.TH epmd 1 "erts 12.2" "Ericsson AB" "User Commands" .SH NAME epmd \- Erlang Port Mapper Daemon .SH DESCRIPTION .RS 2 .TP 2 .B \fIepmd [-d|-debug] [DbgExtra\&.\&.\&.] [-address Addresses] [-port No] [-daemon] [-relaxed_command_check]\fR\&: Starts the port mapper daemon\&. .TP 2 .B \fIepmd [-d|-debug] [-port No] [-names|-kill|-stop Name]\fR\&: Communicates with a running port mapper daemon\&. .RE .LP 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 \fIepmd\fR\& 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 \fIName@Node\fR\&\&. The job of the \fIepmd\fR\& daemon is to keep track of which node name listens on which address\&. Hence, \fIepmd\fR\& maps symbolic node names to machine addresses\&. .LP The TCP/IP \fIepmd\fR\& daemon only keeps track of the \fIName\fR\& (first) part of an Erlang node name\&. The \fIHost\fR\& part (whatever is after the \fI@\fR\&) is implicit in the node name where the \fIepmd\fR\& 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\&. .LP .RS -4 .B Note: .RE On Windows the maximum number of nodes allowed in one epmd instance is 60\&. This is because of limitations in the current implementation\&. If you need more nodes, you should look into using and erlang based epmd implementation such as Erlang EPMD\&. .RS 2 .TP 2 .B Starting the port mapper daemon: The daemon is started automatically by command \fIerl(1)\fR\& 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\&. .RS 2 .LP If argument \fI-daemon\fR\& is not specified, \fIepmd\fR\& 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\&. .RE .RS 2 .LP Regular startup options are described in section Regular Options\&. .RE .RS 2 .LP The \fIDbgExtra\fR\& options are described in section DbgExtra Options\&. .RE .TP 2 .B Communicating with a running port mapper daemon: Communicating with the running \fIepmd\fR\& daemon by the \fIepmd\fR\& program is done primarily for debugging purposes\&. .RS 2 .LP The different queries are described in section Interactive options\&. .RE .RE .SH "REGULAR OPTIONS" .LP These options are available when starting the name server\&. The name server is normally started automatically by command \fIerl(1)\fR\& (if not already available), but it can also be started at system startup\&. .RS 2 .TP 2 .B \fI-address List\fR\&: Lets this instance of \fIepmd\fR\& 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 \fIERL_EPMD_ADDRESS\fR\&; see section Environment Variables\&. .TP 2 .B \fI-port No\fR\&: Lets this instance of \fIepmd\fR\& listen to another TCP port than default 4369\&. This can also be set using environment variable \fIERL_EPMD_PORT\fR\&; see section Environment Variables\&. .TP 2 .B \fI-d | -debug\fR\&: Enables debug output\&. The more \fI-d\fR\& flags specified, the more debug output you will get (to a certain limit)\&. This option is most useful when the \fIepmd\fR\& daemon is not started as a daemon\&. .TP 2 .B \fI-daemon\fR\&: Starts \fIepmd\fR\& detached from the controlling terminal\&. Logging ends up in syslog when available and correctly configured\&. If the \fIepmd\fR\& daemon is started at boot, this option is definitely to be used\&. It is also used when command \fIerl\fR\& automatically starts \fIepmd\fR\&\&. .TP 2 .B \fI-relaxed_command_check\fR\&: Starts the \fIepmd\fR\& program with relaxed command checking (mostly for backward compatibility)\&. This affects the following: .RS 2 .TP 2 * With relaxed command checking, the \fIepmd\fR\& daemon can be killed from the local host with, for example, command \fIepmd -kill\fR\& even if active nodes are registered\&. Normally only daemons with an empty node database can be killed with \fIepmd -kill\fR\&\&. .LP .TP 2 * Command \fIepmd -stop\fR\& (and the corresponding messages to \fIepmd\fR\&, as can be specified using \fIerl_interface:ei(3)\fR\&) 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 \fIepmd\fR\&, which is why command \fIstop\fR\& was only intended for use in debugging situations\&. .RS 2 .LP With relaxed command checking enabled, you can forcibly unregister live nodes\&. .RE .LP .RE .RS 2 .LP Relaxed command checking can also be enabled by setting environment variable \fIERL_EPMD_RELAXED_COMMAND_CHECK\fR\& before starting \fIepmd\fR\&\&. .RE .RS 2 .LP Use relaxed command checking only on systems with very limited interactive usage\&. .RE .RE .SH "DBGEXTRA OPTIONS" .LP .RS -4 .B Note: .RE These options are only for debugging and testing \fIepmd\fR\& clients\&. They are not to be used in normal operation\&. .RS 2 .TP 2 .B \fI-packet_timeout Seconds\fR\&: Sets the number of seconds a connection can be inactive before \fIepmd\fR\& times out and closes the connection\&. Defaults to 60\&. .TP 2 .B \fI-delay_accept Seconds\fR\&: To simulate a busy server, you can insert a delay between when \fIepmd\fR\& gets notified that a new connection is requested and when the connection gets accepted\&. .TP 2 .B \fI-delay_write Seconds\fR\&: Also a simulation of a busy server\&. Inserts a delay before a reply is sent\&. .RE .SH "INTERACTIVE OPTIONS" .LP These options make \fIepmd\fR\& run as an interactive command, displaying the results of sending queries to an already running instance of \fIepmd\fR\&\&. The \fIepmd\fR\& contacted is always on the local node, but option \fI-port\fR\& can be used to select between instances if several are running using different ports on the host\&. .RS 2 .TP 2 .B \fI-port No\fR\&: Contacts the \fIepmd\fR\& listening on the specified TCP port number (default 4369)\&. This can also be set using environment variable \fIERL_EPMD_PORT\fR\&; see section Environment Variables\&. .TP 2 .B \fI-names\fR\&: Lists names registered with the currently running \fIepmd\fR\&\&. .TP 2 .B \fI-kill\fR\&: Kills the currently running \fIepmd\fR\&\&. .RS 2 .LP Killing the running \fIepmd\fR\& is only allowed if \fIepmd -names\fR\& shows an empty database or if \fI-relaxed_command_check\fR\& was specified when the running instance of \fIepmd\fR\& was started\&. .RE .RS 2 .LP Notice that \fI-relaxed_command_check\fR\& is specified when starting the daemon that is to accept killing when it has live nodes registered\&. When running \fIepmd\fR\& interactively, \fI-relaxed_command_check\fR\& 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\&. .RE .TP 2 .B \fI-stop Name\fR\&: Forcibly unregisters a live node from the \fIepmd\fR\& database\&. .RS 2 .LP This command can only be used when contacting \fIepmd\fR\& instances started with flag \fI-relaxed_command_check\fR\&\&. .RE .RS 2 .LP Notice that relaxed command checking must enabled for the \fIepmd\fR\& daemon contacted\&. When running \fIepmd\fR\& interactively, \fI-relaxed_command_check\fR\& has no effect\&. .RE .RE .SH "ENVIRONMENT VARIABLES" .RS 2 .TP 2 .B \fIERL_EPMD_ADDRESS\fR\&: Can be set to a comma-separated list of IP addresses, in which case the \fIepmd\fR\& 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\&. .TP 2 .B \fIERL_EPMD_PORT\fR\&: Can contain the port number \fIepmd\fR\& will use\&. The default port will work fine in most cases\&. A different port can be specified to allow several instances of \fIepmd\fR\&, representing independent clusters of nodes, to co-exist on the same host\&. All nodes in a cluster must use the same \fIepmd\fR\& port number\&. .TP 2 .B \fIERL_EPMD_RELAXED_COMMAND_CHECK\fR\&: If set before start, the \fIepmd\fR\& daemon behaves as if option \fI-relaxed_command_check\fR\& was specified at startup\&. Consequently, if this option is set before starting the Erlang virtual machine, the automatically started \fIepmd\fR\& accepts the \fI-kill\fR\& and \fI-stop\fR\& commands without restrictions\&. .RE .SH "LOGGING" .LP On some operating systems \fIsyslog\fR\& will be used for error reporting when \fIepmd\fR\& runs as a daemon\&. To enable the error logging, you must edit the /etc/syslog\&.conf file and add an entry: .LP .nf !epmd *.*/var/log/epmd.log .fi .LP where \fI\fR\& are at least one real tab character\&. Spaces are silently ignored\&. .SH "ACCESS RESTRICTIONS" .LP The \fIepmd\fR\& 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 \fIepmd\fR\& instance is running on\&. Such requests are considered hostile and the connection is closed immediately\&. .LP The following queries are accepted from remote nodes: .RS 2 .TP 2 * Port queries, that is, on which port the node with a specified name listens .LP .TP 2 * Name listing, that is, gives a list of all names registered on the host .LP .RE .LP To restrict access further, firewall software must be used\&.