Node:Top, Next:Introduction, Previous:(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 1.3.4.

- Introduction:
- Installation:
- Getting started:
- Invoking lsh:
- Invoking lshd:
- 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 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: Selecting algorithms.
- Hostauth options:
- Userauth options:
- Actions: What to do after login.
- Messages: Tuning the amount of messages.

Node:Introduction, Next:Installation, Previous: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...

- Threats:
- Features:
- Related techniques:

Node:Threats, Next:Features, Previous: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.

Node:Features, Next:Related techniques, Previous: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 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. You can also use Thomas Wu's Secure Remote Password Protocol (SRP). Kerberos support is on the wish list but not yet supported (see 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.

Convenient tunneling of X was one of the most impressive features of the original `ssh` programs. The current version of `lsh` implements X-forwarding, although the `lshd` server doesn't provide that service yet.

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, connection 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.

Node:Related techniques, Previous: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.

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

Node:ssh1, Next:ssh2, Previous: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-1.27` which speaks version 1.5 of the protocol. The "free" version of `ssh-1.27` does not allow commercial use without additional licensing, which makes `ssh-1.27` non-free software according to Debian's Free Software Guidelines and the Open Source Definition.

The version 1 protocol has some minor 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.

Node:ssh2, Next:Kerberos, Previous: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. `lsh` implements the required subset of this protocol. It is intended to be compatible with the `ssh2` series of programs distributed by F-Secure Corporation.

However, the existing versions of `ssh2` gets some details of the protocol wrong (probably because it predates the protocol specification), so there is some amount of bug-compatibility required.

Interoperability between independently developed implementations is one necessary condition for the `ssh-2` protocol to become a Proposed Standard.

The license for F-Secure's `ssh2` programs is similar to that for recent versions of `ssh1`, but with a narrower definition of "non-commercial use".

Besides `lsh` there are few free implementations of the `ssh-2` protocols. Since May 2000 it is supported also by OpenSSH.

Node:Kerberos, Next:ipsec, Previous: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.

Node:ipsec, Previous: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 and 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".

Node:Installation, Next:Getting started, Previous: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 it 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.

Node:Getting started, Next:Invoking lsh, Previous: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.

- 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:

Node:lsh basics, Next:tcpip forwarding, Previous:Getting started, 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 between two machines, `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 (see 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 its public hostkey listed in `~/.lsh/known_hosts`.

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 its key is appended to the file `~/.lsh/captured_keys`. You can copy keys you have verified to `~/.lsh/known_hosts`.

You can even use

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

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

You can create fingerprints for the hostkeys you need regularly, and keep with you (see sexp).

    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 sara.lysator.liu.se:/lysator/cvsroot co lsh

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

Node:tcpip forwarding, Next:lshd basics, Previous: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 eachother. 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 (see 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.

Node:lshd basics, Next:public-key, Previous:tcpip forwarding, Up:Getting started

## `lshd` basics

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

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

    lsh-keygen | lsh-writekey -o /etc/lsh_host_key

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`.

Node:public-key, Next:srp, Previous: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

For security reasons, you should keep the private key `~/.lsh/identity` secret. Anybody who can read that 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 a single keypair on each of the systems, and on each system you authorize some or all of your other public keys for login.

Note that `lsh-writekey` does not currently encrypt your private key in any way. That means that you can lose it if a backup tape gets into the wrong hands, and if you use NFS it will likely be sent in the clear across your local network. To encrypt the key using a pass phrase, give the `-c 3des` option to `lsh-writekey`.

Node:srp, Next:sexp, Previous: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`.

Node:sexp, Next:Converting keys, Previous: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/known_hosts` file really contains, try

    sexp-conv -i advanced < ~/.lsh/known_hosts

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 --raw-hash </etc/lsh_host_key.pub

Node:Converting keys, Previous:sexp, Up:Getting started

## Converting keys from `ssh2` and OpenSSH

If you are already using `ssh2` or OpenSSH, and have creating 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.

There are currently no tools for converting private keys.

Node:Invoking lsh, Next:Invoking lshd, Previous: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.

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

Node:Algorithm options, Next:Hostauth options, Previous:Invoking lsh, Up:Invoking lsh

## Algorithm options

Before a packet is sent, each packet can be compressed, encrypted and authenticated, 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 type | Default |
`-z` | Data compression | `none`, `zlib` | The default preference list supports zlib compression, but prefers not to use it.
`-c` | Encryption | `3dec-cbc`, `blowfish-cbc`, `arcfour` `arcfour` | The default encryption algorithm is triple-DES in CBC mode. This seems to be the algorithm of choice among conservative cryptographers. The default list includes only quite old and well studied algorithms. There is a special algorithm name `all` to enable all supported encryption algorithms (except `none`).
`-m` | Message Authentication | `hmac-sha1`, `hmac-md5` | Both supported message 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.

Node:Hostauth options, Next:Userauth options, Previous:Algorithm options, Up:Invoking lsh

## Host authentication options

As described earlier (see 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/known_hosts`. 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/known_hosts

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

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

Node:Userauth options, Next:Action options, Previous: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.

Node:Action options, Next:Verbosity options, Previous: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 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.
`-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`

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.
`-g`
  Remote peers, aka global forwarding. This option applies to the forwarding actions, i.e. `-L` and `-R`. By default, only connections to the loopback interface, ip 127.0.0.1, are forwared. 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.

Node:Verbosity options, Previous: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.

Node:Invoking lshd, Next:Terminology, Previous:Invoking lsh, 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 Algorithm options and 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.
`--interface`
  Network interface to listen on. By default, `lshd` listens on all interfaces.
`-h`
  Location of the server's private key file. By default, `/etc/lsh_host_key`.
`-i`
  Variant of the s-expression syntax to use when reading the host key. Default is to use transport format. Not a terribly useful option.
`--ssh1-fallback`
  This options enables fallback to `ssh1`. `lshd` doesn't implement version 1 of the Secure Shell Protocol. But it can fork an `ssh1` server when an old client connects. Falling back to `ssh1` is inefficient, and requires some special features of the server fallen back to. It should work with the `sshd` daemon supplied with reasonably new versions of F-Secure's `sshd1`, and with OpenSSH.

  The optional argument provides the filename of the ssh1 daemon to use. The default name is `/usr/local/sbin/sshd1`, unless something else was configured at compile time.

`--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`.
`--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`.
`--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. Mainly useful for testing.
`--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.

Node:Terminology, Next:Concept Index, Previous:Invoking lshd, Up:Top

# Terminology

Node:Concept Index, Previous:Terminology, Up:Top

# Concept Index

# Table of Contents

- Introduction
  - Threats
  - Features
  - Related programs and techniques
    - `ssh-1.x`
    - `ssh-2.x`
    - Kerberos
    - IPSEC
- Installation
- Getting started
  - `lsh` basics
  - Port forwarding
  - `lshd` basics
  - Using public-key user authentication
  - Using SRP authentication
  - Examining keys and other sexp files
  - Converting keys from `ssh2` and OpenSSH
- Invoking `lsh`
  - Algorithm options
  - Host authentication options
  - User authentication options
  - Action options
  - Verbosity options
- Invoking `lshd`
- Terminology
- Concept Index