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|
<html lang="en">
<head>
<title>lsh</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<meta name="description" content="lsh">
<meta name="generator" content="makeinfo 4.13">
<link title="Top" rel="top" href="#Top">
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
<meta http-equiv="Content-Style-Type" content="text/css">
<style type="text/css"><!--
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span.roman { font-family:serif; font-weight:normal; }
span.sansserif { font-family:sans-serif; font-weight:normal; }
--></style>
</head>
<body>
<h1 class="settitle">lsh</h1>
<div class="contents">
<h2>Table of Contents</h2>
<ul>
<li><a name="toc_Introduction" href="#Introduction">1 Introduction</a>
<ul>
<li><a href="#Threats">1.1 Threats</a>
<li><a href="#Features">1.2 Features</a>
<li><a href="#Related-techniques">1.3 Related programs and techniques</a>
<ul>
<li><a href="#ssh1">1.3.1 <code>ssh-1.x</code></a>
<li><a href="#ssh2">1.3.2 <code>ssh-2.x</code></a>
<li><a href="#Kerberos">1.3.3 Kerberos</a>
<li><a href="#ipsec">1.3.4 <acronym>IPSEC</acronym></a>
</li></ul>
</li></ul>
<li><a name="toc_Installation" href="#Installation">2 Installation</a>
<li><a name="toc_Getting-started" href="#Getting-started">3 Getting started</a>
<ul>
<li><a href="#lsh_002dmake_002dseed">3.1 Initializing the randomness generator</a>
<li><a href="#lsh-basics">3.2 <samp><span class="command">lsh</span></samp> basics</a>
<li><a href="#tcpip-forwarding">3.3 Port forwarding</a>
<li><a href="#lshd-basics">3.4 <samp><span class="command">lshd</span></samp> basics</a>
<li><a href="#public_002dkey">3.5 Using public-key user authentication</a>
<li><a href="#srp">3.6 Using <acronym>SRP</acronym> authentication</a>
<li><a href="#sexp">3.7 Examining keys and other sexp files</a>
<li><a href="#Converting-keys">3.8 Converting keys from <samp><span class="command">ssh2</span></samp> and OpenSSH</a>
</li></ul>
<li><a name="toc_Invoking-lsh" href="#Invoking-lsh">4 Invoking <samp><span class="command">lsh</span></samp></a>
<ul>
<li><a href="#Algorithm-options">4.1 Algorithm options</a>
<li><a href="#Hostauth-options">4.2 Host authentication options</a>
<li><a href="#Userauth-options">4.3 User authentication options</a>
<li><a href="#Action-options">4.4 Action options</a>
<li><a href="#Verbosity-options">4.5 Verbosity options</a>
</li></ul>
<li><a name="toc_Invoking-lshg" href="#Invoking-lshg">5 Invoking <samp><span class="command">lshg</span></samp></a>
<li><a name="toc_Invoking-lshd" href="#Invoking-lshd">6 Invoking <samp><span class="command">lshd</span></samp></a>
<li><a name="toc_Files-and-environment-variables" href="#Files-and-environment-variables">7 Files and environment variables</a>
<li><a name="toc_Terminology" href="#Terminology">8 Terminology</a>
<li><a name="toc_Concept-Index" href="#Concept-Index">Concept Index</a>
</li></ul>
</div>
<div class="node">
<a name="Top"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Introduction">Introduction</a>,
Previous: <a rel="previous" accesskey="p" href="#dir">(dir)</a>,
Up: <a rel="up" accesskey="u" href="#dir">(dir)</a>
</div>
<!-- node-name, next, previous, up -->
<p>This document describes <samp><span class="command">lsh</span></samp> and related programs. The
<samp><span class="command">lsh</span></samp> suite of programs is intended as a free replacement for
the <samp><span class="command">ssh</span></samp> suite of programs. In turn, <samp><span class="command">ssh</span></samp> was intended
as a secure replacement for the <samp><span class="command">rsh</span></samp> and <samp><span class="command">rlogin</span></samp>
programs for remote login over the Internet.
<p><samp><span class="command">lsh</span></samp> is a component of the <acronym>GNU</acronym> system.
<p>This manual explains how to use and hack <samp><span class="command">lsh</span></samp>; it corresponds to
<samp><span class="command">lsh</span></samp> version 2.0.
<ul class="menu">
<li><a accesskey="1" href="#Introduction">Introduction</a>
<li><a accesskey="2" href="#Installation">Installation</a>
<li><a accesskey="3" href="#Getting-started">Getting started</a>
<li><a accesskey="4" href="#Invoking-lsh">Invoking lsh</a>
<li><a accesskey="5" href="#Invoking-lshg">Invoking lshg</a>
<li><a accesskey="6" href="#Invoking-lshd">Invoking lshd</a>
<li><a accesskey="7" href="#Files-and-environment-variables">Files and environment variables</a>
<li><a accesskey="8" href="#Terminology">Terminology</a>
<li><a accesskey="9" href="#Concept-Index">Concept Index</a>
</li></ul>
<p>--- The Detailed Node Listing ---
<p>Introduction
</p>
<ul class="menu">
<li><a href="#Threats">Threats</a>
<li><a href="#Features">Features</a>
<li><a href="#Related-techniques">Related techniques</a>
</li></ul>
<p>Related programs and techniques
</p>
<ul class="menu">
<li><a href="#ssh1">ssh1</a>: SSH version 1
<li><a href="#ssh2">ssh2</a>: SSH version 2
<li><a href="#Kerberos">Kerberos</a>: Kerberos
<li><a href="#ipsec">ipsec</a>: IP Sec
</li></ul>
<p>Getting started
</p>
<ul class="menu">
<li><a href="#lsh_002dmake_002dseed">lsh-make-seed</a>: Initializing the randomness generator
<li><a href="#lsh-basics">lsh basics</a>: Connection with lsh
<li><a href="#tcpip-forwarding">tcpip forwarding</a>: Forwarding <acronym>TCP/IP</acronym> ports
<li><a href="#lshd-basics">lshd basics</a>: Starting the lshd deamon
<li><a href="#public_002dkey">public-key</a>: Using public-keys
<li><a href="#srp">srp</a>: Using SRP authentication
<li><a href="#sexp">sexp</a>: Examining keys and other S-exp files
<li><a href="#Converting-keys">Converting keys</a>
</li></ul>
<p>Invoking <samp><span class="command">lsh</span></samp>
</p>
<ul class="menu">
<li><a href="#Algorithm-options">Algorithms</a>: Selecting algorithms.
<li><a href="#Hostauth-options">Hostauth options</a>
<li><a href="#Userauth-options">Userauth options</a>
<li><a href="#Action-options">Actions</a>: What to do after login.
<li><a href="#Verbosity-options">Messages</a>: Tuning the amount of messages.
</ul>
<div class="node">
<a name="Introduction"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Installation">Installation</a>,
Previous: <a rel="previous" accesskey="p" href="#Top">Top</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">1 Introduction</h2>
<p>What is this thing called computer security anyway? Why would you want
to use a program like <samp><span class="command">lsh</span></samp>?
<p>This chapter explains the threats <samp><span class="command">lsh</span></samp> tries to protect you from,
and some of the threats that remain. It also describes some of the
technologies used in <samp><span class="command">lsh</span></samp>.
<p>From time to time in this manual, I will speak about the <dfn>enemy</dfn>.
This means anybody who is trying to eavesdrop or disturb your private
communication. This usage is technical, and it does not imply that the
enemy is somehow morally inferior to you: The enemy may be some awful
criminals trying to eavesdrop on you, or it may be the police trying to
eavesdrop on the same criminals.
<p>The enemy can be a criminal, or a competitor, or your boss who's trying
to find out how much you tell collegues at competing firms. It may be
your own or somebody else's national security officials. Or your
ex-boyfriend who happens to be too curious.
<p>So what can the enemy do to your communications and your privacy?
Remember that just because you're paranoid that doesn't mean that nobody
is trying to get you<small class="dots">...</small>
<ul class="menu">
<li><a accesskey="1" href="#Threats">Threats</a>
<li><a accesskey="2" href="#Features">Features</a>
<li><a accesskey="3" href="#Related-techniques">Related techniques</a>
</ul>
<div class="node">
<a name="Threats"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Features">Features</a>,
Previous: <a rel="previous" accesskey="p" href="#Introduction">Introduction</a>,
Up: <a rel="up" accesskey="u" href="#Introduction">Introduction</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">1.1 Threats</h3>
<p>When logging in to some other machine via the Internet, either in the
same building or a few continents away, there are several things that
may be under enemy attack.
<dl>
<dt><dfn>Local attacks</dfn><dd>The enemy controls your local environment. He or she may be looking over
your shoulder. Your local machine might be cracked. Or there may be some
device planted inside your keyboard transmitting everything you type to
the attacker. About the same problems occur if the attacker has taken
control over your target machine, i.e. the remote machine you have
logged in to.
<br><dt><dfn>Denial-of-service attacks</dfn><dd>The enemy has cut your network cable, effectively stopping your
communication. Even without doing physical damage, the enemy may be able
to flood and overload computers or network equipment. Or disrupt network
traffic by sending fake packets to hangup your <acronym>TCP/IP</acronym>
connections.
<br><dt><dfn>Passive eavesdropping</dfn><dd>The enemy may be able to listen to your communication somewhere along
its path. With the global Internet, it's difficult to predict who might
be able to listen. Internet traffic between buildings just a few hundred
meters apart have been observed temporarily being routed through half a
dozen countries, perhaps a few thousand kilometers.
<p>And even without routing anomalies, it is possible that the enemy has
been able to take control of some nearby machine, and can listen in from
there. Of course, passive eavesdropping is most dangerous if you
transmit cleartext passwords. This is the main reason not to use vanilla
telnet to login to remote systems. Use a telnet with support for
<acronym>SSL</acronym> or Kerberos, or use a program like <samp><span class="command">lsh</span></samp> or
<samp><span class="command">ssh</span></samp>.
<p>A passive eavesdropper is assumed not to do anything nasty with your
packets beyond listening to them.
<br><dt><dfn>Name resolution attacks</dfn><dd>The translation from symbolic <acronym>DNS</acronym> names to numeric
ip-addresses may be controlled by the attacker. In this case, you may
think that you are connecting to a friendly machine, when in fact you
are connecting somewhere else.
<br><dt><dfn>Fake packets</dfn><dd>It is fairly easy to fake the source address of an <acronym>IP</acronym>-packet,
although it is more difficult to get hold on the replies to the faked
packets. But even without any replies, this can cause serious
problems.
<br><dt><dfn>Man-in-the-middle attack</dfn><dd>In this attack, the enemy sits between you and the target. When
communicating with you, he pretends to be the target. When communicating
with the target, he pretends to be you. He also passes all information
on more or less unmodified, so that he is invisible to you and the
target. To mount this attack, the enemy either needs physical access to
some network equipment on the path between you and the target, or he has
been able to fool you to connect to him rather than to the target, for
example by manipulating the <acronym>DNS</acronym>-system.
</dl>
<p><samp><span class="command">lsh</span></samp> makes no attempt to protect you from local attacks. You have
to trust the endpoint machines. It seems really difficult to uphold any
security if the local machine is compromised. This is important to keep
in mind in the “visitor”-scenario, where you visit a friend or perhaps an
Internet caf and want to connect to some of the machines at home or at
work. If the enemy has been able to compromize your friend's or the
caf's equipment, you may well be in trouble.
<p>Protection from denial-of-service attacks is also a very difficult
problem, and <samp><span class="command">lsh</span></samp> makes no attempt to protect you from that.
<p>Instead, the aim of <samp><span class="command">lsh</span></samp>, and most serious tools for cryptographic
protection of communications across the net, is to isolate the
vulnerabilities to the communication endpoints. If you know that the
endpoints are safe, the enemy should not be able to compromize your
privacy or communications. Except for denial-of-service attacks (which
at least can't be performed without you noticing it).
<p>First of all, <samp><span class="command">lsh</span></samp> provides protection against passive
eavesdropping. In addition, if you take the appropriate steps to make
sure that hostkeys are properly authenticated, <samp><span class="command">lsh</span></samp> also protects
against man-in-the-middle attacks and in particular against attacks on
the name resolution. In short, you need only trust the security at the
end points: Even if the enemy controls all other network equipment, name
resolution and routing infrastructure, etc, he can't do anything beyond
the denial-of-service attack.
<p>And at last, remember that there is no such thing as absolute security.
You have to estimate the value of that which you are protecting, and
adjust the security measures so that your enemies will not find it worth
the effort to break them.
<div class="node">
<a name="Features"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Related-techniques">Related techniques</a>,
Previous: <a rel="previous" accesskey="p" href="#Threats">Threats</a>,
Up: <a rel="up" accesskey="u" href="#Introduction">Introduction</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">1.2 Features</h3>
<p><samp><span class="command">lsh</span></samp> does not only provide more secure replacements for
<samp><span class="command">telnet</span></samp>, <samp><span class="command">rsh</span></samp> and <samp><span class="command">rlogin</span></samp>, it also provides
some other features to make it convenient to communicate securely. This
section is expected to grow with time, as more features from the
wish-list are added to lsh. One goal for <samp><span class="command">lsh</span></samp> is to make it
reasonable easy to extend it, without messing with the core security
functionality.
<p><samp><span class="command">lsh</span></samp> can also be used in something called gateway mode, in
which you can authenticate once and set up a connection that can
later be used for quickly setting up new sessions with <samp><span class="command">lshg</span></samp>
(see <a href="#Invoking-lshg">Invoking lshg</a>).
<p><samp><span class="command">lsh</span></samp> can be configured to allow login based on a personal
key-pair consisting of a private and a public key, so that you can
execute remote commands without typing your password every time. There
is also experimental support for Thomas Wu's Secure Remote Password
Protocol (<acronym>SRP</acronym>). Kerberos support is on the wish list but not
yet supported (see <a href="#Kerberos">Kerberos</a>).
<p>The public-key authentication methods should also be extended to support
Simple Public Key Infrastructure (<acronym>SPKI</acronym>) certificates,
including some mechanism to delegate restricted logins.
<p>Forwarding of arbitrary <acronym>TCP/IP</acronym> connections is provided. This
is useful for tunneling otherwise insecure protocols, like telnet and
pop, through an encrypted <samp><span class="command">lsh</span></samp> connection.
<p><samp><span class="command">lsh</span></samp> also features a <acronym>SOCKS</acronym>-proxy which also
provides tunneling of <acronym>TCP/IP</acronym> connections, but can be easily
used, e.g. from within popular web browsers like Mozilla and Firefox
for tunneling web traffic. There are also programs like
<samp><span class="command">tsocks</span></samp> that performs transparent redirection of network
access through a <acronym>SOCKS</acronym> proxy.
<p>Convenient tunneling of <acronym>X</acronym> was one of the most impressive
features of the original <samp><span class="command">ssh</span></samp> programs. Both <samp><span class="command">lsh</span></samp> and
<samp><span class="command">lshd</span></samp> support <acronym>X</acronym>-forwarding, although <samp><span class="command">lshg</span></samp>
does not.
<p>Whan <acronym>X</acronym> forwarding is in effect, the remote process is started
in an environment where the <samp><span class="env">DISPLAY</span></samp> variable in the environment
points to a fake <acronym>X</acronym> server, connections to which are forwarded
to the <acronym>X</acronym> server in your local environment. <samp><span class="command">lsh</span></samp> also
creates a new “fake” ‘<samp><span class="samp">MIT-MAGIC-COOKIE-1</span></samp>’ for controlling access
control. Your real <acronym>X</acronym> authentication data is never sent to the
remote machine.
<p>Other kinds of tunneling that may turn out to be useful include
authentication (i.e. <samp><span class="command">ssh-agent</span></samp>), general forwarding of
<acronym>UDP</acronym>, and why not also general <acronym>IP</acronym>-tunneling.
<div class="node">
<a name="Related-techniques"></a>
<p><hr>
Previous: <a rel="previous" accesskey="p" href="#Features">Features</a>,
Up: <a rel="up" accesskey="u" href="#Introduction">Introduction</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">1.3 Related programs and techniques</h3>
<p>This sections describes some other programs and techniques related to
<samp><span class="command">lsh</span></samp>. The ssh family of programs use mostly the same kind of
security as <samp><span class="command">lsh</span></samp>. Kerberos and <acronym>IPSEC</acronym> operate quite
differently, in particular when it comes to protection against
man-in-the-middle attacks.
<ul class="menu">
<li><a accesskey="1" href="#ssh1">ssh1</a>: SSH version 1
<li><a accesskey="2" href="#ssh2">ssh2</a>: SSH version 2
<li><a accesskey="3" href="#Kerberos">Kerberos</a>: Kerberos
<li><a accesskey="4" href="#ipsec">ipsec</a>: IP Sec
</ul>
<div class="node">
<a name="ssh1"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#ssh2">ssh2</a>,
Previous: <a rel="previous" accesskey="p" href="#Related-techniques">Related techniques</a>,
Up: <a rel="up" accesskey="u" href="#Related-techniques">Related techniques</a>
</div>
<!-- node-name, next, previous, up -->
<h4 class="subsection">1.3.1 <code>ssh-1.x</code></h4>
<p>The first of the Secure shell programs was Tatu Ylnen's <samp><span class="command">ssh</span></samp>.
The latest of the version 1 series is <code>ssh-1.33</code> which speaks
version 1.5 of the protocol. The “free” version of <code>ssh-1.33</code>
does not allow commercial use without additional licensing, which makes
<code>ssh-1.33</code> non-free software according to Debian's Free Software
Guidelines and the Open Source Definition.
<p>The version 1 protocol has some subtle weaknesses, in particular, all
support for using stream ciphers was disabled by default a few versions
back, for security reasons.
<p>There also exists free implementations of <code>ssh-1</code>, for both Unix
and Windows. <samp><span class="command">ossh</span></samp> and later OpenSSH are derived from earlier
version av Tatu Ylnen's <samp><span class="command">ssh</span></samp>, and are free software.
<div class="node">
<a name="ssh2"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Kerberos">Kerberos</a>,
Previous: <a rel="previous" accesskey="p" href="#ssh1">ssh1</a>,
Up: <a rel="up" accesskey="u" href="#Related-techniques">Related techniques</a>
</div>
<!-- node-name, next, previous, up -->
<h4 class="subsection">1.3.2 <code>ssh-2.x</code></h4>
<p><samp><span class="command">ssh2</span></samp> implements the next generation of the Secure Shell
protocol, the development of which is supervised by the <acronym>IETF</acronym>
secsh Working Group. Besides <samp><span class="command">lsh</span></samp>, some well known
implementations of this protocol includes
<ul>
<li>OpenSSH (which supports version 2 of the protocol since May 2000).
<li>The <samp><span class="command">ssh2</span></samp> series of proprietary programs sold by the SSH
company. <samp><span class="command">lsh</span></samp> interoperates with current versions of these
programs, but not with version 3.0 and earlier (the older versions get
some details of the protocol wrong, probably because it predates the
protocol specification). The license for the SSH company's
<samp><span class="command">ssh2</span></samp> programs is similar to that for recent versions of
<samp><span class="command">ssh1</span></samp>, but with a narrower definition of “non-commercial
use”.
<li><samp><span class="command">putty</span></samp>, a free <samp><span class="command">ssh</span></samp> implementation for Microsoft
Windows.
</ul>
<p>There a numerous other implementations, both free and proprietary. The
above list is far from complete.
<div class="node">
<a name="Kerberos"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#ipsec">ipsec</a>,
Previous: <a rel="previous" accesskey="p" href="#ssh2">ssh2</a>,
Up: <a rel="up" accesskey="u" href="#Related-techniques">Related techniques</a>
</div>
<!-- node-name, next, previous, up -->
<h4 class="subsection">1.3.3 Kerberos</h4>
<p>Kerberos is a key distribution system originally developed in the late
1980:s as a part of Project Athena at <acronym>MIT</acronym>. Recent development
have been done at The Royal Institute of Technology, Stockholm
(<acronym>KTH</acronym>).
<p>Kerberos uses a central trusted ticket-granting server, and requires
less trust on the local machines in the system. It does not use
public-key technology.
<p>Usually, Kerberos support is compiled into applications such as telnet,
ftp and X-clients. The ssh family of programs, on the other hand, tries
to do all needed magic, for instance to forward <acronym>X</acronym> securely,
and then provides general <acronym>TCP/IP</acronym> forwarding as a kitchen sink.
<p>I believe Kerberos' and lsh's protection against passive eavesdropping
are mostly equivalent. The difference is in the set of machines and
assumptions you have to trust in order to be safe from a
man-in-the-middle attack.
<p>I think the main advantage of <samp><span class="command">lsh</span></samp> over Kerberos is that it is
easier to install and use for on ordinary mortal user. In order to set
up key exchange between two different Kerberos systems (or <dfn>Kerberos
realms</dfn>), the respective system operators need to exchange keys. In the
case of two random users at two random sites, setting up <samp><span class="command">lsh</span></samp> or
some other program in the ssh family is likely easier than to get the
operators to spend time and attention. So <samp><span class="command">lsh</span></samp> should be easier to
use in an anarchistic grass-roots environment.
<p>Another perspective is to combine ssh features like <acronym>X</acronym> and
<acronym>TCP/IP</acronym> forwarding with authentication based on Kerberos. Such
an arrangement may provide the best of two worlds for those who happen
to have an account at a suitable ticket-granting server.
<div class="node">
<a name="ipsec"></a>
<p><hr>
Previous: <a rel="previous" accesskey="p" href="#Kerberos">Kerberos</a>,
Up: <a rel="up" accesskey="u" href="#Related-techniques">Related techniques</a>
</div>
<!-- node-name, next, previous, up -->
<h4 class="subsection">1.3.4 <acronym>IPSEC</acronym></h4>
<p><acronym>IPSEC</acronym> is a set of protocols for protecting general
<acronym>IP</acronym> traffic. It is developed by another <acronym>IETF</acronym> working
group, and is also a required part of <acronym>IP</acronym> version 6.
<p>Again, the main difference between <acronym>IPSEC</acronym>, Kerberos and ssh
is the set of machines that have to be secure and the keys that have to
be exchanged in order to avoid man-in-the-middle attacks.
<p>Current protocols and implementations of <acronym>IPSEC</acronym> only provide
authentication of machines; there's nothing analogous to the user
authentication in ssh or Kerberos.
<p>On the other hand, <acronym>IPSEC</acronym> provides one distinct advantage over
application level encryption. Because <acronym>IP</acronym> and <acronym>TCP</acronym>
headers are authenticated, it provides protection against some
denial-of-service attacks. In particular, it makes attacks that cause
hangup of a <acronym>TCP</acronym> connection considerably more difficult.
<p>So it makes sense to use both <acronym>IPSEC</acronym> and some application
level cryptographic protocol.
<p>Also note that it is possible to use the <dfn>Point-to-Point Protocol</dfn>
(<acronym>PPP</acronym>) to tunnel arbitrary <acronym>IP</acronym> traffic accross an ssh
connection. This arrangement provides some of the functionality of
<acronym>IPSEC</acronym>, and is sometimes referred to as “a poor man's Virtual
Private Network”.
<div class="node">
<a name="Installation"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Getting-started">Getting started</a>,
Previous: <a rel="previous" accesskey="p" href="#Introduction">Introduction</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">2 Installation</h2>
<p>You install <samp><span class="command">lsh</span></samp> with the usual <code>./configure && make &&
make install</code>. For a full listing of the options you can give to
<samp><span class="command">configure</span></samp>, use <code>./configure --help</code>. For example, use
<samp><span class="option">--without-pty</span></samp> to disable pty-support.
<p>The most commonly used option is <samp><span class="option">--prefix</span></samp>, which tells
configure where lsh should be installed. Default prefix is
<samp><span class="file">/usr/local</span></samp>. The <samp><span class="command">lshd</span></samp> server is installed in
<samp><span class="file">$prefix/sbin</span></samp>, all other programs and scripts are installed in
<samp><span class="file">$prefix/bin</span></samp>.
<p>The configure script tries to figure out if the linker needs any special
flags specifying where to find dynamically linked libraries at run time
(one case where this matters is if you have a dynamic libz.so installed
in a non-standard place). Usually, you can use
<pre class="example"> ./configure --with-lib-path=/opt/lib:/other/place
</pre>
<p class="noindent">to specify extra library directories, and the configure script should do
the right thing. If this doesn't work, or you believe that you know your
system better than <samp><span class="command">./configure</span></samp>, just set LDFLAGS and/or
LD_LIBRARY_PATH to the right values instead.
<div class="node">
<a name="Getting-started"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Invoking-lsh">Invoking lsh</a>,
Previous: <a rel="previous" accesskey="p" href="#Installation">Installation</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">3 Getting started</h2>
<p>This section tells you how to perform some common tasks using the
<samp><span class="command">lsh</span></samp> suite of programs, without covering all options and
possibilities.
<ul class="menu">
<li><a accesskey="1" href="#lsh_002dmake_002dseed">lsh-make-seed</a>: Initializing the randomness generator
<li><a accesskey="2" href="#lsh-basics">lsh basics</a>: Connection with lsh
<li><a accesskey="3" href="#tcpip-forwarding">tcpip forwarding</a>: Forwarding <acronym>TCP/IP</acronym> ports
<li><a accesskey="4" href="#lshd-basics">lshd basics</a>: Starting the lshd deamon
<li><a accesskey="5" href="#public_002dkey">public-key</a>: Using public-keys
<li><a accesskey="6" href="#srp">srp</a>: Using SRP authentication
<li><a accesskey="7" href="#sexp">sexp</a>: Examining keys and other S-exp files
<li><a accesskey="8" href="#Converting-keys">Converting keys</a>
</ul>
<div class="node">
<a name="lsh-make-seed"></a>
<a name="lsh_002dmake_002dseed"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#lsh-basics">lsh basics</a>,
Previous: <a rel="previous" accesskey="p" href="#Getting-started">Getting started</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.1 Initializing the randomness generator</h3>
<p>Several of the lsh programs requires a good pseudorandomness generator
for secure operation. The first thing you need to do is to create a
seed file for the generator. To create a personal seed file, stored as
<samp><span class="file">~/.lsh/yarrow-seed-file</span></samp>, run
<pre class="example"> lsh-make-seed
</pre>
<p>To create a seed file for use by <samp><span class="command">lshd</span></samp>, run
<pre class="example"> lsh-make-seed --server
</pre>
<p>as root. The seed file is stored as
<samp><span class="file">/var/spool/lsh/yarrow-seed-file</span></samp>.
<div class="node">
<a name="lsh-basics"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#tcpip-forwarding">tcpip forwarding</a>,
Previous: <a rel="previous" accesskey="p" href="#lsh_002dmake_002dseed">lsh-make-seed</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.2 <samp><span class="command">lsh</span></samp> basics</h3>
<p><samp><span class="command">lsh</span></samp> is the program you use for connection to a remote machine. A
few examples are:
<pre class="example"> lsh sara.lysator.liu.se
</pre>
<p class="noindent">Connects to ‘<samp><span class="samp">sara.lysator.liu.se</span></samp>’ and starts an interactive shell.
In this example, and in the rest of the examples in this section, lsh
will ask for your password, unless you have public-key user
authentication set up.
<p>The first time you try to connect to a new machine, <samp><span class="command">lsh</span></samp>
typically complains about an “unknown host key”. This is because it
has no reason to believe that it was the right machine that answered,
and not a machine controlled by the enemy (see <a href="#Threats">Threats</a>). The default
behaviour is to never ever accept a server that is not properly
authenticated. A machine is considered authentic if it follows the
protocol and has an acl-entry for its public hostkey listed in
<samp><span class="file">~/.lsh/host-acls</span></samp>.
<p>To make lsh less paranoid, use
<pre class="example"> lsh --sloppy-host-authentication sara.lysator.liu.se
</pre>
<p class="noindent">Then <samp><span class="command">lsh</span></samp> will display a <dfn>fingerprint</dfn> of the host key of
the remote machine, and ask you if it is correct. If so, the machine is
considered authentic and a corresponding acl-entry is appended to the
file <samp><span class="file">~/.lsh/captured_keys</span></samp>. You can copy acl-entries you have
verified to <samp><span class="file">~/.lsh/host-acls</span></samp>.
<p>You can even use
<pre class="example"> lsh --sloppy-host-authentication --capture-to ~/.lsh/host-acls
</pre>
<p class="noindent">to get <samp><span class="command">lsh</span></samp> to behave more like the traditional <samp><span class="command">ssh</span></samp>
program.
<!-- You can create fingerprints for the hostkeys you need regularly, and -->
<!-- keep with you (@pxref{sexp}). -->
<pre class="example"> lsh -l omar sara.lysator.liu.se
</pre>
<p class="noindent">Connects, like above, but tries to log in as the user “omar”.
<pre class="example"> lsh sara.lysator.liu.se tar cf - some/dir | (cd /target/dir && tar -xf -)
</pre>
<p>Copies a directory from the remote machine, by executing one remote and
one local <samp><span class="command">tar</span></samp> process and piping them together.
<pre class="example"> CVS_RSH=lsh cvs -d cvs.lysator.liu.se:/cvsroot/lsh co lsh
</pre>
<p class="noindent">Checks out the <samp><span class="command">lsh</span></samp> source code from the <acronym>CVS</acronym>
repository.
<pre class="example"> lsh -G -B sara.lysator.liu.se
</pre>
<p>Opens an ssh connection, creates a “gateway socket”, and forks into
the background.
<pre class="example"> lshg sara.lysator.liu.se
</pre>
<p>creates a new session using an existing gateway socket, without the
overhead for a new key exchange and without asking for any passwords.
<div class="node">
<a name="tcpip-forwarding"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#lshd-basics">lshd basics</a>,
Previous: <a rel="previous" accesskey="p" href="#lsh-basics">lsh basics</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.3 Port forwarding</h3>
<p>One useful feature of <samp><span class="command">lsh</span></samp> and other ssh-like programs is the
ability to forward arbitrary connections inside the encrypted
connection. There are two flavors: “local” and “remote” forwarding.
<p>An example of local forwarding is
<pre class="example"> lsh -L 4000:kom.lysator.liu.se:4894 sara.lysator.liu.se
</pre>
<p class="noindent">This makes <samp><span class="command">lsh</span></samp> listen on port 4000 on the <em>local</em>
machine. When someone connects, <samp><span class="command">lsh</span></samp> asks the server to open a
connection from the <em>remote</em> machine (i.e. ‘<samp><span class="samp">sara</span></samp>’) to port
4894 on another machine (i.e. ‘<samp><span class="samp">kom</span></samp>’). The two connections are piped
together using an encrypted channel.
<p>There are a few things that should be noted here:
<ul>
<li>By default, <samp><span class="command">lsh</span></samp> only listens on the loopback interface, so only
clients on the same machine can use the tunnel. To listen on all
interfaces, use the <samp><span class="option">-g</span></samp> flag.
<li>A connection through the tunnel consists of three parts:
<ol type=1 start=1>
<li>From a client socket to the local port (4000 in this example) that
<samp><span class="command">lsh</span></samp> listens on.
<li>The tunnel itself, from the local machine to the tunnel endpoint,
which is ‘<samp><span class="samp">sara</span></samp>’ in this example.
<li>The connection from the tunnel endpoint to the ultimate target, in this
example from ‘<samp><span class="samp">sara</span></samp>’ to ‘<samp><span class="samp">kom</span></samp>’.
</ol>
<p>Only the middle part is protected by <samp><span class="command">lsh</span></samp>: all data flowing
through the tunnel is sent across the first and last part <em>in the
clear</em>. So forwarding doesn't offer much protection unless the tunnel
endpoint and the ultimate target machine are close to each other. They
should usually be either the same machine, or two machines connected by
a local network that is trusted.
<li>Port forwarding is very useful for traversing firewalls. Of course, you
don't need to use lsh-style forwarding just to get out, there are other
tools like HTTPTunnel for that. But <samp><span class="command">lsh</span></samp> helps you get out
through the firewall in a secure way.
<li>Port forwarding is done in addition to anything else <samp><span class="command">lsh</span></samp> is
doing. In the example above, a tunnel is set up, but <samp><span class="command">lsh</span></samp> will
also start an interactive shell for you. Just as if the <samp><span class="option">-L</span></samp>
option was not present. If this is not what you want, the <samp><span class="option">-N</span></samp> or
<samp><span class="option">-B</span></samp> option is for you (see <a href="#Invoking-lsh">Invoking lsh</a>)
</ul>
<p>Remote forwarding is similar, but asks the <em>remote</em> machine to
listen on a port. An example of remote forwarding is
<pre class="example"> lsh -g -R 8080:localhost:80 sara.lysator.liu.se
</pre>
<p class="noindent">This asks the remote machine to listen on port 8080 (note that you are
probably not authorized to listen on port 80). Whenever someone
connects, the connection is tunnelled to your local machine, and
directed to port 80 on the same machine. Note the use of <samp><span class="option">-g</span></samp>;
the effect is to allow anybody in the world to use the tunnel to connect
to your local webserver.
<p>The same considerations that apply to forwarded local ports apply also to
forwarded remote ports.
<p>At last, you can use any number of <samp><span class="option">-L</span></samp> and <samp><span class="option">-R</span></samp> options
on the same command line.
<div class="node">
<a name="lshd-basics"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#public_002dkey">public-key</a>,
Previous: <a rel="previous" accesskey="p" href="#tcpip-forwarding">tcpip forwarding</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.4 <samp><span class="command">lshd</span></samp> basics</h3>
<p>There are no global configuration files for <samp><span class="command">lshd</span></samp>; all
configuration is done with command line options (see <a href="#Invoking-lshd">Invoking lshd</a>).
<p>To run <samp><span class="command">lshd</span></samp>, you must first create a hostkey, usually stored in
<samp><span class="file">/etc/lsh_host_key</span></samp>. To do this, run
<pre class="example"> lsh-keygen --server | lsh-writekey --server
</pre>
<p class="noindent">This will also create a file <samp><span class="file">/etc/lsh_host_key.pub</span></samp>,
containing the corresponding public key.
<p>A typical command line for starting lshd in daemon mode is simply
<pre class="example"> lshd --daemonic
</pre>
<p>You can find init script for <samp><span class="command">lshd</span></samp> tailored for Debian's and
RedHat's GNU/Linux systems in the <samp><span class="file">contrib</span></samp> directory.
<p>It is also possible to let <samp><span class="command">init</span></samp> start <samp><span class="command">lshd</span></samp>, by
adding it in <samp><span class="file">/etc/inittab</span></samp>.
<div class="node">
<a name="public-key"></a>
<a name="public_002dkey"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#srp">srp</a>,
Previous: <a rel="previous" accesskey="p" href="#lshd-basics">lshd basics</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.5 Using public-key user authentication</h3>
<p>Public-key user authentication is a way to authenticate for login,
without having to type any passwords. There are two steps: Creating a
key pair, and authorizing the public key to the systems where you want
to log in.
<p>To create a keypair, run
<pre class="example"> lsh-keygen | lsh-writekey
</pre>
<p class="noindent">This can take some time, but in the end it creates two files
<samp><span class="file">~/.lsh/identity</span></samp> and <samp><span class="file">~/.lsh/identity.pub</span></samp>.
<p>If you want to use the key to login to some other machine, say
‘<samp><span class="samp">sara</span></samp>’, you can do that by first copying the key,
<pre class="example"> lsh sara.lysator.liu.se '>my-key.pub' < ~/.lsh/identity.pub
</pre>
<p class="noindent">then authorizing it by executing, on ‘<samp><span class="samp">sara</span></samp>’,
<pre class="example"> lsh-authorize my-key.pub
</pre>
<p>By default, <samp><span class="command">lsh-writekey</span></samp> encrypts the private key using a
passphrase. This gives you some protection if a backup tape gets into
the wrong hands, or you use NFS to access the key file in your home
directory. If you want an unencrypted key, pass the flag <samp><span class="option">-c
none</span></samp> to <samp><span class="command">lsh-writekey</span></samp>.
<p>For security reasons, you should keep the private key
<samp><span class="file">~/.lsh/identity</span></samp> secret. This is of course particularly important
if the key is unencrypted; in that case, anybody who can read the file
will be able to login in your name to any machine where the
corresponding public key is registered as an authorized key.
<p>Naturally, you should also make sure not to authorize any keys but your
own. For instance, it is inappropriate to use an insecure mechanism such
as unauthenticated email, <code>ftp</code> or <code>http</code> to transfer your
public key to the machines where you want to authorize it.
<p>If you have accounts on several systems, you usually create one keypair
on each of the systems, and on each system you authorize some or all of
your other public keys for login.
<div class="node">
<a name="srp"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#sexp">sexp</a>,
Previous: <a rel="previous" accesskey="p" href="#public_002dkey">public-key</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.6 Using <acronym>SRP</acronym> authentication</h3>
<p>The Secure Remote Password protocol is a fairly new protocol that
provides mutual authentication based on a password. To use it, you must
first choose a secret password. Next, you create a <dfn>password
verifier</dfn> that is derived from the password. The verifier is stored on
the target machine (i.e. the machine you want to log in to).
<p>To create a verifier, you run the <samp><span class="command">srp-gen</span></samp> program and type
your new password. You have to do it on either the target machine,
redirecting the output to ~/.lsh/srp-verifier, or you can generate it on
some other machine and copy it to the target.
<p>The main advantage of using <acronym>SRP</acronym> is that you use the password
not only to get access to the remote machine, but you also use it to
authenticate the remote machine. I.e. you can use it to connect
securely, <em>without</em> having to know any hostkeys or fingerprints
beforehand!
<p>For instance, you could connect using <acronym>SRP</acronym> to fetch the hostkey
fingerprint for the remote machine, as a kind of bootstrapping
procedure, and then use traditional authentication methods for further
connections.
<p>For this to work, the verifier <em>must</em> be kept <em>secret</em>. If the
enemy gets your verifier, he can mount some attacks:
<ul>
<li>He can mount a <dfn>dictionary attack</dfn> on your password, i.e. generate a
large list of likely password and check if any of them matches yours.
<li>He can impersonate the server. That means that if you try to connect to
the remote machine using <acronym>SRP</acronym>, and the attacker can intercept
your connection (e.g. by attacking the name resolution or routing
system) he can successfully pretend to be the real server.
</ul>
<p>If you use <acronym>SRP</acronym> to get the hostkey or fingerprint for the
remote machine, as outlined above, the impersonation attack destroys
security, you could just as well connect the hostkey presented by the
remote server without verifying it at all.
<p>If you use <acronym>SRP</acronym> exclusively, the situation seems somewhat
different. As far as I can see, an attacker knowing your verifier can
not mount a traditional man-in-the-middle-attack: He can play the
server's part when talking to you, but in order to play your part when
talking to the real server, he needs to know your password as well.
<p><acronym>SRP</acronym> support is disabled by default, but can be enabled by the
<samp><span class="option">--srp-keyexchange</span></samp> option to <samp><span class="command">lshd</span></samp> and <samp><span class="command">lsh</span></samp>
(naturally, it won't be used unless enabled on both sides). At the time
of this writing, <acronym>SRP</acronym> is too new to be trusted by conservative
cryptographers (and remember that conservatism is a virtue when it comes
to security).
<p>And even if <acronym>SRP</acronym> in itself is secure, the way <samp><span class="command">lsh</span></samp>
integrates it into the <code>ssh</code> protocol has not had much peer review.
The bottom line of this disclaimer is that the <acronym>SRP</acronym> support in
<samp><span class="command">lsh</span></samp> should be considered experimental.
<p>As far as I know, using <acronym>SRP</acronym> as a host authentication mechanism
is not supported by any other <code>ssh</code> implementation. The protocol
<samp><span class="command">lsh</span></samp> uses is described in the <samp><span class="file">doc/srp-spec.txt</span></samp>.
Implementations that use <acronym>SRP</acronym> only as a user authentication
mechanism are not compatible with <samp><span class="command">lsh</span></samp>.
<div class="node">
<a name="sexp"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Converting-keys">Converting keys</a>,
Previous: <a rel="previous" accesskey="p" href="#srp">srp</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.7 Examining keys and other sexp files</h3>
<p>Keys and most other objects <samp><span class="command">lsh</span></samp> needs to store on disk are
represented as so called S-expressions or <dfn>sexps</dfn> for short.
S-expressions have their roots in the Lisp world, and a variant of them
in used in the Simple Public Key Infrastructure (<acronym>SPKI</acronym>).
Currently, <samp><span class="command">lsh</span></samp>'s support for <acronym>SPKI</acronym> is quite limited,
but it uses <acronym>SPKI</acronym>'s formats for keys and Access Control Lists
(<acronym>ACL</acronym>:s).
<p>There are several flavours of the sexp syntax:
<ul>
<li>The canonical syntax is somewhere between a text and a binary format,
and is extremely easy for programs to read and write.
<li>The transport syntax, which is suitable when embedding sexps in text
files. It is essentially the canonical representation, encoded using
base64.
<li>The advanced syntax, which is intended for humans to read and write, and
bears some resemblance to Lisp expressions.
</ul>
<p>To see what your <samp><span class="file">~/.lsh/identity.pub</span></samp> file really contains, try
<pre class="example"> sexp-conv < ~/.lsh/identity.pub
</pre>
<p>The <samp><span class="command">sexp-conv</span></samp> program can also be used to computes
fingerprints. The fingerprint of a key (or any sexp, for that matter) is
simply the hash of its canonical representation. For example,
<pre class="example"> sexp-conv --hash </etc/lsh_host_key.pub
</pre>
<p>This flavour of fingerprints is different from the ssh
fingerprint convention, which is based on a hash of the key expressed in
ssh wire format. To produce ssh standard fingerprints, use
‘<samp><span class="samp">lsh-export-key --fingerprint</span></samp>’.
<div class="node">
<a name="Converting-keys"></a>
<p><hr>
Previous: <a rel="previous" accesskey="p" href="#sexp">sexp</a>,
Up: <a rel="up" accesskey="u" href="#Getting-started">Getting started</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">3.8 Converting keys from <samp><span class="command">ssh2</span></samp> and OpenSSH</h3>
<p>If you are already using <samp><span class="command">ssh2</span></samp> or OpenSSH, and have created one
or more personal keypairs, you need to convert the public keys to
<samp><span class="command">lsh</span></samp>'s format before you can authorize them. Use the supplied
<samp><span class="command">ssh-conv</span></samp> script,
<pre class="example"> ssh-conv <openssh-key.pub >new-key.pub
</pre>
<p>You can then use the usual <samp><span class="command">lsh-authorize</span></samp> on the converted
keys. <samp><span class="command">ssh-conv</span></samp> supports both <acronym>DSA</acronym> and <samp><span class="command">RSA</span></samp> keys.
<p>Conversion of keys the other way is also possible, by using the
<samp><span class="command">lsh-export-key</span></samp> program. It reads a public key in
the <acronym>SPKI</acronym> format used by <samp><span class="command">lsh</span></samp> on stdin, and writes the key in
<samp><span class="command">ssh2</span></samp>/OpenSSH format on stdout.
<p>If you want to use your <samp><span class="command">lsh</span></samp> key to log in to another system
running and OpenSSH server, you can do like this:
<pre class="example"> lsh-export-key --openssh < .lsh/identity.pub >sshkey
</pre>
<p>And on the other machine, after having somehow copied the sshkey
file, just add it to the end of your <samp><span class="file">authorized_keys</span></samp> file:
<pre class="example"> cat sshkey >> ~/.ssh/authorized_keys
</pre>
<p><samp><span class="command">lsh-export-key</span></samp> can also be used to check the fingerprint of
keys (just like <samp><span class="command">ssh-keygen</span></samp>).
<pre class="example"> lsh-export-key --fingerprint < /etc/lsh_host_key.pub
</pre>
<p>show the <acronym>MD5</acronym> and Bubble babble
fingerprint of the server public key.
<p>There are currently no tools for converting private keys.
<div class="node">
<a name="Invoking-lsh"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Invoking-lshg">Invoking lshg</a>,
Previous: <a rel="previous" accesskey="p" href="#Getting-started">Getting started</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">4 Invoking <samp><span class="command">lsh</span></samp></h2>
<p><a name="lsh_002dusage"></a>You use <samp><span class="command">lsh</span></samp> to login to a remote machine. Basic usage is
<p>‘<samp><span class="samp">lsh [-p </span><var>port number</var><span class="samp">] sara.lysator.liu.se</span></samp>’
<p>which attempts to connect, login, and start an interactive shell on the
remote machine. Default <var>port number</var> is whatever your system's
<samp><span class="file">/etc/services</span></samp> lists for <samp><span class="command">ssh</span></samp>. Usually, that is port 22.
<p>There is a plethora of options to <samp><span class="command">lsh</span></samp>, to let you configure where
and how to connect, how to authenticate, and what you want to do once
properly logged in to the remote host. Many options have both long and
short forms. This manual does not list all variants; for a full listing
of supported options, use ‘<samp><span class="samp">lsh --help</span></samp>’.
<p>Note that for many of the options to <samp><span class="command">lsh</span></samp>, the ordering of the
options on the command line is important.
<!-- FIXME: Say something about the escape char mechanism here -->
<ul class="menu">
<li><a accesskey="1" href="#Algorithm-options">Algorithms</a>: Selecting algorithms.
<li><a accesskey="2" href="#Hostauth-options">Hostauth options</a>
<li><a accesskey="3" href="#Userauth-options">Userauth options</a>
<li><a accesskey="4" href="#Action-options">Actions</a>: What to do after login.
<li><a accesskey="5" href="#Verbosity-options">Messages</a>: Tuning the amount of messages.
</ul>
<div class="node">
<a name="Algorithm-options"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Hostauth-options">Hostauth options</a>,
Previous: <a rel="previous" accesskey="p" href="#Invoking-lsh">Invoking lsh</a>,
Up: <a rel="up" accesskey="u" href="#Invoking-lsh">Invoking lsh</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">4.1 Algorithm options</h3>
<p>Before a packet is sent, each packet can be compressed, authenticated,
and encrypted, in that order. When the packet is received, it is first
decrypted, next it is checked that it is authenticated properly, and
finally it is decompressed. The algorithms used for this are negotiated
with the peer at the other end of the connection, as a part of the
initial handshake and key exchange.
<p>Each party provides a list of supported algorithms, and the first
algorithm listed by the client, which is also found on the server's
list, is selected. Note that this implies that order in which algorithms
are listed on the server's list doesn't matter: if several algorithms
are present on both the server's and the client's lists, it's the
client's order that determines which algorithm is selected.
<p>Algorithms of different types, e.g. data compression and message
authentication, are negotiated independently. Furthermore, algorithms
used for transmission from the client to the server are independent of
the algorithms used for transmission from the server to the client.
There are therefore no less than six different lists that could be
configured at each end.
<p>The command line options for lsh and lshd don't let you specify
arbitrary lists. For instance, you can't specify different preferences
for sending and receiving.
<p>There is a set of default algorithm preferences. When you use a command
line option to say that you want to use <var>algorithm</var> for one of the
algorithms, the default list is replaced with a list containing the
single element <var>algorithm</var>. For example, if you use <samp><span class="option">-c
arcfour</span></samp> to say that you want to use <code>arcfour</code> as the encryption
algorithm, the connection will either end up using <code>arcfour</code>, or
algorithm negotiation will fail because the peer doesn't support
<code>arcfour</code>.
<p><table summary=""><tr align="left"><td valign="top" width="10%">Option
</td><td valign="top" width="20%">Algorithm type </td><td valign="top" width="20%">Default </td><td valign="top" width="50%">
<br></td></tr><tr align="left"><td valign="top" width="10%"><samp><span class="option">-z</span></samp> </td><td valign="top" width="20%">Data compression
</td><td valign="top" width="20%"><code>none</code>, <code>zlib</code>
<p></td><td valign="top" width="50%">The default preference list supports zlib compression, but
prefers not to use it.
<p><br></td></tr><tr align="left"><td valign="top" width="10%"><samp><span class="option">-c</span></samp> </td><td valign="top" width="20%">Encryption
</td><td valign="top" width="20%"><code>aes256-cbs</code>, <code>3dec-cbc</code>, <code>blowfish-cbc</code>, <code>arcfour</code>
<p></td><td valign="top" width="50%">The default encryption algorithm is aes256. The default list
includes only quite old and well studied algorithms. There is a special
algorithm name <code>all</code> to enable all supported encryption algorithms
(except <code>none</code>).
<p><br></td></tr><tr align="left"><td valign="top" width="10%"><samp><span class="option">-m</span></samp> </td><td valign="top" width="20%">Message Authentication
</td><td valign="top" width="20%"><code>hmac-sha1</code>, <code>hmac-md5</code>
<p></td><td valign="top" width="50%">Both supported message authentication algorithms are of the
<acronym>HMAC</acronym> family.
<br></td></tr></table>
<p>As a special case, <samp><span class="option">-z</span></samp> with no argument changes the compression
algorithm list to <code>zlib</code>, <code>none</code>, which means that you want to
use <code>zlib</code> if the other end supports it. This is different from
<samp><span class="option">-zzlib</span></samp> which causes the negotiation to fail if the other end
doesn't support <code>zlib</code>. A somewhat unobvious consequence of
<samp><span class="option">-z</span></samp> having an <em>optional</em> argument is that if you provide an
argument, it must follow directly after the option letter, no spaces
allowed.
<div class="node">
<a name="Hostauth-options"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Userauth-options">Userauth options</a>,
Previous: <a rel="previous" accesskey="p" href="#Algorithm-options">Algorithm options</a>,
Up: <a rel="up" accesskey="u" href="#Invoking-lsh">Invoking lsh</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">4.2 Host authentication options</h3>
<p>As described earlier (see <a href="#Threats">Threats</a>), proper authentication of the
remote host is crucial to protect the connection against
man-in-the-middle attacks. By default, <samp><span class="command">lsh</span></samp> verifies the server's
claimed host key against the <dfn>Access Control Lists</dfn> in
<samp><span class="file">~/.lsh/host-acls</span></samp>. If the remote host cannot be authenticated,
the connection is dropped.
<p>The options that change this behaviour are
<dl>
<dt><samp><span class="option">--host-db</span></samp><dd>Specifies the location of the <acronym>ACL</acronym> file.
<br><dt><samp><span class="option">--sloppy-host-authentication</span></samp><dd>Tell <samp><span class="command">lsh</span></samp> not to drop the connection if the server's key can not
be authenticated. Instead, it displays the fingerprint of the key, and
asks if it is trusted. The received key is also appended to the file
<samp><span class="file">~/.lsh/captured_keys</span></samp>. If run in quiet mode, ‘<samp><span class="samp">lsh -q
--sloppy-host-authentication</span></samp>’, <samp><span class="command">lsh</span></samp> connects to any host, no
questions asked.
<br><dt><samp><span class="option">--strict-host-authentication</span></samp><dd>Disable sloppy operation (this is the default behaviour).
<br><dt><samp><span class="option">--capture-to</span></samp><dd>Use some other file than <samp><span class="file">~/.lsh/captured_keys</span></samp>. For example,
<pre class="example"> lsh --sloppy-host-authentication --capture-to ~/.lsh/host-acls
</pre>
<p class="noindent">makes <samp><span class="command">lsh</span></samp> behave more like the <samp><span class="command">ssh</span></samp> program.
<br><dt><samp><span class="option">--srp-keyexchange</span></samp><dd>Try using <acronym>SRP</acronym> for keyexchange and mutual authentication.
</dl>
<div class="node">
<a name="Userauth-options"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Action-options">Action options</a>,
Previous: <a rel="previous" accesskey="p" href="#Hostauth-options">Hostauth options</a>,
Up: <a rel="up" accesskey="u" href="#Invoking-lsh">Invoking lsh</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">4.3 User authentication options</h3>
<dl>
<dt><samp><span class="option">-l</span></samp><dd>Provide a name to use when logging in. By default, the value of the
<samp><span class="env">LOGNAME</span></samp> environment variable is used.
<br><dt><samp><span class="option">-i</span></samp><dd>Try the keys from this file to log in. By default, <samp><span class="command">lsh</span></samp> uses
<samp><span class="file">~/.lsh/identity</span></samp>, if it exists. It ought to be possible to use
several <samp><span class="option">-i</span></samp> options to use more than one file, but that is
currently not implemented.
<br><dt><samp><span class="option">--no-publickey</span></samp><dd>Don't attempt to log in using public key authentication.
</dl>
<div class="node">
<a name="Action-options"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Verbosity-options">Verbosity options</a>,
Previous: <a rel="previous" accesskey="p" href="#Userauth-options">Userauth options</a>,
Up: <a rel="up" accesskey="u" href="#Invoking-lsh">Invoking lsh</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">4.4 Action options</h3>
<p>There are many things <samp><span class="command">lsh</span></samp> can do once you are logged in. There
are two types of options that control this: <dfn>actions</dfn> and
<dfn>action modifiers</dfn>. For short options, actions use uppercase letters
and modifiers use lowercase.
<p>For each modifier <samp><span class="option">--foo</span></samp> there's also a negated form
<samp><span class="option">--no-foo</span></samp>. Options can also be negated by preceding it with the
special option <samp><span class="option">-n</span></samp>. This is mainly useful for negating short
options. For instance, use <samp><span class="option">-nt</span></samp> to tell <samp><span class="command">lsh</span></samp> not to
request a remote pseudo terminal. Each modifier and its negation can be
used several times on the command line. For each action, the latest
previous modifier of each pair apply.
<p>First, the actions:
<dl>
<dt><samp><span class="option">-L</span></samp><dd>Requests forwarding of a local port. This option takes a mandatory
argument of the form
<var>listen-port</var>:<var>target-host</var>:<var>target-port</var>. This option tells
<samp><span class="command">lsh</span></samp> to listen on <var>listen-port</var> on the local machine. When
someone conects to that port, <samp><span class="command">lsh</span></samp> asks the remote server to open
a connection to <var>target-port</var> on <var>target-host</var>, and if it
succeeds, the two connections are joined together through an the
<samp><span class="command">lsh</span></samp> connection. Both port numbers should be given in decimal.
<br><dt><samp><span class="option">-R</span></samp><dd>Requests forwarding of a remote port. It takes one mandatory argument,
just like <samp><span class="option">-L</span></samp>. But in this case <samp><span class="command">lsh</span></samp> asks the
<em>remote</em> server to listen on <var>listen-port</var>. When someone
connects to the remote hosts, the server will inform the local
<samp><span class="command">lsh</span></samp>. The local <samp><span class="command">lsh</span></samp> then connects to <var>target-port</var> on
<var>target-host</var>.
<br><dt><samp><span class="option">-D</span></samp><dd>Requests SOCKS-style forwarding. It takes one optional argument, the
port number to use for the SOCKS proxy (default is 1080). Other
applications can then use socks version 4 or version 5, to open
outgoing connections which are forwarded via the SSH connection. Note
that for short options the port number must be in the same argument if given
(i.e. ‘<samp><span class="samp">-D1080</span></samp>’ is correct, ‘<samp><span class="samp">-D 1080</span></samp>’ is not).
<br><dt><samp><span class="option">-E</span></samp><dd>This option takes one mandatory argument, which is a command line to be
executed on the remote machine.
<br><dt><samp><span class="option">-S</span></samp><dd>Start an interactive shell on the remote machine.
<br><dt><samp><span class="option">-G</span></samp><dd>Open a gateway on the local machine. A gateway is a local socket,
located under /tmp, that can be used for controlling and using the ssh
connection. It is protected using the ordinary file permissions.
<br><dt><samp><span class="option">-N</span></samp><dd>This is a no-operation action. It inhibits the default action, which is
to start an interactive shell on the remote machine. It is useful if you
want to set up a few forwarded tunnels or a gateway, and nothing more.
<br><dt><samp><span class="option">-B</span></samp><dd>Put the client into the background after key exchange and
user authentication. Implies <samp><span class="option">-N</span></samp>
<br><dt><samp><span class="option">--subsystem</span></samp><dd>Specifies a subsystem to connect to, implies <samp><span class="option">--no-pty</span></samp>. Example usage:
‘<samp><span class="samp">--subsystem=sftp</span></samp>’
</dl>
<p>If there are trailing arguments after the name of the remote system,
this is equivalent to a <samp><span class="option">-E</span></samp> option, with a command string
constructed by catenating all the remaining arguments, separated by
spaces. This implies that the arguments are usually expanded first by
the local shell, and then the resulting command string is interpreted
again by the remote system.
<p>If there are no trailing arguments after the name of the remote system,
and the <samp><span class="option">-N</span></samp> option is not given, the default action is to start
a shell on the remote machine. I.e. this is equivalent to the
<samp><span class="option">-S</span></samp> option.
<p>There are a few supported modifiers:
<dl>
<dt><samp><span class="option">-t</span></samp><dd>Request a pseudo terminal. <samp><span class="command">lsh</span></samp> asks the remote system to allocate
a pseudo terminal. If it succeeds, the local terminal is set to raw
mode. The default behaviour is to request a pty if and only if the
local <samp><span class="command">lsh</span></samp> process has a controlling terminal. This modifier
applies to actions that create remote processes, i.e. <samp><span class="option">-E</span></samp> and
<samp><span class="option">-S</span></samp>, as well as the default actions.
<p>Currently, this option is ignored if there is no local terminal.
<br><dt><samp><span class="option">-x</span></samp><dd>Request <acronym>X</acronym> forwarding. Applies to the <samp><span class="option">-E</span></samp> and
<samp><span class="option">-S</span></samp> and the default actions.
<br><dt><samp><span class="option">--stdin</span></samp><dd>Redirect the stdin of a remote process from a given, local, file.
Default is to use <samp><span class="command">lsh</span></samp>'s stdin for the first process, and
<samp><span class="file">/dev/null</span></samp> for the rest. This option applies to the <samp><span class="option">-E</span></samp>
and <samp><span class="option">-S</span></samp> options as well as to the default actions. The option
applies to only one process; as soon as it is used it is reset to the
default.
<br><dt><samp><span class="option">--stdout</span></samp><dd>Redirect the stdout of a remote process to a given, local, file. Default
is to use <samp><span class="command">lsh</span></samp>'s stdout. Like <samp><span class="option">--stdin</span></samp>, it is reset
after it is used.
<br><dt><samp><span class="option">--stderr</span></samp><dd>Redirect the stdout of a remote process to a given, local, file.
Analogous to the <samp><span class="option">--stdout</span></samp> option.
<br><dt><samp><span class="option">--detach</span></samp><dd>Detach from terminal at session end.
<br><dt><samp><span class="option">--write-pid</span></samp><dd>Applies to <samp><span class="option">-E</span></samp>. Write PID of backgrounded process to stdout.
<br><dt><samp><span class="option">-e</span></samp><dd>Set the escape character (use “none”) to disable. Default is “~”
if a tty is allocated and “none” otherwise.
<br><dt><samp><span class="option">-g</span></samp><dd>Remote peers, aka global forwarding. This option applies to the
forwarding actions, i.e. <samp><span class="option">-L</span></samp>, <samp><span class="option">-R</span></samp> and <samp><span class="option">-D</span></samp>. By
default, only connections to the loopback interface, ip 127.0.0.1, are
forwarded. This implies that only processes on the same machine can use
the forwarded tunnel directly. If the -g modifier is in effect, the
forwarding party will listen on <em>all</em> network interfaces.
</dl>
<div class="node">
<a name="Verbosity-options"></a>
<p><hr>
Previous: <a rel="previous" accesskey="p" href="#Action-options">Action options</a>,
Up: <a rel="up" accesskey="u" href="#Invoking-lsh">Invoking lsh</a>
</div>
<!-- node-name, next, previous, up -->
<h3 class="section">4.5 Verbosity options</h3>
<p>These options determines what messages <samp><span class="command">lsh</span></samp> writes on
its stderr.
<dl>
<dt><samp><span class="option">-q</span></samp><dd>Quiet mode. Disables all messages and all questions, except password
prompts and fatal internal errors.
<br><dt><samp><span class="option">-v</span></samp><dd>Verbose mode. Makes <samp><span class="command">lsh</span></samp> a little more verbose. The intention is
to provide information that is useful for ordinary trouble shooting,
and makes sense also to those not familiar with <samp><span class="command">lsh</span></samp> internals.
<br><dt><samp><span class="option">--trace</span></samp><dd>Trace mode. Prints some internal information to aid tracking
<samp><span class="command">lsh</span></samp>'s flow of control.
<br><dt><samp><span class="option">--debug.</span></samp><dd>Debug mode. Dumps <em>a lot</em> of information, including dumps of all
sent and received packets. It tries to avoid dumping highly sensitive data,
such as private keys and the contents of <code>SSH_MSG_USERAUTH_REQUEST</code>
messages, but you should still use it with care.
<br><dt><samp><span class="option">--log-file</span></samp><dd>This option redirects all messages to a file. Takes one mandatory
argument: The name of that file.
</dl>
<p>Note that all these options are orthogonal. If you use <samp><span class="option">--trace</span></samp>,
you usually want to add <samp><span class="option">-v</span></samp> as well; <samp><span class="option">--trace</span></samp> does not
do that automatically.
<div class="node">
<a name="Invoking-lshg"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Invoking-lshd">Invoking lshd</a>,
Previous: <a rel="previous" accesskey="p" href="#Invoking-lsh">Invoking lsh</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">5 Invoking <samp><span class="command">lshg</span></samp></h2>
<p><a name="lshg_002dusage"></a>You use <samp><span class="command">lshg</span></samp> to login to a remote machine to which you have
previously used <samp><span class="command">lsh</span></samp> to set up a gateway (see <a href="#Action-options">Action options</a>). Its usage is very similar to that of <samp><span class="command">lsh</span></samp>
(see <a href="#Invoking-lsh">Invoking lsh</a>), except that some options are not available.
<p>Basic usage is
<p>‘<samp><span class="samp">lshg [-l </span><var>username</var><span class="samp">] </span><var>host</var></samp>’
<p>which attempts to connect to the gateway that should previously have
been established by running ‘<samp><span class="samp">lsh [-l </span><var>username</var><span class="samp">] -G </span><var>host</var></samp>’)
<p>The <var>username</var> and <var>host</var> are used to locate the gateway. The
default value for <var>username</var> is determined in the same way as for
<samp><span class="command">lsh</span></samp> (see <a href="#Invoking-lsh">Invoking lsh</a>).
<p>As <samp><span class="command">lshg</span></samp> uses almost the same options as <samp><span class="command">lsh</span></samp>
(see <a href="#Invoking-lsh">Invoking lsh</a>), only options that are not available or have
a different meaning in <samp><span class="command">lshg</span></samp> are listed here.
<p>The algorithm options (see <a href="#Algorithm-options">Algorithm options</a>) as well as most of
the userauth (see <a href="#Userauth-options">Userauth options</a>) and hostauth (see <a href="#Hostauth-options">Hostauth options</a>) are not available in <samp><span class="command">lshg</span></samp> as they are only used by
session setup, which is already handled by <samp><span class="command">lsh</span></samp>.
<p>Due to technical reasons, <acronym>X11</acronym>-forwarding cannot be performed by
<samp><span class="command">lshg</span></samp>, thus the <samp><span class="option">--x11-forward</span></samp> option (see <a href="#Action-options">Action options</a>) is not
available.
<p>To summarize, these are the options that are new, not available or
that have different meanings:
<dl>
<dt><samp><span class="option">-G</span></samp><dd>For <samp><span class="command">lsh</span></samp> <samp><span class="option">-G</span></samp> requests a gateway to be set up. For
<samp><span class="command">lshg</span></samp> it means that if no usable gateway is found
<samp><span class="command">lsh</span></samp> should be launched with the same arguments instead.
<br><dt><samp><span class="option">--send-debug</span></samp><dd>Not available in <samp><span class="command">lsh</span></samp>. Sends a ‘<samp><span class="samp">debug</span></samp>’ message to the remote machine.
<br><dt><samp><span class="option">--send-ignore</span></samp><dd>Not available in <samp><span class="command">lsh</span></samp>. Sends a ‘<samp><span class="samp">ignore</span></samp>’ message to the remote machine.
<br><dt><samp><span class="option">-x</span></samp><dd>(<samp><span class="option">--x11-forward</span></samp>) Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">-c</span></samp><dd>(<samp><span class="option">--crypto</span></samp>) Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">-z</span></samp><dd>(<samp><span class="option">--compression</span></samp>) Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">-m</span></samp><dd>(<samp><span class="option">--mac</span></samp>) Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--hostkey-algorithm</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--capture-to</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--strict-host-authentication</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--sloppy-host-authentication</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--host-db</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--publickey</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--no-publickey</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--dh-keyexchange</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--no-dh-keyexchange</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--srp-keyexchange</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">--no-srp-keyexchange</span></samp><dd>Not available in <samp><span class="command">lshg</span></samp>.
<br><dt><samp><span class="option">-i</span></samp><dd><samp><span class="option">--identity</span></samp> Not available in <samp><span class="command">lshg</span></samp>.
</dl>
<div class="node">
<a name="Invoking-lshd"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Files-and-environment-variables">Files and environment variables</a>,
Previous: <a rel="previous" accesskey="p" href="#Invoking-lshg">Invoking lshg</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">6 Invoking <samp><span class="command">lshd</span></samp></h2>
<p><a name="lshd_002dusage"></a><samp><span class="command">lshd</span></samp> is a server that accepts connections from clients
speaking the Secure Shell Protocol. It is usually started automatically
when the systems boots, and runs with root privileges. However, it is
also possible to start <samp><span class="command">lshd</span></samp> manually, and with user
privileges.
<p>There are currently no configuration files. Instead, command line options
are used to tell <samp><span class="command">lshd</span></samp> what to do. Many options have <samp><span class="option">--foo</span></samp>
and <samp><span class="option">--no-foo</span></samp> variants. Options specifying the default behaviour
are not listed here.
<p>Some of the options are the shared with <samp><span class="command">lsh</span></samp>. In particular, see
<a href="#Algorithm-options">Algorithm options</a> and <a href="#Verbosity-options">Verbosity options</a>.
<p>Options specific to the <samp><span class="command">lshd</span></samp> server are:
<dl>
<dt><samp><span class="option">-p</span></samp><dd>Port to listen to. The mandatory argument is a decimal port number or a
service name. Default is "ssh", usually port 22.
<p>It should also be possible to use several -p options as a convenient way
to make lshd listen on several ports on each specified (or default)
interface, but that is not yet implemented.
<p>Note that if you use both <samp><span class="option">-p</span></samp> and <samp><span class="option">--interface</span></samp>, the
order matters.
<br><dt><samp><span class="option">--interface</span></samp><dd>Network interface to listen on. By default, <samp><span class="command">lshd</span></samp> listens on all
interfaces. An interface can be specified as a DNS name, a literal IPv4
address, or a literal IPv6 address enclosed in square brackets. It can
optionally be followed by a colon and a port number or service name. If
no port number or service is specified, the default or the value from a
<em>preceding</em> <samp><span class="option">-p</span></samp> is used.
<p>Some examples: <code>--interface=localhost</code>,
<code>--interface=1.2.3.4:443</code>, <code>--interface=[aaaa::bbbb]:4711</code>. To
make <samp><span class="command">lshd</span></samp> listen on several ports and interfaces at the same
time, just use several <samp><span class="option">--interface</span></samp> options on the command line.
<br><dt><samp><span class="option">-h</span></samp><dd>Location of the server's private key file. By default,
<samp><span class="file">/etc/lsh_host_key</span></samp>.
<br><dt><samp><span class="option">--daemonic</span></samp><dd>Enables daemonic mode. <samp><span class="command">lshd</span></samp> forks into the background,
redirects its stdio file descriptors to <samp><span class="file">/dev/null</span></samp>, changes its
working directory to <samp><span class="file">/</span></samp>, and redirects any diagnostic or debugging
messages via syslog.
<p><samp><span class="command">lshd</span></samp> should be able to deal with the environment it inherits
if it is started by <samp><span class="command">init</span></samp> or <samp><span class="command">inetd</span></samp>, but this is not
really tested.
<br><dt><samp><span class="option">--pid-file</span></samp><dd>Creates a locked pid file, to make it easier to write start and stop
scripts for <samp><span class="command">lshd</span></samp>. The mandatory argument provides the filename.
This option is enabled by default when operating in daemonic mode, and
the default filename is <samp><span class="file">/var/run/lshd.pid</span></samp>.
<br><dt><samp><span class="option">--no-syslog</span></samp><dd>Disable the use of the syslog facility. Makes sense only together with
<samp><span class="option">--daemonic</span></samp>
<br><dt><samp><span class="option">--enable-core</span></samp><dd>By default, <samp><span class="command">lshd</span></samp> disables core dumps, to avoid leaking sensitive
information. This option changes that behaviour, and allows <samp><span class="command">lshd</span></samp>
to dump core on fatal errors.
<br><dt><samp><span class="option">--no-password</span></samp><dd>Disable the "password" user authentication mechanism.
<br><dt><samp><span class="option">--no-publickey</span></samp><dd>Disable the "publickey" user authentication mechanism.
<br><dt><samp><span class="option">--root-login</span></samp><dd>Enable root login. By default, root can not log in using <samp><span class="command">lshd</span></samp>.
<br><dt><samp><span class="option">--login-auth-mode</span></samp><dd>This option is highly experimental. Bypass <samp><span class="option">lshd</span></samp>'s user
authentication, and allow users to spawn their login-shell without any
authentication. Usually combined with <samp><span class="option">--login-shell</span></samp>, to set the
login shell to a program that performce password authentication.
<br><dt><samp><span class="option">--kerberos-passwords</span></samp><dd>Verify passwords against the kerberos database. This is implemented
using the <samp><span class="command">lsh-krb-checkpw</span></samp> helper program. Note that this does
<em>not</em> use the Kerberos infrastructure in the Right Way. Experimental.
<br><dt><samp><span class="option">--password-helper</span></samp><dd>Tells <samp><span class="command">lshd</span></samp> to use a helper program for verifying passwords.
This is a generalization of <samp><span class="option">--kerberos-passwords</span></samp>, and it could
be used for verifying passwords against any password database. See the
source files <samp><span class="file">lsh-krb-checkpw.c</span></samp> and <samp><span class="file">unix_user.c</span></samp> for
details.
<br><dt><samp><span class="option">--login-shell</span></samp><dd>Use the specified program as the login shell for all users, overriding
the login shell in the passwd database.
<br><dt><samp><span class="option">--srp-keyexchange</span></samp><dd>Enable <acronym>SRP</acronym> keyexchange and user authentication.
<br><dt><samp><span class="option">--no-pty-support</span></samp><dd>Disable support for pseudo terminals.
<br><dt><samp><span class="option">--no-tcp-forward</span></samp><dd>Disable support for tcp forwarding, in both directions.
<br><dt><samp><span class="option">--subsystems</span></samp><dd>Specifies a list of subsystems and corresponding
programs.
Example usage: ‘<samp><span class="samp">--subsystems=sftp=/usr/sbin/sftp-server,foosystem=/usr/bin/foo</span></samp>’
</dl>
<div class="node">
<a name="Files-and-environment-variables"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Terminology">Terminology</a>,
Previous: <a rel="previous" accesskey="p" href="#Invoking-lshd">Invoking lshd</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">7 Files and environment variables</h2>
<p>This chapters describes all files and all environment variables that
are used by <samp><span class="command">lsh</span></samp>, <samp><span class="command">lshd</span></samp>, and related programs.
<p>There are a few environment variables that modifies the behaviour of
the <samp><span class="command">lsh</span></samp> programs. And there are also a handful of variables
that are setup by <samp><span class="command">lshd</span></samp> when starting user processes.
<dl>
<dt><samp><span class="env">DISPLAY</span></samp><dd>When <acronym>X</acronym>-forwarding is enabled, <samp><span class="env">DISPLAY</span></samp> specifies the
local display. Used by <samp><span class="command">lsh</span></samp>.
<br><dt><samp><span class="env">HOME</span></samp><dd>User's home directory. Determines where client programs looks for the
<samp><span class="file">~/.lsh</span></samp> directory. When <samp><span class="command">lshd</span></samp> starts a user program, it
sets <samp><span class="env">HOME</span></samp> from the value in the <samp><span class="file">/etc/passwd</span></samp> file, except
if <samp><span class="command">lshd</span></samp> is running as an ordinary user process. In the
latter case, the new process inherits <samp><span class="command">lsh</span></samp>'s own value of
<samp><span class="env">HOME</span></samp>.
<br><dt><samp><span class="env">LOGNAME</span></samp><dd>The user's log in name. Used as the default name for logging into
remote systems. Set by <samp><span class="command">lshd</span></samp> when starting new processes.
<br><dt><samp><span class="env">LSH_YARROW_SEED_FILE</span></samp><dd>If set, it points out the location of the seed-file for the randomness
generator. Recognized both by <samp><span class="command">lshd</span></samp> and the client programs.
<br><dt><samp><span class="env">LSHFLAGS</span></samp><dd>If set, lsh will parse any options as had they been given on the
command line.
<br><dt><samp><span class="env">LSHGFLAGS</span></samp><dd>If set, lshg will parse any options as had they been given on the
command line.
<br><dt><samp><span class="env">POSIXLY_CORRECT</span></samp><dd>Affects the command line parsing of programs which by default accept
options mixed with arguments.
<br><dt><samp><span class="env">SEXP_CONV</span></samp><dd>The location of the <samp><span class="command">sexp-conv</span></samp> program. If not set, the default
<samp><span class="file">$prefix/bin/sexp-conv</span></samp> is used.
<br><dt><samp><span class="env">SSH_CLIENT</span></samp><dd>This variable may be set by lshd for established sessions. If it is set
it consists of three parts separated by whitespace, the first part
contains the address of the connecting client. The second part
contains the tcp port used on the connecting client and the third part
contains the tcp port used on the server.
<br><dt><samp><span class="env">SSH_TTY</span></samp><dd>This variable may be set by lshd for established sessions. If it is set
it is the name of the tty allocated.
<br><dt><samp><span class="env">SHELL</span></samp><dd>User's login shell. When <samp><span class="command">lshd</span></samp> starts a user process, it sets
<samp><span class="env">SHELL</span></samp> to the value in <samp><span class="file">/etc/passwd</span></samp>, unless overridden by
the <samp><span class="option">--login-shell</span></samp> command line option.
<br><dt><samp><span class="env">TERM</span></samp><dd>The type of the local terminal. If the client requests a pty for a
remote process, the value of <samp><span class="env">TERM</span></samp> is transferred from client to
server.
<br><dt><samp><span class="env">TMPDIR</span></samp><dd>Determines where the unix socket used by <samp><span class="command">lshg</span></samp> is located in
the filesystem.
<br><dt><samp><span class="env">TZ</span></samp><dd>Time zone. Processes started by <samp><span class="command">lshd</span></samp> inherit the value of
this variable from the server process.
<!-- used by xlib @item XAUTHORITY -->
</dl>
<p>Files used by the lsh client, stored in the <samp><span class="file">~/lsh</span></samp> directory:
<dl>
<dt><samp><span class="file">captured_keys</span></samp><dd>Keys for remote hosts, saved when running ‘<samp><span class="samp">lsh
--sloppy-host-authentication</span></samp>’. Or more precicely, each key is stored
together with an as SPKI (Simple Public Key Intrastructure) ACL:s
(Access Control Lists).
<br><dt><samp><span class="file">identity</span></samp><dd>Your private key file. Usually created by ‘<samp><span class="samp">lsh-keygen |
lsh-writekey</span></samp>’. Read by <samp><span class="command">lsh</span></samp>. Should be kept secret.
<br><dt><samp><span class="file">identity.pub</span></samp><dd>The corresponding public key. You can copy this file to other systems
in order to authorize the private key to login (see <a href="#Converting-keys">Converting keys</a>).
<br><dt><samp><span class="file">host-acls</span></samp><dd>Host keys (or more precisely, ACL:s) that lsh considers authentic.
Entries have the same format as in <samp><span class="file">captured_keys</span></samp>.
<br><dt><samp><span class="file">yarrow-seed-file</span></samp><dd>The seed file for the randomness generator. Should be kept secret.
</dl>
<p>Files used by <samp><span class="command">lshd</span></samp>, some of which are read from user home
directories:
<dl>
<dt><samp><span class="file">/etc/lsh_host_key</span></samp><dd>The server's private host key.
<br><dt><samp><span class="file">/etc/lsh_host_key.pub</span></samp><dd>The corresponding public key.
<br><dt><samp><span class="file">/var/spool/lsh/yarrow-seed-file</span></samp><dd>The seed-file for <samp><span class="command">lshd</span></samp>'s randomness generator.
<br><dt><samp><span class="file">~/.lsh/authorized_keys</span></samp><dd>This is a directory that keeps a “database” of keys authorized for
login. With the current implementation, a key is authorized for login
if and only if this directory contains a file with a name which is the
SHA1 hash of the key. The usual way to create files is by running the
script <samp><span class="command">lsh-authorize</span></samp>.
<br><dt><samp><span class="file">~/.lsh/srp-verifier</span></samp><dd>If you use the experimental support for <acronym>SRP</acronym> (see <a href="#srp">srp</a>),
the server reads a user's <acronym>SRP</acronym> verifier from this file.
</dl>
<div class="node">
<a name="Terminology"></a>
<p><hr>
Next: <a rel="next" accesskey="n" href="#Concept-Index">Concept Index</a>,
Previous: <a rel="previous" accesskey="p" href="#Files-and-environment-variables">Files and environment variables</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="chapter">8 Terminology</h2>
<div class="node">
<a name="Concept-Index"></a>
<p><hr>
Previous: <a rel="previous" accesskey="p" href="#Terminology">Terminology</a>,
Up: <a rel="up" accesskey="u" href="#Top">Top</a>
</div>
<!-- node-name, next, previous, up -->
<h2 class="unnumbered">Concept Index</h2>
<ul class="index-cp" compact>
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