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Newsgroups: comp.security.unix,comp.security.misc
Subject: SSH (Secure Shell) FAQ - Frequently Asked Questions
MIME-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 8bit
Archive-name: computer-security/ssh-faq
Url: http://www.uni-karlsruhe.de/~ig25/ssh-faq/
Posting-frequency: every 14 days
-----BEGIN PGP SIGNED MESSAGE-----
Ssh (Secure Shell) FAQ - Frequently asked questions
by Thomas K÷nig Thomas.Koenig@ciw.uni-karlsruhe.de
$Date: 1996/03/27 14:17:53 $
This document is a list of Frequently Asked Questions (plus hopefully
correct answers) about the Secure Shell, ssh. IMPORTANT: There is a
security bug in all versions of ssh before 1.2.12.92. Upgrade to a
newer version is recommended.
1. Meta-questions
1.1. Where do I get this document?
1.2. Where do I send questions, corrections etc. about this document?
2. Ssh basics
2.1. What is ssh?
2.2. Why should I use it?
2.3. What kinds of attacks does ssh protect against?
2.4. What kind of attacks does ssh not protect against?
2.5. How does it work?
3. Obtaining and installing ssh
3.1. What is the latest version of ssh?
3.2. May I legally run ssh?
3.3. What about commercial use of ssh?
3.4. Where can I obtain ssh?
3.5. How do I install it?
3.6. Where do I get help?
3.7. Are there any versions for other operating systems than UNIX?
3.8. What about administration of ssh?
4. Ssh Applications
4.1. Can I run backups over ssh?
4.2. Should I turn encryption off, for performance reasons?
4.3. Can I use ssh to communicate across a firewall?
4.4. Can I distribute files with ssh, as with rdist?
4.5. Can I use ssh to securely connect two subnets across the
Internet?
4.6. Can I use ssh to securely forward UDP-based services, such as NFS
or NIS?
4.7. Can I forward SGI GL connections over ssh?
4.8. Can I use ssh to protect services like ftp or POP?
5. Problems
5.1. ssh otherhost xclient & does not work!
5.2. Ssh fails with "Resource temporarily unavailable" for Solaris 2.4
5.3. X11 forwarding does not work for an SCO binary with the iBCS2
emulator under Linux.
5.4. Ssh is doing wrong things for multi-homed hosts!
5.5. Userid swapping is broken under AIX!
5.6. ssh-keygen dumps core on Alpha OSF!
5.7. ssh-keygen dumps core on Solaris or SunOS
5.8. On Linux, compilation aborts with some error message about
libc.so.4
5.9. X authorization sometimes fails.
5.10. Ssh on Irix 5 tells me "You don't exist, go away!"
5.11. Ssh asks me for passwords despite .rhosts!
5.12. Why does ssh loop with "Secure connection refused'?
5.13. ssh-agent does not work with rxvt! rxvt closes all file
descriptors when starting up, including the one used by
6. Miscellaneous
6.1. How widespread is use of ssh?
6.2. Credits
1. Meta-questions
1.1. Where do I get this document?
The latest version of this document is available from http://www.uni-
karlsruhe.de/~ig25/ssh-faq/. It will also be posted, on a regular
basis, to the Usenet newsgroups comp.security.misc,
comp.security.unix, comp.answers and news.answers. This version is
PGP-signed, and will be available from
ftp://rtfm.mit.edu/pub/usenet/news.answers/computer-security/ssh-faq
and from http://www.uni-karlsruhe.de/~ig25/ssh-faq/ssh-faq.faq.
The original SGML file is at http://www.uni-karlsruhe.de/~ig25/ssh-
faq/ssh-faq.sgml.
Also of interest is the ssh home page, at http://www.cs.hut.fi/ssh/.
1.2. Where do I send questions, corrections etc. about this document?
Please send them to the maintainer, Thomas.Koenig@ciw.uni-karlsruhe.de
2. Ssh basics
2.1. What is ssh?
To quote the README file:
Ssh (Secure Shell) is a program to log into another computer over a
network, to execute commands in a remote machine, and to move files
from one machine to another. It provides strong authentication and
secure communications over insecure channels. It is intended as a
replacement for rlogin, rsh, and rcp.
Additionally, ssh provides secure X connections and secure forwarding
of arbitrary TCP connections.
2.2. Why should I use it?
The traditional BSD 'r' - commmands (rsh, rlogin, rcp) are vulnerable
to different kinds of attacks. Somebody who has root access to
machines on the network, or physical access to the wire, can gain
unauthorized access to systems in a variety of ways. It is also
possible for such a person to log all the traffic to and from your
system, including passwords (which ssh never sends in the clear).
The X Window System also has a number of severe vulnerabilities. With
ssh, you can create secure remote X sessions which are transparent to
the user. As a side effect, using remote X clients with ssh is more
convenient for users.
Users can continue to use old .rhosts and /etc/hosts.equiv files;
changing over to ssh is mostly transparent for them. If a remote site
does not support ssh, a fallback mechanism to rsh is included.
2.3. What kinds of attacks does ssh protect against?
Ssh protects against:
o IP spoofing, where a remote host sends out packets which pretend to
come from another, trusted host. Ssh even protects against a
spoofer on the local network, who can pretend he is your router to
the outside.
o IP source routing, where a host can pretend that an IP packet comes
from another, trusted host.
o DNS spoofing, where an attacker forges name server records
o Interception of cleartext passwords and other data by intermediate
hosts.
o Manipulation of data by people in control of intermediate hosts
o Attacks based on listening to X authentication data and spoofed
connection to the X11 server.
In other words, ssh never trusts the net; somebody hostile who has
taken over the network can only force ssh to disconnect, but cannot
decrypted or play back the traffic, or hijack the connection.
The above only holds if you actually use encryption. Ssh does have an
option to use encryption of type "none" this is only for debugging
purposes, and should not be used.
2.4. What kind of attacks does ssh not protect against?
Ssh will not help you with anything that compromises your host's
security in some other way. Once an attacker has gained root access to
a machine, he can then subvert ssh, too.
If somebody malevolent has access to your home directory, then
security is nonexistent. This is very much the case if your home
directory is exported via NFS.
2.5. How does it work?
For more extensive information, please refer to the README and RFC
files in the ssh directory. The proposed RFC is also available as an
Internet Draft, as draft-ylonen-ssh-protocol-00.txt.
All communications are encrypted using IDEA or one of several other
ciphers (three-key triple-DES, DES, RC4-128, TSS). Encryption keys are
exchanged using RSA, and data used in the key exchange is destroyed
every hour (keys are not saved anywhere). Every host has an RSA key
which is used to authenticate the host. Encryption is used to protect
against IP-spoofing; public key authentication is used to protect
against DNS and routing spoofing.
RSA keys are also used to authenticate hosts.
3. Obtaining and installing ssh
3.1. What is the latest version of ssh?
The latest officially released version is 1.2.0. The latest
development version is 1.2.13.
ssh-1.2.12.92 and later versions contain an important security fix.
Using it instead of earlier versions is recommended. The fix here is
intended to be temporary, and upgrading to the next release after that
is highly recommended when it becomes available. The temporary fix
has some drawbacks. For more information, read the ssh mailing list
archive at
http://www.cs.hut.fi/ssh/ssh-archive/.
Ssh currently runs on UNIX or related systems, plus under OS/2. Ports
have been successful to all "mainstream" UNIX systems. The current
Windows version is believed to be unstable.
At present, there are no known working versions for other operating
systems (but see below).
3.2. May I legally run ssh?
Ssh is free software, and can be freely used by anyone for any
purpose.
However, in some countries, particularly France, Russia, Iraq, and
Pakistan, it may be illegal to use any encryption at all without a
special permit.
If you are in the United States, you should be aware that, while ssh
was written outside the United States using information publicly
available everywhere, the US Government may consider it a criminal
offence to export this software from the US once it has been imported,
including putting it on a ftp site. Contact the Office of Defence
Trade Controls if you need more information.
The algorithms RSA and IDEA, which are used by ssh, are claimed as
patented in different countries, including the US. Linking against the
RSAREF2 library, which is possible, may or may not make it legal to
use ssh for non-commercial purposes in the US. You may need to obtain
licenses for commercial use of IDEA; ssh can be configured to work
without it. Ssh works perfectly fine without IDEA, however.
For more detail, refer to the file COPYING in the ssh source
distribution.
For information on software patents in general, see the Leauge for
Programming Freedom's homepage at http://lpf.org/.
3.3. What about commercial use of ssh?
Ssh has been freely available in the Unix environment, and almost
certainly will remain to be so in future.
Tatu Yl÷nen, the original author of ssh, has started a company, SSH
Communications Security Oy, that will provide commercial support and
licenses for ssh. He is working on licensing the patents to be able
to provide fully licensed commercial versions. The negotiations are
still underway, and it will take a couple more weeks before it is
known what the situation will really be.
About commercial issues, it is best to contact him directly for now.
I can best be reached by e-mail as
ylo@cs.hut.fi, or by fax at +358-0-4354 3206.
3.4. Where can I obtain ssh?
The central site for distributing ssh is ftp://ftp.cs.hut.fi/pub/ssh/.
Official releases are PGP-signed, with the key ID
DCB9AE01 1995/04/24 Ssh distribution key <ylo@cs.hut.fi>
Key fingerprint = C8 90 C8 5A 08 F0 F5 FD 61 AF E6 FF CF D4 29 D9
The latest development version is available from
ftp://ftp.cs.hut.fi/pub/ssh/snapshots/.
Ssh is also available via anonymous ftp from the following sites:
Australia:
ftp://coombs.anu.edu.au/pub/security/tools
Chile:
ftp://ftp.inf.utfsm.cl/pub/security/ssh
Finland:
ftp://ftp.funet.fi/pub/unix/security/login/ssh
Germany:
ftp://ftp.cert.dfn.de/pub/tools/net/ssh
Hungary:
ftp://ftp.kfki.hu/pub/packages/security/ssh
Ireland:
ftp://odyssey.ucc.ie/pub/ssh
Poland:
ftp://ftp.agh.edu.pl/pub/security/ssh
Portugal:
ftp://ftp.ci.uminho.pt/pub/security/ssh
Russia:
ftp://ftp.kiae.su/unix/crypto
Slovenia:
ftp://ftp.arnes.si/security/ssh
United Kingdom:
ftp://ftp.exweb.com/pub/security/ssh
United States:
ftp://ftp.net.ohio-state.edu/pub/security/ssh
United States:
ftp://ftp.gw.com/pub/unix/ssh
Some mirrors may not have the most recent snapshots available.
3.5. How do I install it?
Get the file from a site near you, then unpack it with
gzip -c -d ssh-1.2.13.tar.gz | tar xvf -
then change into the directory ssh-1.2.13, read the file INSTALL, and
follow the directions in it.
3.6. Where do I get help?
First of all, read the documentation, this document :-) and the ssh
home page, at http://www.cs.hut.fi/ssh/.
If this doesn't help, you can send mail to the mailing list for ssh
users at ssh@clinet.fi. To subscribe, send mail to
majordomo@clinet.fi with
subscribe ssh
in the body of the message.
Before subscribing, you might like to take a look at the archives of
the mailing list, at http://www.cs.hut.fi/ssh/ssh-archive.
3.7. Are there any versions for other operating systems than UNIX?
Heikki Suonsivu (hsu@clinet.fi) and Michael Henits (moi@dio.com) each
offered a US$ 100 reward for the first stable, freely redistributable
version for either Windows or MacOS.
There is a preliminary version for Windows by Cedomir.Igaly@srce.hr,
available from http://public.srce.hr/~cigaly/ssh/; you might want to
test this. A mirror is avialable from
ftp://ftp.cs.hut.fi/pub/ssh/windows/cigaly/; the filename is
ssh-1-2-.zip.
Tatu Yl÷nen, the original author of ssh, is currently working on a
Windows version.
Bernt.Budde@udac.uu.se is working on a Mac port.
A port to VMS, by Mark Martinec (Mark.Martinec@nsc.ijs.si), is being
worked on.
A port to OS/2 can be obtained from ftp://ftp.cs.hut.fi/pub/ssh/os2/.
The current maintainer, Robert Muchsel (rmuchsel@iiic.ethz.ch) does
not have much time to continue supporting it; if you're interested in
doing so, please drop him a line.
There is a special mailing list for the OS/2 version of ssh. To
subscribe, send mail to majordomo@clinet.fi with
subscribe ssh-os2
in the body of the message.
3.8. What about administration of ssh?
The central problem of administering ssh is the management of host
keys. You can collect them automatically each night using either
make-ssh-known-hosts.pl (distributed with the ssh source distribution)
or with the much faster ssh-keyscan, from
ftp://cag.lcs.mit.edu/pub/dm/ (also available from
ftp://ftp.cs.hut.fi/ssh/contrib/).
Thomas K÷nig has written a script to process output from one of these
utilities, check for new keys, warn about hosts which have changed
their keys (which could be an indication of a man in the middle
attack) and generate a complete new file. This script is available
from http://www.uni-karlsruhe.de/~ig25/ssh-faq/comp-host-list.
With these utilities, you can write scripts to verify public keys on a
regular basis. When new machines are running ssh or people have
changed public keys, you may want to contact the people in question
directly, to make sure there were no man in the middle attacks (to
which these utilities are vulnerable).
A fingerprint scheme (equivalent to PGP fingerprints) has been
proposed to make this easier; it will probably be implemented in the
next release.
4. Ssh Applications
4.1. Can I run backups over ssh?
Yes. Since ssh is a drop-in replacement for rsh, backup scripts should
continue to work. If you use rdist, see below.
4.2. Should I turn encryption off, for performance reasons?
No; you should keep it turned on, for security reasons.
Today's CPUs are fast enough that performance losses (if any) only are
noticable for local Ethernet speeds, or faster.
You might want to specify RC4 encryption instead of the default, IDEA,
with -c rc4, for faster operation.
Following are some measurements where the different encryption methods
were applied between a P5/90 and a 486/100, both running Linux, for
copying files with scp across a lightly loaded Ethernet.
The model chosen was t=a+x/b; a is the startup time in seconds, and b
the sustainable transfer rate in kB/s. Also given are the 68.3%
confidence intervals for the data, as determined by the Levenberg-
Marquardt algorithm as implemented a pre-3.6 version of gnuplot.
Encryption a[s] da[s] b[kB/s] db[kB/s]
none 2.37 0.37 386.1 5.8
rc4 1.96 0.27 318.2 2.9
tss 2.33 0.37 298.5 3.5
des 2.07 0.19 218.8 1.0
idea 2.25 0.45 169.6 1.3
3des 1.92 0.11 118.2 0.2
Across a heavily loaded Ethernet, rc4 encryption together with
compression may actually be faster than using rcp.
If you don't encrypt your sessions, you are vulnerable to all the
attacks which are open on the "r" suite of utilities, and you might as
well not use ssh.
4.3. Can I use ssh to communicate across a firewall?
Yes; you can use TCP forwarding for that, by using its secure TCP
forwarding features.
4.4. Can I distribute files with ssh, as with rdist?
Stock rdist 6.1.0 does not work together with ssh, due to bugs in it.
The 6.1.1 versions of rdist and later versions are believed to work.
If you use rdist, don't forget to compile the path to ssh into it.
Alternatively, you may specify the -P option so rdist uses ssh, and
not rsh.
4.5. Can I use ssh to securely connect two subnets across the Inter¡
net?
This has been discussed on the ssh mailing list. A proposed solution
was to run ppp with TCP forwarding; however, this has not been
implemented yet.
Another solution, proposed and executed by anthony.baxter@aaii.oz.au,
is to use tunnel devices.
If you have details of anything that works, please forward them to the
ssh mailing list.
4.6. Can I use ssh to securely forward UDP-based services, such as
NFS or NIS?
There is a general working solution for RPC-based services, such as
NIS. You can download it from ftp://ftp.tu-
chemnitz.de/pub/Local/informatik/sec_rpc/. NIS, in particular, is
working.
In principle, this could also be adapted for NFS; this has not been
done yet.
Services which are based purely on UDP, such as DNS, have not been
secured with ssh yet, although it is possible in principle.
4.7. Can I forward SGI GL connections over ssh?
It is not likely that this will be implemented. GL uses a totally
different protocol from X, and at least gld would have to be replaced.
OpenGL, when run as an X server extension, should pose no problem.
4.8. Can I use ssh to protect services like ftp or POP?
If you want to avoid sending ftp passwords in cleartext over the net,
you can use ssh to encrypt your command channel. Suppose you are on a
host called myhost and want to initiate a ftp connection to ftphost.
On mymachine, you do
mymachine$ ssh -L 1234:ftphost.do.main:21 ftphost
This logs you on to ftphost and also forwards connections to 1234 on
mymachine to ftphost.
Then, in another window, you to
mymachine$ ftp mymachine 1234
220 ftphost FTP server (Foonix 08/15) ready.
Name: (mymachine:yourname):
331 Password required for yourname
Password:
230 User yourname logged in.
For POP, Stephane Bortzmeyer (bortzmeyer@pasteur.fr) has written a
script which protects the mail transfer and passwords ussing ssh. It
requires no modification to existing POP servers or clients, and is
available from ftp://ftp.pasteur.fr/pub/Network/gwpop/.
Other services could be secured by similar means.
5. Problems
If you don't find your problem listed below, please submit a bug
report to ssh-bugs@clinet.fi giving full details of
o Version number of ssh and (if different) sshd
o What you expected ssh to do
o What ssh did instead (including all error messages)
o The system you use (for example, the output of uname -a), and the
output of config.guess.
o The compiler you used, plus any compilation flags
o The output of ssh -v
o The output of the sshd daemon when run in debug mode, as sshd -d
Please try the latest snapshot from
ftp://ftp.cs.hut.fi/pub/ssh/snapshots/ before reporting any bug.
5.1. ssh otherhost xclient & does not work!
No, it doesn't. Use "ssh -f otherhost xclient" instead, or "ssh -n
otherhost xclient &" if you want a script to be compatible with rsh.
5.2. Ssh fails with "Resource temporarily unavailable" for Solaris
2.4
This is a kernel bug in Solaris. Get the patch 101945-34 to fix it.
5.3. X11 forwarding does not work for an SCO binary with the iBCS2
emulator under Linux.
You need to set the hostname to the fully qualified domain name for
this to work. Some Linux distributions set the hostname to the first
part of the FQDN only.
5.4. Ssh is doing wrong things for multi-homed hosts!
Check whether gethostbyname() really returns the complete lists of
possible IP addresses (you might, for example, have your system
configured to search /etc/hosts first, which might contain only one of
the IP addresses).
5.5. Userid swapping is broken under AIX!
This is a bug in AIX 3.2.5, reported as APAR IX38941, and fixed by
patches U435001, U427862, U426915, and a few others. Contact your IBM
representative for details.
5.6. ssh-keygen dumps core on Alpha OSF!
For Alpha OSF/1 1.3.2, this is due to a bug in the vendor-supplied
compiler with maximum optimization.
Turn off all optimization for ssh-keygen, or use gcc.
5.7. ssh-keygen dumps core on Solaris or SunOS
This is a bug in gcc 2.7.0, which causes it to generated incorrect
code without optimization. Supply the "-O" or "-O -g" options to gcc
when compiling. Alternatively, upgrade to gcc 2.7.2.
5.8. On Linux, compilation aborts with some error message about
libc.so.4
This is an incorrectly configured Linux system; do a "cd /usr/lib; ln
-s libc.sa libg.sa" as root to remedy this.
5.9. X authorization sometimes fails.
This is believed to be a bug in HP-UX 9 xauth, SR 5003209619. Patch
PHSS_5568 is believed to fix this problem.
If this occurs for any other platform, please mail details to
ssh-bugs@clinet.fi.
5.10. Ssh on Irix 5 tells me "You don't exist, go away!"
As of 1.2.12, you need to remove -lnsl in the Makefile. A later
version of ssh might fix this.
5.11. Ssh asks me for passwords despite .rhosts!
There are several possibilities why this could be the case; common
ones include
o The client host key is not stored in the known_hosts file. Note
that this has to be the canonical (usually, the fully qualified)
domain name.
o The host does not have a reverse mapping in the name servers. Note
that ssh requires that it has both a reverse mapping, and a forward
mapping that contains the original IP address.
o A multi-homed host does not have all of its IP addresses listed in
the DNS entry. Note that versions prior to 1.2.12 have bugs in
handling multi-homed hosts.
o User's home directory or ~/.rhosts is world or group-writable (see
StrictModes server configuration option).
o On some machines, if the home directory is on an NFS volume,
~/.rhosts and your home directory may need to be world-readable.
o The root account has to use ~/.rhosts or ~/.shosts;
/etc/shosts.equiv and /etc/hosts.equiv are disregarded for root.
o Confusion between RhostsRSAAuthentication and RSAAuthentication.
RhostsRSAAuthentication is a functional replacement for the 'r'
utilities; this requires the ssh program to be setuid root, a
secret key in /etc/host_key file on the client, a corresponding
public key entry in /etc/ssh_known_hosts, plus entries in
~/.[sr]hosts or /etc/[s]hosts.equiv.
RSAAuthentication is done on a per-user basis and requires a
~/.ssh/identity file on the client side (to be generated with ssh-
keygen), plus a matching ~/.ssh/authorized_keys on the server side.
5.12. Why does ssh loop with "Secure connection refused'?
This is a configuration problem.
Ssh attempts to fall back to the "r" commands when it cannot connect
to an ssh daemon on the remote host. It does this by execing your old
rsh to use the old protocol.
There are two possibilities why this could be:
o You probably have installed ssh as rsh, and forgotten to give the
--with-rsh=PATH option to configure the second time. When ssh is
looking for rsh, it keeps executing itself (or an older version of
itself). To solve this, recompile ssh with the correct place for
rsh.
o You moved the old rsh and rlogin into a different directory and
correctly are calling the old rsh. The old rsh has a hard-coded
path to the old rlogin program, so you wind up execing the old rsh
which in turn execs the new replacement (ssh)rlogin.
In that case, you might want to move the old rsh and rlogin
binaries into /usr/old, patch the old rsh binary by running the
Perl script
perl -pi.orig -e 's+/usr/(bin|ucb)/rlogin+/usr/old/rlogin+g ;' /usr/old/rsh
which will generate a patched version of rsh and save the old one in
/usr/old/rsh.orig.
Reconfigure ssh with --with-rsh=/usr/old/rsh.
5.13. rxvt closes all file descriptors when starting up, including
the one used by ssh-agent. Use xterm, or look at the mailing list
archives at http://www.cs.hut.fi/ssh/ssh-archive/ for Timo Rinne's
rxvt patch. ssh-agent does not work with rxvt!
6. Miscellaneous
6.1. How widespread is use of ssh?
As with every piece of freely available software, this is difficult to
find out. The best current estimates are that at least 1000
insitutions in 40 countries use it. This estimate is based on
o The number of people on the ssh mailing list, around 600, from 40
different countries and several hundred domains
o Each week, the ssh home pages are accessed from roughly 5000
different machines, many of them web caches; also, these machines
often are different from week to week.
6.2. Credits
Most of the credit, of course, goes to Tatu Yl÷nen for writing ssh and
making it available to the public. I have also used parts of his text
from the documentation accompanying the ssh source distribution.
Thanks also for his corrections for this FAQ.
Also of invaluable help were corrections and additions from members of
the ssh mailing list and the Usenet newsgroups, by Mark Martinec,
Pedro Melo, Michael Soukas, Adrian Colley, Kenneth J. Hendrickson,
Adam Hammer, Olaf Titz, David Mazieres and Wayne Schroeder.
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