epmd(1)
NAME
epmd - Erlang Port Mapper Daemon
DESCRIPTION
epmd [-d|-debug] [DbgExtra...] [-address Addresses] [-port No]
[-daemon] [-relaxed_command_check]:
Starts the port mapper daemon.
epmd [-d|-debug] [-port No] [-names|-kill|-stop Name]:
Communicates with a running port mapper daemon.
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 epmd 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 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 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 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 command erl(1) 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 Environment Variables.
If argument -daemon is not specified, epmd 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.
Regular startup options are described in section Regular Options.
The DbgExtra options are described in section DbgExtra Options.
Communicating with a running port mapper daemon:
Communicating with the running epmd daemon by the epmd program is
done primarily for debugging purposes.
The different queries are described in section Interactive options.
REGULAR OPTIONS
These options are available when starting the name server. The name server is normally started automatically by command erl(1) (if not already available), but it can also be started at system startup. -address List: Lets 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 environment variable ERL_EPMD_ADDRESS; see section Environment Variables. -port No: Lets this instance of epmd listen to another TCP port than default 4369. This can also be set using environment variable ERL_EPMD_PORT; see section Environment Variables. -d | -debug: Enables debug output. The more -d flags specified, 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: Starts epmd detached from the controlling terminal. Logging ends up in syslog when available and correctly configured. If the epmd daemon is started at boot, this option is definitely to be used. It is also used when command erl automatically starts epmd. -relaxed_command_check: Starts 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 local host with, for example, command epmd -kill even if active nodes are registered. Normally only daemons with an empty node database can be killed with epmd -kill. * Command epmd -stop (and the corresponding messages to epmd, as can be specified using erl_interface:ei(3erl)) 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 epmd, which is why command stop 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 environment variable ERL_EPMD_RELAXED_COMMAND_CHECK before starting epmd. Use relaxed command checking only on systems with very limited interactive usage.
DBGEXTRA OPTIONS
Note:
These options are only for debugging and testing epmd clients. They are
not to be used in normal operation.
-packet_timeout Seconds:
Sets the number of seconds a connection can be inactive before epmd
times out and closes the connection. Defaults to 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 option -port 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 specified TCP port number
(default 4369). This can also be set using environment variable
ERL_EPMD_PORT; see section Environment Variables.
-names:
Lists names registered with the currently running epmd.
-kill:
Kills the currently running epmd.
Killing the running epmd is only allowed if epmd -names shows an
empty database or if -relaxed_command_check was specified when the
running instance of epmd was started.
Notice that -relaxed_command_check is specified 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
must be killed using, for example, signals or some other OS-
specific method if it has active clients registered.
-stop Name:
Forcibly unregisters a live node from the epmd database.
This command can only be used when contacting epmd instances
started with flag -relaxed_command_check.
Notice that relaxed command checking must enabled for the epmd
daemon contacted. When running epmd interactively,
-relaxed_command_check has no effect.
ENVIRONMENT VARIABLES
ERL_EPMD_ADDRESS:
Can 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 behavior is to listen on
all available IP addresses.
ERL_EPMD_PORT:
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 before start, the epmd daemon behaves as if option
-relaxed_command_check was specified at startup. Consequently, if
this option is set before starting the Erlang virtual machine, the
automatically started epmd accepts the -kill and -stop commands
without restrictions.
LOGGING
On some operating systems syslog will be used for error reporting when
epmd runs as a daemon. To enable the error logging, you must edit the
/etc/syslog.conf file and add an entry:
!epmd
*.*<TABs>/var/log/epmd.log
where <TABs> are at least one real tab character. Spaces are silently
ignored.
ACCESS RESTRICTIONS
The epmd 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 epmd instance is running on. Such requests are considered hostile
and the connection is closed immediately.
The following queries are accepted from remote nodes:
* Port queries, that is, on which port the node with a specified name
listens
* Name listing, that is, gives a list of all names registered on the
host
To restrict access further, firewall software must be used.
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
