# net_kernel

###
Module

net_kernel

###
Module Summary

Erlang networking kernel.

###
Description

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, #{name_domain => shortnames}).
    {ok,<0.64.0>}
    (foobar@gringotts)2>

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.

Warning

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.

###
Exports

####
allow(Nodes) -> ok | error

### Types

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

### Types

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

### Types

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

Returns currently used net tick time in seconds. For more information see the net_ticktime kernel(6) parameter.

Defined return values (Res):

NetTicktime

  net_ticktime is NetTicktime seconds.

{ongoing_change_to, NetTicktime}

  net_kernel is currently changing net_ticktime to NetTicktime seconds.

ignored

  The local node is not alive.

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

### Types

Node = node()
Options = [inet:socket_getopt()]
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.

####
get_state() ->
#{started => no | static | dynamic,
name => atom(),
name_type => static | dynamic,
name_domain => shortnames | longnames}
OTP 25.0

Get the current state of the distribution for the local node.

Returns a map with (at least) the following key-value pairs:

started => Started

  Valid values for Started:

  no

    The distribution is not started. In this state none of the other keys below are present in the map.

  static

    The distribution was started with command line arguments -name or -sname.

  dynamic

    The distribution was started with net_kernel:start/1 and can be stopped with net_kernel:stop/0.

name => Name

  The name of the node. Same as returned by erlang:node/0 except when name_type is dynamic in which case Name may be undefined (instead of nonode@nohost).

name_type => NameType

  Valid values for NameType:

  static

    The node has a static node name set by the node itself.

  dynamic

    The distribution was started in dynamic node name mode, and will get its node name assigned from the first node it connects to. If key name has value undefined that has not happened yet.

name_domain => NameDomain

  Valid values for NameDomain:

  shortnames

    The distribution was started to use node names with a short host portion (not fully qualified).

  longnames

    The distribution was started to use node names with a long fully qualified host portion.

####
monitor_nodes(Flag) -> ok | Error

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

### Types

Flag = boolean()
Options = OptionsList | OptionsMap
OptionsList = [ListOption]
ListOption =
connection_id | {node_type, NodeType} | nodedown_reason
OptionsMap =
#{connection_id => boolean(),
node_type => NodeType,
nodedown_reason => boolean()}
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.

Delivery guarantees of nodeup/nodedown messages:

- nodeup messages are delivered before delivery of any signals from the remote node through the newly established connection.

- nodedown messages are delivered after all the signals from the remote node over the connection have been delivered.

- nodeup messages are delivered after the corresponding node appears in results from erlang:nodes().

- nodedown messages are delivered after the corresponding node has disappeared in results from erlang:nodes().

- As of OTP 23.0, a nodedown message for a connection being taken down will be delivered before a nodeup message due to a new connection to the same node. Prior to OTP 23.0, this was not guaranteed to be the case.

The format of the node status change messages depends on Options. If Options is the empty list or if net_kernel:monitor_nodes/1 is called, the format is as follows:

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

When Options is the empty map or empty list, the caller will only subscribe for status change messages for visible nodes. That is, only nodes that appear in the result of erlang:nodes/0.

If Options equals anything other than the empty list, the format of the status change messages is as follows:

    {nodeup, Node, Info} | {nodedown, Node, Info}
      Node = node()
      Info = #{Tag => Val} | [{Tag, Val}]

Info is either a map or a list of 2-tuples. Its content depends on Options. If Options is a map, Info will also be a map. If Options is a list, Info will also be a list.

When Options is a map, currently the following associations are allowed:

connection_id => boolean()

  If the value of the association equals true, a connection_id => ConnectionId association will be included in the Info map where ConnectionId is the connection identifier of the connection coming up or going down. For more info about this connection identifier see the documentation of erlang:nodes/2.

node_type => NodeType

  Valid values for NodeType:

  visible

    Subscribe to node status change messages for visible nodes only. The association node_type => visible will be included in the Info map.

  hidden

    Subscribe to node status change messages for hidden nodes only. The association node_type => hidden will be included in the Info map.

  all

    Subscribe to node status change messages for both visible and hidden nodes. The association node_type => visible | hidden will be included in the Info map.

  If no node_type => NodeType association is included in the Options map, the caller will subscribe for status change messages for visible nodes only, but no node_type => visible association will be included in the Info map.

nodedown_reason => boolean()

  If the value of the association equals true, a nodedown_reason => Reason association will be included in the Info map for 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 one of the following:

  connection_setup_failed

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

  no_network

    No network is available.

  net_kernel_terminated

    The net_kernel process terminated.

  shutdown

    Unspecified connection shutdown.

  connection_closed

    The connection was closed.

  disconnect

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

  net_tick_timeout

    Net tick time-out.

  send_net_tick_failed

    Failed to send net tick over the connection.

  get_status_failed

    Status information retrieval from the Port holding the connection failed.

When Options is a list, currently ListOption can be one of the following:

connection_id

  A {connection_id, ConnectionId} tuple will be included in Info where ConnectionId is the connection identifier of the connection coming up or going down. For more info about this connection identifier see the documentation of erlang:nodes/2.

{node_type, NodeType}

  Valid values for NodeType:

  visible

    Subscribe to node status change messages for visible nodes only. The tuple {node_type, visible} will be included in the Info list.

  hidden

    Subscribe to node status change messages for hidden nodes only. The tuple {node_type, hidden} will be included in the Info list.

  all

    Subscribe to node status change messages for both visible and hidden nodes. The tuple {node_type, visible | hidden} will be included in the Info list.

  If no {node_type, NodeType} option has been given. The caller will subscribe for status change messages for visible nodes only, but no {node_type, visible} tuple will be included in the Info list.

nodedown_reason

  The tuple {nodedown_reason, Reason} will be included in the Info list for nodedown messages.

  See the documentation of the nodedown_reason => boolean() association above for information about possible Reason values.

Example:

    (a@localhost)1> net_kernel:monitor_nodes(true, #{connection_id=>true, node_type=>all, nodedown_reason=>true}).
    ok
    (a@localhost)2> flush().
    Shell got {nodeup,b@localhost,
                      #{connection_id => 3067552,node_type => visible}}
    Shell got {nodeup,c@localhost,
                      #{connection_id => 13892107,node_type => hidden}}
    Shell got {nodedown,b@localhost,
                        #{connection_id => 3067552,node_type => visible,
                          nodedown_reason => connection_closed}}
    Shell got {nodedown,c@localhost,
                        #{connection_id => 13892107,node_type => hidden,
                          nodedown_reason => net_tick_timeout}}
    Shell got {nodeup,b@localhost,
                      #{connection_id => 3067553,node_type => visible}}
    ok
    (a@localhost)3>

####
set_net_ticktime(NetTicktime) -> Res

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

### Types

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.

Note

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:

unchanged

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

change_initiated

  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

### Types

Node = node() | new
Options = [inet:socket_setopt()]
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 configuration parameters inet_dist_listen_options and inet_dist_connect_options.

Returns ignored if the local node is not alive.

####
start(Name, Options) -> {ok, pid()} | {error, Reason}
OTP 24.3

### Types

Options =
#{name_domain => NameDomain,
net_ticktime => NetTickTime,
net_tickintensity => NetTickIntensity,
dist_listen => boolean(),
hidden => boolean()}
Name = atom()
NameDomain = shortnames | longnames
NetTickTime = integer() >= 1
NetTickIntensity = 4..1000
Reason = {already_started, pid()} | term()

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

If Name is set to undefined the distribution will be started to request a dynamic node name from the first node it connects to. See Dynamic Node Name. Setting Name to undefined implies options dist_listen => false and hidden => true.

Currently supported options:

name_domain => NameDomain

  Determines the host name part of the node name. If NameDomain equals longnames, fully qualified domain names will be used which also is the default. If NameDomain equals shortnames, only the short name of the host will be used.

net_ticktime => NetTickTime

  Net tick time to use in seconds. Defaults to the value of the net_ticktime kernel(6) parameter. For more information about net tick time, see the kernel parameter. However, note that if the value of the kernel parameter is invalid, it will silently be replaced by a valid value, but if an invalid NetTickTime value is passed as option value to this function, the call will fail.

net_tickintensity => NetTickIntensity

  Net tick intensity to use. Defaults to the value of the net_tickintensity kernel(6) parameter. For more information about net tick intensity, see the kernel parameter. However, note that if the value of the kernel parameter is invalid, it will silently be replaced by a valid value, but if an invalid NetTickIntensity value is passed as option value to this function, the call will fail.

dist_listen => boolean()

  Enable or disable listening for incoming connections. Defaults to the value of the -dist_listen erl command line argument. Note that dist_listen => false implies hidden => true.

  If undefined has been passed as Name, the dist_listen option will be overridden with dist_listen => false.

hidden => boolean()

  Enable or disable hidden node. Defaults to true if the -hidden erl command line argument has been passed; otherwise false.

  If undefined has been passed as Name, or the option dist_listen equals false, the hidden option will be overridden with hidden => true.

####
start(Options) -> {ok, pid()} | {error, Reason}

### Types

Options = [Name | NameDomain | TickTime, ...]
Name = atom()
NameDomain = shortnames | longnames
TickTime = integer() >= 1
Reason = {already_started, pid()} | term()
Warning

start/1 is deprecated. Use start/2 instead.

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

Options list can only be exactly one of the following lists (order is imporant):

[Name]

  The same as net_kernel:start([Name, longnames, 15000]).

[Name, NameDomain]

  The same as net_kernel:start([Name, NameDomain, 15000]).

[Name, NameDomain, TickTime]

  The same as net_kernel:start(Name, #{name_domain => NameDomain, net_ticktime => ((TickTime*4-1) div 1000) + 1, net_tickintensity => 4}). Note that TickTime is not the same as net tick time expressed in milliseconds. TickTime is the time between ticks when net tick intensity equals 4.

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

### Types

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/2, otherwise {error, not_allowed} is returned. Returns {error, not_found} if the local node is not alive.