mandos

DESCRIPTION

mandos is a server daemon which handles incoming requests for passwords for a pre-defined list of client host computers. For an introduction, see intro(8mandos). 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.

--debuglevel LEVEL

Set the debugging log level. LEVEL is a string, one of “CRITICAL”, “ERROR”, “WARNING”, “INFO”, or “DEBUG”, in order of increasing verbosity. The default level is “WARNING”.

--priority PRIORITY

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.

--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-dbus

This option controls whether the server will provide a D-Bus system bus interface. The default is to provide such an interface.

--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.

--no-restore

This option controls whether the server will restore its state from the last time it ran. Default is to restore last state.

--statedir DIRECTORY

Directory to save (and restore) state in. Default is “/var/lib/mandos”.

--socket FD

If this option is used, the server will not create a new network socket, but will instead use the supplied file descriptor. By default, the server will create a new network socket.

--foreground

This option will make the server run in the foreground and not write a PID file. The default is to not run in the foreground, except in debug mode, which implies this option.

--no-zeroconf

This option controls whether the server will announce its existence using Zeroconf. Default is to use Zeroconf. If Zeroconf is not used, a port number or a socket is required.

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 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 a TLS public key, and the key ID of this public key 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”
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, extended timeout, checker program, and interval between checks can be configured both globally and per client; see mandos-clients.conf(5).

APPROVAL

The server can be configured to require manual approval for a client before it is sent its secret. The delay to wait for such approval and the default action (approve or deny) can be configured both globally and per client; see mandos-clients.conf(5). By default all clients will be approved immediately without delay.

This can be used to deny a client its secret if not manually approved within a specified time. It can also be used to make the server delay before giving a client its secret, allowing optional manual denying of this specific client.

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.

PERSISTENT STATE

Client settings, initially read from clients.conf, are persistent across restarts, and run-time changes will override settings in clients.conf. However, if a setting is changed (or a client added, or removed) in clients.conf, this will take precedence.

D-BUS INTERFACE

The server will by default provide a D-Bus system bus interface. This interface will only be accessible by the root user or a Mandos-specific user, if such a user exists. For documentation of the D-Bus API, see the file DBUS-API.

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.
/run/mandos.pid: The file containing the process id of the mandos process started last. Note: If the /run directory does not exist, /var/run/mandos.pid will be used instead.
/var/lib/mandos: Directory where persistent state will be saved. Change this with the --statedir option. See also the --no-restore option.
/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.

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

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

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 correct key ID of the stored key ID. This is guaranteed by the fact that the client sends its public key in the TLS handshake; this ensures it to be genuine. The server computes the key ID of the key itself and looks up the key ID 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.

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

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