<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE comref SYSTEM "comref.dtd">
<comref>
<header>
<copyright>
<year>1996</year><year>2016</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
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http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
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<title>epmd</title>
<prepared>Claes Wikström</prepared>
<responsible></responsible>
<docno>1</docno>
<approved></approved>
<checked></checked>
<date>1998-01-05</date>
<rev>A</rev>
<file>epmd.xml</file>
</header>
<com>epmd</com>
<comsummary>Erlang Port Mapper Daemon</comsummary>
<description>
<taglist>
<tag><c><![CDATA[epmd [-d|-debug] [DbgExtra...] [-address Addresses]
[-port No] [-daemon] [-relaxed_command_check]]]></c></tag>
<item>
<p>Starts the port mapper daemon.</p>
</item>
<tag><c><![CDATA[epmd [-d|-debug] [-port No]
[-names|-kill|-stop Name]]]></c></tag>
<item>
<p>Communicates with a running port mapper daemon.</p>
</item>
</taglist>
<p>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 address are sent to the
<c><![CDATA[epmd]]></c> daemon running on the local host.
In a TCP/IP environment, the address consists
of the IP address and a port number. The node name is
an atom on the form of <c><![CDATA[Name@Node]]></c>.
The job of the <c><![CDATA[epmd]]></c> daemon is to keep track of which
node name listens on which address. Hence, <c><![CDATA[epmd]]></c> maps
symbolic node names to machine addresses.</p>
<p>The TCP/IP <c>epmd</c> daemon only keeps track of
the <c>Name</c> (first) part of an Erlang node name. The <c>Host</c>
part (whatever is after the <c><![CDATA[@]]></c>) is implicit in the
node name where the <c>epmd</c> daemon was 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.</p>
<taglist>
<tag>Starting the port mapper daemon</tag>
<item>
<p>The daemon is started automatically by command
<seealso marker="erl"><c>erl(1)</c></seealso>
if the node is to be distributed and no running
instance is present. If automatically launched
environment variables must be used to change the behavior
of the daemon; see section
<seealso marker="#environment_variables">Environment
Variables</seealso>.</p>
<p>If argument <c>-daemon</c> is not specified,
<c><![CDATA[epmd]]></c> runs as a normal program with the
controlling terminal of the shell in which it is
started. Normally, it is to be run as a daemon.</p>
<p>Regular startup options are described in section
<seealso marker="#daemon_flags">Regular Options</seealso>.</p>
<p>The <c>DbgExtra</c> options are described in section
<seealso marker="#debug_flags">DbgExtra Options</seealso>.</p>
</item>
<tag>Communicating with a running port mapper daemon</tag>
<item>
<p>Communicating with the running <c>epmd</c> daemon by the
<c>epmd</c> program is done primarily for debugging purposes.</p>
<p>The different queries are described in section <seealso
marker="#interactive_flags">Interactive options</seealso>.</p>
</item>
</taglist>
</description>
<section>
<marker id="daemon_flags"></marker>
<title>Regular Options</title>
<p>These options are available when starting the name server. The name
server is normally started automatically by command
<seealso marker="erl"><c>erl(1)</c></seealso> (if not already available),
but it can also be started at system startup.</p>
<taglist>
<tag><c><![CDATA[-address List]]></c></tag>
<item>
<p>Lets this instance of <c>epmd</c> 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 environment variable
<c><![CDATA[ERL_EPMD_ADDRESS]]></c>; see section <seealso
marker="#environment_variables">Environment Variables</seealso>.</p>
</item>
<tag><c><![CDATA[-port No]]></c></tag>
<item>
<p>Lets this instance of <c>epmd</c> listen to another TCP port than
default 4369. This can also be set using environment variable
<c><![CDATA[ERL_EPMD_PORT]]></c>; see section <seealso
marker="#environment_variables">Environment Variables</seealso>.</p>
</item>
<tag><c><![CDATA[-d | -debug]]></c></tag>
<item>
<p>Enables debug output. The more <c>-d</c> flags specified, the more
debug output you will get (to a certain limit). This option is most
useful when the <c>epmd</c> daemon is not started as a daemon.</p>
</item>
<tag><c><![CDATA[-daemon]]></c></tag>
<item>
<p>Starts <c>epmd</c> detached from the controlling terminal. Logging
ends up in syslog when available and correctly configured. If the
<c>epmd</c> daemon is started at boot, this option is definitely
to be used. It is also used when command <c>erl</c> automatically
starts <c>epmd</c>.</p>
</item>
<tag><c><![CDATA[-relaxed_command_check]]></c></tag>
<item>
<p>Starts the <c>epmd</c> program with relaxed command checking
(mostly for backward compatibility). This affects the following:</p>
<list type="bulleted">
<item>
<p>With relaxed command checking, the <c>epmd</c> daemon can be
killed from the local host with, for example, command
<c>epmd -kill</c> even if active nodes are registered. Normally
only daemons with an empty node database can be killed with
<c>epmd -kill</c>.</p>
</item>
<item>
<p>Command <c>epmd -stop</c> (and the corresponding messages to
<c>epmd</c>, as can be specified using <seealso
marker="erl_interface:ei"><c>erl_interface:ei(3)</c></seealso>) is
normally always ignored. This because it can cause a strange
situation where two nodes of the same name can be alive at the
same time. A node unregisters itself by only closing the
connection to <c>epmd</c>, which is why command <c>stop</c>
was only intended for use in debugging situations.</p>
<p>With relaxed command checking enabled, you can forcibly
unregister live nodes.</p>
</item>
</list>
<p>Relaxed command checking can also be enabled by setting environment
variable <c>ERL_EPMD_RELAXED_COMMAND_CHECK</c> before starting
<c>epmd</c>.</p>
<p>Use relaxed command checking only on systems with very limited
interactive usage.</p>
</item>
</taglist>
</section>
<section>
<marker id="debug_flags"></marker>
<title>DbgExtra Options</title>
<note>
<p>These options are only for debugging and testing <c>epmd</c> clients.
They are not to be used in normal operation.</p>
</note>
<taglist>
<tag><c><![CDATA[-packet_timeout Seconds]]></c></tag>
<item>
<p>Sets the number of seconds a connection can be
inactive before <c>epmd</c> times out and closes the
connection. Defaults to 60.</p>
</item>
<tag><c><![CDATA[-delay_accept Seconds]]></c></tag>
<item>
<p>To simulate a busy server, you can insert a delay between when
<c>epmd</c> gets notified that a new connection is requested and
when the connection gets accepted.</p>
</item>
<tag><c><![CDATA[-delay_write Seconds]]></c></tag>
<item>
<p>Also a simulation of a busy server. Inserts
a delay before a reply is sent.</p>
</item>
</taglist>
</section>
<section>
<marker id="interactive_flags"></marker>
<title>Interactive Options</title>
<p>These options make <c>epmd</c> run as an interactive command,
displaying the results of sending queries to an already running
instance of <c>epmd</c>. The <c>epmd</c> contacted is always on the
local node, but option <c>-port</c> can be used to select between
instances if several are running using different ports on the host.</p>
<taglist>
<tag><c><![CDATA[-port No]]></c></tag>
<item>
<p>Contacts the <c>epmd</c> listening on the specified TCP port
number (default 4369). This can also be set using environment
variable <c><![CDATA[ERL_EPMD_PORT]]></c>; see section <seealso
marker="#environment_variables">Environment Variables</seealso>.</p>
</item>
<tag><c><![CDATA[-names]]></c></tag>
<item>
<p>Lists names registered with the currently running <c>epmd</c>.</p>
</item>
<tag><c><![CDATA[-kill]]></c></tag>
<item>
<p>Kills the currently running <c>epmd</c>.</p>
<p>Killing the running <c>epmd</c> is only allowed if
<c>epmd -names</c> shows an empty database or if
<c>-relaxed_command_check</c> was specified when the running
instance of <c>epmd</c> was started.</p>
<p>Notice that <c>-relaxed_command_check</c> is specified when
starting the daemon that is to accept killing when it has live
nodes registered. When running <c>epmd</c> interactively,
<c>-relaxed_command_check</c> has no effect. A daemon that is
started without relaxed command checking must be killed using,
for example, signals or some other OS-specific method if it has
active clients registered.</p>
</item>
<tag><c><![CDATA[-stop Name]]></c></tag>
<item>
<p>Forcibly unregisters a live node from the <c>epmd</c> database.</p>
<p>This command can only be used when contacting <c>epmd</c>
instances started with flag <c>-relaxed_command_check</c>.</p>
<p>Notice that relaxed command checking must enabled for the
<c>epmd</c> daemon contacted. When running <c>epmd</c>
interactively, <c>-relaxed_command_check</c> has no effect.</p>
</item>
</taglist>
</section>
<section>
<marker id="environment_variables"></marker>
<title>Environment Variables</title>
<taglist>
<tag><c><![CDATA[ERL_EPMD_ADDRESS]]></c></tag>
<item>
<p>Can be set to a comma-separated
list of IP addresses, in which case the <c>epmd</c> 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 behavior is to listen on
all available IP addresses.</p>
</item>
<tag><c><![CDATA[ERL_EPMD_PORT]]></c></tag>
<item>
<p>Can contain the port number <c>epmd</c> will use.
The default port will work fine in most cases. A different port can
be specified to allow several instances of <c>epmd</c>, representing
independent clusters of nodes, to co-exist on the same host.
All nodes in a cluster must use the same <c>epmd</c> port number.</p>
</item>
<tag><c><![CDATA[ERL_EPMD_RELAXED_COMMAND_CHECK]]></c></tag>
<item>
<p>If set before start, the <c>epmd</c> daemon behaves
as if option <c>-relaxed_command_check</c> was specified at
startup. Consequently, if this option is set before starting
the Erlang virtual machine, the automatically started
<c>epmd</c> accepts the <c>-kill</c> and <c>-stop</c>
commands without restrictions.</p>
</item>
</taglist>
</section>
<section>
<title>Logging</title>
<p>On some operating systems <em>syslog</em> will be used for
error reporting when <c>epmd</c> runs as a daemon. To enable
the error logging, you must edit the
<path unix="" windows="">/etc/syslog.conf</path> file and add an
entry:</p>
<code type="none"><![CDATA[
!epmd
*.*<TABs>/var/log/epmd.log
]]></code>
<p>where <c><TABs></c> are at least one real tab character.
Spaces are silently ignored.</p>
</section>
<section>
<title>Access Restrictions</title>
<p>The <c>epmd</c> daemon accepts messages from both the local host 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 node name if the client is not a process
on the same host as the <c>epmd</c> instance is running on. Such
requests are considered hostile and the connection is closed
immediately.</p>
<p>The following queries are accepted from remote nodes:</p>
<list type="bulleted">
<item>
<p>Port queries, that is, on which port the node with a specified
name listens</p>
</item>
<item>
<p>Name listing, that is, gives a list of all names registered on
the host</p>
</item>
</list>
<p>To restrict access further, firewall software must be used.</p>
</section>
</comref>
