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mandos

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NAME

mandos - Gives encrypted passwords to authenticated Mandos clients

SYNOPSIS

mandos [--interface NAME | -i NAME]

[--address ADDRESS | -a ADDRESS]

[--port PORT | -p PORT]

[--priority PRIORITY]

[--servicename NAME]

[--configdir DIRECTORY]

[--debug]

[--no-ipv6]

mandos {--help | -h}

mandos --version

mandos --check

DESCRIPTION

mandos

is a server daemon which handles incoming request for passwords for a pre-defined list of client host computers. The Mandos server uses Zeroconf to announce itself on the local network, and uses TLS to communicate securely with and to authenticate the clients. The Mandos server uses IPv6 to allow Mandos clients to use IPv6 link-local addresses, since the clients will probably not have any other addresses configured (see the section called "OVERVIEW"). Any authenticated client is then given the stored pre-encrypted password for that specific client.

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

--help, -h

Show a help message and exit

--interface NAME, -i NAME

If this is specified, the server will only announce the service and listen to requests on the specified network interface. Default is to use all available interfaces. Note: a failure to bind to the specified interface is not considered critical, and the server will not exit, but instead continue normally.

--address ADDRESS, -a ADDRESS

If this option is used, the server will only listen to the specified IPv6 address. If a link-local address is specified, an interface should be set, since a link-local address is only valid on a single interface. By default, the server will listen to all available addresses. If set, this must normally be an IPv6 address; an IPv4 address can only be specified using IPv4-mapped IPv6 address syntax: "::FFFF:192.0.2.3". (Only if IPv6 usage is disabled (see below) must this be an IPv4 address.)

--port PORT, -p PORT

If this option is used, the server will bind to that port. By default, the server will listen to an arbitrary port given by the operating system.

--check

Run the server's self-tests. This includes any unit tests, etc.

--debug

If the server is run in debug mode, it will run in the foreground and print a lot of debugging information. The default is to not run in debug mode.

--priority PRIORITY

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.

--servicename NAME

Zeroconf service name. The default is "Mandos". This only needs to be changed if for some reason is would be necessary to run more than one server on the same host. This would not normally be useful. If there are name collisions on the same network, the newer server will automatically rename itself to "Mandos #2", and so on; therefore, this option is not needed in that case.

--configdir DIRECTORY

Directory to search for configuration files. Default is "/etc/mandos". See mandos.conf(5) and mandos-clients.conf(5).

--version

Prints the program version and exit.

--no-ipv6

This option controls whether the server will use IPv6 sockets and addresses. The default is to use IPv6. This option should never normally be turned off, even in IPv4-only environments. This is because mandos-client(8mandos) will normally use IPv6 link-local addresses, and will not be able to find or connect to the server if this option is turned off. Only advanced users should consider changing this option.

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 server part. It is a normal server program and will run in a normal system environment, not in an initial RAM disk environment.

NETWORK PROTOCOL

The Mandos server announces itself as a Zeroconf service of type "_mandos._tcp". The Mandos client connects to the announced address and port, and sends a line of text where the first whitespace-separated field is the protocol version, which currently is "1". The client and server then start a TLS protocol handshake with a slight quirk: the Mandos server program acts as a TLS "client" while the connecting Mandos client acts as a TLS "server". The Mandos client must supply an OpenPGP certificate, and the fingerprint of this certificate is used by the Mandos server to look up (in a list read from clients.conf at start time) which binary blob to give the client. No other authentication or authorization is done by the server.

Table 1. Mandos Protocol (Version 1)

Mandos Client	Direction	Mandos Server
Connect	->
"1\r\n"	->
TLS handshake as TLS "server"	<->	TLS handshake as TLS "client"
OpenPGP public key (part of TLS handshake)	->
	<-	Binary blob (client will assume OpenPGP data)
	<-	Close

CHECKING

The server will, by default, continually check that the clients are still up. If a client has not been confirmed as being up for some time, the client is assumed to be compromised and is no longer eligible to receive the encrypted password. (Manual intervention is required to re-enable a client.) The timeout, checker program, and interval between checks can be configured both globally and per client; see mandos-clients.conf(5). A client successfully receiving its password will also be treated as a successful checker run.

LOGGING

The server will send log message with various severity levels to /dev/log. With the --debug option, it will log even more messages, and also show them on the console.

EXIT STATUS

The server will exit with a non-zero exit status only when a critical error is encountered.

ENVIRONMENT

PATH

To start the configured checker (see the section called "CHECKING"), the server uses /bin/sh, which in turn uses PATH to search for matching commands if an absolute path is not given. See sh(1).

FILES

Use the --configdir option to change where mandos looks for its configurations files. The default file names are listed here.

/etc/mandos/mandos.conf

Server-global settings. See mandos.conf(5) for details.

/etc/mandos/clients.conf

List of clients and client-specific settings. See mandos-clients.conf(5) for details.

/var/run/mandos.pid

The file containing the process id of mandos.

/dev/log

The Unix domain socket to where local syslog messages are sent.

/bin/sh

This is used to start the configured checker command for each client. See mandos-clients.conf(5) for details.

BUGS

This server might, on especially fatal errors, emit a Python backtrace. This could be considered a feature.

Currently, if a client is declared "invalid" due to having timed out, the server does not record this fact onto permanent storage. This has some security implications, see the section called "CLIENTS".

There is currently no way of querying the server of the current status of clients, other than analyzing its syslog output.

There is no fine-grained control over logging and debug output.

Debug mode is conflated with running in the foreground.

The console log messages do not show a time stamp.

This server does not check the expire time of clients' OpenPGP keys.

EXAMPLE

Normal invocation needs no options:

mandos

Run the server in debug mode, read configuration files from the ~/mandos directory, and use the Zeroconf service name "Test" to not collide with any other official Mandos server on this host:

mandos --debug --configdir ~/mandos --servicename Test

Run the server normally, but only listen to one interface and only on the link-local address on that interface:

mandos --interface eth7 --address fe80::aede:48ff:fe71:f6f2

SECURITY

SERVER

Running this mandos server program should not in itself present any security risk to the host computer running it. The program switches to a non-root user soon after startup.

CLIENTS

The server only gives out its stored data to clients which does have the OpenPGP key of the stored fingerprint. This is guaranteed by the fact that the client sends its OpenPGP public key in the TLS handshake; this ensures it to be genuine. The server computes the fingerprint of the key itself and looks up the fingerprint in its list of clients. The clients.conf file (see mandos-clients.conf(5)) must be made non-readable by anyone except the user starting the server (usually root).

As detailed in the section called "CHECKING", the status of all client computers will continually be checked and be assumed compromised if they are gone for too long.

If a client is compromised, its downtime should be duly noted by the server which would therefore declare the client invalid. But if the server was ever restarted, it would re-read its client list from its configuration file and again regard all clients therein as valid, and hence eligible to receive their passwords. Therefore, be careful when restarting servers if it is suspected that a client has, in fact, been compromised by parties who may now be running a fake Mandos client with the keys from the non-encrypted initial RAM image of the client host. What should be done in that case (if restarting the server program really is necessary) is to stop the server program, edit the configuration file to omit any suspect clients, and restart the server program.

For more details on client-side security, see mandos-client(8mandos).

SEE ALSO

mandos-clients.conf(5), mandos.conf(5), mandos-client(8mandos), sh(1)

m[blue]Zeroconfm[][1]

Zeroconf is the network protocol standard used by clients for finding this Mandos server on the local network.

m[blue]Avahim[][2]

Avahi is the library this server calls to implement Zeroconf service announcements.

m[blue]GnuTLSm[][3]

GnuTLS is the library this server uses to implement TLS for communicating securely with the client, and at the same time confidently get the client's public OpenPGP key.

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

The clients use 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 sent to clients is binary encrypted OpenPGP data.

RFC 5081: Using OpenPGP Keys for Transport Layer Security

This is implemented by GnuTLS and used by this server so that OpenPGP keys can be used.

COPYRIGHT

Copyright © 2008-2009 Teddy Hogeborn, Björn Påhlsson

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 m[blue]http://www.gnu.org/licenses/m[].

NOTES

1.

Zeroconf

http://www.zeroconf.org/

2.

Avahi

http://www.avahi.org/

3.

GnuTLS

http://www.gnu.org/software/gnutls/