description = [[ 
Extracts a list of Citrix servers from the ICA Browser service.
]]

---
-- @usage sudo ./nmap -sU --script=citrix-enum-servers -p 1604 
--
-- @output
-- PORT     STATE SERVICE
-- 1604/udp open  unknown
-- | citrix-enum-servers:  
-- |   CITRIXSRV01  
-- |_  CITRIXSRV02
--

-- Version 0.2

-- Created 11/26/2009 - v0.1 - created by Patrik Karlsson <patrik@cqure.net>
-- Revised 11/26/2009 - v0.2 - minor packet documentation


author = "Patrik Karlsson"
license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
categories = {"discovery", "safe"}

require "comm"
require "shortport"

portrule = shortport.portnumber(1604, "udp")

--
-- process the response from the server
-- @param response string, complete server response
-- @return string row delimited with \n containing all published applications
--
function process_server_response(response)
    
    local pos, packet_len = bin.unpack("SS", response)
    local server_name
    local server_list = {}

    if packet_len < 40 then
		return
    end

    -- the list of published applications starts at offset 40
    offset = 41

    while offset < packet_len do
        pos, server_name = bin.unpack("z", response:sub(offset))
		offset = offset + pos - 1
		table.insert(server_list, server_name)
    end

    return server_list

end


action = function(host, port)

	local packet, counter, socket
	local query = {}
	local server_list = {}

	--
	-- Packets were intercepted from the Citrix Program Neighborhood client
	-- They are used to query a server for it's list of published applications
	--
	-- We're really not interested in the responses to the first two packets
	-- The third response contains the list of published applications
	-- I couldn't find any documentation on this protocol so I'm providing
	-- some brief information for the bits and bytes this script uses.
	--
	-- Spec. of response to query[2] that contains a list of published apps
	-- 
	-- offset	size	content
	-- -------------------------
	-- 0		16-bit	Length
	-- 12		32-bit	Server IP (not used here)
	-- 30		8-bit	Last packet	(1), More packets(0)	
	-- 40		-		null-separated list of applications
	--
	query[0] = string.char(
				0x1e, 0x00, -- Length: 30
				0x01, 0x30, 0x02, 0xfd, 0xa8, 0xe3, 0x00, 0x00,
				0x00, 0x00, 0x00, 0x00, 0x00, 0x00,	0x00, 0x00, 
				0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
				0x00, 0x00, 0x00, 0x00
				)

	query[1] = string.char(
				0x2a, 0x00, -- Length: 42
				0x01, 0x32, 0x02, 0xfd, 0xa8, 0xe3, 0x00, 0x00, 
				0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
				0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 
				0x00, 0x00, 0x01, 0x00, 0x02, 0x00,	0x00, 0x00, 
				0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
				)
	
	counter = 0

	socket = nmap.new_socket()
	socket:set_timeout(5000)

	try = nmap.new_try(function() socket:close() end)
	try(socket:connect(host, port))

	-- send the two first packets and never look back
	repeat
	    try(socket:send(query[counter]))
	    packet = try(socket:receive())
	    counter = counter + 1
	until (counter>#query)

	-- process the first response
  	server_list =  process_server_response( packet )

	--
	-- the byte at offset 31 in the response has a really magic function
	-- if it is set to zero (0) we have more response packets to process
	-- if it is set to one (1) we have arrived at the last packet of our journey
	--
	while packet:sub(31,31) ~= string.char(0x01) do
	    packet = try( socket:receive() )
	    local tmp_table = process_server_response( packet )	
	
		for _, v in ipairs(tmp_table) do
			table.insert(server_list, v)
		end
	end

	if #server_list>0 then
	    nmap.set_port_state(host, port, "open")
	end
	
	socket:close()

	return stdnse.format_output(true, server_list)

end