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<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE comref SYSTEM "comref.dtd">
<comref>
<header>
<copyright>
<year>1996</year><year>2013</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>epmd</title>
<prepared>Claes Wikström</prepared>
<responsible></responsible>
<docno>1</docno>
<approved></approved>
<checked></checked>
<date>98-01-05</date>
<rev>A</rev>
<file>epmd.xml</file>
</header>
<com>epmd</com>
<comsummary>
<p>Erlang Port Mapper Daemon</p>
<taglist>
<tag><c><![CDATA[epmd [-d|-debug] [DbgExtra...] [-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>
</comsummary>
<description>
<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 the 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 name of the node 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> map
symbolic node names to machine addresses.</p>
<p>The TCP/IP <c>epmd</c> daemon actually 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 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.</p>
<taglist>
<tag>Starting the port mapper daemon</tag>
<item>
<p>The daemon is started automatically by the <c>erl</c>
command if the node is to be distributed and there is no
running instance present. If automatically launched,
environment variables has to be used to alter the behavior of
the daemon. See the <seealso
marker="#environment_variables">Environment
variables</seealso> section below.</p>
<p>If the -daemon argument is not given, the
<c><![CDATA[epmd]]></c> runs as a normal program with the
controlling terminal of the shell in which it is
started. Normally, it should run as a daemon.</p>
<p>Regular start-up options are described in the
<seealso marker="#daemon_flags">Regular options</seealso>
section below.</p>
<p>The <c>DbgExtra</c> options are described in the
<seealso marker="#debug_flags">DbgExtra options</seealso>
section below.</p>
</item>
<tag>Communicating with a running port mapper daemon</tag>
<item>
<p>Communicating with the running epmd daemon by means of the
<c>epmd</c> program is done primarily for debugging
purposes.</p>
<p>The different queries are described in the <seealso
marker="#interactive_flags">Interactive options</seealso>
section below.</p>
</item>
</taglist>
</description>
<section>
<marker id="daemon_flags"></marker>
<title>Regular options</title>
<p>These options are available when starting the actual name server. The name server is normally started automatically by the <c>erl</c> command (if not already available), but it can also be started at i.e. system start-up.</p>
<taglist>
<tag><c><![CDATA[-address List]]></c></tag>
<item>
<p>Let 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 the
<c><![CDATA[ERL_EPMD_ADDRESS]]></c> environment variable, see the
section <seealso marker="#environment_variables">Environment
variables</seealso> below.</p>
</item>
<tag><c><![CDATA[-port No]]></c></tag>
<item>
<p>Let this instance of epmd listen to another TCP port than
default 4369. This can also be set using the
<c><![CDATA[ERL_EPMD_PORT]]></c> environment variable, see the
section <seealso marker="#environment_variables">Environment
variables</seealso> below</p>
</item>
<tag><c><![CDATA[-d | -debug]]></c></tag>
<item>
<p>Enable debug output. The more <c>-d</c> 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.</p>
</item>
<tag><c><![CDATA[-daemon]]></c></tag>
<item>
<p>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 <c>erl</c> command automatically starts <c>epmd</c>.</p>
</item>
<tag><c><![CDATA[-relaxed_command_check]]></c></tag>
<item>
<p>Start the epmd 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 localhost with i.e. <c>epmd -kill</c> even if there are active nodes registered. Normally only daemons with an empty node database can be killed with the <c>epmd -kill</c> command.</p>
</item>
<item>
<p>The <c>epmd -stop</c> command (and the corresponding messages to epmd, as can be given using <c>erl_interface/ei</c>) is normally always ignored, as it opens up for strange situation when two nodes of the same name can be alive at the same time. A node unregisters itself by just closing the connection to epmd, why the <c>stop</c> command 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 the environment variable <c>ERL_EPMD_RELAXED_COMMAND_CHECK</c> prior to starting <c>epmd</c>.</p>
<p>Only use relaxed command checking on systems with very limited interactive usage.</p>
</item>
</taglist>
</section>
<section>
<marker id="debug_flags"></marker>
<title>DbgExtra options</title>
<p>These options are purely for debugging and testing epmd clients, they should not be used in normal operation.</p>
<taglist>
<tag><c><![CDATA[-packet_timeout Seconds]]></c></tag>
<item>
<p>Set the number of seconds a connection can be
inactive before epmd times out and closes the
connection (default 60).</p>
</item>
<tag><c><![CDATA[-delay_accept Seconds]]></c></tag>
<item>
<p>To simulate a busy server you can insert a delay between epmd
gets notified about that a new connection is requested and
when the connections 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 ta an already running instance of <c>epmd</c>. The epmd contacted is always on the local node, but the <c>-port</c> option can be used to select between instances if several are running using different port on the host.</p>
<taglist>
<tag><c><![CDATA[-port No]]></c></tag>
<item>
<p>Contacts the <c>epmd</c> listening on the given TCP port number
(default 4369). This can also be set using the
<c><![CDATA[ERL_EPMD_PORT]]></c> environment variable, see the
section <seealso marker="#environment_variables">Environment
variables</seealso> below</p>
</item>
<tag><c><![CDATA[-names]]></c></tag>
<item>
<p>List names registered with the currently running epmd</p>
</item>
<tag><c><![CDATA[-kill]]></c></tag>
<item>
<p>Kill the currently running <c>epmd</c>.</p>
<p>Killing the running <c>epmd</c> is only allowed if <c>epmd
-names</c> show an empty database or
<c>-relaxed_command_check</c> was given when the running
instance of <c>epmd</c> was started. Note that
<c>-relaxed_command_check</c> is given when starting the
daemon that is to accept killing when it has live nodes
registered. When running epmd interactively,
<c>-relaxed_command_check</c> 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.</p>
</item>
<tag><c><![CDATA[-stop Name]]></c></tag>
<item>
<p>Forcibly unregister a live node from <c>epmd</c>'s database</p>
<p>This command can only be used when contacting <c>epmd</c>
instances started with the <c>-relaxed_command_check</c>
flag. Note that relaxed command checking has to be enabled for
the <c>epmd</c> daemon contacted, When running epmd
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>This environment variable may 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 behaviour is to listen on
all available IP addresses.</p>
</item>
<tag><c><![CDATA[ERL_EPMD_PORT]]></c></tag>
<item>
<p>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.</p>
</item>
<tag><c><![CDATA[ERL_EPMD_RELAXED_COMMAND_CHECK]]></c></tag>
<item>
<p>If set prior to start, the <c>epmd</c> daemon will behave
as if the <c>-relaxed_command_check</c> option was given at
start-up. If consequently setting this option before starting
the Erlang virtual machine, the automatically started
<c>epmd</c> will accept 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 epmd runs as an daemon. To enable
the error logging you have to edit <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 <TABs> are at least one real tab character. Spaces will
silently be ignored.
</p>
</section>
<section>
<title>Access restrictions</title>
<p>The <c>epmd</c> 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 <c>epmd</c> instance is running on,
why such requests are considered hostile and the connection is
immediately closed.</p>
<p>The queries accepted from remote nodes are:</p>
<list type="bulleted">
<item>
<p>Port queries - i.e. on which port does the node with a given
name listen</p>
</item>
<item>
<p>Name listing - i.e. give a list of all names registered on
the host</p>
</item>
</list>
<p>To restrict access further, firewall software has to be used.</p>
</section>
</comref>
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