CODEWITHSUNDEEP
luawiresharkwireshark-dissector
Michael
Michael

Reputation: 133

Writing a wireshark dissector in lua and setting it on a dynamic port


I have a client server protocol that works the following way.
Client sends udp broadcast with servers id (to a fixed port).
Server receives the datagram and if its matches his id he sends the client the port that he is listening to.
Then the client opens a tcp connection to that port.
I'm writing a Wireshark dissector in Lua for this thing and I need to setup the port for the tcp connection dynamically (I don't know in advance on witch port the servers listens).

I tried something like that:

-- declare our protocol
myproto_udp_proto = Proto("myproto_UDP","myproto UDP Protocol")
myproto_tcp_proto = Proto("myproto_TCP","myproto TCP Protocol")
-- create a function to dissect it
function myproto_tcp_proto.dissector(buffer,pinfo,tree)
        pinfo.cols.protocol = "myproto TCP"
        local subtree = tree:add(myproto_tcp_proto,buffer(),"myproto TCP Protocol Data")
        if buffer(0,2):uint() == 0xF00D then
            subtree:add(buffer(0,2),"Magic(F00D)")
        else
            subtree:add(buffer(0,2),"Bad Magic")
        end
end
function myproto_udp_proto.dissector(buffer,pinfo,tree)
    pinfo.cols.protocol = "myproto UDP"
    local subtree = tree:add(myproto_udp_proto,buffer(),"myproto UDP Protocol Data")
    if buffer(0,2):uint() == 0xF00D then
        subtree:add(buffer(0,2),"Magic(F00D)")
        local command;
        local port = -1;
        if buffer(2,1):uint() == 01 then
            command = "Searching for server"
        elseif buffer(2,1):uint() == 02 then
            command = "I'm server"
            port = buffer(7,2):uint()
        else
            command = "unknown";
        end
        subtree:add(buffer(2,1),command)
        subtree:add(buffer(3,4),"Server id: " .. buffer(3,4):uint())
        if port ~= -1 then
            subtree:add(buffer(7,2),"Server listening port: " .. buffer(7,2):uint())
            subtree:add(buffer(9,4),"check bytes")
            myproto_tcp_init(port)
        end
    else
        subtree:add(buffer(0,2),"Bad Magic")
    end
end
-- load the udp.port table
udp_table = DissectorTable.get("udp.port")
-- register our protocol to handle udp port 1338
udp_table:add(1338,myproto_udp_proto)

function myproto_tcp_init(port)
    -- load the tcp.port table
    tcp_table = DissectorTable.get("tcp.port")
    -- register our protocol to handle tcp port !DYNAMIC!
    tcp_table:add(port,myproto_tcp_proto)
end

What I'm missing?
Thanks in advance

Upvotes: 2

Views: 4874

Answers (1)

Michael
Michael

Reputation: 133

The mechanism presented here works. The problem was that the port number was taken from the wrong bytes (from buffer(4,2) instead of buffer(7,2)).

Upvotes: 2

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