FYI. Please regard this as an announcement; there won't be any repostings of that particular FAQ on bugtraq (at least not by me :-) Newsgroups: comp.security.unix,comp.security.misc Subject: SSH (Secure Shell) FAQ - Frequently Asked Questions 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 Koenig, Thomas.Koenig@ciw.uni-karlsruhe.de $Id: ssh-faq.sgml,v 1.16 1995/12/07 10:54:21 ig25 Exp $ This document is a list of Frequently Asked Questions (plus hopefully correct answers) about the Secure Shell, ssh. 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. What systems does ssh run on? 3.3. May I legally run 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? 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 OpenGL connections over ssh? 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 fails for HP-UX 9.05 6. Miscellaneous 6.1. 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. 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. 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. The 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 1.2.12. 3.2. What systems does ssh run on? Ssh currently runs on UNIX or related system. Ports have been successful to all "mainstream" systems. At present, there are no known working versions for other operating systems (but see below). 3.3. 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/. 3.4. 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 =3D 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 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-1.2.12.tar.gz | tar xvf - then change into the directory ssh-1.2.12, read the file INSTALL, and follow the directions. 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 the message 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. 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. 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. At an actual measurement, this dropped sustainable transfer speed between a P90 and a 486/100 (not the fastest CPUs around) from 386 kb/s (for no encryption) to 318 kb/s. 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. You can use the Linux version of rdist (which should compile on any system for which rdist also works), available from ftp://sunsite.unc.edu/pub/Linux/system/Network/file-transfer/ as rdist-6.1.0-linuxpl2.tar.gz. 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. 4.6. Can I use ssh to securely forward UDP-based services, such as NFS or NIS? Forwarding UDP packets has been proposed, but has not been implemented. There are two problems with this: o Some UDP-based programs use the IP address of the incoming packet and the port it was sent from as a form of authorization. Forwarding such packets from local ports would tend to confuse these (badly written :-) programs. o UDP-based programs usually use a retransmit strategy if they do not receive an answer for a predetermined time. This leads to ineffiency if packets are forwarded across a reliable connection, such as TCP. Somebody would have to implement lossy UDP forwarding to avoid this. 4.7. Can I forward SGI OpenGL connections over ssh? It is not likely that this will be implemented. OpenGL uses a totally different protocol from X, and at least gld would have to be replaced. 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 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 101945-32. 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 fails for HP-UX 9.05 This one is known, but a fix is not available yet. If you can supply any additional data, please send it to ssh-bugs@clinet.fi. The symptoms, as known so far, are: When the target machine is running HP-UX 9.05, it is most likely that X authorization fails if the xauth list produces some lines of output like "this_host:1 this_host:2 this_host:4", with gaps in the lettering. X authorization keeps failing until a local display number is higher than the highest already present number. Removing all xauth data does not seem to help. 6. Miscellaneous 6.1. Credits Most of the credit, of course, goes to Tatu Ylonen 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, by Mark Martinec, Pedro Melo, Michael Soukas, Adrian Colley, and Kenneth J. Hendrickson. -----BEGIN PGP SIGNATURE----- Version: 2.6.2i iQCVAwUBMMbl5/Bu+cbJcKCVAQHGegQAmwrCuVd44aVYiQZE9/R5NEMOwOZmPVsJ KVHMsF49tPcA70zl1+KkZji00LtSFQTi9Lw5ts8xMqEmrWtkA9YAVFM7i6FqKApr yzvFUljNvH7yJFa152f0TXO78fA/yd5EFrNSjDY6gsmf6Nitg488p5fZGyH+X/3U wbzx2fswdoc=3D =3DszVP -----END PGP SIGNATURE----- -- Thomas K=F6nig, Thomas.Koenig@ciw.uni-karlsruhe.de, ig25@dkauni2.bitnet. The joy of engineering is to find a straight line on a double logarithmic diagram.