NAME
mandos-client - Client for Mandos
SYNOPSIS
mandos-client [--connect ADDRESS:PORT | -c ADDRESS:PORT]
[--interface NAME | -i NAME]
[--pubkey FILE | -p FILE]
[--seckey FILE | -s FILE]
[--priority STRING]
[--dh-bits BITS]
[--delay SECONDS]
[--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
a network interface, uses the interface's IPv6 link-local address 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 is received. If no servers are
found, or after all servers have been tried, it waits indefinitely for
new servers to appear.
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.
This option is normally only useful for testing and debugging.
--interface=NAME, -i NAME
Network interface that will be brought up and scanned for Mandos
servers to connect to. The default is "eth0".
If the --connect option is used, 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 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.
NAME can be the empty string; this will not use any specific
interface, and will not bring up an interface on startup. 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". See
gnutls_priority_init(3) for the syntax. Warning: changing this may
make the TLS handshake fail, making server-client communication
impossible.
--dh-bits=BITS
Sets the number of bits to use for the prime number in the TLS
Diffie-Hellman key exchange. Default is 1024.
--delay=SECONDS
After bringing the 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.
--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 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 new Mandos servers as they appear, trying to
get a decryptable password and print it.
ENVIRONMENT
This program does not use any environment variables, not even the ones
provided by cryptsetup(8).
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.
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 interface is "eth0":
mandos-client
Search for Mandos servers (and connect to them) using another
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, 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.
SEE ALSO
cryptsetup(8), crypttab(5), mandos(8), password-prompt(8mandos),
plugin-runner(8mandos)
Zeroconf[1]
Zeroconf is the network protocol standard used for finding Mandos
servers on the local network.
Avahi[2]
Avahi is the library this program calls to find Zeroconf services.
GnuTLS[3]
GnuTLS is the library this client uses to implement TLS for
communicating securely with the server, and at the same time send
the public OpenPGP key to the server.
GPGME[4]
GPGME is the library used to decrypt the OpenPGP data sent by the
server.
RFC 4291: IP Version 6 Addressing Architecture
Section 2.2: Text Representation of Addresses
Section 2.5.5.2: IPv4-Mapped IPv6 Address
Section 2.5.6, Link-Local IPv6 Unicast Addresses
This client uses IPv6 link-local addresses, which are
immediately usable since a link-local addresses is
automatically assigned to a network interfaces when it is
brought up.
RFC 4346: The Transport Layer Security (TLS) Protocol Version 1.1
TLS 1.1 is the protocol implemented by GnuTLS.
RFC 4880: OpenPGP Message Format
The data received from the server is binary encrypted OpenPGP data.
RFC 5081: Using OpenPGP Keys for Transport Layer Security
This is implemented by GnuTLS and used by this program so that
OpenPGP keys can be used.
COPYRIGHT
Copyright (C) 2008-2009 Teddy Hogeborn, Bjorn Pahlsson
This manual page 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.
This manual page 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 this program. If not, see http://www.gnu.org/licenses/.
NOTES
1. Zeroconf
http://www.zeroconf.org/
2. Avahi
http://www.avahi.org/
3. GnuTLS
http://www.gnu.org/software/gnutls/
4. GPGME
http://www.gnupg.org/related_software/gpgme/