;;; dns.el --- Domain Name Service lookups  -*- lexical-binding:t -*-

;; Copyright (C) 2002-2021 Free Software Foundation, Inc.

;; Author: Lars Magne Ingebrigtsen <larsi@gnus.org>
;; Keywords: network comm

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;;; Code:

(require 'cl-lib)

(defvar dns-timeout 5
  "How many seconds to wait when doing DNS queries.")

(defvar dns-servers nil
  "List of DNS servers to query.
If nil, /etc/resolv.conf and nslookup will be consulted.")

(defvar dns-servers-valid-for-interfaces nil
  "The return value of `network-interface-list' when `dns-servers' was set.
If the set of network interfaces and/or their IP addresses
change, then presumably the list of DNS servers needs to be
updated.  Set this variable to t to disable the check.")

;;; Internal code:

(defvar dns-query-types
  '((A 1)
    (NS 2)
    (MD 3)
    (MF 4)
    (CNAME 5)
    (SOA 6)
    (MB 7)
    (MG 8)
    (MR 9)
    (NULL 10)
    (WKS 11)
    (PTR 12)
    (HINFO 13)
    (MINFO 14)
    (MX 15)
    (TXT 16)
    (AAAA 28) ; RFC3596
    (SRV 33) ; RFC2782
    (AXFR 252)
    (MAILB 253)
    (MAILA 254)
    (* 255))
  "Names of query types and their values.")

(defvar dns-classes
  '((IN 1)
    (CS 2)
    (CH 3)
    (HS 4))
  "Classes of queries.")

(defun dns-write-bytes (value &optional length)
  (let (bytes)
    (dotimes (_ (or length 1))
      (push (% value 256) bytes)
      (setq value (/ value 256)))
    (dolist (byte bytes)
      (insert byte))))

(defun dns-read-bytes (length)
  (let ((value 0))
    (dotimes (_ length)
      (setq value (logior (* value 256) (following-char)))
      (forward-char 1))
    value))

(defun dns-get (type spec)
  (cadr (assq type spec)))

(defun dns-inverse-get (value spec)
  (let ((found nil))
    (while (and (not found)
		spec)
      (if (eq value (cadr (car spec)))
	  (setq found (caar spec))
	(pop spec)))
    found))

(defun dns-write-name (name)
  (dolist (part (split-string name "\\."))
    (dns-write-bytes (length part))
    (insert part))
  (dns-write-bytes 0))

(defun dns-read-string-name (string buffer)
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (insert string)
    (goto-char (point-min))
    (dns-read-name buffer)))

(defun dns-read-name (&optional buffer)
  (let ((ended nil)
	(name nil)
	length)
    (while (not ended)
      (setq length (dns-read-bytes 1))
      (if (= 192 (logand length (ash 3 6)))
	  (let ((offset (+ (* (logand 63 length) 256)
			   (dns-read-bytes 1))))
	    (save-excursion
	      (when buffer
		(set-buffer buffer))
	      (goto-char (1+ offset))
	      (setq ended (dns-read-name buffer))))
	(if (zerop length)
	    (setq ended t)
	  (push (buffer-substring (point)
				  (progn (forward-char length) (point)))
		name))))
    (if (stringp ended)
	(if (null name)
	    ended
	  (concat (mapconcat 'identity (nreverse name) ".") "." ended))
      (mapconcat 'identity (nreverse name) "."))))

(defun dns-write (spec &optional tcp-p)
  "Write a DNS packet according to SPEC.
If TCP-P, the first two bytes of the packet will be the length field."
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (dns-write-bytes (dns-get 'id spec) 2)
    (dns-write-bytes
     (logior
      (ash (if (dns-get 'response-p spec) 1 0) 7)
      (ash
       (cond
	((eq (dns-get 'opcode spec) 'query) 0)
	((eq (dns-get 'opcode spec) 'inverse-query) 1)
	((eq (dns-get 'opcode spec) 'status) 2)
	(t (error "No such opcode: %s" (dns-get 'opcode spec))))
       3)
      (ash (if (dns-get 'authoritative-p spec) 1 0) 2)
      (ash (if (dns-get 'truncated-p spec) 1 0) 1)
      (ash (if (dns-get 'recursion-desired-p spec) 1 0) 0)))
    (dns-write-bytes
     (cond
      ((eq (dns-get 'response-code spec) 'no-error) 0)
      ((eq (dns-get 'response-code spec) 'format-error) 1)
      ((eq (dns-get 'response-code spec) 'server-failure) 2)
      ((eq (dns-get 'response-code spec) 'name-error) 3)
      ((eq (dns-get 'response-code spec) 'not-implemented) 4)
      ((eq (dns-get 'response-code spec) 'refused) 5)
      (t 0)))
    (dns-write-bytes (length (dns-get 'queries spec)) 2)
    (dns-write-bytes (length (dns-get 'answers spec)) 2)
    (dns-write-bytes (length (dns-get 'authorities spec)) 2)
    (dns-write-bytes (length (dns-get 'additionals spec)) 2)
    (dolist (query (dns-get 'queries spec))
      (dns-write-name (car query))
      (dns-write-bytes (cadr (assq (or (dns-get 'type query) 'A)
				   dns-query-types)) 2)
      (dns-write-bytes (cadr (assq (or (dns-get 'class query) 'IN)
				   dns-classes)) 2))
    (dolist (slot '(answers authorities additionals))
      (dolist (resource (dns-get slot spec))
	(dns-write-name (car resource))
      (dns-write-bytes (cadr (assq (dns-get 'type resource) dns-query-types))
		       2)
      (dns-write-bytes (cadr (assq (dns-get 'class resource) dns-classes))
		       2)
      (dns-write-bytes (dns-get 'ttl resource) 4)
      (dns-write-bytes (length (dns-get 'data resource)) 2)
      (insert (dns-get 'data resource))))
    (when tcp-p
      (goto-char (point-min))
      (dns-write-bytes (buffer-size) 2))
    (buffer-string)))

(defun dns-read (packet &optional tcp-p)
  (with-temp-buffer
    (set-buffer-multibyte nil)
    (let ((spec nil)
          queries answers authorities additionals)
      (insert packet)
      ;; When using TCP we have a 2 byte length field to ignore.
      (goto-char (+ (point-min)
                    (if tcp-p 2 0)))
      (push (list 'id (dns-read-bytes 2)) spec)
      (let ((byte (dns-read-bytes 1)))
        (push (list 'response-p (if (zerop (logand byte (ash 1 7))) nil t))
              spec)
        (let ((opcode (logand byte (ash 7 3))))
          (push (list 'opcode
                      (cond ((eq opcode 0) 'query)
                            ((eq opcode 1) 'inverse-query)
                            ((eq opcode 2) 'status)))
                spec))
        (push (list 'authoritative-p (if (zerop (logand byte (ash 1 2)))
                                         nil t)) spec)
        (push (list 'truncated-p (if (zerop (logand byte (ash 1 2))) nil t))
              spec)
        (push (list 'recursion-desired-p
                    (if (zerop (logand byte (ash 1 0))) nil t)) spec))
      (let ((rc (logand (dns-read-bytes 1) 15)))
        (push (list 'response-code
                    (cond
                     ((eq rc 0) 'no-error)
                     ((eq rc 1) 'format-error)
                     ((eq rc 2) 'server-failure)
                     ((eq rc 3) 'name-error)
                     ((eq rc 4) 'not-implemented)
                     ((eq rc 5) 'refused)))
              spec))
      (setq queries (dns-read-bytes 2))
      (setq answers (dns-read-bytes 2))
      (setq authorities (dns-read-bytes 2))
      (setq additionals (dns-read-bytes 2))
      (let ((qs nil))
        (dotimes (_ queries)
          (push (list (dns-read-name)
                      (list 'type (dns-inverse-get (dns-read-bytes 2)
                                                   dns-query-types))
                      (list 'class (dns-inverse-get (dns-read-bytes 2)
                                                    dns-classes)))
                qs))
        (push (list 'queries qs) spec))
      (cl-loop for (slot length) in `((answers ,answers)
                                      (authorities ,authorities)
                                      (additionals ,additionals))
               do (let ((qs nil)
                        type)
                    (dotimes (_ length)
                      (push (list (dns-read-name)
                                  (list 'type
                                        (setq type (dns-inverse-get
                                                    (dns-read-bytes 2)
                                                    dns-query-types)))
                                  (list 'class (dns-inverse-get
                                                (dns-read-bytes 2)
                                                dns-classes))
                                  (list 'ttl (dns-read-bytes 4))
                                  (let ((length (dns-read-bytes 2)))
                                    (list 'data
                                          (dns-read-type
                                           (buffer-substring
                                            (point)
                                            (progn (forward-char length)
                                                   (point)))
                                           type))))
                            qs))
                    (push (list slot qs) spec)))
      (nreverse spec))))

(defun dns-read-int32 ()
  (declare (obsolete nil "28.1"))
  (number-to-string (dns-read-bytes 4)))

(defun dns-read-type (string type)
  (let ((buffer (current-buffer))
	(point (point)))
    (prog1
        (with-temp-buffer
	  (set-buffer-multibyte nil)
          (insert string)
          (goto-char (point-min))
          (cond
           ((eq type 'A)
            (let ((bytes nil))
              (dotimes (_ 4)
                (push (dns-read-bytes 1) bytes))
              (mapconcat 'number-to-string (nreverse bytes) ".")))
           ((eq type 'AAAA)
            (let (hextets)
              (dotimes (_ 8)
                (push (dns-read-bytes 2) hextets))
              (mapconcat (lambda (n) (format "%x" n))
                         (nreverse hextets) ":")))
           ((eq type 'SOA)
            (list (list 'mname (dns-read-name buffer))
                  (list 'rname (dns-read-name buffer))
		  (list 'serial (dns-read-bytes 4))
		  (list 'refresh (dns-read-bytes 4))
		  (list 'retry (dns-read-bytes 4))
		  (list 'expire (dns-read-bytes 4))
		  (list 'minimum (dns-read-bytes 4))))
           ((eq type 'SRV)
            (list (list 'priority (dns-read-bytes 2))
                  (list 'weight (dns-read-bytes 2))
                  (list 'port (dns-read-bytes 2))
                  (list 'target (dns-read-name buffer))))
           ((eq type 'MX)
            (cons (dns-read-bytes 2) (dns-read-name buffer)))
           ((or (eq type 'CNAME) (eq type 'NS) (eq type 'PTR))
            (dns-read-string-name string buffer))
           (t string)))
      (goto-char point))))

(declare-function network-interface-list "process.c")

(defun dns-servers-up-to-date-p ()
  "Return false if we need to recheck the list of DNS servers."
  (and dns-servers
       (or (eq dns-servers-valid-for-interfaces t)
	   (equal dns-servers-valid-for-interfaces
		  (network-interface-list)))))

(defun dns-set-servers ()
  "Set `dns-servers' to a list of DNS servers or nil if none are found.
Parses \"/etc/resolv.conf\" or calls \"nslookup\"."
  (setq dns-servers nil)
  (or (when (file-exists-p "/etc/resolv.conf")
	(with-temp-buffer
	  (insert-file-contents "/etc/resolv.conf")
	  (goto-char (point-min))
	  (while (re-search-forward "^nameserver[\t ]+\\([^ \t\n]+\\)" nil t)
	    (push (match-string 1) dns-servers))
	  (setq dns-servers (nreverse dns-servers))))
      (when (executable-find "nslookup")
	(with-temp-buffer
	  (call-process "nslookup" nil t nil "localhost")
	  (goto-char (point-min))
          (when (re-search-forward
	   "^Address:[ \t]*\\([0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+\\|[[:xdigit:]:]*\\)" nil t)
	      (setq dns-servers (list (match-string 1)))))))
  (setq dns-servers-valid-for-interfaces (network-interface-list)))

(defun dns-read-txt (string)
  (if (> (length string) 1)
      (substring string 1)
    string))

(defun dns-get-txt-answer (answers)
  (let ((result "")
	(do-next nil))
    (dolist (answer answers)
      (dolist (elem answer)
	(when (consp elem)
	  (cond
	   ((eq (car elem) 'type)
	    (setq do-next (eq (cadr elem) 'TXT)))
	   ((eq (car elem) 'data)
	    (when do-next
	      (setq result (concat result (dns-read-txt (cadr elem))))))))))
    result))

;;; Interface functions.
(defvar dns-cache (make-vector 4096 0))

(defun dns-query-cached (name &optional type fullp reversep)
  (let* ((key (format "%s:%s:%s:%s" name type fullp reversep))
	 (sym (intern-soft key dns-cache)))
    (if (and sym
	     (boundp sym))
	(symbol-value sym)
      (let ((result (dns-query name type fullp reversep)))
	(set (intern key dns-cache) result)
	result))))

(defun dns-query-asynchronous (name callback &optional type full reverse)
  "Query a DNS server for NAME of TYPE.
CALLBACK will be called with a single parameter: The result.

If there's no result, or `dns-timeout' has passed, CALLBACK will
be called with nil as the parameter.

If FULL, return the entire record.
If REVERSE, look up an IP address."
  (setq type (or type 'A))
  (unless (dns-servers-up-to-date-p)
    (dns-set-servers))

  (when reverse
    (setq name (concat
		(mapconcat 'identity (nreverse (split-string name "\\.")) ".")
		".in-addr.arpa")
	  type 'PTR))

  (if (not dns-servers)
      (progn
        (message "No DNS server configuration found")
        nil)
    (dns--lookup name callback type full)))

(defun dns--lookup (name callback type full)
  (with-current-buffer (generate-new-buffer " *dns*")
    (set-buffer-multibyte nil)
    (let* ((tcp nil)
           (process
            (condition-case ()
                (let ((server (car dns-servers))
	              (coding-system-for-read 'binary)
	              (coding-system-for-write 'binary))
                  (if (featurep 'make-network-process '(:type datagram))
	              (make-network-process
	               :name "dns"
	               :coding 'binary
	               :buffer (current-buffer)
	               :host server
	               :service "domain"
	               :type 'datagram)
                    ;; On MS-Windows datagram sockets are not
                    ;; supported, so we fall back on opening a TCP
                    ;; connection to the DNS server.
                    (progn
                      (setq tcp t)
                      (open-network-stream "dns" (current-buffer)
                                           server "domain"))))
              (error
               (message
                "dns: Got an error while trying to talk to %s"
                (car dns-servers))
               nil)))
           (triggered nil)
           (buffer (current-buffer))
           timer)
      (if (not process)
          (progn
            (kill-buffer buffer)
            (funcall callback nil))
        ;; Call the callback if we don't get any response at all.
        (setq timer (run-at-time dns-timeout nil
                                 (lambda ()
                                   (unless triggered
                                     (setq triggered t)
                                     (delete-process process)
                                     (kill-buffer buffer)
                                     (funcall callback nil)))))
        (process-send-string
         process
         (dns-write `((id ,(random 65000))
                      (opcode query)
                      (queries ((,name (type ,type))))
                      (recursion-desired-p t))
                    tcp))
        (set-process-filter
         process
         (lambda (process string)
           (with-current-buffer (process-buffer process)
             (goto-char (point-max))
             (insert string)
             (goto-char (point-min))
             ;; If this is DNS, then we always get the full data in
             ;; one packet.  If it's TCP, we may only get part of the
             ;; data, but the first two bytes says how long the data
             ;; is supposed to be.
             (when (or (not tcp)
                       (>= (buffer-size) (dns-read-bytes 2)))
               (setq triggered t)
               (cancel-timer timer)
               (dns--filter process callback type full tcp)))))
        ;; In case we the process is deleted for some reason, then do
        ;; a failure callback.
        (set-process-sentinel
         process
         (lambda (_ state)
           (when (and (eq state 'deleted)
                      ;; Ensure we don't trigger this callback twice.
                      (not triggered))
             (setq triggered t)
             (cancel-timer timer)
             (kill-buffer buffer)
             (funcall callback nil))))))))

(defun dns--filter (process callback type full tcp)
  (let ((message (buffer-string)))
    (when (process-live-p process)
      (delete-process process))
    (kill-buffer (current-buffer))
    (when (>= (length message) 2)
      (let ((result (dns-read message tcp)))
        (funcall callback
                 (if full
                     result
                   (let ((answer (car (dns-get 'answers result))))
                     (when (eq type (dns-get 'type answer))
                       (if (eq type 'TXT)
                           (dns-get-txt-answer (dns-get 'answers result))
                         (dns-get 'data answer))))))))))

(defun dns-query (name &optional type full reverse)
  "Query a DNS server for NAME of TYPE.
If FULL, return the entire record returned.
If REVERSE, look up an IP address."
  (let ((result nil))
    (dns-query-asynchronous
     name
     (lambda (response)
       (setq result (list response)))
     type full reverse)
    ;; Loop until we get the callback.
    (while (not result)
      (sleep-for 0.01))
    (car result)))

(provide 'dns)

;;; dns.el ends here