This is /home/nisse/hack/lsh/doc/lsh.info, produced by makeinfo version
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INFO-DIR-SECTION GNU Packages
START-INFO-DIR-ENTRY
* LSH: (lsh).           Secure Shell and related utilities.
END-INFO-DIR-ENTRY

   Draft manual for LSH. This manual corresponds to `lsh' version
2.0.

   Copyright 2000, 2004 Niels Möller

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided also
that the sections entitled "Copying" and "GNU General Public License"
are included exactly as in the original, and provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Free Software Foundation.


File: lsh.info,  Node: Top,  Next: Introduction,  Prev: (dir),  Up: (dir)



   This document describes `lsh' and related programs. The `lsh' suite
of programs is intended as a free replacement for the `ssh' suite of
programs. In turn, `ssh' was intended as a secure replacement for the
`rsh' and `rlogin' programs for remote login over the Internet.

   `lsh' is a component of the GNU system.

   This manual explains how to use and hack `lsh'; it corresponds to
`lsh' version 2.0.

* Menu:

* Introduction::
* Installation::
* Getting started::
* Invoking lsh::
* Invoking lshg::
* Invoking lshd::
* Files and environment variables::
* Terminology::
* Concept Index::

 --- The Detailed Node Listing ---

Introduction

* Threats::
* Features::
* Related techniques::

Related programs and techniques

* ssh1::                        SSH version 1
* ssh2::                        SSH version 2
* Kerberos::                    Kerberos
* ipsec::                       IP Sec

Getting started

* lsh-make-seed::               Initializing the randomness generator
* lsh basics::                  Connection with lsh
* tcpip forwarding::            Forwarding TCP/IP ports
* lshd basics::                 Starting the lshd deamon
* public-key::                  Using public-keys
* srp::                         Using SRP authentication
* sexp::                        Examining keys and other S-exp files
* Converting keys::

Invoking `lsh'

* Algorithms: Algorithm options.  Selecting algorithms.
* Hostauth options::
* Userauth options::
* Actions: Action options.      What to do after login.
* Messages: Verbosity options.  Tuning the amount of messages.


File: lsh.info,  Node: Introduction,  Next: Installation,  Prev: Top,  Up: Top

Introduction
************

What is this thing called computer security anyway? Why would you want
to use a program like `lsh'?

   This chapter explains the threats `lsh' tries to protect you from,
and some of the threats that remain. It also describes some of the
technologies used in `lsh'.

   From time to time in this manual, I will speak about the "enemy".
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.

   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.

   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...

* Menu:

* Threats::
* Features::
* Related techniques::


File: lsh.info,  Node: Threats,  Next: Features,  Prev: Introduction,  Up: Introduction

Threats
=======

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.

"Local attacks"
     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.

"Denial-of-service attacks"
     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
     TCP/IP connections.

"Passive eavesdropping"
     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.

     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 SSL or Kerberos, or use a program like
     `lsh' or `ssh'.

     A passive eavesdropper is assumed not to do anything nasty with
     your packets beyond listening to them.

"Name resolution attacks"
     The translation from symbolic DNS 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.

"Fake packets"
     It is fairly easy to fake the source address of an IP-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.

"Man-in-the-middle attack"
     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 DNS-system.


   `lsh' 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 café 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
café's equipment, you may well be in trouble.

   Protection from denial-of-service attacks is also a very difficult
problem, and `lsh' makes no attempt to protect you from that.

   Instead, the aim of `lsh', 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).

   First of all, `lsh' provides protection against passive
eavesdropping. In addition, if you take the appropriate steps to make
sure that hostkeys are properly authenticated, `lsh' 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.

   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.


File: lsh.info,  Node: Features,  Next: Related techniques,  Prev: Threats,  Up: Introduction

Features
========

`lsh' does not only provide more secure replacements for `telnet',
`rsh' and `rlogin', 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 `lsh' is to make it reasonable easy to extend it, without
messing with the core security functionality.

   `lsh' 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 `lshg' (*note Invoking
lshg::).

   `lsh' 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
(SRP). Kerberos support is on the wish list but not yet supported
(*note Kerberos::).

   The public-key authentication methods should also be extended to
support Simple Public Key Infrastructure (SPKI) certificates, including
some mechanism to delegate restricted logins.

   Forwarding of arbitrary TCP/IP connections is provided. This is
useful for tunneling otherwise insecure protocols, like telnet and pop,
through an encrypted `lsh' connection.

   `lsh' also features a SOCKS-proxy which also provides tunneling of
TCP/IP 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 `tsocks' that performs transparent redirection
of network access through a SOCKS proxy.

   Convenient tunneling of X was one of the most impressive features of
the original `ssh' programs. Both `lsh' and `lshd' support
X-forwarding, although `lshg' does not.

   Whan X forwarding is in effect, the remote process is started in an
environment where the `DISPLAY' variable in the environment points to a
fake X server, connections to which are forwarded to the X server in
your local environment. `lsh' also creates a new "fake"
`MIT-MAGIC-COOKIE-1' for controlling access control. Your real X
authentication data is never sent to the remote machine.

   Other kinds of tunneling that may turn out to be useful include
authentication (i.e. `ssh-agent'), general forwarding of UDP, and why
not also general IP-tunneling.


File: lsh.info,  Node: Related techniques,  Prev: Features,  Up: Introduction

Related programs and techniques
===============================

This sections describes some other programs and techniques related to
`lsh'. The ssh family of programs use mostly the same kind of security
as `lsh'. Kerberos and IPSEC operate quite differently, in particular
when it comes to protection against man-in-the-middle attacks.

* Menu:

* ssh1::                        SSH version 1
* ssh2::                        SSH version 2
* Kerberos::                    Kerberos
* ipsec::                       IP Sec


File: lsh.info,  Node: ssh1,  Next: ssh2,  Prev: Related techniques,  Up: Related techniques

`ssh-1.x'
---------

The first of the Secure shell programs was Tatu Ylönen's `ssh'.  The
latest of the version 1 series is `ssh-1.33' which speaks version 1.5
of the protocol. The "free" version of `ssh-1.33' does not allow
commercial use without additional licensing, which makes `ssh-1.33'
non-free software according to Debian's Free Software Guidelines and
the Open Source Definition.

   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.

   There also exists free implementations of `ssh-1', for both Unix and
Windows. `ossh' and later OpenSSH are derived from earlier version av
Tatu Ylönen's `ssh', and are free software.


File: lsh.info,  Node: ssh2,  Next: Kerberos,  Prev: ssh1,  Up: Related techniques

`ssh-2.x'
---------

`ssh2' implements the next generation of the Secure Shell protocol, the
development of which is supervised by the IETF secsh Working Group.
Besides `lsh', some well known implementations of this protocol includes
   * OpenSSH (which supports version 2 of the protocol since May 2000).

   * The `ssh2' series of proprietary programs sold by the SSH company.
     `lsh' 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 `ssh2'
     programs is similar to that for recent versions of `ssh1', but
     with a narrower definition of "non-commercial use".

   * `putty', a free `ssh' implementation for Microsoft Windows.


   There a numerous other implementations, both free and proprietary.
The above list is far from complete.


File: lsh.info,  Node: Kerberos,  Next: ipsec,  Prev: ssh2,  Up: Related techniques

Kerberos
--------

Kerberos is a key distribution system originally developed in the late
1980:s as a part of Project Athena at MIT. Recent development have been
done at The Royal Institute of Technology, Stockholm (KTH).

   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.

   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 X securely,
and then provides general TCP/IP forwarding as a kitchen sink.

   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.

   I think the main advantage of `lsh' 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 "Kerberos
realms"), the respective system operators need to exchange keys. In the
case of two random users at two random sites, setting up `lsh' or some
other program in the ssh family is likely easier than to get the
operators to spend time and attention. So `lsh' should be easier to use
in an anarchistic grass-roots environment.

   Another perspective is to combine ssh features like X and TCP/IP
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.


File: lsh.info,  Node: ipsec,  Prev: Kerberos,  Up: Related techniques

IPSEC
-----

IPSEC is a set of protocols for protecting general IP traffic. It is
developed by another IETF working group, and is also a required part of
IP version 6.

   Again, the main difference between IPSEC, 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.

   Current protocols and implementations of IPSEC only provide
authentication of machines; there's nothing analogous to the user
authentication in ssh or Kerberos.

   On the other hand, IPSEC provides one distinct advantage over
application level encryption. Because IP and TCP headers are
authenticated, it provides protection against some denial-of-service
attacks. In particular, it makes attacks that cause hangup of a TCP
connection considerably more difficult.

   So it makes sense to use both IPSEC and some application level
cryptographic protocol.

   Also note that it is possible to use the "Point-to-Point Protocol"
(PPP) to tunnel arbitrary IP traffic accross an ssh connection. This
arrangement provides some of the functionality of IPSEC, and is
sometimes referred to as "a poor man's Virtual Private Network".


File: lsh.info,  Node: Installation,  Next: Getting started,  Prev: Introduction,  Up: Top

Installation
************

You install `lsh' with the usual `./configure && make && make install'.
For a full listing of the options you can give to `configure', use
`./configure --help'. For example, use `--without-pty' to disable
pty-support.

   The most commonly used option is `--prefix', which tells configure
where lsh should be installed. Default prefix is `/usr/local'. The
`lshd' server is installed in `$prefix/sbin', all other programs and
scripts are installed in `$prefix/bin'.

   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

     ./configure --with-lib-path=/opt/lib:/other/place

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 `./configure', just set LDFLAGS and/or
LD_LIBRARY_PATH to the right values instead.


File: lsh.info,  Node: Getting started,  Next: Invoking lsh,  Prev: Installation,  Up: Top

Getting started
***************

This section tells you how to perform some common tasks using the `lsh'
suite of programs, without covering all options and possibilities.

* Menu:

* lsh-make-seed::               Initializing the randomness generator
* lsh basics::                  Connection with lsh
* tcpip forwarding::            Forwarding TCP/IP ports
* lshd basics::                 Starting the lshd deamon
* public-key::                  Using public-keys
* srp::                         Using SRP authentication
* sexp::                        Examining keys and other S-exp files
* Converting keys::


File: lsh.info,  Node: lsh-make-seed,  Next: lsh basics,  Prev: Getting started,  Up: Getting started

Initializing the randomness generator
=====================================

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
`~/.lsh/yarrow-seed-file', run

     lsh-make-seed

   To create a seed file for use by `lshd', run

     lsh-make-seed --server

   as root. The seed file is stored as
`/var/spool/lsh/yarrow-seed-file'.


File: lsh.info,  Node: lsh basics,  Next: tcpip forwarding,  Prev: lsh-make-seed,  Up: Getting started

`lsh' basics
============

`lsh' is the program you use for connection to a remote machine. A few
examples are:

     lsh sara.lysator.liu.se

Connects to `sara.lysator.liu.se' 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.

   The first time you try to connect to a new machine, `lsh' 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 (*note Threats::). 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
`~/.lsh/host-acls'.

   To make lsh less paranoid, use

     lsh --sloppy-host-authentication sara.lysator.liu.se

Then `lsh' will display a "fingerprint" 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
`~/.lsh/captured_keys'. You can copy acl-entries you have verified to
`~/.lsh/host-acls'.

   You can even use

     lsh --sloppy-host-authentication --capture-to ~/.lsh/host-acls

to get `lsh' to behave more like the traditional `ssh' program.

     lsh -l omar sara.lysator.liu.se

Connects, like above, but tries to log in as the user "omar".

     lsh sara.lysator.liu.se tar cf - some/dir | (cd /target/dir && tar -xf -)

   Copies a directory from the remote machine, by executing one remote
and one local `tar' process and piping them together.

     CVS_RSH=lsh cvs -d cvs.lysator.liu.se:/cvsroot/lsh co lsh

Checks out the `lsh' source code from the CVS repository.

     lsh -G -B sara.lysator.liu.se

   Opens an ssh connection, creates a "gateway socket", and forks into
the background.

     lshg sara.lysator.liu.se

   creates a new session using an existing gateway socket, without the
overhead for a new key exchange and without asking for any passwords.


File: lsh.info,  Node: tcpip forwarding,  Next: lshd basics,  Prev: lsh basics,  Up: Getting started

Port forwarding
===============

One useful feature of `lsh' and other ssh-like programs is the ability
to forward arbitrary connections inside the encrypted connection. There
are two flavors: "local" and "remote" forwarding.

   An example of local forwarding is

     lsh -L 4000:kom.lysator.liu.se:4894 sara.lysator.liu.se

This makes `lsh' listen on port 4000 on the _local_ machine. When
someone connects, `lsh' asks the server to open a connection from the
_remote_ machine (i.e. `sara') to port 4894 on another machine (i.e.
`kom'). The two connections are piped together using an encrypted
channel.

   There are a few things that should be noted here:

   * By default, `lsh' only listens on the loopback interface, so only
     clients on the same machine can use the tunnel. To listen on all
     interfaces, use the `-g' flag.

   * A connection through the tunnel consists of three parts:

       1. From a client socket to the local port (4000 in this example)
          that `lsh' listens on.

       2. The tunnel itself, from the local machine to the tunnel
          endpoint, which is `sara' in this example.

       3. The connection from the tunnel endpoint to the ultimate
          target, in this example from `sara' to `kom'.


     Only the middle part is protected by `lsh': all data flowing
     through the tunnel is sent across the first and last part _in the
     clear_. 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.

   * 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 `lsh'
     helps you get out through the firewall in a secure way.

   * Port forwarding is done in addition to anything else `lsh' is
     doing. In the example above, a tunnel is set up, but `lsh' will
     also start an interactive shell for you. Just as if the `-L'
     option was not present. If this is not what you want, the `-N' or
     `-B' option is for you (*note Invoking lsh::)

   Remote forwarding is similar, but asks the _remote_ machine to
listen on a port. An example of remote forwarding is

     lsh -g -R 8080:localhost:80 sara.lysator.liu.se

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 `-g'; the
effect is to allow anybody in the world to use the tunnel to connect to
your local webserver.

   The same considerations that apply to forwarded local ports apply
also to forwarded remote ports.

   At last, you can use any number of `-L' and `-R' options on the same
command line.


File: lsh.info,  Node: lshd basics,  Next: public-key,  Prev: tcpip forwarding,  Up: Getting started

`lshd' basics
=============

There are no global configuration files for `lshd'; all configuration
is done with command line options (*note Invoking lshd::).

   To run `lshd', you must first create a hostkey, usually stored in
`/etc/lsh_host_key'. To do this, run

     lsh-keygen --server | lsh-writekey --server

This will also create a file `/etc/lsh_host_key.pub', containing the
corresponding public key.

   A typical command line for starting lshd in daemon mode is simply

     lshd --daemonic

   You can find init script for `lshd' tailored for Debian's and
RedHat's GNU/Linux systems in the `contrib' directory.

   It is also possible to let `init' start `lshd', by adding it in
`/etc/inittab'.


File: lsh.info,  Node: public-key,  Next: srp,  Prev: lshd basics,  Up: Getting started

Using public-key user authentication
====================================

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.

   To create a keypair, run

     lsh-keygen | lsh-writekey

This can take some time, but in the end it creates two files
`~/.lsh/identity' and `~/.lsh/identity.pub'.

   If you want to use the key to login to some other machine, say
`sara', you can do that by first copying the key,

     lsh sara.lysator.liu.se '>my-key.pub' < ~/.lsh/identity.pub

then authorizing it by executing, on `sara',

     lsh-authorize my-key.pub

   By default, `lsh-writekey' 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 `-c none' to
`lsh-writekey'.

   For security reasons, you should keep the private key
`~/.lsh/identity' 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.

   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, `ftp' or `http' to transfer
your public key to the machines where you want to authorize it.

   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.


File: lsh.info,  Node: srp,  Next: sexp,  Prev: public-key,  Up: Getting started

Using SRP authentication
========================

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 "password verifier"
that is derived from the password. The verifier is stored on the target
machine (i.e. the machine you want to log in to).

   To create a verifier, you run the `srp-gen' 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.

   The main advantage of using SRP 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, _without_ having to know any hostkeys or fingerprints
beforehand!

   For instance, you could connect using SRP to fetch the hostkey
fingerprint for the remote machine, as a kind of bootstrapping
procedure, and then use traditional authentication methods for further
connections.

   For this to work, the verifier _must_ be kept _secret_. If the enemy
gets your verifier, he can mount some attacks:

   * He can mount a "dictionary attack" on your password, i.e. generate
     a large list of likely password and check if any of them matches
     yours.

   * He can impersonate the server. That means that if you try to
     connect to the remote machine using SRP, 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.

   If you use SRP 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.

   If you use SRP 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.

   SRP support is disabled by default, but can be enabled by the
`--srp-keyexchange' option to `lshd' and `lsh' (naturally, it won't be
used unless enabled on both sides). At the time of this writing, SRP is
too new to be trusted by conservative cryptographers (and remember that
conservatism is a virtue when it comes to security).

   And even if SRP in itself is secure, the way `lsh' integrates it
into the `ssh' protocol has not had much peer review.  The bottom line
of this disclaimer is that the SRP support in `lsh' should be
considered experimental.

   As far as I know, using SRP as a host authentication mechanism is
not supported by any other `ssh' implementation. The protocol `lsh'
uses is described in the `doc/srp-spec.txt'.  Implementations that use
SRP only as a user authentication mechanism are not compatible with
`lsh'.


File: lsh.info,  Node: sexp,  Next: Converting keys,  Prev: srp,  Up: Getting started

Examining keys and other sexp files
===================================

Keys and most other objects `lsh' needs to store on disk are
represented as so called S-expressions or "sexps" for short.
S-expressions have their roots in the Lisp world, and a variant of them
in used in the Simple Public Key Infrastructure (SPKI).  Currently,
`lsh''s support for SPKI is quite limited, but it uses SPKI's formats
for keys and Access Control Lists (ACL:s).

   There are several flavours of the sexp syntax:

   * The canonical syntax is somewhere between a text and a binary
     format, and is extremely easy for programs to read and write.

   * The transport syntax, which is suitable when embedding sexps in
     text files. It is essentially the canonical representation,
     encoded using base64.

   * The advanced syntax, which is intended for humans to read and
     write, and bears some resemblance to Lisp expressions.

   To see what your `~/.lsh/identity.pub' file really contains, try

     sexp-conv < ~/.lsh/identity.pub

   The `sexp-conv' 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,

     sexp-conv --hash </etc/lsh_host_key.pub

   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 `lsh-export-key
--fingerprint'.


File: lsh.info,  Node: Converting keys,  Prev: sexp,  Up: Getting started

Converting keys from `ssh2' and OpenSSH
=======================================

If you are already using `ssh2' or OpenSSH, and have created one or
more personal keypairs, you need to convert the public keys to `lsh''s
format before you can authorize them. Use the supplied `ssh-conv'
script,

     ssh-conv <openssh-key.pub >new-key.pub

   You can then use the usual `lsh-authorize' on the converted keys.
`ssh-conv' supports both DSA and `RSA' keys.

   Conversion of keys the other way is also possible, by using the
`lsh-export-key' program. It reads a public key in the  SPKI format
used by `lsh' on stdin, and writes the key in `ssh2'/OpenSSH format on
stdout.

   If you want to use your `lsh' key to log in to another system
running and OpenSSH server, you can do like this:

     lsh-export-key --openssh < .lsh/identity.pub >sshkey

   And on the other machine, after having somehow copied the sshkey
file, just add it to the end of your `authorized_keys' file:

     cat sshkey >> ~/.ssh/authorized_keys

   `lsh-export-key' can also be used to check the fingerprint of keys
(just like `ssh-keygen').

     lsh-export-key --fingerprint < /etc/lsh_host_key.pub

   show the MD5 and Bubble babble fingerprint of the server public key.

   There are currently no tools for converting private keys.


File: lsh.info,  Node: Invoking lsh,  Next: Invoking lshg,  Prev: Getting started,  Up: Top

Invoking `lsh'
**************

You use `lsh' to login to a remote machine. Basic usage is

   `lsh [-p PORT NUMBER] sara.lysator.liu.se'

   which attempts to connect, login, and start an interactive shell on
the remote machine. Default PORT NUMBER is whatever your system's
`/etc/services' lists for `ssh'. Usually, that is port 22.

   There is a plethora of options to `lsh', 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 `lsh --help'.

   Note that for many of the options to `lsh', the ordering of the
options on the command line is important.

* Menu:

* Algorithms: Algorithm options.  Selecting algorithms.
* Hostauth options::
* Userauth options::
* Actions: Action options.      What to do after login.
* Messages: Verbosity options.  Tuning the amount of messages.


File: lsh.info,  Node: Algorithm options,  Next: Hostauth options,  Prev: Invoking lsh,  Up: Invoking lsh

Algorithm options
=================

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.

   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.

   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.

   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.

   There is a set of default algorithm preferences. When you use a
command line option to say that you want to use ALGORITHM for one of the
algorithms, the default list is replaced with a list containing the
single element ALGORITHM. For example, if you use `-c arcfour' to say
that you want to use `arcfour' as the encryption algorithm, the
connection will either end up using `arcfour', or algorithm negotiation
will fail because the peer doesn't support `arcfour'.

Option  Algorithm      Default        
        type                          
`-z'    Data           `none', `zlib' The default preference list
        compression                   supports zlib compression, but
                                      prefers not to use it.
`-c'    Encryption     `aes256-cbs',  The default encryption algorithm is
                       `3dec-cbc',    aes256. The default list includes
                       `blowfish-cbc',only quite old and well studied
                       `arcfour'      algorithms. There is a special
                                      algorithm name `all' to enable all
                                      supported encryption algorithms
                                      (except `none').
`-m'    Message        `hmac-sha1',   Both supported message
        Authentication `hmac-md5'     authentication algorithms are of the
                                      HMAC family.

   As a special case, `-z' with no argument changes the compression
algorithm list to `zlib', `none', which means that you want to use
`zlib' if the other end supports it. This is different from `-zzlib'
which causes the negotiation to fail if the other end doesn't support
`zlib'. A somewhat unobvious consequence of `-z' having an _optional_
argument is that if you provide an argument, it must follow directly
after the option letter, no spaces allowed.


File: lsh.info,  Node: Hostauth options,  Next: Userauth options,  Prev: Algorithm options,  Up: Invoking lsh

Host authentication options
===========================

As described earlier (*note Threats::), proper authentication of the
remote host is crucial to protect the connection against
man-in-the-middle attacks. By default, `lsh' verifies the server's
claimed host key against the "Access Control Lists" in
`~/.lsh/host-acls'. If the remote host cannot be authenticated, the
connection is dropped.

   The options that change this behaviour are

`--host-db'
     Specifies the location of the ACL file.

`--sloppy-host-authentication'
     Tell `lsh' 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 `~/.lsh/captured_keys'. If run in quiet mode, `lsh -q
     --sloppy-host-authentication', `lsh' connects to any host, no
     questions asked.

`--strict-host-authentication'
     Disable sloppy operation (this is the default behaviour).

`--capture-to'
     Use some other file than `~/.lsh/captured_keys'. For example,

          lsh --sloppy-host-authentication --capture-to ~/.lsh/host-acls

     makes `lsh' behave more like the `ssh' program.

`--srp-keyexchange'
     Try using SRP for keyexchange and mutual authentication.



File: lsh.info,  Node: Userauth options,  Next: Action options,  Prev: Hostauth options,  Up: Invoking lsh

User authentication options
===========================

`-l'
     Provide a name to use when logging in. By default, the value of the
     `LOGNAME' environment variable is used.

`-i'
     Try the keys from this file to log in. By default, `lsh' uses
     `~/.lsh/identity', if it exists. It ought to be possible to use
     several `-i' options to use more than one file, but that is
     currently not implemented.

`--no-publickey'
     Don't attempt to log in using public key authentication.



File: lsh.info,  Node: Action options,  Next: Verbosity options,  Prev: Userauth options,  Up: Invoking lsh

Action options
==============

There are many things `lsh' can do once you are logged in. There are
two types of options that control this: "actions" and "action
modifiers". For short options, actions use uppercase letters and
modifiers use lowercase.

   For each modifier `--foo' there's also a negated form `--no-foo'.
Options can also be negated by preceding it with the special option
`-n'. This is mainly useful for negating short options. For instance,
use `-nt' to tell `lsh' 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.

   First, the actions:

`-L'
     Requests forwarding of a local port. This option takes a mandatory
     argument of the form LISTEN-PORT:TARGET-HOST:TARGET-PORT. This
     option tells `lsh' to listen on LISTEN-PORT on the local machine.
     When someone conects to that port, `lsh' asks the remote server to
     open a connection to TARGET-PORT on TARGET-HOST, and if it
     succeeds, the two connections are joined together through an the
     `lsh' connection. Both port numbers should be given in decimal.

`-R'
     Requests forwarding of a remote port. It takes one mandatory
     argument, just like `-L'. But in this case `lsh' asks the _remote_
     server to listen on LISTEN-PORT. When someone connects to the
     remote hosts, the server will inform the local `lsh'. The local
     `lsh' then connects to TARGET-PORT on TARGET-HOST.

`-D'
     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. `-D1080' is correct, `-D 1080' is not).

`-E'
     This option takes one mandatory argument, which is a command line
     to be executed on the remote machine.

`-S'
     Start an interactive shell on the remote machine.

`-G'
     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.

`-N'
     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.

`-B'
     Put the client into the background after key exchange and user
     authentication. Implies `-N'

`--subsystem'
     Specifies a subsystem to connect to, implies `--no-pty'. Example
     usage: `--subsystem=sftp'


   If there are trailing arguments after the name of the remote system,
this is equivalent to a `-E' 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.

   If there are no trailing arguments after the name of the remote
system, and the `-N' option is not given, the default action is to start
a shell on the remote machine. I.e. this is equivalent to the `-S'
option.

   There are a few supported modifiers:

`-t'
     Request a pseudo terminal. `lsh' 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 `lsh' process has a controlling terminal. This modifier
     applies to actions that create remote processes, i.e. `-E' and
     `-S', as well as the default actions.

     Currently, this option is ignored if there is no local terminal.

`-x'
     Request X forwarding. Applies to the `-E' and `-S' and the default
     actions.

`--stdin'
     Redirect the stdin of a remote process from a given, local, file.
     Default is to use `lsh''s stdin for the first process, and
     `/dev/null' for the rest. This option applies to the `-E' and `-S'
     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.

`--stdout'
     Redirect the stdout of a remote process to a given, local, file.
     Default is to use `lsh''s stdout. Like `--stdin', it is reset
     after it is used.

`--stderr'
     Redirect the stdout of a remote process to a given, local, file.
     Analogous to the `--stdout' option.

`--detach'
     Detach from terminal at session end.

`--write-pid'
     Applies to `-E'. Write PID of backgrounded process to stdout.

`-e'
     Set the escape character (use "none") to disable. Default is "~"
     if a tty is allocated and "none" otherwise.

`-g'
     Remote peers, aka global forwarding. This option applies to the
     forwarding actions, i.e. `-L', `-R' and `-D'. 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 _all_ network
     interfaces.



File: lsh.info,  Node: Verbosity options,  Prev: Action options,  Up: Invoking lsh

Verbosity options
=================

These options determines what messages `lsh' writes on its stderr.

`-q'
     Quiet mode. Disables all messages and all questions, except
     password prompts and fatal internal errors.

`-v'
     Verbose mode. Makes `lsh' 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 `lsh'
     internals.

`--trace'
     Trace mode. Prints some internal information to aid tracking
     `lsh''s flow of control.

`--debug.'
     Debug mode. Dumps _a lot_ 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
     `SSH_MSG_USERAUTH_REQUEST' messages, but you should still use it
     with care.

`--log-file'
     This option redirects all messages to a file. Takes one mandatory
     argument: The name of that file.


   Note that all these options are orthogonal. If you use `--trace',
you usually want to add `-v' as well; `--trace' does not do that
automatically.


File: lsh.info,  Node: Invoking lshg,  Next: Invoking lshd,  Prev: Invoking lsh,  Up: Top

Invoking `lshg'
***************

You use `lshg' to login to a remote machine to which you have
previously used `lsh' to set up a gateway (*note Action options::). Its
usage is very similar to that of `lsh' (*note Invoking lsh::), except
that some options are not available.

   Basic usage is

   `lshg [-l USERNAME] HOST'

   which attempts to connect to the gateway that should previously have
been established by running `lsh [-l USERNAME] -G HOST')

   The USERNAME and HOST are used to locate the gateway. The default
value for USERNAME is determined in the same way as for `lsh' (*note
Invoking lsh::).

   As `lshg' uses almost the same options as `lsh' (*note Invoking
lsh::), only options that are not available or have a different meaning
in `lshg' are listed here.

   The algorithm options (*note Algorithm options::) as well as most of
the userauth (*note Userauth options::) and hostauth (*note Hostauth
options::) are not available in `lshg' as they are only used by session
setup, which is already handled by `lsh'.

   Due to technical reasons, X11-forwarding cannot be performed by
`lshg', thus the `--x11-forward' option (*note Action options::) is not
available.

   To summarize, these are the options that are new, not available or
that have different meanings:

`-G'
     For `lsh' `-G' requests a gateway to be set up. For `lshg' it
     means that if no usable gateway is found `lsh' should be launched
     with the same arguments instead.

`--send-debug'
     Not available in `lsh'. Sends a `debug' message to the remote
     machine.

`--send-ignore'
     Not available in `lsh'.  Sends a `ignore' message to the remote
     machine.

`-x'
     (`--x11-forward') Not available in `lshg'.

`-c'
     (`--crypto') Not available in `lshg'.

`-z'
     (`--compression') Not available in `lshg'.

`-m'
     (`--mac') Not available in `lshg'.

`--hostkey-algorithm'
     Not available in `lshg'.

`--capture-to'
     Not available in `lshg'.

`--strict-host-authentication'
     Not available in `lshg'.

`--sloppy-host-authentication'
     Not available in `lshg'.

`--host-db'
     Not available in `lshg'.

`--publickey'
     Not available in `lshg'.

`--no-publickey'
     Not available in `lshg'.

`--dh-keyexchange'
     Not available in `lshg'.

`--no-dh-keyexchange'
     Not available in `lshg'.

`--srp-keyexchange'
     Not available in `lshg'.

`--no-srp-keyexchange'
     Not available in `lshg'.

`-i'
     `--identity' Not available in `lshg'.



File: lsh.info,  Node: Invoking lshd,  Next: Files and environment variables,  Prev: Invoking lshg,  Up: Top

Invoking `lshd'
***************

`lshd' 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 `lshd' manually, and with user privileges.

   There are currently no configuration files. Instead, command line
options are used to tell `lshd' what to do. Many options have `--foo'
and `--no-foo' variants. Options specifying the default behaviour are
not listed here.

   Some of the options are the shared with `lsh'. In particular, see
*Note Algorithm options:: and *Note Verbosity options::.

   Options specific to the `lshd' server are:

`-p'
     Port to listen to. The mandatory argument is a decimal port number
     or a service name. Default is "ssh", usually port 22.

     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.

     Note that if you use both `-p' and `--interface', the order
     matters.

`--interface'
     Network interface to listen on. By default, `lshd' 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 _preceding_ `-p' is used.

     Some examples: `--interface=localhost', `--interface=1.2.3.4:443',
     `--interface=[aaaa::bbbb]:4711'. To make `lshd' listen on several
     ports and interfaces at the same time, just use several
     `--interface' options on the command line.

`-h'
     Location of the server's private key file. By default,
     `/etc/lsh_host_key'.

`--daemonic'
     Enables daemonic mode. `lshd' forks into the background, redirects
     its stdio file descriptors to `/dev/null', changes its working
     directory to `/', and redirects any diagnostic or debugging
     messages via syslog.

     `lshd' should be able to deal with the environment it inherits if
     it is started by `init' or `inetd', but this is not really tested.

`--pid-file'
     Creates a locked pid file, to make it easier to write start and
     stop scripts for `lshd'. The mandatory argument provides the
     filename.  This option is enabled by default when operating in
     daemonic mode, and the default filename is `/var/run/lshd.pid'.

`--no-syslog'
     Disable the use of the syslog facility. Makes sense only together
     with `--daemonic'

`--enable-core'
     By default, `lshd' disables core dumps, to avoid leaking sensitive
     information. This option changes that behaviour, and allows `lshd'
     to dump core on fatal errors.

`--no-password'
     Disable the "password" user authentication mechanism.

`--no-publickey'
     Disable the "publickey" user authentication mechanism.

`--root-login'
     Enable root login. By default, root can not log in using `lshd'.

`--login-auth-mode'
     This option is highly experimental. Bypass `lshd''s user
     authentication, and allow users to spawn their login-shell without
     any authentication. Usually combined with `--login-shell', to set
     the login shell to a program that performce password
     authentication.

`--kerberos-passwords'
     Verify passwords against the kerberos database. This is implemented
     using the `lsh-krb-checkpw' helper program. Note that this does
     _not_ use the Kerberos infrastructure in the Right Way.
     Experimental.

`--password-helper'
     Tells `lshd' to use a helper program for verifying passwords.
     This is a generalization of `--kerberos-passwords', and it could
     be used for verifying passwords against any password database. See
     the source files `lsh-krb-checkpw.c' and `unix_user.c' for details.

`--login-shell'
     Use the specified program as the login shell for all users,
     overriding the login shell in the passwd database.

`--srp-keyexchange'
     Enable SRP keyexchange and user authentication.

`--no-pty-support'
     Disable support for pseudo terminals.

`--no-tcp-forward'
     Disable support for tcp forwarding, in both directions.

`--subsystems'
     Specifies a list of subsystems and corresponding programs.
     Example usage:
     `--subsystems=sftp=/usr/sbin/sftp-server,foosystem=/usr/bin/foo'



File: lsh.info,  Node: Files and environment variables,  Next: Terminology,  Prev: Invoking lshd,  Up: Top

Files and environment variables
*******************************

This chapters describes all files and all environment variables that
are used by `lsh', `lshd', and related programs.

   There are a few environment variables that modifies the behaviour of
the `lsh' programs. And there are also a handful of variables that are
setup by `lshd' when starting user processes.

`DISPLAY'
     When X-forwarding is enabled, `DISPLAY' specifies the local
     display. Used by `lsh'.

`HOME'
     User's home directory. Determines where client programs looks for
     the `~/.lsh' directory. When `lshd' starts a user program, it sets
     `HOME' from the value in the `/etc/passwd' file, except if `lshd'
     is running as an ordinary user process. In the latter case, the
     new process inherits `lsh''s own value of `HOME'.

`LOGNAME'
     The user's log in name. Used as the default name for logging into
     remote systems. Set by `lshd' when starting new processes.

`LSH_YARROW_SEED_FILE'
     If set, it points out the location of the seed-file for the
     randomness generator. Recognized both by `lshd' and the client
     programs.

`LSHFLAGS'
     If set, lsh will parse any options as had they been given on the
     command line.

`LSHGFLAGS'
     If set, lshg will parse any options as had they been given on the
     command line.

`POSIXLY_CORRECT'
     Affects the command line parsing of programs which by default
     accept options mixed with arguments.

`SEXP_CONV'
     The location of the `sexp-conv' program. If not set, the default
     `$prefix/bin/sexp-conv' is used.

`SSH_CLIENT'
     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.

`SSH_TTY'
     This variable may be set by lshd for established sessions. If it
     is set it is the name of the tty allocated.

`SHELL'
     User's login shell. When `lshd' starts a user process, it sets
     `SHELL' to the value in `/etc/passwd', unless overridden by the
     `--login-shell' command line option.

`TERM'
     The type of the local terminal. If the client requests a pty for a
     remote process, the value of `TERM' is transferred from client to
     server.

`TMPDIR'
     Determines where the unix socket used by `lshg' is located in the
     filesystem.

`TZ'
     Time zone. Processes started by `lshd' inherit the value of this
     variable from the server process.


   Files used by the lsh client, stored in the `~/lsh' directory:

`captured_keys'
     Keys for remote hosts, saved when running `lsh
     --sloppy-host-authentication'. Or more precicely, each key is
     stored together with an as SPKI (Simple Public Key Intrastructure)
     ACL:s (Access Control Lists).

`identity'
     Your private key file. Usually created by `lsh-keygen |
     lsh-writekey'. Read by `lsh'. Should be kept secret.

`identity.pub'
     The corresponding public key. You can copy this file to other
     systems in order to authorize the private key to login (*note
     Converting keys::).

`host-acls'
     Host keys (or more precisely, ACL:s) that lsh considers authentic.
     Entries have the same format as in `captured_keys'.

`yarrow-seed-file'
     The seed file for the randomness generator. Should be kept secret.


   Files used by `lshd', some of which are read from user home
directories:

`/etc/lsh_host_key'
     The server's private host key.

`/etc/lsh_host_key.pub'
     The corresponding public key.

`/var/spool/lsh/yarrow-seed-file'
     The seed-file for `lshd''s randomness generator.

`~/.lsh/authorized_keys'
     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 `lsh-authorize'.

`~/.lsh/srp-verifier'
     If you use the experimental support for SRP (*note srp::), the
     server reads a user's SRP verifier from this file.


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Terminology
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Concept Index
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Ref: lsh-usage33662
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