Ubuntu Manpage: mandos-client - Client for Mandos

Provided by: mandos-client_1.8.15-1_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]
                     [--tls-privkey FILE | -t FILE]
                     [--tls-pubkey FILE | -T 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 a raw public
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”.

--tls-pubkey=FILE, -T FILE
TLS raw public key file name. The default name is
“/conf/conf.d/mandos/tls-pubkey.pem”.

--tls-privkey=FILE, -t FILE
TLS secret key file name. The default name is “/conf/conf.d/mandos/tls-privkey.pem”.

--priority=STRING
GnuTLS priority string for the TLS handshake. The default is

“SECURE128:!CTYPE-X.509:+CTYPE-RAWPK:!RSA:!VERS-ALL:+VERS-TLS1.3:%PROFILE_ULTRA” when
using raw public keys in TLS, and “SECURE256:!CTYPE-X.509:+CTYPE-OPENPGP:!RSA
:+SIGN-DSA-SHA256” when using OpenPGP keys in TLS,. 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 GnuTLS security
profile set in its priority string. 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 a TLS 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 a separate 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
           purposely 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.

       /conf/conf.d/mandos/tls-pubkey.pem, /conf/conf.d/mandos/tls-privkey.pem
           Public and private raw key files, in “PEM” format. These are the default file names,
           they can be changed with the --tls-pubkey and --tls-privkey 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 custom keys:

       mandos-client --debug --pubkey keydir/pubkey.txt --seckey keydir/seckey.txt --tls-pubkey
       keydir/tls-pubkey.pem --tls-privkey keydir/tls-privkey.pem

       Run in debug mode, with custom keys, 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 --tls-pubkey
       keydir/tls-pubkey.pem --tls-privkey keydir/tls-privkey.pem --connect
       fe80::aede:48ff:fe71:f6f2:4711 --interface eth2

SECURITY

This program assumes that it is set-uid to root, and 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 and TLS 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 © 2008-2019 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
           https://www.avahi.org/

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

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