SensibleRednetRepeat

--[[
For reasons outlined here (https://wiki.computercraft.cc/Network_security), Rednet is not really a good choice for new networking setups, apart from its capability to relay messages (obsoleted by ender modems).

However, if you do want to keep using it, without the risk of it crashing due to the exploits I have identified (https://pastebin.com/pJnfSDcL), you can use this convenient, patched repeater. It doesn't alleviate the fundamental issues with Rednet, though.
]]

-- Find modems
local modems = {peripheral.find "modem"}

local comp_ID = os.getComputerID()

print(("%d modem(s) found"):format(#modems))

local function for_each_modem(fn)
    for _, m in pairs(modems) do
        fn(m)
    end
end

local function open(channel)
    for_each_modem(function(m) m.open(channel) end)
end

local function close(channel)
    for_each_modem(function(m) m.close(channel) end)
end

-- Open channels
open(rednet.CHANNEL_REPEAT)

-- Main loop (terminate to break)
local ok, error = pcall(function()
    local received_messages = {}
    local received_message_timers = {}
    local transmitted_messages = 0

    while true do
        local event, modem, channel, reply_channel, message, distance = os.pullEvent()
        if event == "modem_message" then
            -- Got a modem message, rebroadcast it if it's a rednet thing
            if channel == rednet.CHANNEL_REPEAT and type(message) == "table" then
                local id = message.nMessageID -- unfortunately we must keep the stupid rednet identifiers SOMEWHERE...
                local recipient = message.nRecipient
                local route = message.route -- protocol extension
                if type(route) ~= "table" then route = { reply_channel } end
                table.insert(route, comp_ID)
                message.route = route
                if id and recipient and (type(id) == "number" or type(id) == "string") and type(recipient) == "number" and recipient >= 0 and recipient <= 65535 then
                    if not received_messages[id] then
                        -- Ensure we only repeat a message once per 30 seconds
                        received_messages[id] = true
                        received_message_timers[os.startTimer(30)] = id

                        -- Send on all other open modems, to the target and to other repeaters
                        for_each_modem(function(m)
                            m.transmit(rednet.CHANNEL_REPEAT, reply_channel, message)
                            m.transmit(recipient, reply_channel, message)
                        end)

                        -- Log the event
                        transmitted_messages = transmitted_messages + 1
                        term.clear()
                        term.setCursorPos(1, 1)
                        print(("%d message(s) repeated"):format(transmitted_messages))
                        print(string.format("%s\nfrom %d to %s dist %d", tostring(message.message), reply_channel, tostring(recipient), tostring(distance) or "[n/a]"))
                    end
                end
            end

        elseif event == "timer" then
            -- Got a timer event, use it to clear the message history
            local timer = modem
            local id = received_message_timers[timer]
            if id then
                received_message_timers[timer] = nil
                received_messages[timer] = nil
            end

        end
    end
end)
if not ok then
    printError(error)
end

-- Close channels
close(rednet.CHANNEL_REPEAT)