Ubuntu Manpage: mandos-client - Client for Mandos

name
synopsis
description
purpose
options
overview
exit status
environment
network hooks
files
bugs
example
security
see also
copyright
notes

Provided by: mandos-client_1.7.19-1_amd64

NAME

       mandos-client - Client for Mandos

SYNOPSIS

       mandos-client [--connect ADDRESS:PORT | -c ADDRESS:PORT]
                     [--interface NAME[,NAME...] | -i NAME[,NAME...]...]
                     [--pubkey FILE | -p FILE]
                     [--seckey FILE | -s FILE]
                     [--priority STRING]
                     [--dh-bits BITS]
                     [--dh-params FILE]
                     [--delay SECONDS]
                     [--retry SECONDS]
                     [--network-hook-dir DIR]
                     [--debug]

       mandos-client {--help | -?}

       mandos-client --usage

       mandos-client {--version | -V}

DESCRIPTION

mandos-client is a client program that communicates with mandos(8) to get a password. In slightly more
detail, this client program brings up network interfaces, uses the interfaces’ IPv6 link-local addresses
to get network connectivity, uses Zeroconf to find servers on the local network, and communicates with
servers using TLS with an OpenPGP key to ensure authenticity and confidentiality. This client program
keeps running, trying all servers on the network, until it receives a satisfactory reply or a TERM
signal. After all servers have been tried, all servers are periodically retried. If no servers are found
it will wait indefinitely for new servers to appear.

The network interfaces are selected like this: If any interfaces are specified using the --interface
option, those interface are used. Otherwise, mandos-client will use all interfaces that are not loopback
interfaces, are not point-to-point interfaces, are capable of broadcasting and do not have the NOARP flag
(see netdevice(7)). (If the --connect option is used, point-to-point interfaces and non-broadcast
interfaces are accepted.) If any used interfaces are not up and running, they are first taken up (and
later taken down again on program exit).

Before network interfaces are selected, all “network hooks” are run; see the section called “NETWORK
HOOKS”.

This program is not meant to be run directly; it is really meant to run as a plugin of the Mandos plugin-
runner(8mandos), which runs in the initial RAM disk environment because it is specified as a “keyscript”
in the crypttab(5) file.

PURPOSE

       The purpose of this is to enable remote and unattended rebooting of client host computer with an
       encrypted root file system. See the section called “OVERVIEW” for details.

OPTIONS

This program is commonly not invoked from the command line; it is normally started by the Mandos plugin
runner, see plugin-runner(8mandos). Any command line options this program accepts are therefore normally
provided by the plugin runner, and not directly.

--connect=ADDRESS:PORT, -c ADDRESS:PORT
Do not use Zeroconf to locate servers. Connect directly to only one specified Mandos server. Note
that an IPv6 address has colon characters in it, so the last colon character is assumed to separate
the address from the port number.

Normally, Zeroconf would be used to locate Mandos servers, in which case this option would only be
used when testing and debugging.

--interface=NAME[,NAME...], -i NAME[,NAME...]
Comma separated list of network interfaces that will be brought up and scanned for Mandos servers to
connect to. The default is the empty string, which will automatically use all appropriate interfaces.

If the --connect option is used, and exactly one interface name is specified (except “none”), this
specifies the interface to use to connect to the address given.

Note that since this program will normally run in the initial RAM disk environment, the interface
must be an interface which exists at that stage. Thus, the interface can normally not be a
pseudo-interface such as “br0” or “tun0”; such interfaces will not exist until much later in the boot
process, and can not be used by this program, unless created by a “network hook” — see the section
called “NETWORK HOOKS”.

NAME can be the string “none”; this will make mandos-client only bring up interfaces specified before
this string. This is not recommended, and only meant for advanced users.

--pubkey=FILE, -p FILE
OpenPGP public key file name. The default name is “/conf/conf.d/mandos/pubkey.txt”.

--seckey=FILE, -s FILE
OpenPGP secret key file name. The default name is “/conf/conf.d/mandos/seckey.txt”.

--priority=STRING
GnuTLS priority string for the TLS handshake. The default is
“SECURE256:!CTYPE-X.509:+CTYPE-OPENPGP:!RSA :+SIGN-DSA-SHA256”. See gnutls_priority_init(3) for the
syntax. Warning: changing this may make the TLS handshake fail, making server-client communication
impossible. Changing this option may also make the network traffic decryptable by an attacker.

--dh-bits=BITS
Sets the number of bits to use for the prime number in the TLS Diffie-Hellman key exchange. The
default value is selected automatically based on the OpenPGP key. Note that if the --dh-params option
is used, the values from that file will be used instead.

--dh-params=FILE
Specifies a PEM-encoded PKCS#3 file to read the parameters needed by the TLS Diffie-Hellman key
exchange from. If this option is not given, or if the file for some reason could not be used, the
parameters will be generated on startup, which will take some time and processing power. Those using
servers running under time, power or processor constraints may want to generate such a file in
advance and use this option.

--delay=SECONDS
After bringing a network interface up, the program waits for the interface to arrive in a “running”
state before proceeding. During this time, the kernel log level will be lowered to reduce clutter on
the system console, alleviating any other plugins which might be using the system console. This
option sets the upper limit of seconds to wait. The default is 2.5 seconds.

--retry=SECONDS
All Mandos servers are tried repeatedly until a password is received. This value specifies, in
seconds, how long between each successive try for the same server. The default is 10 seconds.

--network-hook-dir=DIR
Network hook directory. The default directory is “/lib/mandos/network-hooks.d”.

--debug
Enable debug mode. This will enable a lot of output to standard error about what the program is
doing. The program will still perform all other functions normally.

It will also enable debug mode in the Avahi and GnuTLS libraries, making them print large amounts of
debugging output.

--help, -?
Gives a help message about options and their meanings.

--usage
Gives a short usage message.

--version, -V
Prints the program version.

OVERVIEW

       This is part of the Mandos system for allowing computers to have encrypted root file systems and at the
       same time be capable of remote and/or unattended reboots. The computers run a small client program in the
       initial RAM disk environment which will communicate with a server over a network. All network
       communication is encrypted using TLS. The clients are identified by the server using an OpenPGP key; each
       client has one unique to it. The server sends the clients an encrypted password. The encrypted password
       is decrypted by the clients using the same OpenPGP key, and the password is then used to unlock the root
       file system, whereupon the computers can continue booting normally.

       This program is the client part. It is a plugin started by plugin-runner(8mandos) which will run in an
       initial RAM disk environment.

       This program could, theoretically, be used as a keyscript in /etc/crypttab, but it would then be
       impossible to enter a password for the encrypted root disk at the console, since this program does not
       read from the console at all. This is why a separate plugin runner (plugin-runner(8mandos)) is used to
       run both this program and others in in parallel, one of which (password-prompt(8mandos)) will prompt for
       passwords on the system console.

EXIT STATUS

       This program will exit with a successful (zero) exit status if a server could be found and the password
       received from it could be successfully decrypted and output on standard output. The program will exit
       with a non-zero exit status only if a critical error occurs. Otherwise, it will forever connect to any
       discovered Mandos servers, trying to get a decryptable password and print it.

ENVIRONMENT

       MANDOSPLUGINHELPERDIR
           This environment variable will be assumed to contain the directory containing any helper executables.
           The use and nature of these helper executables, if any, is purposefully not documented.

       This program does not use any other environment variables, not even the ones provided by cryptsetup(8).

NETWORK HOOKS

If a network interface like a bridge or tunnel is required to find a Mandos server, this requires the
interface to be up and running before mandos-client starts looking for Mandos servers. This can be
accomplished by creating a “network hook” program, and placing it in a special directory.

Before the network is used (and again before program exit), any runnable programs found in the network
hook directory are run with the argument “start” or “stop”. This should bring up or down, respectively,
any network interface which mandos-client should use.

REQUIREMENTS
A network hook must be an executable file, and its name must consist entirely of upper and lower case
letters, digits, underscores, periods, and hyphens.

A network hook will receive one argument, which can be one of the following:

start
This should make the network hook create (if necessary) and bring up a network interface.

stop
This should make the network hook take down a network interface, and delete it if it did not exist
previously.

files
This should make the network hook print, one file per line, all the files needed for it to run.
(These files will be copied into the initial RAM filesystem.) Typical use is for a network hook which
is a shell script to print its needed binaries.

It is not necessary to print any non-executable files already in the network hook directory, these
will be copied implicitly if they otherwise satisfy the name requirements.

modules
This should make the network hook print, on separate lines, all the kernel modules needed for it to
run. (These modules will be copied into the initial RAM filesystem.) For instance, a tunnel interface
needs the “tun” module.

The network hook will be provided with a number of environment variables:

MANDOSNETHOOKDIR
The network hook directory, specified to mandos-client by the --network-hook-dir option. Note: this
should always be used by the network hook to refer to itself or any files in the hook directory it
may require.

DEVICE
The network interfaces, as specified to mandos-client by the --interface option, combined to one
string and separated by commas. If this is set, and does not contain the interface a hook will bring
up, there is no reason for a hook to continue.

MODE
This will be the same as the first argument; i.e. “start”, “stop”, “files”, or “modules”.

VERBOSITY
This will be the “1” if the --debug option is passed to mandos-client, otherwise “0”.

DELAY
This will be the same as the --delay option passed to mandos-client. Is only set if MODE is “start”
or “stop”.

CONNECT
This will be the same as the --connect option passed to mandos-client. Is only set if --connect is
passed and MODE is “start” or “stop”.

A hook may not read from standard input, and should be restrictive in printing to standard output or
standard error unless VERBOSITY is “1”.

FILES

       /conf/conf.d/mandos/pubkey.txt, /conf/conf.d/mandos/seckey.txt
           OpenPGP public and private key files, in “ASCII Armor” format. These are the default file names, they
           can be changed with the --pubkey and --seckey options.

       /lib/mandos/network-hooks.d
           Directory where network hooks are located. Change this with the --network-hook-dir option. See the
           section called “NETWORK HOOKS”.

BUGS

       Please report bugs to the Mandos development mailing list: <mandos-dev@recompile.se> (subscription
       required). Note that this list is public. The developers can be reached privately at
       <mandos@recompile.se> (OpenPGP key fingerprint 153A 37F1 0BBA 0435 987F 2C4A 7223 2973 CA34 C2C4 for
       encrypted mail).

EXAMPLE

       Note that normally, command line options will not be given directly, but via options for the Mandos
       plugin-runner(8mandos).

       Normal invocation needs no options, if the network interfaces can be automatically determined:

       mandos-client

       Search for Mandos servers (and connect to them) using one specific interface:

       mandos-client --interface eth1

       Run in debug mode, and use a custom key:

       mandos-client --debug --pubkey keydir/pubkey.txt --seckey keydir/seckey.txt

       Run in debug mode, with a custom key, and do not use Zeroconf to locate a server; connect directly to the
       IPv6 link-local address “fe80::aede:48ff:fe71:f6f2”, port 4711, using interface eth2:

       mandos-client --debug --pubkey keydir/pubkey.txt --seckey keydir/seckey.txt --connect
       fe80::aede:48ff:fe71:f6f2:4711 --interface eth2

SECURITY

This program is set-uid to root, but will switch back to the original (and presumably non-privileged)
user and group after bringing up the network interface.

To use this program for its intended purpose (see the section called “PURPOSE”), the password for the
root file system will have to be given out to be stored in a server computer, after having been encrypted
using an OpenPGP key. This encrypted data which will be stored in a server can only be decrypted by the
OpenPGP key, and the data will only be given out to those clients who can prove they actually have that
key. This key, however, is stored unencrypted on the client side in its initial RAM disk image file
system. This is normally readable by all, but this is normally fixed during installation of this program;
file permissions are set so that no-one is able to read that file.

The only remaining weak point is that someone with physical access to the client hard drive might turn
off the client computer, read the OpenPGP keys directly from the hard drive, and communicate with the
server. To safeguard against this, the server is supposed to notice the client disappearing and stop
giving out the encrypted data. Therefore, it is important to set the timeout and checker interval values
tightly on the server. See mandos(8).

It will also help if the checker program on the server is configured to request something from the client
which can not be spoofed by someone else on the network, like SSH server key fingerprints, and unlike
unencrypted ICMP echo (“ping”) replies.

Note: This makes it completely insecure to have Mandos clients which dual-boot to another operating
system which is not trusted to keep the initial RAM disk image confidential.

COPYRIGHT

       Copyright © 2008-2018 Teddy Hogeborn, Björn Påhlsson

       This manual page is part of Mandos.

       Mandos is free software: you can redistribute it and/or modify it under the terms of the GNU General
       Public License as published by the Free Software Foundation, either version 3 of the License, or (at your
       option) any later version.

       Mandos is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the
       implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public
       License for more details.

       You should have received a copy of the GNU General Public License along with Mandos. If not, see
       http://www.gnu.org/licenses/.

NOTES

        1. Zeroconf
           http://www.zeroconf.org/

        2. Avahi
           http://www.avahi.org/

        3. GnuTLS
           https://www.gnutls.org/

        4. GPGME
           https://www.gnupg.org/related_software/gpgme/