1epmd(1) User Commands epmd(1)
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6 epmd - Erlang Port Mapper Daemon
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9 epmd [-d|-debug] [DbgExtra...] [-address Addresses] [-port No] [-dae‐
10 mon] [-relaxed_command_check]:
11 Starts the port mapper daemon.
12 epmd [-d|-debug] [-port No] [-names|-kill|-stop Name]:
14 Communicates with a running port mapper daemon.
15 This daemon acts as a name server on all hosts involved in distributed
17 Erlang computations. When an Erlang node starts, the node has a name
18 and it obtains an address from the host OS kernel. The name and address
19 are sent to the epmd daemon running on the local host. In a TCP/IP
20 environment, the address consists of the IP address and a port number.
21 The node name is an atom on the form of Name@Node. The job of the epmd
22 daemon is to keep track of which node name listens on which address.
23 Hence, epmd maps symbolic node names to machine addresses.
24 The TCP/IP epmd daemon only keeps track of the Name (first) part of an
26 Erlang node name. The Host part (whatever is after the @) is implicit
27 in the node name where the epmd daemon was contacted, as is the IP
28 address where the Erlang node can be reached. Consistent and correct
29 TCP naming services are therefore required for an Erlang network to
30 function correctly.
31 Starting the port mapper daemon:
33 The daemon is started automatically by command erl(1) if the node
34 is to be distributed and no running instance is present. If auto‐
35 matically launched environment variables must be used to change the
36 behavior of the daemon; see section Environment Variables.
37 If argument -daemon is not specified, epmd runs as a normal program
39 with the controlling terminal of the shell in which it is started.
40 Normally, it is to be run as a daemon.
41 Regular startup options are described in section Regular Options.
43 The DbgExtra options are described in section DbgExtra Options.
45 Communicating with a running port mapper daemon:
47 Communicating with the running epmd daemon by the epmd program is
48 done primarily for debugging purposes.
49 The different queries are described in section Interactive options.
51
53 These options are available when starting the name server. The name
54 server is normally started automatically by command erl(1) (if not
55 already available), but it can also be started at system startup.
56 -address List:
58 Lets this instance of epmd listen only on the comma-separated list
59 of IP addresses and on the loopback address (which is implicitly
60 added to the list if it has not been specified). This can also be
61 set using environment variable ERL_EPMD_ADDRESS; see section Envi‐
62 ronment Variables.
63 -port No:
65 Lets this instance of epmd listen to another TCP port than default
66 4369. This can also be set using environment variable
67 ERL_EPMD_PORT; see section Environment Variables.
68 -d | -debug:
70 Enables debug output. The more -d flags specified, the more debug
71 output you will get (to a certain limit). This option is most use‐
72 ful when the epmd daemon is not started as a daemon.
73 -daemon:
75 Starts epmd detached from the controlling terminal. Logging ends up
76 in syslog when available and correctly configured. If the epmd dae‐
77 mon is started at boot, this option is definitely to be used. It is
78 also used when command erl automatically starts epmd.
79 -relaxed_command_check:
81 Starts the epmd program with relaxed command checking (mostly for
82 backward compatibility). This affects the following:
83 * With relaxed command checking, the epmd daemon can be killed from
85 the local host with, for example, command epmd -kill even if
86 active nodes are registered. Normally only daemons with an empty
87 node database can be killed with epmd -kill.
88 * Command epmd -stop (and the corresponding messages to epmd, as
90 can be specified using erl_interface:ei(3)) is normally always
91 ignored. This because it can cause a strange situation where two
92 nodes of the same name can be alive at the same time. A node
93 unregisters itself by only closing the connection to epmd, which
94 is why command stop was only intended for use in debugging situa‐
95 tions.
96 With relaxed command checking enabled, you can forcibly unregis‐
98 ter live nodes.
99 Relaxed command checking can also be enabled by setting environment
101 variable ERL_EPMD_RELAXED_COMMAND_CHECK before starting epmd.
102 Use relaxed command checking only on systems with very limited
104 interactive usage.
105
107 Note:
108 These options are only for debugging and testing epmd clients. They are
109 not to be used in normal operation.
110 -packet_timeout Seconds:
113 Sets the number of seconds a connection can be inactive before epmd
114 times out and closes the connection. Defaults to 60.
115 -delay_accept Seconds:
117 To simulate a busy server, you can insert a delay between when epmd
118 gets notified that a new connection is requested and when the con‐
119 nection gets accepted.
120 -delay_write Seconds:
122 Also a simulation of a busy server. Inserts a delay before a reply
123 is sent.
124
126 These options make epmd run as an interactive command, displaying the
127 results of sending queries to an already running instance of epmd. The
128 epmd contacted is always on the local node, but option -port can be
129 used to select between instances if several are running using different
130 ports on the host.
131 -port No:
133 Contacts the epmd listening on the specified TCP port number
134 (default 4369). This can also be set using environment variable
135 ERL_EPMD_PORT; see section Environment Variables.
136 -names:
138 Lists names registered with the currently running epmd.
139 -kill:
141 Kills the currently running epmd.
142 Killing the running epmd is only allowed if epmd -names shows an
144 empty database or if -relaxed_command_check was specified when the
145 running instance of epmd was started.
146 Notice that -relaxed_command_check is specified when starting the
148 daemon that is to accept killing when it has live nodes registered.
149 When running epmd interactively, -relaxed_command_check has no
150 effect. A daemon that is started without relaxed command checking
151 must be killed using, for example, signals or some other OS-spe‐
152 cific method if it has active clients registered.
153 -stop Name:
155 Forcibly unregisters a live node from the epmd database.
156 This command can only be used when contacting epmd instances
158 started with flag -relaxed_command_check.
159 Notice that relaxed command checking must enabled for the epmd dae‐
161 mon contacted. When running epmd interactively, -relaxed_com‐
162 mand_check has no effect.
163
165 ERL_EPMD_ADDRESS:
166 Can be set to a comma-separated list of IP addresses, in which case
167 the epmd daemon will listen only on the specified address(es) and
168 on the loopback address (which is implicitly added to the list if
169 it has not been specified). The default behavior is to listen on
170 all available IP addresses.
171 ERL_EPMD_PORT:
173 Can contain the port number epmd will use. The default port will
174 work fine in most cases. A different port can be specified to allow
175 several instances of epmd, representing independent clusters of
176 nodes, to co-exist on the same host. All nodes in a cluster must
177 use the same epmd port number.
178 ERL_EPMD_RELAXED_COMMAND_CHECK:
180 If set before start, the epmd daemon behaves as if option
181 -relaxed_command_check was specified at startup. Consequently, if
182 this option is set before starting the Erlang virtual machine, the
183 automatically started epmd accepts the -kill and -stop commands
184 without restrictions.
185
187 On some operating systems syslog will be used for error reporting when
188 epmd runs as a daemon. To enable the error logging, you must edit the
189 /etc/syslog.conf file and add an entry:
190 !epmd
192 *.*<TABs>/var/log/epmd.log
193 where <TABs> are at least one real tab character. Spaces are silently
196 ignored.
197
199 The epmd daemon accepts messages from both the local host and remote
200 hosts. However, only the query commands are answered (and acted upon)
201 if the query comes from a remote host. It is always an error to try to
202 register a node name if the client is not a process on the same host as
203 the epmd instance is running on. Such requests are considered hostile
204 and the connection is closed immediately.
205 The following queries are accepted from remote nodes:
207 * Port queries, that is, on which port the node with a specified name
209 listens
210 * Name listing, that is, gives a list of all names registered on the
212 host
213 To restrict access further, firewall software must be used.
215Ericsson AB erts 10.5.6 epmd(1)