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# net_kernel (kernel v10.4)

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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, #{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.

# Summary

## Types

connection_state()
connection_type()

## Functions

allow(Nodes)

Permits access to the specified set of nodes.

allowed()

Returns a list of nodes that are explicitly allowed to connect to the node by calling `allow/1`. If empty list is returned, it means that any node using the same cookie will be able to connect.

connect_node(Node)

Establishes a connection to `Node`.

get_net_ticktime()

Returns currently used net tick time in seconds.

get_state()

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

getopts(Node, Options)

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

monitor_nodes(Flag)

Equivalent to `monitor_nodes(Flag, [])`.

monitor_nodes(Flag, Options)

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.

set_net_ticktime(NetTicktime)

Equivalent to `set_net_ticktime(NetTicktime, 60)`.

set_net_ticktime(NetTicktime, TransitionPeriod)

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

setopts(Node, Options)

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.

start(Options) deprecated

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

start(Name, Options)

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

stop()

Turns a distributed node into a non-distributed node.

# Types

# connection_state()

(not exported)

    -type connection_state() :: check_pending | pending | up | up_pending.

# connection_type()

(not exported)

    -type connection_type() :: normal | hidden.

# Functions

# allow(Nodes)

    -spec allow(Nodes) -> ok | error | ignored when 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.

Disallowing an already connected node will not cause it to be disconnected. It will, however, prevent any future reconnection attempts.

Passing `Nodes` as an empty list has never any affect at all.

Returns `error` if any element in `Nodes` is not an atom, and `ignored` if the local node is not alive.

# allowed()

(since OTP 28.0)

    -spec allowed() -> {ok, Nodes} | ignored when Nodes :: [node()].

Returns a list of nodes that are explicitly allowed to connect to the node by calling `allow/1`. If empty list is returned, it means that any node using the same cookie will be able to connect.

# connect_node(Node)

    -spec connect_node(Node) -> boolean() | ignored when 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()

    -spec get_net_ticktime() -> Res
                              when
                                  Res :: NetTicktime | {ongoing_change_to, NetTicktime} | ignored,
                                  NetTicktime :: pos_integer().

Returns currently used net tick time in seconds.

For more information see the `net_ticktime` `Kernel` 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.

# get_state()

(since OTP 25.0)

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

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.

# getopts(Node, Options)

(since OTP 19.1)

    -spec getopts(Node, Options) -> {ok, OptionValues} | {error, Reason} | ignored
                     when
                         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.

# monitor_nodes(Flag)

    -spec monitor_nodes(Flag) -> ok | Error when Flag :: boolean(), Error :: error | {error, term()}.

Equivalent to `monitor_nodes(Flag, [])`.

# monitor_nodes(Flag, Options)

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

    -spec set_net_ticktime(NetTicktime) -> Res
                              when
                                  NetTicktime :: pos_integer(),
                                  Res :: unchanged | change_initiated | {ongoing_change_to, NewNetTicktime},
                                  NewNetTicktime :: pos_integer().

Equivalent to `set_net_ticktime(NetTicktime, 60)`.

# set_net_ticktime(NetTicktime, TransitionPeriod)

    -spec set_net_ticktime(NetTicktime, TransitionPeriod) -> Res
                              when
                                  NetTicktime :: pos_integer(),
                                  TransitionPeriod :: non_neg_integer(),
                                  Res :: unchanged | change_initiated | {ongoing_change_to, NewNetTicktime},
                                  NewNetTicktime :: pos_integer().

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 `MTTI`s 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)

(since OTP 19.1)

    -spec setopts(Node, Options) -> ok | {error, Reason} | ignored
                     when
                         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(Options)

This function is deprecated. Use start/2 instead.

    -spec start(Options) -> {ok, pid()} | {error, Reason}
                   when
                       Options :: [Name | NameDomain | TickTime, ...],
                       Name :: atom(),
                       NameDomain :: shortnames | longnames,
                       TickTime :: pos_integer(),
                       Reason :: {already_started, pid()} | term().

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`.

# start(Name, Options)

(since OTP 24.3)

    -spec start(Name, Options) -> {ok, pid()} | {error, Reason}
                   when
                       Options ::
                           #{name_domain => NameDomain,
                             net_ticktime => NetTickTime,
                             net_tickintensity => NetTickIntensity,
                             dist_listen => boolean(),
                             hidden => boolean()},
                       Name :: atom(),
                       NameDomain :: shortnames | longnames,
                       NetTickTime :: pos_integer(),
                       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`.

# stop()

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