epmd - Online in the Cloud

PROGRAM:

NAME

epmd - Erlang Port Mapper Daemonepmd [-d|-debug] [DbgExtra...] [-address Addresses] [-port
No] [-daemon] [-relaxed_command_check]Starts the port mapper daemonepmd [-d|-debug] [-port
No] [-names|-kill|-stop Name]Communicates with a running port mapper daemon

DESCRIPTION

This daemon acts as a name server on all hosts involved in distributed Erlang
computations. When an Erlang node starts, the node has a name and it obtains an address
from the host OS kernel. The name and the address are sent to the epmd daemon running on
the local host. In a TCP/IP environment, the address consists of the IP address and a port
number. The name of the node is an atom on the form of Name@Node. The job of the epmd
daemon is to keep track of which node name listens on which address. Hence, epmd maps
symbolic node names to machine addresses.

The TCP/IP epmd daemon actually only keeps track of the Name (first) part of an Erlang
node name. The Host part (whatever is after the @) is implicit in the node name where the
epmd daemon was actually contacted, as is the IP address where the Erlang node can be
reached. Consistent and correct TCP naming services are therefore required for an Erlang
network to function correctly.

Starting the port mapper daemon:
The daemon is started automatically by the erl command if the node is to be
distributed and there is no running instance present. If automatically launched,
environment variables have to be used to alter the behavior of the daemon. See the
Environment variables section below.

If the -daemon argument is not given, epmd runs as a normal program with the
controlling terminal of the shell in which it is started. Normally, it should run as a
daemon.

Regular start-up options are described in the Regular options section below.

The DbgExtra options are described in the DbgExtra options section below.

Communicating with a running port mapper daemon:
Communicating with the running epmd daemon by means of the epmd program is done
primarily for debugging purposes.

The different queries are described in the Interactive options section below.

REGULAR OPTIONS

These options are available when starting the actual name server. The name server is
normally started automatically by the erl command (if not already available), but it can
also be started at i.e. system start-up.

-address List:
Let this instance of epmd listen only on the comma-separated list of IP addresses and
on the loopback address (which is implicitly added to the list if it has not been
specified). This can also be set using the ERL_EPMD_ADDRESS environment variable. See
the section Environment variables below.

-port No:
Let this instance of epmd listen to another TCP port than default 4369. This can also
be set using the ERL_EPMD_PORT environment variable. See the section Environment
variables below

-d | -debug:
Enable debug output. The more -d flags given, the more debug output you will get (to a
certain limit). This option is most useful when the epmd daemon is not started as a
daemon.

-daemon:
Start epmd detached from the controlling terminal. Logging will end up in syslog when
available and correctly configured. If the epmd daemon is started at boot, this option
should definitely be used. It is also used when the erl command automatically starts
epmd.

-relaxed_command_check:
Start the epmd program with relaxed command checking (mostly for backward
compatibility). This affects the following:

* With relaxed command checking, the epmd daemon can be killed from the localhost with
i.e. epmd -kill even if there are active nodes registered. Normally only daemons
with an empty node database can be killed with the epmd -kill command.

* The epmd -stop command (and the corresponding messages to epmd, as can be given
using erl_interface/ei) is normally always ignored, as it opens up the possibility
of a strange situation where two nodes of the same name can be alive at the same
time. A node unregisters itself by just closing the connection to epmd, which is why
the stop command was only intended for use in debugging situations.

With relaxed command checking enabled, you can forcibly unregister live nodes.

Relaxed command checking can also be enabled by setting the environment variable
ERL_EPMD_RELAXED_COMMAND_CHECK prior to starting epmd.

Only use relaxed command checking on systems with very limited interactive usage.

DBGEXTRA OPTIONS

These options are purely for debugging and testing epmd clients. They should not be used
in normal operation.

-packet_timeout Seconds:
Set the number of seconds a connection can be inactive before epmd times out and
closes the connection (default 60).

-delay_accept Seconds:
To simulate a busy server you can insert a delay between when epmd gets notified that
a new connection is requested and when the connection gets accepted.

-delay_write Seconds:
Also a simulation of a busy server. Inserts a delay before a reply is sent.

INTERACTIVE OPTIONS

These options make epmd run as an interactive command, displaying the results of sending
queries to an already running instance of epmd. The epmd contacted is always on the local
node, but the -port option can be used to select between instances if several are running
using different ports on the host.

-port No:
Contacts the epmd listening on the given TCP port number (default 4369). This can also
be set using the ERL_EPMD_PORT environment variable. See the section Environment
variables below.

-names:
List names registered with the currently running epmd

-kill:
Kill the currently running epmd.

Killing the running epmd is only allowed if epmd -names shows an empty database or
-relaxed_command_check was given when the running instance of epmd was started. Note
that -relaxed_command_check is given when starting the daemon that is to accept
killing when it has live nodes registered. When running epmd interactively,
-relaxed_command_check has no effect. A daemon that is started without relaxed command
checking has to be killed using i.e. signals or some other OS specific method if it
has active clients registered.

-stop Name:
Forcibly unregister a live node from epmd's database

This command can only be used when contacting epmd instances started with the
-relaxed_command_check flag. Note that relaxed command checking has to be enabled for
the epmd daemon contacted. When running epmd interactively, -relaxed_command_check has
no effect.

ENVIRONMENT VARIABLES

ERL_EPMD_ADDRESS:
This environment variable may be set to a comma-separated list of IP addresses, in
which case the epmd daemon will listen only on the specified address(es) and on the
loopback address (which is implicitly added to the list if it has not been specified).
The default behaviour is to listen on all available IP addresses.

ERL_EPMD_PORT:
This environment variable can contain the port number epmd will use. The default port
will work fine in most cases. A different port can be specified to allow several
instances of epmd, representing independent clusters of nodes, to co-exist on the same
host. All nodes in a cluster must use the same epmd port number.

ERL_EPMD_RELAXED_COMMAND_CHECK:
If set prior to start, the epmd daemon will behave as if the -relaxed_command_check
option was given at start-up. Consequently, if this option is set before starting the
Erlang virtual machine, the automatically started epmd will accept the -kill and -stop
commands without restrictions.

LOGGING

On some operating systems syslog will be used for error reporting when epmd runs as an
daemon. To enable the error logging you have to edit /etc/syslog.conf file and add an
entry

!epmd
*.*<TABs>/var/log/epmd.log

where <TABs> are at least one real tab character. Spaces will silently be ignored.

ACCESS RESTRICTIONS

The epmd daemon accepts messages from both localhost and remote hosts. However, only the
query commands are answered (and acted upon) if the query comes from a remote host. It is
always an error to try to register a nodename if the client is not a process located on
the same host as the epmd instance is running on- such requests are considered hostile and
the connection is immediately closed.

The queries accepted from remote nodes are:

* Port queries - i.e. on which port does the node with a given name listen

* Name listing - i.e. give a list of all names registered on the host

To restrict access further, firewall software has to be used.

Use epmd online using onworks.net services