
91 OpenSSH still doesnt support configurable chrooting for users OR
92 disallowing interactive login.
94 NOV 2004 ADDENDUM:
95 OpenSSH still doesnt support configurable chrooting for users OR
96 disallowing interactive login.
98 DEC 2005 ADDENDUM:
99 OpenSSH still doesnt support configurable chrooting for users OR
100 disallowing interactive login.