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Ssh (Secure Shell) FAQ - Frequently asked questions
by Thomas Koenig Thomas.Koenig@ciw.uni-karlsruhe.de
$Date: 1997/06/06 17:19:04 $
This document is a list of Frequently Asked Questions (plus hopefully
correct answers) about the Secure Shell, ssh.
______________________________________________________________________
Table of Contents:
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. Does it make sense to install ssh as non-root under UNIX?
3.7. Where do I get help?
3.8. Are there any versions for other operating systems than UNIX?
3.9. 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 use rdist with ssh?
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?
4.9. Can I use ssh across a Socks firewall?
4.10. Is there ssh support for AFS/Kerberos?
5. Problems
5.1. ssh otherhost xclient & does not work!
5.2. Ssh fails with "Resource temporarily unavailable" for Solaris
5.3. Sshd hangs under Solaris 2.5!
5.4. X11 forwarding does not work for an SCO binary with the iBCS2
emulator under Linux.
5.5. Ssh is doing wrong things for multi-homed hosts!
5.6. Userid swapping is broken under AIX!
5.7. ssh-keygen dumps core on Alpha OSF!
5.8. ssh-keygen dumps core on Solaris or SunOS
5.9. On Linux, compilation aborts with some error message about
libc.so.4
5.10. X authorization sometimes fails.
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!
5.14. X authorization always fails.
5.15. ssh hangs when forwarding multiple TCP connections.
5.16. What does Warning: remote host denied X11 forwarding mean?
5.17. I still see cleartext packages on the net when I run ssh!
5.18. I have problems with RSAREF, something to do with too many
bits!
5.19. Compiling fails with some error messages from the assembler.
5.20. Compiling with Solaris 2.5 fails!
5.21. Ssh suddenly drops connections!
5.22. Connections are forwarded as root by ssh!
6. Miscellaneous
6.1. What known security bugs exist in which versions of ssh?
6.2. How widespread is use of ssh?
6.3. I don't like the commercial aspects of ssh.
6.4. 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, sci.crypt, comp.answers, sci.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. You can download a gzipped PostScript version from
http://www.uni-karlsruhe.de/~ig25/ssh-faq/ssh-faq.ps.gz.
If your link to Germany is slow, you might get better connectivity at
http://aleph1.mit.edu/ssh-faq/.
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 unsecure 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 from ftp://ftp.ietf.org/internet-drafts/draft-ietf-tls-
ssh-00.txt.
All communications are encrypted using IDEA or one of several other
ciphers (three-key triple-DES, DES, RC4-128, TSS, Blowfish).
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 when RSA
host authentication is used. 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 is 1.2.20.
Ssh currently runs on UNIX or related systems, plus under OS/2. Ports
have been successful to all "mainstream" UNIX systems. There are two
versions for MS-Windows. There is a free beta version by Cedomir
Igaly, which can be obtained from http://public.srce.hr/~cigaly/ssh
or, preferably, from a mirror at
ftp://hotline.pvt.net/pub/win_utils/winsock/ssh/.
There's also a commercial version by Tatu Yloenen, the original author
of ssh. There's also a beta version for the Mac, available from
Datafellows.
3.2. May I legally run ssh?
The UNIX version of ssh 1.2.20 may be used and distributed freely, but
must not be sold commercially as a separate product, as part of a
bigger product or project, or otherwise used for financial gain
without a separate license.
Earlier versions of ssh had a less restrictive license; see the file
COPYING in the accompanying source distributions.
Tatu Yloenen's MS-Windows version of ssh is a commercial product,
which requires licensing.
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
RSAREF 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 Yloenen, the original author of ssh, has started a company, SSH
Communications Security Oy, that will provide commercial support and
licenses for ssh. This company is working together with Data Fellows,
who are the sole contact for licensing ssh. More information can be
found at http://www.europe.datafellows.com/ and http://www.ssh.fi/.
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-1.2.20.tar.gz | tar xvf -
then change into the directory ssh-1.2.20, read the file INSTALL, and
follow the directions in it.
3.6. Does it make sense to install ssh as non-root under UNIX?
You can install and run a ssh binary, which you can use to log into
another system on which sshd is running.
If you want to log in to the remote system without typing in your
password, you'll have to generate a private key in your home directory
using ssh-keygen, then put your public key into
$HOME/.ssh/authorized_keys.
You can also start up sshd yourself as non-root, supplying the -p
option so it binds to a non-privileged port (>1024), and then connect
from another system with ssh -p. This will only allow connections to
your own account, and sshd will, as a rule, not be restarted when your
machine reboots.
You will have to decide wether this is useful for you or not.
3.7. Where do I get help?
First of all, read the documentation, this document :-) and the ssh
home page, at http://www.cs.hut.fi/ssh/.
For users, there is an introduction at
http://www.tac.nyc.ny.us/~kim/ssh/.
If these resources don't help, you can post to the Usenet newsgroup
comp.security.ssh or send mail to the gatewayed mailing list for ssh
users at ssh@clinet.fi. To subscribe, send mail to
majordomo@clinet.fi with
subscribe ssh
in the body of the message.
Before subscribing, you might like to take a look at the archives of
the mailing list, at http://www.cs.hut.fi/ssh/ssh-archive.
3.8. Are there any versions for other operating systems than UNIX?
Heikki Suonsivu (hsu@clinet.fi) and Michael Henits (moi@dio.com) each
offered a US$ 100 reward for the first stable, freely redistributable
version for either MS-Windows or MacOS.
There was a preliminary version for MS-Windows by Cedomir Igaly.
Unfortunately, it does not appear to be available any more. You can
try out archie; look for the filename ssh-1-2-.zip.
The commercial version by Tatu Yloenen, the original author of ssh, is
available from http://www.europe.datafellows.com/f-secure/fssh-
reg.htm.
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/.
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.
3.9. What about administration of ssh?
The central problem of administering ssh is the management of host
keys. To allow a client to connect to a remote host with RSA host
authentication, the server needs to know the client's public key.
You can collect these 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 Koenig 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 blowfish encryption instead of the default,
IDEA, with -c blowfish, 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 use rdist with ssh?
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.
If you use password authentication with rdist 6.1.2 or 6.1.3, you will
need to apply the following patch to rdist to make it work:
--- src/rshrcmd.c.orig Tue Jun 11 16:51:21 1996
+++ src/rshrcmd.c Tue Jun 11 16:52:05 1996
@@ -63,7 +63,7 @@
/* child. we use sp[1] to be stdin/stdout, and close
sp[0]. */
(void) close(sp[0]);
- if (dup2(sp[1], 0) < 0 || dup2(0,1) < 0 || dup2(0, 2) < 0) {
+ if (dup2(sp[1], 0) < 0 || dup2(0,1) < 0) {
error("dup2 failed: %s.", SYSERR);
_exit(255);
}
<p>
This also applies if you get a "Warning: Denied agent forwarding
because the other end has too old version." error (which occurs if
your client is 1.2.17 or later, and it connects to an older server).
Another alternative would be to use rsync, a rdist replacement, which
was designed to work with ssh, and makes better use of bandwidth. More
information can be found at ftp://samba.anu.edu.au/pub/rsync or
ftp://sunsite.auc.dk/pub/unix/rsync.
4.5. Can I use ssh to securely connect two subnets across the Inter-
net?
You can run PPP over a regular ssh connection. See
http://www.inka.de/~bigred/sw/ssh-ppp-new.txt for a working solution.
It's a good idea to enable compression for this.
However, this may cause problems for forwarding TCP connections,
because both the TCP connection over which ssh runs and a TCP
connection forwarded over the PPP/ssh tunnel may retransmit at the
same time. In this case, it is better to use encrypted IP tunneling
via UDP. A possible implementation of this is
http://www.inka.de/~bigred/devel/cipe.html.
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.
You may need to set the environment variable GLFORCEDIRECT=no.
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. This will still
leave your data channel open to all attacks on TCP, and will not work
through a firewall.
Suppose you are on a host called myhost and want to initiate a ftp
connection to ftphost. On myhost, you do
myhost$ ssh -L 1234:ftphost.do.main:21 ftphost
This logs you on to ftphost and also forwards connections to 1234 on
myhost to ftphost.
Then, in another window, you do
myhost$ ftp mymachine 1234
220 ftphost FTP server (Foonix 08/15) ready.
Name: (myhost:yourname):
331 Password required for yourname
Password:
230 User yourname logged in.
This works if the remote ftp daemon accepts PORT commands which
specify a different host from the one the command channel appears to
come from, and if the ftp client always uses PORT. This is true for
vanilla UNIX ftp client and ftpd servers; it may not work for more
advanced ftpds, such as wu-ftpd.
For servers which do not accept this, you can see wether you ftp
client supports passive mode, and wether the ftp server accepts PASV.
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. Note, however, that
unencrypted ftp data connections are still vulnerable to session
hijacking and snooping.
4.9. Can I use ssh across a Socks firewall?
Socks 5 support should work in 1.2.16 or later.
4.10. Is there ssh support for AFS/Kerberos?
At the moment, not in the main sources. There's an AFS patch
available from http://www-personal.umich.edu/~dugsong/ssh-afs-
kerberos.html which should make it into the contrib directory shortly.
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 For a compilation problem, the contents of the file config.log
(generated by configure)
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
For Solaris 2.4, this s a kernel bug. Get the patch 101945-37 to fix
it. Please note that at least one earlier version, 101945-36, seems
to have reintroduced the bug.
If you experience the same problem with Solaris 2.5.1, upgrade to ssh
1.2.14 or later, which should have solved the problem.
5.3. Sshd hangs under Solaris 2.5!
This is a problem with the Solaris shared library code, which causes a
hang with some name server functions.
Get Patch 103187-02 (for x86, 103188-02) to fix this. This problem
may or may not be fixed in Solaris 2.5.1.
5.4. 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.
5.5. 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).
5.6. 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.
5.7. 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. Gcc 2.7.2 is
known to have problems on the Alpha, however.
5.8. 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.
5.9. 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.
5.10. 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.
5.11. 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 client 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 client or 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. ssh-agent does not work with rxvt!
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.
5.14. X authorization always fails.
This can happen if the xauth program was not found at configure time.
Correct the path, reconfigure and recompile.
5.15. ssh hangs when forwarding multiple TCP connections.
This is due to a known race condition in the ssh protocol before
1.2.13.
Some changes have been made to the protocol in 1.2.14 to prevent this.
Unfortunately, these changes may also cause hangs when using TCP
forwarding between 1.2.14 and earlier versions. In these cases,
upgrade to 1.2.14 or later at both ends is recommended.
5.16. What does Warning: remote host denied X11 forwarding mean?
Either the remote end has disabled X11 forwarding (ForwardX11 No in
the config file), or either the xauth command or the X11 libraries
were not found when compiling the server.
5.17. I still see cleartext packages on the net when I run ssh!
It is very likely that you are looking at a telnet, rlogin or X
session to the machine that you run ssh on. Check that those packets
really are ssh packets (for example by checking their port number;
sshd listens on port 22).
5.18. I have problems with RSAREF, something to do with too many
bits!
This is a limitation in the RSAREF library. You should set a host key
with at most 896 bits.
5.19. Compiling fails with some error messages from the assembler.
For several operating systems there were bugs in the gmp assembler
routines. Try
make distclean
configure --disable-asm
to compile.
5.20. Compiling with Solaris 2.5 fails!
Set the CPP environment variable to "cc -E -Xs" before running
configure.
5.21. Ssh suddenly drops connections!
This is a problem which has been reported by several people for SunOS
4, Solaris 2, Linux, and HP-UX 9 and 10, with 1.2.16 and 1.2.17. It
happens with scp, when transferring large amounts of data via ssh's
stdin, or when forwarding an X connection which receives a large
amount of graphics data (such as a MPEG movie).
Try to apply the following patch to 1.2.16 or 1.2.17 for a fix. This
is in 1.2.18 or later.
--- serverloop.c.orig Tue Jan 21 14:38:25 1997
+++ serverloop.c. Tue Jan 21 14:37:54 1997
@@ -405,7 +405,7 @@
buffer_len(&stdin_buffer));
if (len <= 0)
{
- if (errno != EWOULDBLOCK)
+ if ((errno != EWOULDBLOCK) && (errno != EAGAIN))
{
if (fdin == fdout)
shutdown(fdin, 1); /* We will no longer send. */
5.22. Connections are forwarded as root by ssh!
When a client connects, sshd forks a child that does the protocol
handling, and this child forks a second child for the user shell or
command. The problem is that the setuid() call to the correct user
appears only in the second child, so the first child keeps running as
root.
Among other potential problems this means that connections redirected
with -Lx:host:port will be made from the root uid to host:port, since
the first child does them. This means that when the target host does
an ident query, it gets back only "root" and no indication of the
actual user.
This has been reported as a bug; it is not known wether this will be
fixed in a future release.
6. Miscellaneous
6.1. What known security bugs exist in which versions of ssh?
All versions of ssh prior to 1.2.12.92 had a security flaw which
allowed local users to get access to the secret host key. This is
fixed in 1.2.13 and later.
If you run ssh 1.2.13 on Alpha OSF 1.3 or SCO in C2 security mode,
local users can gain root access. This is fixed by applying
ftp://ftp.cs.hut.fi/pub/ssh/ssh-osf1-c2-setluid.patch or by upgrading
to 1.2.14 or later.
Versions of ssh prior to 1.2.17 had problems with authentication agent
handling on some machines. There is a chance (a race condition) that
a malicious user could steal another user's credentials. This should
be fixed in 1.2.17.
The arcfour cipher is used in a way which makes it susceptible in
version 1 of the ssh protocol. Therefore, its use has been disabled
in 1.2.18 and later.
6.2. 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.
6.3. I don't like the commercial aspects of ssh.
The protocols ssh uses are freely available. There are no
restrictions if anybody wants to write a version that is available
under different conditions and is interoperable with existing ssh
installations.
Ssh is also on the Internet Standards Track. This means that a
second, independent implementation is required.
You will have to be aware of patent (RSA, IDEA) and export control
issues before writing a second implementation.
6.4. Credits
Most of the credit, of course, goes to Tatu Yloenen 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, Axel Boldt and Wayne
Schroeder.
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--
Thomas Koenig, Thomas.Koenig@ciw.uni-karlsruhe.de, ig25@dkauni2.bitnet.
The joy of engineering is to find a straight line on a double
logarithmic diagram.
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