Router

-- GLOBALS
parent = -1
myID = os.getComputerID()

-- Functions & Stuff

function processRequest(request, senderId)
    -- Standard responses for identifying and adding to networks.
    if request == "$HELLO" then
        print("Responding to network discovery.")
        rednet.send(senderId, "$HELLO")
        return
    elseif request == "$ADDME" then
        response = "$MASTER$ADDROUTER-"..senderId.."-"..myID
        print("Sending request to master, awaiting response.")
        rednet.send(parent, response)
        return
    end

    -- If we find either a server or master request, we'll just immediatly send it to the next router in the chain.
    if string.find(request, "$SERVER") or string.find(request, "$MASTER") then
        print("Forwarding message to parent.")
        rednet.send(parent, request)
        return
    end

    -- If we find that the request is targeted at an end-user, we'll find out which router to send it next to and send it.
    print("Finding - in "..request)
    local s, e, cap = request:find("(.-)".."-", 1)
    print("Found - at index "..s)
    if not cap or not tonumber(cap) then
        print("Invalid message.")
        return
    end
    print("Sending message to next router.")
    request = string.sub(request, e + 1)
    rednet.send(tonumber(cap), request)
end

-- Main Logic

rednet.open("top")

local id, message

while true do
    print("Trying to connect...")
    rednet.broadcast("$HELLO")
    id, message = rednet.receive(5)
    if message then break end
end

print("Received message: "..message.." from "..id)
rednet.send(id, "$ADDME")

while true do
    id, message = rednet.receive(5)
    if not message then
        print("No network routers nearby, couldn't connect to network.")
        return
    end

    if message == "$ACK" then break end
end

print("Added to network, ID: "..myID)
parent = id
print("All messages are being sent to: "..parent)
print("Check master server for confirmation.")

while true do
    print("Listening for messages...")
    id, message = rednet.receive()
    print("Received message: "..message.." from "..id)
    processRequest(message, id)
end

rednet.close()