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Man Pages


Manual Reference Pages  -  SAFESH (1)

NAME

safesh - safe key manager for OpenSSH

CONTENTS

Synopsis
Description
Basic Concept Description
Simple Howto
Utility Commands
Files
Authors
See Also
Known Issues
Missing Features
Variable Replacement In Descriptions

SYNOPSIS

safesh [user@host] [
ssh-parameters ... ] safeshinstall [user@host] cvs-safesh [user@host] [command] scpsh [user@host]

DESCRIPTION

NOTE: This text often refers to $VARIABLE in description. What each of the references will be replaced with when safesh runs is described at the end of the manpage.

The safesh utility automatically creates one DSA key (called an identity) for each host you connect to, and stores this in a separate agent for each host. It is also capable of adding keys for other hosts to this agent, so you can use it for restricted forwarding of authentication. Because each host uses its own ssh-agent(1), the hosts you forward authentication to can only get at the authentication for the hosts you specifically say it should be able to get at.

When run, safesh:

  1. Normalizes the hostname you are talking about, using the $HOME/.safesh/map file.
  2. Checks if the user and host has an SSH DSA key in $HOME/.safesh, and creates one using ssh-keygen(1) if it does not. The DSA key is stored in $HOME/.safesh/$USER@$HOST-$PORT/dsa_id. You will be asked for a passphrase when the key is created. Note that if you use the same passphrase for all safesh keys, you will only be asked for the passphrase once per host you connect to. If you use different passphrases, you will be asked once per forwarded key for each host you connect to (after a machine startup).
  3. Checks if you have the ssh-agent(1) for this host running, and starts it if not.
  4. Checks what keys you are supposed to have active when connecting to this host (the key for the host and any keys listed in $HOME/.safesh/$USER@$HOST-$PORT/extra_keys), and which of these are missing from the active agent.
  5. If any identities were missing from the agent, it executes ssh-add(1) to add them to the agent.
  6. Executes ssh(1) with either $USER@$HOST or the extra command line supplied by the user.

BASIC CONCEPT DESCRIPTION

The safesh utility is an authentication manager for OpenSSH. It is an attempt at making it easy to use the built-in authentication features of OpenSSH securely. By default, the SSH security model is that all hosts the user connects to are trusted, and are given complete access, including the ability to authenticate as the user towards other hosts if the user is running ssh-agent(1). OpenSSH has improved this security model somewhat by not forwarding SSH authentication by default, but still allows the host that you connect to to grab your credentials and authenticate as you to anybody else when you do authentication forwarding to it.

SIMPLE HOWTO

Starting to make use of safesh is trivial:
  1. Do "safeshinstall <[user@]hostname>".

    This will ask for a passphrase (three times), create a directory $HOME/.safesh/<user>@<hostname>-22, which contains authentication data for your user at <hostname>, and add the contents of $HOME/.safesh/<user>@<hostname>-22/id_dsa.pub to $HOME/.ssh/authorized_keys2 on the host you connect to. The latter will result in ssh(1) asking for authentication in the fashion you already use (most likely by asking for your password).

  2. Log in with "safesh <hostname>" from now on.

    This will ask you for a passphrase if you have not logged into that host this session, and otherwise just let right in.

UTILITY COMMANDS

The safesh utility ships with two utility hacks to work around the fact that it is not a complete ssh(1) replacement, scpsh and cvs-safesh.

The scpsh utility is for supporting use of scp(1) with safesh. The scpsh [user@host] command will start a new interactive shell (using the SHELL environment variable to determine which it should be), with the environment variables for using ssh-agent(1) to authenticate to [user@host] already set. This allows use of scp(1) without having to type passwords to authenticate.

The cvs-safesh utility makes it easy to use safesh along with cvs(1) and other programs that use rsh(1) or ssh(1) with the format "ssh <user@host> <command>" or "ssh <host> <command>". To use with cvs(1), just set CVS_RSH to "cvs-safesh" instead of "ssh".

FILES

$HOME/.safesh/
  Directory containing information for safesh.
$HOME/.safesh/map
  Mapping file for safesh, describing how to map host names to their canonical form. This is usually used to map short names to their long form. The format of the file is one mapping per line, what it is mapped from as the first word, what it is mapped to as the second.

It is also possible to use this to map domain names to their safe form by having the name of the host as the first parameter, and the name of the host with a period (‘.’) at the end as the second parameter, e.g., "freefall.freebsd.org freefall.freebsd.org.".

$HOME/.safesh/$USER@$HOST-$PORT/
  Directory with data for a particular hostname. Automatically generated on first connect to a host with safesh.
$HOME/.safesh/$USER@$HOST-$PORT/dsa_id
  Private key for use to authenticate as $USER@$HOST. Automatically generated on first connect to a host with safesh.
$HOME/.safesh/$USER@$HOST-$PORT/dsa_id.pub
  Public key for use by $HOST to authenticate $USER. To connect to $HOST as $USER using safesh without giving a password, add the contents of this file to the end of $HOME/.ssh/authorized_keys2. Automatically generated on first connect to a host with safesh.
$HOME/.safesh/$USER@$HOST-$PORT/$AUTHTARGET
  Private key for use when $HOST authenticates towards $AUTHTARGET. This is used in preference to $HOME/.safesh/$AUTHTARGET/dsa_id for authentication forwarding through $HOST to $AUTHTARGET. The file is only used if $AUTHTARGET is listed in $HOME/.safesh/$USER@$HOST-$PORT/extra_keys. This file is not generated automatically by safesh. It is only present if you have generated it using ssh-keygen(1). Note that it is usually more than useless (can pose a security risk) to copy a key used for other authentication to this location.

The use of explict authentication files for authentication forwarding is primarily for protection against the case where the machine you run safesh on is compromised. Using this file, you can use a separate passphrase from the one used for the key for connecting directly to $AUTHTARGET; that key need not even exist. By using IP restrictions in the authorized_keys file for the key, you can make sure that the host safesh runs on cannot connect to $AUTHTARGET using the authentication forwarding key. The use of a separate forwarding key can also be used in combination with a modified SSH to log which key was used where, and thus track key propagation.

$HOME/.safesh/$USER@$HOST-$PORT/$AUTHTARGET.pub
  Public key corresponding to the private key described above.
$HOME/.safesh/$USER@$HOST-$PORT/extra_keys
  List of extra keys to make available for this host. Each line in the file is first attempted matched against the host/user/port database in $HOME/.safesh/. Username and/or port is added if just the hostname is specified extra_keys, and the hostname is always normalized using the map file. If a key exists in $HOME/.safesh/, safesh attempts to add that. Otherwise, it first tries to look for the line as a file relative to /, then relative to $HOME. If it does not find either of these, safesh will exit with an error message. If it finds one, it will add it using ssh-add(1).
$HOME/.safesh/$USER@$HOST-$PORT/activeagent-$YOURHOST.sh
  Bourne shell (see sh(1), bash(1), zsh(1)) script for setting up the environment variables for a particular ssh-agent(1) process used for this host. Only valid if safesh has been run against that host as this user since the machine safesh runs on was last booted. Note that this file must be source’d, not just run as a shell script.
$HOME/.safesh/$USER@$HOST-$PORT/activeagent-$YOURHOST.csh
  CSH (see csh(1), tcsh(1)) script for setting up the environment variables for a particular ssh-agent(1) process used for this host. Only valid if safesh has been run against that host as this user since the machine safesh runs on was last booted. Note that this file must be source’d, not just run as a shell script.

AUTHORS

The safesh utility was written by
.An Eivind Eklund Aq eivind@FreeBSD.org .

SEE ALSO

ssh(1), ssh-add(1), ssh-agent(1), ssh-keygen(1)

KNOWN ISSUES

The safesh utility does not handle whitespace in filenames specified in extra_keys correctly.

The ssh-agent(1) processes that are started by safesh will hang around until next reboot unless you put the "killall ssh-agent" command in .logout or similar. This allows any login to your account to use your authentication towards machines you have connected to (including anybody with root on the box), persisting after you log out. You must always assume that root can grab your authentication at the moment you run do it, so this is only an issue in that the authentication stays available longer. This is not resolvable without rewriting ssh-agent(1).

MISSING FEATURES

Two-step secure SSH with an untrusted host in the middle
  It is possible to use the port forwarding capability of ssh(1) to forward authentication through another server, without allowing the other server to indepently authenticate to a third party, and without allowing it to see what is going on in your connection. This is based on just forwarding a tunnel through the untrusted host, and doing direct authentication to the server on the other side. With the present version of OpenSSH, this has the problem of leaving the actual port forwarding open while the tunnel is open, allowing other users to set up their own tunnels, and weakening another side of the security model.
Read out fingerprints
  The safesh utility should make it trivial to retrieve the fingerprint for:
  1. The host it is running on.

    This must presently be done with "ssh-keygen -l /etc/ssh/ssh_host_key.pub" (to get the fingerprint for SSH 1) and "ssh-keygen -l -f /etc/ssh/ssh_host_dsa_key" (for SSH 2).

  2. Other hosts, as registered in the known_host file on the host it is running on.

    This must presently be done by manual inspection.

Merge known_hosts
  The safesh utility should make it trivial to merge known_hosts and known_hosts2 with ones from another host, including retrieving and uploading known_hosts as appropriate.
Manage .ssh/authorized_keys2
  The safesh utility should be able to automatically remove keys from the authorized_keys2 file on other machines, to make everything about the safesh process self-contained.
Manage setup of key limitations
  When managing authorized_keys2, it is also reasonable to manage key limitation in this. IP restrictions ("from=") should be handled to make it easy to create setups where the local machine does not have direct access to a target. Command restrictions etc. would be good to have just for completeness.
Emulate the entire ssh 1 syntax
  Presently, the safesh utility has a fairly weird syntax. This is because it is a fairly quick hack, just made to be usable. Later, it would be nice to rewrite it to be fully compatible with ssh(1). This would allow use as a drop-in replacement.
Description of the trust/threat/security model
  It would be nice to have a complete description of the normal SSH threat model as well as the safesh threat model, in order to make people fully conscious of their own model.
Emulate scp(1)
  The scp(1) utility is very useful, and the only way to use it with safesh at the moment is through a subshell created by scpsh. An ideal safesh implementation would include wrapping of scp(1), too.

VARIABLE REPLACEMENT IN DESCRIPTIONS

$HOME Path to your home directory.
$HOST The name of the host you are running safesh towards. This is the machine you are ssh 1 ’ing into.
$YOURHOST The name of the host you are running safesh on, as output by hostname(1). This is the name of the machine you are ssh 1 ’ing from. The use of $YOURHOST makes safesh safe to use with NFS-mounted home directories.
$AUTHTARGET
  The authentication target for an authentication forwarding. This is not the same as $HOST. $AUTHTARGET is a machine you are ssh 1 ’ing to from $HOST. The format of $AUTHTARGET is
.Sm off <user@<somehost-<someport>>>,
.Sm on where <<user>> defaults to the username you run safesh as, and <<someport>> default to 22 (and it is not possible to set anything else at this time).
$USER The username used on $HOST; defaults to the same as the username you have on $YOURHOST, but will be different if you do "safesh user@host" instead of just "safesh host".
$PORT The port used on $HOST. Presently always 22.
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