rednet

Communicate with other computers by using modems. rednet provides a layer of abstraction on top of the main modem peripheral, making it slightly easier to use.

Basic usage

In order to send a message between two computers, each computer must have a modem on one of its sides (or in the case of pocket computers and turtles, the modem must be equipped as an upgrade). The two computers should then call rednet.open, which sets up the modems ready to send and receive messages.

Once rednet is opened, you can send messages using rednet.send and receive them using rednet.receive. It's also possible to send a message to every rednet-using computer using rednet.broadcast.

Protocols and hostnames

Several rednet messages accept "protocol"s - simple string names describing what a message is about. When sending messages using rednet.send and rednet.broadcast, you can optionally specify a protocol for the message. This same protocol can then be given to rednet.receive, to ignore all messages not using this protocol.

It's also possible to look-up computers based on protocols, providing a basic system for service discovery and DNS. A computer can advertise that it supports a particular protocol with rednet.host, also providing a friendly "hostname". Other computers may then find all computers which support this protocol using rednet.lookup.

See also

• rednet_message Queued when a rednet message is received.
• modem Rednet is built on top of the modem peripheral. Modems provide a more bare-bones but flexible interface.

Changes

• New in version 1.2

CHANNEL_BROADCAST = 65535Source

The channel used by the Rednet API to broadcast messages.

CHANNEL_REPEAT = 65533Source

The channel used by the Rednet API to repeat messages.

MAX_ID_CHANNELS = 65500Source

The number of channels rednet reserves for computer IDs. Computers with IDs greater or equal to this limit wrap around to 0.

open(modem)Source

Opens a modem with the given peripheral name, allowing it to send and receive messages over rednet.

This will open the modem on two channels: one which has the same ID as the computer, and another on the broadcast channel.

Parameters

1. modem string The name of the modem to open.

Throws

• If there is no such modem with the given name

Usage

• Open rednet on the back of the computer, allowing you to send and receive rednet messages using it.

  rednet.open("back")

• Open rednet on all attached modems. This abuses the "filter" argument to peripheral.find.

  peripheral.find("modem", rednet.open)

See also

• rednet.close
• rednet.isOpen

close([modem])Source

Close a modem with the given peripheral name, meaning it can no longer send and receive rednet messages.

Parameters

1. modem? string The side the modem exists on. If not given, all open modems will be closed.

Throws

• If there is no such modem with the given name

See also

• rednet.open

isOpen([modem])Source

Determine if rednet is currently open.

Parameters

1. modem? string Which modem to check. If not given, all connected modems will be checked.

Returns

1. boolean If the given modem is open.

See also

• rednet.open

Changes

• New in version 1.31

send(recipient, message [, protocol])Source

Allows a computer or turtle with an attached modem to send a message intended for a computer with a specific ID. At least one such modem must first be opened before sending is possible.

Assuming the target was in range and also had a correctly opened modem, the target computer may then use rednet.receive to collect the message.

Parameters

1. recipient number The ID of the receiving computer.
2. message The message to send. Like with modem.transmit, this can contain any primitive type (numbers, booleans and strings) as well as tables. Other types (like functions), as well as metatables, will not be transmitted.
3. protocol? string The "protocol" to send this message under. When using rednet.receive one can filter to only receive messages sent under a particular protocol.

Returns

1. boolean If this message was successfully sent (i.e. if rednet is currently open). Note, this does not guarantee the message was actually received.

Usage

• Send a message to computer #2.

  rednet.send(2, "Hello from rednet!")

See also

• rednet.receive

Changes

• Changed in version 1.6: Added protocol parameter.
• Changed in version 1.82.0: Now returns whether the message was successfully sent.

broadcast(message [, protocol])Source

Broadcasts a string message over the predefined CHANNEL_BROADCAST channel. The message will be received by every device listening to rednet.

Parameters

1. message The message to send. This should not contain coroutines or functions, as they will be converted to nil.
2. protocol? string The "protocol" to send this message under. When using rednet.receive one can filter to only receive messages sent under a particular protocol.

Usage

• Broadcast the words "Hello, world!" to every computer using rednet.

  rednet.broadcast("Hello, world!")

See also

• rednet.receive

Changes

• Changed in version 1.6: Added protocol parameter.

receive([protocol_filter [, timeout]])Source

Wait for a rednet message to be received, or until nTimeout seconds have elapsed.

Parameters

1. protocol_filter? string The protocol the received message must be sent with. If specified, any messages not sent under this protocol will be discarded.
2. timeout? number The number of seconds to wait if no message is received.

Returns

1. number The computer which sent this message
2. The received message
3. string | nil The protocol this message was sent under.

Or

1. nil If the timeout elapsed and no message was received.

Usage

• Receive a rednet message.

  local id, message = rednet.receive()
  print(("Computer %d sent message %s"):format(id, message))

• Receive a message, stopping after 5 seconds if no message was received.

  local id, message = rednet.receive(nil, 5)
  if not id then
      printError("No message received")
  else
      print(("Computer %d sent message %s"):format(id, message))
  end

• Receive a message from computer #2.

  local id, message
  repeat
      id, message = rednet.receive()
  until id == 2
  
  print(message)

See also

• rednet.broadcast
• rednet.send

Changes

• Changed in version 1.6: Added protocol filter parameter.

host(protocol, hostname)Source

Register the system as "hosting" the desired protocol under the specified name. If a rednet lookup is performed for that protocol (and maybe name) on the same network, the registered system will automatically respond via a background process, hence providing the system performing the lookup with its ID number.

Multiple computers may not register themselves on the same network as having the same names against the same protocols, and the title localhost is specifically reserved. They may, however, share names as long as their hosted protocols are different, or if they only join a given network after "registering" themselves before doing so (eg while offline or part of a different network).

Parameters

1. protocol string The protocol this computer provides.
2. hostname string The name this computer exposes for the given protocol.

Throws

• If trying to register a hostname which is reserved, or currently in use.

See also

• rednet.unhost
• rednet.lookup

Changes

• New in version 1.6

unhost(protocol)Source

Stop hosting a specific protocol, meaning it will no longer respond to rednet.lookup requests.

Parameters

1. protocol string The protocol to unregister your self from.

Changes

• New in version 1.6

lookup(protocol [, hostname])Source

Search the local rednet network for systems hosting the desired protocol and returns any computer IDs that respond as "registered" against it.

If a hostname is specified, only one ID will be returned (assuming an exact match is found).

Parameters

1. protocol string The protocol to search for.
2. hostname? string The hostname to search for.

Returns

1. number... A list of computer IDs hosting the given protocol.

Or

1. number | nil The computer ID with the provided hostname and protocol, or nil if none exists.

Usage

• Find all computers which are hosting the "chat" protocol.

  local computers = {rednet.lookup("chat")}
  print(#computers .. " computers available to chat")
  for _, computer in pairs(computers) do
    print("Computer #" .. computer)
  end

• Find a computer hosting the "chat" protocol with a hostname of "my_host".

  local id = rednet.lookup("chat", "my_host")
  if id then
    print("Found my_host at computer #" .. id)
  else
    printError("Cannot find my_host")
  end

Changes

• New in version 1.6

run()Source

Listen for modem messages and converts them into rednet messages, which may then be received.

This is automatically started in the background on computer startup, and should not be called manually.