Reference Manual

Version 6.3

Table of Contents




Module Summary

Erlang networking kernel.


The net kernel is a system process, registered as net_kernel, which must be operational for distributed Erlang to work. The purpose of this process is to implement parts of the BIFs spawn/4 and spawn_link/4, and to provide monitoring of the network.

An Erlang node is started using command-line flag -name or -sname:

$ erl -sname foobar

It is also possible to call net_kernel:start([foobar]) directly from the normal Erlang shell prompt:

1> net_kernel:start([foobar, shortnames]).

If the node is started with command-line flag -sname, the node name is foobar@Host, where Host is the short name of the host (not the fully qualified domain name). If started with flag -name, the node name is foobar@Host, where Host is the fully qualified domain name. For more information, see erl.

Normally, connections are established automatically when another node is referenced. This functionality can be disabled by setting Kernel configuration parameter dist_auto_connect to never, see kernel(6). In this case, connections must be established explicitly by calling connect_node/1.

Which nodes that are allowed to communicate with each other is handled by the magic cookie system, see section Distributed Erlang in the Erlang Reference Manual.


Starting a distributed node without also specifying -proto_dist inet_tls will expose the node to attacks that may give the attacker complete access to the node and in extension the cluster. When using un-secure distributed nodes, make sure that the network is configured to keep potential attackers out. See the Using SSL for Erlang Distribution User's Guide for details on how to setup a secure distributed node.


allow(Nodes) -> ok | error


Nodes = [node()]

Permits access to the specified set of nodes.

Before the first call to allow/1, any node with the correct cookie can be connected. When allow/1 is called, a list of allowed nodes is established. Any access attempts made from (or to) nodes not in that list will be rejected.

Subsequent calls to allow/1 will add the specified nodes to the list of allowed nodes. It is not possible to remove nodes from the list.

Returns error if any element in Nodes is not an atom.

connect_node(Node) -> boolean() | ignored


Node = node()

Establishes a connection to Node. Returns true if a connection was established or was already established or if Node is the local node itself. Returns false if the connection attempt failed, and ignored if the local node is not alive.

get_net_ticktime() -> Res


Res = NetTicktime | {ongoing_change_to, NetTicktime} | ignored
NetTicktime = integer() >= 1

Gets net_ticktime (see kernel(6)).

Defined return values (Res):


net_ticktime is NetTicktime seconds.

{ongoing_change_to, NetTicktime}

net_kernel is currently changing net_ticktime to NetTicktime seconds.


The local node is not alive.

getopts(Node, Options) ->
           {ok, OptionValues} | {error, Reason} | ignored
OTP 19.1


Node = node()
OptionValues = [inet:socket_setopt()]
Reason = inet:posix() | noconnection

Get one or more options for the distribution socket connected to Node.

If Node is a connected node the return value is the same as from inet:getopts(Sock, Options) where Sock is the distribution socket for Node.

Returns ignored if the local node is not alive or {error, noconnection} if Node is not connected.

monitor_nodes(Flag) -> ok | Error
monitor_nodes(Flag, Options) -> ok | Error


Flag = boolean()
Options = [Option]
Option = {node_type, NodeType} | nodedown_reason
NodeType = visible | hidden | all
Error = error | {error, term()}

The calling process subscribes or unsubscribes to node status change messages. A nodeup message is delivered to all subscribing processes when a new node is connected, and a nodedown message is delivered when a node is disconnected.

If Flag is true, a new subscription is started. If Flag is false, all previous subscriptions started with the same Options are stopped. Two option lists are considered the same if they contain the same set of options.

As from Kernel version 2.11.4, and ERTS version 5.5.4, the following is guaranteed:

  • nodeup messages are delivered before delivery of any message from the remote node passed through the newly established connection.

  • nodedown messages are not delivered until all messages from the remote node that have been passed through the connection have been delivered.

Notice that this is not guaranteed for Kernel versions before 2.11.4.

As from Kernel version 2.11.4, subscriptions can also be made before the net_kernel server is started, that is, net_kernel:monitor_nodes/[1,2] does not return ignored.

As from Kernel version 2.13, and ERTS version 5.7, the following is guaranteed:

  • nodeup messages are delivered after the corresponding node appears in results from erlang:nodes/X.

  • nodedown messages are delivered after the corresponding node has disappeared in results from erlang:nodes/X.

Notice that this is not guaranteed for Kernel versions before 2.13.

The format of the node status change messages depends on Options. If Options is [], which is the default, the format is as follows:

{nodeup, Node} | {nodedown, Node}
  Node = node()

If Options is not [], the format is as follows:

{nodeup, Node, InfoList} | {nodedown, Node, InfoList}
  Node = node()
  InfoList = [{Tag, Val}]

InfoList is a list of tuples. Its contents depends on Options, see below.

Also, when OptionList == [], only visible nodes, that is, nodes that appear in the result of erlang:nodes/0, are monitored.

Option can be any of the following:

{node_type, NodeType}

Valid values for NodeType:


Subscribe to node status change messages for visible nodes only. The tuple {node_type, visible} is included in InfoList.


Subscribe to node status change messages for hidden nodes only. The tuple {node_type, hidden} is included in InfoList.


Subscribe to node status change messages for both visible and hidden nodes. The tuple {node_type, visible | hidden} is included in InfoList.


The tuple {nodedown_reason, Reason} is included in InfoList in nodedown messages.

Reason can, depending on which distribution module or process that is used be any term, but for the standard TCP distribution module it is any of the following:


The connection setup failed (after nodeup messages were sent).


No network is available.


The net_kernel process terminated.


Unspecified connection shutdown.


The connection was closed.


The connection was disconnected (forced from the current node).


Net tick time-out.


Failed to send net tick over the connection.


Status information retrieval from the Port holding the connection failed.

set_net_ticktime(NetTicktime) -> Res
set_net_ticktime(NetTicktime, TransitionPeriod) -> Res


NetTicktime = integer() >= 1
TransitionPeriod = integer() >= 0
Res =
    unchanged |
    change_initiated |
    {ongoing_change_to, NewNetTicktime}
NewNetTicktime = integer() >= 1

Sets net_ticktime (see kernel(6)) to NetTicktime seconds. TransitionPeriod defaults to 60.

Some definitions:

Minimum transition traffic interval (MTTI)

minimum(NetTicktime, PreviousNetTicktime)*1000 div 4 milliseconds.

Transition period

The time of the least number of consecutive MTTIs to cover TransitionPeriod seconds following the call to set_net_ticktime/2 (that is, ((TransitionPeriod*1000 - 1) div MTTI + 1)*MTTI milliseconds).

If NetTicktime < PreviousNetTicktime, the net_ticktime change is done at the end of the transition period; otherwise at the beginning. During the transition period, net_kernel ensures that there is outgoing traffic on all connections at least every MTTI millisecond.


The net_ticktime changes must be initiated on all nodes in the network (with the same NetTicktime) before the end of any transition period on any node; otherwise connections can erroneously be disconnected.

Returns one of the following:


net_ticktime already has the value of NetTicktime and is left unchanged.


net_kernel initiated the change of net_ticktime to NetTicktime seconds.

{ongoing_change_to, NewNetTicktime}

The request is ignored because net_kernel is busy changing net_ticktime to NewNetTicktime seconds.

setopts(Node, Options) -> ok | {error, Reason} | ignored
OTP 19.1


Node = node() | new
Reason = inet:posix() | noconnection

Set one or more options for distribution sockets. Argument Node can be either one node name or the atom new to affect the distribution sockets of all future connected nodes.

The return value is the same as from inet:setopts/2 or {error, noconnection} if Node is not a connected node or new.

If Node is new the Options will then also be added to kernel configration parameters inet_dist_listen_options and inet_dist_connect_options.

Returns ignored if the local node is not alive.


Name = atom()
NameType = shortnames | longnames
Reason = {already_started, pid()} | term()

Turns a non-distributed node into a distributed node by starting net_kernel and other necessary processes.

Notice that the argument is a list with exactly one, two, or three arguments. NameType defaults to longnames and Ticktime to 15000.

stop() -> ok | {error, Reason}


Reason = not_allowed | not_found

Turns a distributed node into a non-distributed node. For other nodes in the network, this is the same as the node going down. Only possible when the net kernel was started using start/1, otherwise {error, not_allowed} is returned. Returns {error, not_found} if the local node is not alive.