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SSH (Secure Shell) FAQ - Frequently Asked Questions


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Archive-name: computer-security/ssh-faq
Url: http://www.uni-karlsruhe.de/~ig25/ssh-faq/
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-----BEGIN PGP SIGNED MESSAGE-----

  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/
  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/

     Poland:
	ftp://ftp.agh.edu.pl/pub/security/ssh

     Portugal:
	ftp://ftp.ci.uminho.pt/pub/security/

     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/ 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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Last Update March 27 2014 @ 02:11 PM