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.