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(defpackage :cl-mdns/packet
(:use :cl)
(:local-nicknames (:nb :nibbles))
(:export :build-query-packet))
(in-package :cl-mdns/packet)
(defun encode-name (name-string)
"Converts local domain name to encoded byte array.
E.g. converts 'name.local' to #(4 110 97 109 101 5 108 111 99 97 108 0)"
(let ((parts (uiop:split-string name-string :separator "."))
(output (make-array 0 :element-type '(unsigned-byte 8) :adjustable t :fill-pointer 0)))
(dolist (part parts)
(vector-push-extend (length part) output)
(loop for char across part do
(vector-push-extend (char-code char) output)))
(vector-push-extend 0 output) ; Terminate
output))
(defun make-question-header ()
(let ((buffer (make-array 12 :element-type '(unsigned-byte 8) :initial-element 0)))
;; Transaction ID (2 bytes): 0 for mDNS
;; Flags (2 bytes): 0x0000 basic query
;; QDCOUNT (2 bytes): 1
(setf (nb:ub16ref/be buffer 4) 1)
buffer))
(defun make-question-tail ()
(let ((buffer (make-array 4 :element-type '(unsigned-byte 8))))
(setf (nibbles:ub16ref/be buffer 0) #x01) ; Type: AA
(setf (nibbles:ub16ref/be buffer 2) 1) ; Class: IN
buffer))
(defun build-query-packet (service-name)
"Creates 'question' packet for given SERVICE-NAME."
(let ((header (make-question-header))
(name (encode-name service-name))
(tail (make-question-tail)))
(concatenate '(vector (unsigned-byte 8)) header name tail)))
(defun make-dns-header ()
(let ((buffer (make-array 12 :element-type '(unsigned-byte 8) :initial-element 0)))
;; Header
(setf (nb:ub16ref/be buffer 0) 0)
(setf (nb:ub16ref/be buffer 2) #x8400) ; Flags: Standard response, AA
(setf (nb:ub16ref/be buffer 4) 0) ; Questions
(setf (nb:ub16ref/be buffer 6) 1) ; Answer RRs
(setf (nb:ub16ref/be buffer 8) 0) ; Authority RRs
(setf (nb:ub16ref/be buffer 10) 0) ; Additional RRs
buffer))
(defun ip-addr-list-p (x)
(and (listp x)
(= (length x) 4)
(every #'integerp x)))
(deftype ip-addr-list ()
'(satisfies ip-addr-list-p))
(defun make-ip-body (ip)
(destructuring-bind (ip-a ip-b ip-c ip-d) ip
(let ((buffer (make-array 14 :element-type '(unsigned-byte 8) :initial-element 0))
(pos 0))
;; Type A (1), Class IN (1)
(setf (nb:ub16ref/be buffer pos) 1) (incf pos 2)
(setf (nb:ub16ref/be buffer pos) 1) (incf pos 2)
;; TTL (120 seconds)
(setf (nb:ub32ref/be buffer pos) 120) (incf pos 4)
;; Data Length (4 bytes for IPv4)
(setf (nb:ub16ref/be buffer pos) 4) (incf pos 2)
(setf (aref buffer pos) ip-a) (incf pos)
(setf (aref buffer pos) ip-b) (incf pos)
(setf (aref buffer pos) ip-c) (incf pos)
(setf (aref buffer pos) ip-d)
buffer)))
(defun build-response (domain-name ip)
(check-type ip ip-addr-list)
(let ((header (make-dns-header))
(name (encode-name domain-name))
(ip-part (make-ip-body ip)))
(concatenate '(vector (unsigned-byte 8)) header name ip-part)))
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