; 
; dns.scm
;
; Implementation of the RFC1035 

;;; This file is part of the Scheme Untergrund Networking package.

;;; Copyright (c) 2002 by Marcus Crestani.
;;; For copyright information, see the file COPYING which comes with
;;; the distribution.

; domain names - implementation and specification
; based on the PLT-implementation.
; 
;
; TODO: 
;       - test, test, test
;       - types from newer RFCs (41, unknown)
;       - more documentation
;
; ---
; sample usage & documentation:
;
;  <ip-address32> is a 32bit integer internet->address, shortly address32.
;  <ip-string> is a string in standard dot notation "xxx.xxx.xxx.xxx".
;  <name> is a string
;
;  <nameserver> can either be a domainname, an ip-string or an ip-address32.
;  if it is a domainname, its ip is looked up on a nameserver listed in
;  /etc/resolv.conf.
;
;  (dns-find-nameserver) --> <ip-address32>
;  this parses the /etc/resolv.conf file and returns the first found 
;  nameserver in address32 format.
;
;
;  
;  (dns-lookup-name <name> [nameserver]) --> <ip-address32>
;  (dns-lookup-ip <ip-string | ip-address32> [nameserver]) --> <name>
;  (dns-lookup-nameserver <name> [nameserver]) 
;                     --> <list of ip-address32s of authoritative nameservers>
;  (dns-lookup-mail-exchanger <name> [nameserver]) 
;                     --> <list of names of mail-exchangers>
; 
;  dns-lookup-name, dns-lookup-ip, dns-lookup-nameserver and 
;  dns-lookup-mail-exchanger are "simple lookup functions",
;  they return the wanted information or #f.
;  dns-lookup-ip can either be given an ip-string or an ip-address32.
;   
;  concurrent dns lookup:
;  if a list of nameservers is given to the optional <nameserver> argument,
;  a concurrent lookup to all nameservers in this list is started.
;  The nameservers in this list could either be ip-strings or ip-address32s.
;  example: (dns-lookup-name "www.uni-tuebingen.de" (dns-find-nameserver-list))
;           starts an concurrent lookup which contacts all nameservers in 
;           /etc/resolv.conf.
;
;
;  (dns-lookup <name | ip-string | ip-address32> <type> [nameserver]) 
;                     --> <dns-message>
;  (show-dns-message <dns-message) --> the whole message, human readable
;
;  a <dns-message> is a record, with several entries, which holds the whole
;  query/response dialog. the simplest way to get detailed information about
;  the record structure is to view the result of show-dns-message.
;
;  dns-lookup returns much more information than the simple lookup functions, 
;  only useful in very special cases.
;  
;
;  some lookups return a hostname (e.g. mx). 
;  many applications need instead of a hostname a ip address.
;  force-ip and force-ip-list guarantee that a ip address is
;  returned.
;
;  (force-ip <name>) --> <ip>
;  (force-ip-list <list of names>) --> <list of ips>
;
;
;  useful converters:
;
;  (address32->ip-string <ip-address32>) -> <ip-string>
;  (ip-string->address32 <ip-string>) -> <ip-address32>




;;; should debug-msgs be printed out?
(define *debug* #f)


;; --- error conditions

;; supertype of all errors signaled by this library
(define-condition-type 'dns-error '(error))
(define dns-error? (condition-predicate 'dns-error))

(define-condition-type 'invalid-type '(dns-error))
(define invalid-type? (condition-predicate 'invalid-type))

(define-condition-type 'invalid-class '(dns-error))
(define invalid-class? (condition-predicate 'invalid-class))

(define-condition-type 'parse-error '(dns-error))
(define parse-error? (condition-predicate 'parse))

(define-condition-type 'unexpected-eof-from-server '(dns-error))
(define unexpected-eof-from-server? (condition-predicate 'unexpected-eof-from-server))

(define-condition-type 'bad-address '(dns-error))
(define bad-address? (condition-predicate 'bad-address))

(define-condition-type 'no-nameservers '(dns-error))
(define no-nameservers? (condition-predicate 'no-nameservers))

(define-condition-type 'bad-nameserver '(dns-error))
(define bad-nameserver? (condition-predicate 'bad-nameserver))

(define-condition-type 'not-a-hostname '(dns-error))
(define not-a-hostname? (condition-predicate 'not-a-hostname))

(define-condition-type 'not-a-ip '(dns-error))
(define not-a-ip? (condition-predicate 'not-a-ip))

;; supertype of all errors signaled if the dns server returned a non-sero
;; reply code
(define-condition-type 'dns-server-error '(dns-error))
(define dns-server-error? (condition-predicate 'dns-server-error))

(define-condition-type 'dns-format-error '(dns-server-error))
(define dns-format-error? (condition-predicate 'dns-format-error))

(define-condition-type 'dns-server-failure '(dns-server-error))
(define dns-server-failure? (condition-predicate 'dns-server-failure))

(define-condition-type 'dns-name-error '(dns-server-error))
(define dns-name-error? (condition-predicate 'dns-name-error))

(define-condition-type 'dns-not-implemented '(dns-server-error))
(define dns-not-implemented? (condition-predicate 'dns-not-implemented))

(define-condition-type 'dns-refused '(dns-server-error))
(define dns-refused? (condition-predicate 'dns-refused))

(define (dns-error condition . stuff)
  (apply signal condition (dns-error->string condition) stuff))

(define (dns-error->string condition)
  (string-append 
   "dns-error: "
   (case condition
     ((invalid-type)
      "make-octet-question: invalid DNS query type")
     ((invalid-class)
      "make-octet-question: invalid DNS query class")
     ((parse-error)
      "parse: error parsing server message")
     ((unexpected-eof-from-server)
      "send-receive-message: unexpected EOF from server")
     ((bad-address)
      "dns-get-information: bad address (in combination with query type)")
     ((no-nameservers)
      "dns-find-nameserver: no nameservers found in /etc/resolv.conf")
     ((bad-nameserver)
      "send-receive-message: nameserver refused connection")
     ((not-a-hostname)
      "no hostname given")
     ((not-a-ip)
      "no ip given")
     ((dns-format-error) 
      "error from server: (1) format error")
     ((dns-server-failure) 
      "error from server: (2) server failure")
     ((dns-name-error) 
      "error from server: (3) name error")
     ((dns-not-implemented) 
      "error from server: (4) not implemented")
     ((dns-refused) 
      "error from server: (5) refused")
     (else (error "Unknown dns-error" condition)))))
  

;;; -- globals and types
;; off
(define *nul* (ascii->char 0))

;; on
(define *on* (ascii->char 1))

;; message types
(define types
  '((unknown 0); types, which are not yet implemented
    (a 1)      ; a host address 
    (ns 2)     ; an authoritative name server
    (md 3)     ; (obsolete)
    (mf 4)     ; (obsolete)
    (cname 5)  ; the canonical name for an alias
    (soa 6)    ; marks the start of a zone of authority
    (mb 7)     ; (experimental)
    (mg 8)     ; (experimental)
    (mr 9)     ; (experimental)
    (null 10)  ; (experimental)
    (wks 11)   ; a well known service description
    (ptr 12)   ; a domain name pointer
    (hinfo 13) ; host information
    (minfo 14) ; (experimental)
    (mx 15)    ; mail exchange
    (txt 16))) ; text strings

;; message classes
(define classes
  '((in 1)     ; the Internet
    (cs 2)     ; (obsolete)
    (ch 3)     ; the CHAOS class
    (hs 4)))   ; Hesoid


;;; -- useful stuff

;; assoc the other way round
(define (cossa i l)
  (if *debug* (display "cossa\n"))
  (cond
   ((null? l) 'unknown)
   ((equal? (cadar l) i)
    (car l))
   (else (cossa i (cdr l)))))

;; encodes numbers (16bit) to octets
(define (number->octet-pair n)
  (if *debug* (display "number->octet-pair\n"))
  (list (ascii->char (arithmetic-shift n -8))
	(ascii->char (modulo n 256))))

;; decodes octets to numbers (16bit)
(define (octet-pair->number a b)
  (if *debug* (display "octet-pair->number\n"))
  (+ (arithmetic-shift (char->ascii a) 8)
     (char->ascii b)))

;; encodes numbers (32bit) to octets, needed for ttl
(define (number->octet-quad n)
  (if *debug* (display "number->octet-quad\n"))
  (list (ascii->char (arithmetic-shift n -24))
	(ascii->char (modulo (arithmetic-shift n -16) 256))
	(ascii->char (modulo (arithmetic-shift n -8) 256))
	(ascii->char (modulo n 256))))

;; decodes octets to numbers, needed for 32bit ttl
(define (octet-quad->number a b c d)
  (if *debug* (display "octet-quad->number\n"))
  (+ (arithmetic-shift (char->ascii a) 24)
     (arithmetic-shift (char->ascii b) 16)
     (arithmetic-shift (char->ascii c) 8)
     (char->ascii d)))

;; encodes a domain-name string to octets
(define (name->octets s)
  (define (encode-portion s)
    (cons
     (ascii->char (string-length s))
     (string->list s)))

  (if *debug* (display "name->octets\n"))
  (let loop ((s s))
    (cond
     ((regexp-search (rx (: bos (submatch (* (~ "."))) "." (submatch (* any))))
		     s)
      => (lambda (match)
	   (append
	    (encode-portion (match:substring match 1))
	    (loop (match:substring match 2)))))
     (else
      (if (= 0 (string-length s))
	  (list *nul*)
	  (append
	   (encode-portion s)
	   (list *nul*)))))))


;; for tcp: message has to be tagged with its length
(define (add-size-tag m)
  (if *debug* (display "add-size-tag\n"))
  (append (number->octet-pair (length m)) m))

;; converts an octeted-ip to a 32bit integer internet-address
(define (octet-ip->address32 ip)
  (if *debug* (display "octet-ip->address32\n"))
  (+ (arithmetic-shift (char->ascii (list-ref ip 0)) 24)
     (arithmetic-shift (char->ascii (list-ref ip 1)) 16)
     (arithmetic-shift (char->ascii (list-ref ip 2)) 8)
     (char->ascii (list-ref ip 3))))

;; converts a 32 bit integer internet-address to an octeted-ip
(define (address32->octet-ip ip)
  (if *debug* (display "number->octet-quad\n"))
  (list (arithmetic-shift ip -24)
	(modulo (arithmetic-shift ip -16) 256)
	(modulo (arithmetic-shift ip -8) 256)
	(modulo ip 256)))

;; converts an ip-string to an 32bit integer internet-address
(define (ip-string->address32 ip)
  (if *debug* (display "ip-string->address32\n"))
  (octet-ip->address32 (string->octet-ip ip)))

;; converts an ip-string to an 32bit integer internet-address
(define (address32->ip-string ip)
;  (if *debug* (display "address32->ip-string\n"))
  (format #f
	  "~a.~a.~a.~a" 
	  (arithmetic-shift ip -24)
	  (modulo (arithmetic-shift ip -16) 256)
	  (modulo (arithmetic-shift ip -8) 256)
	  (modulo ip 256)))

;; converts an octeted-ip to an human readable ip-string
(define (octet-ip->string s)
  (if *debug* (display "octet-ip->string\n"))
  (format #f
	  "~a.~a.~a.~a" 
	  (char->ascii (list-ref s 0))
	  (char->ascii (list-ref s 1))
	  (char->ascii (list-ref s 2))
	  (char->ascii (list-ref s 3))))

(define ip-string-regexp (rx (: bos 
			  (submatch (** 1 3 digit)) "." 
			  (submatch (** 1 3 digit)) "." 
			  (submatch (** 1 3 digit)) "." 
			  (submatch (** 1 3 digit)) 
			  eos)))

;; converts an ip-string to octets
(define (string->octet-ip s)
  (cond
   ((regexp-search ip-string-regexp  s)
    => (lambda (match)
	 (list
	  (ascii->char (string->number (match:substring match 1)))
	  (ascii->char (string->number (match:substring match 2)))
	  (ascii->char (string->number (match:substring match 3)))
	  (ascii->char (string->number (match:substring match 4))))))
   (else
    (error "invalid ip-string" s))))

;; calculates a "random" number, needed for message-ids
(define random
  (let ((crank (make-random (modulo (time) (- (expt 2 27) 1)))))
    (lambda (limit)
      (quotient (* (modulo (crank) 314159265)
		   limit)
		314159265))))

;; checks if a string is a ip
(define (ip-string? s)
  (define (byte-as-string? string) 
    (let ((number (string->number string)))
      (and number
	   (>= number 0)
	   (< number 256))))
  (cond
   ((regexp-search ip-string-regexp  s)
    => (lambda (match)
	 (and (byte-as-string? (match:substring match 1))
	      (byte-as-string? (match:substring match 2))
	      (byte-as-string? (match:substring match 3))
	      (byte-as-string? (match:substring match 4)))))
   (else #f)))

    
;; checks if v is a address32
(define (address32? v)
  (and (number? v)
       (<= 0 v #xffffffff)))

;; returns a in-addr.arpa name-string or #f (needed to resolve hostname by ip)
(define (ip-string->in-addr s)
  (cond
   ((regexp-search ip-string-regexp s)
    => (lambda (match)
	 (string-append
	  (match:substring match 4) "."
	  (match:substring match 3) "."
	  (match:substring match 2) "."
	  (match:substring match 1) "."
	  "in-addr.arpa")))
   (else #f)))
      
;; filters types in a list of rrs 
(define (filter-type list type)
  (if *debug* (display "ip-string->in-addr\n"))
  (filter (lambda (rr) 
	    (eq? (rr:type rr) type))
	  list))

;; sorts a mx-rr-list by preference. needed for dns-lookup-mail-exchanger.
(define (sort-by-preference mx-list)
  (sort-list mx-list 
	     (lambda (a b) 
	       (< (rr-data-mx:preference (rr:data a)) (rr-data-mx:preference (rr:data b))))))


;; returns a IP if available (additonal type-a processing)
(define (force-ip name)
  (let loop ((result (dns-lookup-name name)))
    (if (ip-string? result)
	result
	(loop (dns-lookup-name result)))))

;; returns a list of IPs (additional type-a processing)
(define (force-ip-list names)
  (map (lambda (elem) (force-ip elem)) names))


;;; -- message constructors: encode to octet-messages

;; makes an message header
(define (make-octet-header id qr opcode aa tc rd ra z rcode qdcount ancount nscount arcount)
  (if *debug* (display "make-octet-header\n"))
  (let* ((header-id (number->octet-pair id))
	 (header-flags (list 
			(ascii->char (+ (arithmetic-shift qr 7)
					(arithmetic-shift opcode 3)
					(arithmetic-shift aa 2)
					(arithmetic-shift tc 1)
					rd))
			(ascii->char (+ (arithmetic-shift ra 7)
				        (arithmetic-shift z 4)
					rcode))))
	 (header-qdcount (number->octet-pair qdcount))
	 (header-ancount (number->octet-pair ancount))
	 (header-nscount (number->octet-pair nscount))
	 (header-arcount (number->octet-pair arcount)))
    (append header-id
	    header-flags
	    header-qdcount
	    header-ancount
	    header-nscount
	    header-arcount)))


;; a standard query header, usefull for most queries
(define (make-std-octet-query-header id question-count)
  (if *debug* (display "make-std-octet-query-header\n"))
  (let* ((qr 0)        ; querytype: query 0, response 1
	 (opcode 0)    ; opcode: query 0, iquery 1 (OBSOLETE), status 2
	 (aa 0)        ; authorative answer (in answers only)
	 (tc 0)        ; truncation (size matters only with UDP)
	 (rd 1)        ; recursion desired: nameserver pursues the query recursivly (optional)
	 (ra 0)        ; recursion available (in answers only)
	 (z 0)         ; future use
         (rcode 0)     ; response code: error conditions (in answers only)
	 (qdcount question-count)
	 (ancount 0)   ; answer count (in answers only)
	 (nscount 0)   ; name server resources (in answers only)
	 (arcount 0))  ; additional records (in answers only)

    (make-octet-header id qr opcode aa tc rd ra z rcode qdcount ancount nscount arcount)))


;; makes a question (name, type, class)
(define (make-octet-question name type class)
  (if *debug* (display "make-octet-question\n"))
  (if (not (assoc type types))
      (dns-error 'invalid-type))
  (if (not (assoc class classes))
      (dns-error 'invalid-class))

  (let* ((qname   (name->octets name))
	 (qtype   (number->octet-pair (cadr (assoc type types))))
	 (qclass  (number->octet-pair (cadr (assoc class classes)))))
    (append qname qtype qclass)))


;; makes a query-message (header and question only)
(define (make-octet-query-message id name type class)
  (if *debug* (display "make-octet-query-message\n"))
  (append
   (make-std-octet-query-header id 1)
   (make-octet-question name type class)))


;; makes a resource record for ans, nss, ars (name, type, class, ttl, data)
(define (make-octet-rr name type class ttl rdata)
  (if *debug* (display "make-octet-rr\n"))
  (let* ((name (name->octets name))
	 (type (number->octet-pair (cadr (assoc type types))))
	 (class (number->octet-pair (cadr (assoc class classes))))
	 (ttl (number->octet-quad ttl))
	 (rdlength (number->octet-pair (length rdata)))
	 (rdata rdata))
    (append name type class ttl rdlength rdata)))



;;; -- parsed message records

;;; -- dns-message: complete data-structure of an dns-lookup
(define-record dns-message
  query
  reply
  cache?
  protocol
  tried-nameservers)

;; message
(define-record message
  header
  questions
  answers
  nameservers
  additionals
  source)

;; header
(define-record header
  id
  flags
  qdc
  anc
  nsc
  arc)

;; flags
(define-record flags
  querytype
  opcode
  auth
  trunc
  recursiondesired
  recursionavailable
  z
  rcode)

;; question
(define-record question
  name
  type
  class)

;; rr
(define-record rr
  name
  type
  class
  ttl
  data)

;; cache
(define-record cache
  answer
  ttl
  time)

;;; -- message parser

;; parses a domain-name in an message. returns the name and the rest of the message.
(define (parse-name start message)
  (if *debug* (display "parse-name\n"))
  (let ((v (char->ascii (car start))))
    (cond
     ((zero? v)
      ;; End of name
      (values #f (cdr start)))
     ((zero? (bitwise-and #xc0 v))
      ;; Normal label
      (let loop ((len v)
		 (start (cdr start))
		 (accum '()))
	(cond
	 ((zero? len)
	  (call-with-values
	   (lambda () (parse-name start message))
	   (lambda (s start)
	     (let ((s0 (list->string (reverse! accum))))
	       (values (if s
			   (string-append s0 "." s)
			   s0)
		       start)))))
	 (else (loop (- len 1)
		     (cdr start)
		     (cons (car start) accum))))))
     (else
      ;; Compression offset
      (let ((offset (+ (arithmetic-shift (bitwise-and #x3f v) 8)
		       (char->ascii (cadr start)))))
	(call-with-values
	 (lambda () (parse-name (list-tail message offset) message))
	 (lambda (s ignore-start)
	   (values s (cddr start)))))))))

;; parses a question in a message. returns the question and the rest of the message.
(define (parse-question start message)
  (if *debug* (display "parse-question\n"))
  (call-with-values
   (lambda () (parse-name start message))
   (lambda (name start)
     (let ((type (car (cossa (octet-pair->number (car start) (cadr start)) types)))
	   (start (cddr start)))
       (let ((class (car (cossa (octet-pair->number (car start) (cadr start)) classes)))
	     (start (cddr start)))
	 (values (make-question name type class) start))))))

;; parses a resourcerecord in a message. returns the rr and the rest of the message.
(define (parse-rr start message)
  (if *debug* (display "parse-rr\n"))
  (call-with-values
   (lambda () (parse-name start message))
   (lambda (name start)
     (let ((type (car (cossa (octet-pair->number (car start) (cadr start)) types)))
	   (start (cddr start)))
       (let ((class (car (cossa (octet-pair->number (car start) (cadr start)) classes)))
	     (start (cddr start)))
	 (let ((ttl (octet-quad->number (car start) (cadr start)
					(caddr start) (cadddr start)))
	       (start (cddddr start)))
	   (let ((len (octet-pair->number (car start) (cadr start)))
		 (start (cddr start)))
	     ;; Extract next len bytes of data:
	     (let loop ((len len)
			(start start)
			(accum '()))
	       (if (zero? len)
		   (values (make-rr name type class ttl (parse-rr-data type class (reverse! accum) message)) start)
		   (loop (- len 1)
			 (cdr start)
			 (cons (car start) accum)))))))))))

;;; -- rr-data-type records

(define-record rr-data-a
  ip)

(define-record rr-data-ns
  name)

(define-record rr-data-cname
  name)

;; ###
;; hinfo not correctly implemented, trying to find examples 
(define-record rr-data-hinfo  
  data)                       

(define-record rr-data-mx
  preference
  exchanger)

(define-record rr-data-ptr
  name)

(define-record rr-data-soa
  mname
  rname
  serial
  refresh
  retry
  expire
  minimum)

;; ### same as hinfo
(define-record rr-data-txt
  text)

;; ### same as hinfo and txt
(define-record rr-data-wks
  data)

;;

(define (parse-rr-data type class data message)
  (if *debug* (display "parse-rr-data\n"))
  (cond
   ((eq? type 'a) 
    (make-rr-data-a (octet-ip->address32 data)))
   
   ((eq? type 'ns) 
    (make-rr-data-ns (call-with-values 
		      (lambda () (parse-name data message))
		      (lambda (name rest) name))))

   ((eq? type 'cname) 
    (make-rr-data-cname (call-with-values
			 (lambda () (parse-name data message))
			 (lambda (name rest) name))))

   ((eq? type 'mx)
    (make-rr-data-mx (octet-pair->number (car data) (cadr data))
		     (call-with-values
		      (lambda ()(parse-name (cddr data) message))
		      (lambda (name rest) name))))

   ((eq? type 'ptr)
    (make-rr-data-ptr (call-with-values
		       (lambda () (parse-name data message))
		       (lambda (name rest) name))))
   
   ((eq? type 'soa)
    (call-with-values
     (lambda () (parse-name data message))
     (lambda (mname rest)
       (call-with-values
	(lambda () (parse-name rest message))
	(lambda (rname rest)
	  (let ((serial (octet-quad->number (car rest) (cadr rest) (caddr rest) (cadddr rest)))
		(rest (cddddr rest)))
	    (let ((refresh (octet-quad->number (car rest) (cadr rest) (caddr rest) (cadddr rest)))
		  (rest (cddddr rest)))
	      (let ((retry (octet-quad->number (car rest) (cadr rest) (caddr rest) (cadddr rest)))
		    (rest (cddddr rest)))
		(let ((expire (octet-quad->number (car rest) (cadr rest) (caddr rest) (cadddr rest)))
		      (rest (cddddr rest)))
		  (let ((minimum (octet-quad->number (car rest) (cadr rest) (caddr rest) (cadddr rest)))
			(rest (cddddr rest)))
		    (make-rr-data-soa mname rname serial refresh retry expire minimum)))))))))))

   ((eq? type 'hinfo)
    (make-rr-data-hinfo (list->string data)))

   ((eq? type 'txt)
    (make-rr-data-txt (list->string data)))
   
   ((eq? type 'wks)
    (make-rr-data-wks data))

   (else (list data))))

;; parses n-times a message with parse. returns a list of parse-returns.
(define (parse-n parse start message n)
  (if *debug* (display "parse-n\n"))
  (let loop ((n n) (start start) (accum '()))
    (if (zero? n)
	(values (reverse! accum) start)
	(call-with-values
	 (lambda () (parse start message))
	 (lambda (rr start)
	   (loop (- n 1) start (cons rr accum)))))))

;; parses a message-headers flags. returns the flags.
(define (parse-flags message)
  (if *debug* (display "parse-flags\n"))
  (let ((v0 (list-ref message 2))
	(v1 (list-ref message 3)))
    ;; Check for error code:
    (let ((rcode  (bitwise-and #xf (char->ascii v1)))
	  (z      (arithmetic-shift (bitwise-and 112 (char->ascii v1)) -4))
	  (ra     (arithmetic-shift (bitwise-and 64 (char->ascii v1)) -7))
	  (rd     (bitwise-and 1 (char->ascii v0)))
	  (tc     (arithmetic-shift (bitwise-and 2 (char->ascii v0)) -1))
	  (aa     (arithmetic-shift (bitwise-and 4 (char->ascii v0)) -2))
	  (opcode (arithmetic-shift (bitwise-and 120 (char->ascii v0)) -3))
	  (qr     (arithmetic-shift (bitwise-and 128 (char->ascii v0)) -7))) 
      (make-flags qr opcode aa tc rd ra z rcode))))


;; parses a message-header. returns the header.
(define (parse-header message)
  (if *debug* (display "parse-header\n"))
  (let ((id       (octet-pair->number (list-ref message 0) (list-ref message 1)))
	(flags    (parse-flags message))
	(qd-count (octet-pair->number (list-ref message 4) (list-ref message 5)))
	(an-count (octet-pair->number (list-ref message 6) (list-ref message 7)))
	(ns-count (octet-pair->number (list-ref message 8) (list-ref message 9)))
	(ar-count (octet-pair->number (list-ref message 10) (list-ref message 11))))
    (make-header id flags qd-count an-count ns-count ar-count)))


;; parses a message. returns the parsed message.
(define (parse message)
  (if *debug* (display "parse\n"))
  (let* ((header (parse-header message))
	 (start (list-tail message 12)))
    (call-with-values
     (lambda () (parse-n parse-question start message (header:qdc header)))
     (lambda (qds start)
       (call-with-values
	(lambda () (parse-n parse-rr start message (header:anc header)))
	(lambda (ans start)
	  (call-with-values
	   (lambda () (parse-n parse-rr start message (header:nsc header)))
	   (lambda (nss start)
	     (call-with-values
	      (lambda () (parse-n parse-rr start message (header:arc header)))
	      (lambda (ars start)
		(if (not (null? start))
		    (dns-error 'parse-error))
		(make-message header qds ans nss ars message)))))))))))



;;; -- send, receive and validate message

;; checks if the received reply is valid. returns #t or error-msg.
(define (reply-acceptable? reply query)
  (if *debug* (display "reply-acceptable?\n"))
  ;; Check correct id
  (if (not (and (char=? (car reply) (car query))
		(char=? (cadr reply) (cadr query))))
      (display "send-receive-message: bad reply id from server"))
  ;; Check for error code:
  (let ((rcode (flags:rcode (parse-flags reply))))
    (if (not (zero? rcode))
	(case rcode
	  ((1) (dns-error 'dns-format-error))
	  ((2) (dns-error 'dns-server-failure))
	  ((3) (dns-error 'dns-name-error))
	  ((4) (dns-error 'dns-not-implemented))
	  ((5) (dns-error 'dns-refused))))))

;; #t if message is truncated (could happen via UDP)
(define (truncated? reply)
  (let ((trunc (flags:trunc (parse-flags reply))))
    (= trunc 1)))

;; connects to nameserver and sends and receives messages. returns the reply.
;; here: via TCP
(define (send-receive-message-tcp nameservers query)
  (receive (reply hit-ns other-nss)
      (let ((sockets (map (lambda (nameserver)
			    (let ((sock (create-socket protocol-family/internet
						       socket-type/stream))
				  (addr (internet-address->socket-address
					 nameserver 53)))
			      ;; we ignore the return value and select
			      ;; unconditionally later
			      (connect-socket-no-wait sock addr)
			      sock))
			  nameservers)))
	(let* ((ws (map socket:outport sockets))
	       (wsv (list->vector ws))
	       (wport-nameserver-alist (map cons ws nameservers))
	       (wport-socket-alist (map cons ws sockets)))
	  (dynamic-wind
	   (lambda () #f)
	   (lambda ()
	     (receive (ignore-r write-ready ignore-e)
		 ;;; TODO this should circumvent the buffer
		 (select '#() wsv '#())
	       (let* ((w (vector-ref write-ready 0))
		      (hit-ns (cdr (assoc w wport-nameserver-alist)))
		      (sock (cdr (assoc w wport-socket-alist))))
		 (if (not (connect-socket-successful? sock))
		     (dns-error 'bad-nameserver hit-ns))
		 (let ((query-string (list->string query))
		       (r (socket:inport sock)))
		   (display (list->string (add-size-tag query)) w)
		   (force-output w)
		   (let ((a (read-char r))
			 (b (read-char r)))
		     (let ((len (octet-pair->number a b)))
		       (let ((s (read-string len r)))
			 (if (not (= len (string-length s)))
			     (dns-error 'unexpected-eof-from-server))
			 (values (string->list s)
				 hit-ns
				 (delete hit-ns nameservers)))))))))
	   (lambda ()
	     (for-each close-socket sockets)))))
    (reply-acceptable? reply query)
    (values (parse reply)
	    hit-ns
	    other-nss)))

;; here: via UDP
(define (send-receive-message-udp nameservers query)
  (if *debug* (display "send-receive-message\n"))
  (receive (reply hit-ns other-nss)
      (let ((sockets (map (lambda (nameserver)
			    (let ((sock (create-socket protocol-family/internet
						       socket-type/datagram))
				  (addr (internet-address->socket-address
					 nameserver 53)))
			      (connect-socket sock addr)
			      sock))
			  nameservers)))
	(let ((rs (map socket:inport sockets))
	      (ws (map socket:outport sockets)))
	  (dynamic-wind
	   (lambda ()
	     'nothing-to-be-done-before)
	   (lambda ()
	     (let ((query-string (list->string query))
		   (rsv (list->vector rs))
		   (rport-nameserver-alist (map cons rs nameservers))
		   (rport-socket-alist (map cons rs sockets)))
	       (for-each (lambda (w) (display query-string w)) ws)
	       (for-each force-output ws)
	       (receive (read-ready ignore-w ignore-e)
		 ;;; TODO this should circumvent the buffer
		   (select rsv '#() '#())
		 (let* ((r (vector-ref read-ready 0))
			(hit-ns (cdr (assoc r rport-nameserver-alist))))
		   (if (not (connect-socket-successful? (cdr (assoc r rport-socket-alist))))
		       (dns-error 'bad-nameserver hit-ns))
		           ;;; 512 is the maximum udp-message size:
		   (values (string->list (read-string/partial 512 r)) 
			   hit-ns
			   (delete hit-ns nameservers))))))
	   (lambda ()
	     (for-each close-socket sockets)))))
    (reply-acceptable? reply query)
    (if (truncated? reply)
	(send-receive-message-tcp nameservers query)
	(values (parse reply)
		hit-ns
		other-nss))))


;;; -- cache

;; creates the cache, an empty string-table
(define cache (make-string-table))

;; resets the cache
(define (dns-clear-cache)
  (set! cache (make-string-table)))

;; searches in a dns-msg for the shortest ttl. this is needed for cache-management.
(define (find-shortest-ttl dns-msg)
  (if *debug* (display "find-shortest-ttl\n"))
  (letrec ((minimum #f)
	   (find-shortest-ttl-1
	    (lambda (dns-msg)
	      (cond
	       ((dns-message? dns-msg) 
		(find-shortest-ttl-1 (dns-message:reply dns-msg)))
	       ((message? dns-msg)
		(for-each (lambda (x) (find-shortest-ttl-1 x)) (message:answers dns-msg))
		(for-each (lambda (x) (find-shortest-ttl-1 x)) (message:nameservers dns-msg))
		(for-each (lambda (x) (find-shortest-ttl-1 x)) (message:additionals dns-msg))
		minimum)
	       ((rr? dns-msg)
		(cond
		 ((not minimum)  (set! minimum (rr:ttl dns-msg)))
		 (else
		  (if (and (not minimum) (> minimum (rr:ttl dns-msg)))
		      (set! minimum (rr:ttl dns-msg))))))))))
    (find-shortest-ttl-1 dns-msg)))


(define (make-key qds nameserver)
  (let*;; cache-key relevant data
      ((name (question:name (car qds)))
       (type (question:type (car qds)))
       (class (question:class (car qds))))
    (format #f "~a;~a;~a;~a" nameserver name type class)))

(define (lookup-cache qds nameserver)
  (let* ((key (make-key qds nameserver))
	 (found-data (table-ref cache key)))
    (cond
     ((and found-data
	   ;; checks if cached-data is still valid
	   (< (time) (+ (cache:time found-data) (cache:ttl found-data))))
      found-data)
     (else #f))))

(define (update-cache! key entry)
  (table-set! cache key entry))

(define (dns-query-no-cache question protocol nameservers tried)
	 ;; returns new retrieved data
	 (receive (dns-msg hit-ns nss-with-no-reply)
	     (send-receive-message nameservers question protocol)
	   (values 
	    (make-dns-message (parse question) dns-msg  #f protocol (reverse tried))
	    hit-ns
	    nss-with-no-reply)))

(define (dns-query-with-cache question protocol nameservers tried)
  (let ((qds (message:questions (parse question))))
    (let lp ((ns nameservers))
      (if (null? ns)
	  (receive (reply-msg hit-ns nss-with-no-reply)
	      (send-receive-message nameservers question protocol)
	    (update-cache! (make-key qds hit-ns) 
			   (make-cache reply-msg (find-shortest-ttl reply-msg) (time)))
	    ;; returns new retrieved data and updates cache
	    (values (make-dns-message (parse question) reply-msg #f protocol (reverse tried))
		    hit-ns
		    nss-with-no-reply))
	  (cond ((lookup-cache qds (car ns))
		 => (lambda (found-data)
		      ;; returns cached data
		      (values (make-dns-message (parse question) (cache:answer found-data) #t protocol '())
			      #f
			      nameservers)))
		(else (lp (cdr ns))))))))

(define (send-receive-message nameservers question protocol)
  ((cond 
    ((eq? protocol 'tcp) send-receive-message-tcp)
    ((eq? protocol 'udp) send-receive-message-udp))
   nameservers question))
  
;; makes a dns-query. optional cache-check. 
;; returns a dns-message with cache-flag and either cache-data or new received data.
(define (dns-query/cache question use-cache? protocol nameservers tried)
      (if use-cache?
	  (dns-query-with-cache question protocol nameservers tried)
	  (dns-query-no-cache question protocol nameservers tried)))

;; dns and recursion
;;  recursion means, if the demanded information is not available from the
;;  nameserver, another nameserver (usualy an authority) has to be contacted.
;;  normally the recursion is done for us by the nameserver istself, but
;;  this feature is technically optional (RFC 1035).
;;  dns-get-information implements the resovler-side recursion.
;;  it returns a dns-message
(define (dns-get-information question use-cache? protocol nameservers check-answer)
  (let lp ((tried '()) (nss nameservers))
    (if (null? nss)
	(dns-error 'bad-address)
	(receive (dns-msg hit-ns nss-with-no-reply)
	    (dns-query/cache question use-cache? protocol nss tried)
	  (if (check-answer dns-msg)
	      dns-msg
	      (let ((auth? (not 
			    (zero? 
			     (flags:auth (header:flags 
					  (message:header 
					   (dns-message:reply dns-msg))))))))
		(if auth?
		    (dns-error 'bad-address)
		    ;; other nameservers names are found in the nameserver-part,
		    ;; but their ip-adresses are found in the additonal-rrs
		    (let ((other-nameservers 
			   (filter (lambda (elem) (eq? (rr:type elem) 'a))
				   (message:additionals (dns-message:reply dns-msg)))))
		      (lp (if (not (member hit-ns tried)) (cons hit-ns tried) tried)
			  (lset-union equal?
				      nss-with-no-reply
				      (lset-difference equal? other-nameservers tried)))))))))))

;; parses the resolv.conf file and returns a list of found nameserver
(define (dns-find-nameserver-list)
  (with-input-from-file "/etc/resolv.conf"
    (lambda ()
      (let loop ((ns '()))
	(let ((l (read-line)))
	  (cond
	   ((eof-object? l)
	    (if (null? ns)
		(dns-error 'no-nameservers)
		ns))
	   ((regexp-search
	     (rx (: "nameserver" (+ (| " " "\t") 
				    (submatch (** 1 3 digit) 
					      (= 3 (: "." (** 1 3 digit)))))))
	     l)
	    => (lambda (match)
		 (loop (append ns (list (ip-string->address32 (match:substring match 1)))))))
	   (else
	    (loop ns))))))))


;; returns the first found nameserver
(define (dns-find-nameserver)
  (let ((ns (dns-find-nameserver-list)))
    (if (null? ns)
	(dns-error 'no-nameservers)
	(car ns))))


;; checks the nameservers argument of the lookup functions.
;; if a nameserver-name is given and not a nameserver-ip
;; (dns-lookup-name nameserver) is called.
(define (check-args args)
  (if (null? args) 
      (dns-find-nameserver-list)
      (map (lambda (nameserver)
	     (cond
	      ((address32? nameserver) nameserver)
	      ((ip-string? nameserver) (ip-string->address32 nameserver))
	      (else (dns-lookup-name nameserver (dns-find-nameserver-list)))))
	   (car args))))

;; dns-lookup with more options than dns-lookup-*
(define (dns-lookup name type . nameservers)
  (let* ((maybe-ip-string (if (address32? name) 
			      (ip-string->in-addr (address32->ip-string name))
			      (ip-string->in-addr name)))
	 (question (if maybe-ip-string	; if name is a ip-addr, the query is a in-addr.arpa address
		       (make-octet-query-message (random 256) maybe-ip-string type 'in)
		       (make-octet-query-message (random 256) name type 'in)))
	 (use-cache? #t)
	 (protocol 'udp)
	 (nameservers (check-args nameservers))
	 (check-answer (lambda (dns-msg) #t))
	 (dns-msg (dns-get-information question use-cache? protocol nameservers check-answer))
	 (answers (message:answers (dns-message:reply dns-msg))))
    (if (not (null? answers))
	(for-each (lambda (x) (show-dns-message x)(newline)) answers)
	;;; TODO remove display
	(display "no answers received - but resolved information in other sections.\n"))
    dns-msg))


;; looks up a hostname, returns an ip.
;; (dns-lookup-name <name> nameservers)
(define (dns-lookup-name name . nameservers)
  (let* ((maybe-ip-string (if (address32? name) 
			      (ip-string->in-addr (address32->ip-string name))
			      (ip-string->in-addr name)))
	 (question (if maybe-ip-string	; if name is a ip-addr, the query is a in-addr.arpa address
		       (dns-error 'not-a-hostname)
		       (make-octet-query-message (random 256) name 'a 'in)))
	 (use-cache? #t)
	 (protocol 'udp)
	 (nameservers (check-args nameservers))
	 (check-answer (lambda (dns-msg) 
			 (let* ((reply (dns-message:reply dns-msg))
				(answers (message:answers reply)))
			   (not (null? (filter-type answers 'a))))))
	 (dns-msg (dns-get-information question use-cache? protocol nameservers check-answer))
	 (answers (filter-type (message:answers (dns-message:reply dns-msg)) 'a)))
    (rr-data-a:ip (rr:data (car answers)))))

;; looks up an ip, returns a hostname
;; (dns-inverse-lookup <name> [nameserver])
(define (dns-lookup-ip ip . nameservers)
  (let* ((maybe-ip-string (if (address32? ip) 
			      (ip-string->in-addr (address32->ip-string ip))
			      (ip-string->in-addr ip)))
	 (question (if maybe-ip-string	; if name is a ip-addr, the query is a in-addr.arpa address
		       (make-octet-query-message (random 256) maybe-ip-string 'ptr 'in)
		       (dns-error 'not-a-ip)))
	 (use-cache? #t)
	 (protocol 'udp)
	 (nameservers (check-args nameservers))
	 (check-answer (lambda (dns-msg) 
			 (let* ((reply (dns-message:reply dns-msg))
				(answers (message:answers reply)))
			   (not (null? (filter-type answers 'ptr))))))
	 (dns-msg (dns-get-information question use-cache? protocol nameservers check-answer))
	 (answers (filter-type (message:answers (dns-message:reply dns-msg)) 'ptr)))
    (rr-data-ptr:name (rr:data (car answers)))))

(define dns-inverse-lookup dns-lookup-ip)

;; looks up an authoritative nameserver for a hostname
;; returns a list of nameservers
;; (dns-lookup-nameserver <name> [nameserver])
(define (dns-lookup-nameserver name . nameservers)
  (let* ((maybe-ip-string (if (address32? name) 
			      (ip-string->in-addr (address32->ip-string name))
			      (ip-string->in-addr name)))
	 (question (if maybe-ip-string	; if name is a ip-addr, the query is a in-addr.arpa address
		       (dns-error 'not-a-hostname)
		       (make-octet-query-message (random 256) name 'ns 'in)))
	 (use-cache? #t)
	 (protocol 'udp)
	 (nameservers (check-args nameservers))
	 (check-answer (lambda (dns-msg) 
			 (let* ((reply (dns-message:reply dns-msg))
				(answers (message:answers reply))
				(nameservers (message:nameservers reply)))
			   (or (not (null? (filter-type nameservers 'soa)))
			       (not (null? (filter-type answers 'ns)))))))
	 (dns-msg (dns-get-information question use-cache? protocol nameservers check-answer))
	 (reply (dns-message:reply dns-msg))
	 (soa (filter-type (message:nameservers reply) 'soa))
	 (nss (filter-type (message:answers reply) 'ns))
	 (add (filter-type (message:additionals reply) 'a)))
    (if (null? nss)
	(list (dns-lookup-name (rr-data-soa:mname (rr:data (car soa)))))
	(map (lambda (elem) (rr-data-a:ip (rr:data elem))) add))))

;; looks up a mail-exchanger for a hostname.
;; returns a list of mail-exchanger, sorted by their preference
;; if there are no mx-records in the answer-section, 
;; implementation based on RFC2821
;; (dns-lookup-mail-exchanger <name> [nameserver])
(define (dns-lookup-mail-exchanger name . nameservers)
  (let* ((ip-string (if (address32? name) 
			(ip-string->in-addr (address32->ip-string name))
			(ip-string->in-addr name)))
	 (question (if ip-string	; if name is a ip-addr, the query is a in-addr.arpa address
		       (dns-error 'not-a-hostname)
		       (make-octet-query-message (random 256) name 'mx 'in)))
	 (use-cache? #t)
	 (protocol 'tcp)
	 (nameservers (check-args nameservers))
	 (check-answer (lambda (dns-msg) 
			 (let* ((reply (dns-message:reply dns-msg))
				(answers (message:answers reply))
				(nameservers (message:nameservers reply)))
			   (or (not (null? (filter-type answers 'mx)))
			       (not (null? (filter-type answers 'cname)))
			       (not (null? (filter-type answers 'a)))))))
	 (dns-msg (dns-get-information question use-cache? protocol nameservers check-answer))
	 (reply (dns-message:reply dns-msg))
	 (mx (filter-type (message:answers reply) 'mx))
	 (soa (filter-type (message:nameservers reply) 'soa))
	 (cname (filter-type (message:answers reply) 'cname))
	 (a (filter-type (message:answers reply) 'a)))

    (cond
     ((not (null? a)) 
      (list (rr-data-a:ip (rr:data (car a)))))
     ((not (null? cname))
      (dns-lookup-mail-exchanger (rr-data-cname:name (rr:data (car cname)))))
     ((null? mx) 
      (list (rr-data-soa:rname (rr:data (car soa)))))
     (else
      (map (lambda (elem) (rr-data-mx:exchanger (rr:data elem))) (sort-by-preference mx))))))

;;; pretty-prints a dns-msg
(define (show-dns-message dns-msg)
  (let* ((d 
	  (lambda (n s1 s2)
	    (letrec ((loop (lambda (n)
			     (if (zero? n)
				 ""
				 (string-append " " (loop (- n 1)))))))
	      (display (loop n))
	      (display s1)
	      (display ": ")
	      (display s2)
	      (newline)))))
    
    (cond
     ((dns-message? dns-msg) 
      (begin
	(d 0 "DNS-MESSAGE" "")
	(d 1 "QUERY" "")(show-dns-message (dns-message:query dns-msg))(newline)
	(d 1 "REPLY" "")(show-dns-message (dns-message:reply dns-msg))(newline)
	(d 1 "CACHE?" (if (dns-message:cache? dns-msg)
			  "found in cache"
			  "not found in cache"))
	(d 1 "PROTOCOL" (let ((protocol (dns-message:protocol dns-msg)))
			  (cond
			   ((eq? protocol 'tcp) "TCP")
			   ((eq? protocol 'udp) "UDP"))))
	(d 1 "TRIED-NAMESERVERS" (if (> (length (dns-message:tried-nameservers dns-msg)) 1)
				     (begin
				       (display " had perform recursion: ")
				       (dns-message:tried-nameservers dns-msg))
				     (begin
				       (display " without recursion: ")
				       (dns-message:tried-nameservers dns-msg))))))
     ((message? dns-msg)
      (begin
	(d 2 "MESSAGE" "")
	(d 3 "Header     " "")(show-dns-message (message:header dns-msg))
	(d 3 "Questions  " "")(for-each (lambda (x) (show-dns-message x)(newline)) (message:questions dns-msg))
	(d 3 "Answers    " "")(for-each (lambda (x) (show-dns-message x)(newline)) (message:answers dns-msg))
	(d 3 "Nameservers" "")(for-each (lambda (x) (show-dns-message x)(newline)) (message:nameservers dns-msg))
	(d 3 "Additionals" "")(for-each (lambda (x) (show-dns-message x)(newline)) (message:additionals dns-msg))))
     ((header? dns-msg)
      (begin
	(d 4 "id" (header:id dns-msg))
	(d 4 "Flags" "")(show-dns-message (header:flags dns-msg))
	(d 4 "question-count    " (header:qdc dns-msg))
	(d 4 "answer-count      " (header:anc dns-msg))
	(d 4 "nameserver-count  " (header:nsc dns-msg))
	(d 4 "additional-count  " (header:arc dns-msg))))
     ((flags? dns-msg)
      (begin 
	(d 5 "querytype" (flags:querytype dns-msg))
	(d 5 "opcode" (flags:opcode dns-msg))
	(d 5 "auth" (flags:auth dns-msg))
	(d 5 "trunc" (flags:trunc dns-msg))
	(d 5 "recursiondesired" (flags:recursiondesired dns-msg))
	(d 5 "recursionavailable" (flags:recursionavailable dns-msg))
	(d 5 "z" (flags:z dns-msg))
	(d 5 "rcode" (flags:rcode dns-msg))))
     ((question? dns-msg)
      (begin
	(d 4 "name " (question:name dns-msg))
	(d 4 "type " (question:type dns-msg))
	(d 4 "class" (question:class dns-msg))))
     ((rr? dns-msg)
      (begin
	(d 4 "name " (rr:name dns-msg))
	(d 4 "type " (rr:type dns-msg))
	(d 4 "class" (rr:class dns-msg))
	(d 4 "ttl  " (rr:ttl dns-msg))
	(d 4 "data " "") (show-dns-message (rr:data dns-msg))))
     ((rr-data-a? dns-msg)
      (d 5 "ip " (rr-data-a:ip dns-msg)))
     ((rr-data-ns? dns-msg)
      (d 5 "name " (rr-data-ns:name dns-msg)))
     ((rr-data-cname? dns-msg)
      (d 5 "name " (rr-data-cname:name dns-msg)))
     ((rr-data-mx? dns-msg)
      (begin
	(d 5 "preference " (rr-data-mx:preference dns-msg))
	(d 5 "exchanger  " (rr-data-mx:exchanger dns-msg))))
     ((rr-data-ptr? dns-msg) 
      (d 5 "name " (rr-data-ptr:name dns-msg)))
     ((rr-data-soa? dns-msg)
      (begin
	(d 5 "mname   " (rr-data-soa:mname dns-msg))
	(d 5 "rname   " (rr-data-soa:rname dns-msg))
	(d 5 "serial  " (rr-data-soa:serial dns-msg))
	(d 5 "refresh " (rr-data-soa:refresh dns-msg))
	(d 5 "expire  " (rr-data-soa:expire dns-msg))
	(d 5 "minimum " (rr-data-soa:expire dns-msg))))
     ;; ###
     ((rr-data-hinfo? dns-msg)
      (d 5 "data " (rr-data-hinfo:data dns-msg)))
     ((rr-data-txt? dns-msg)
      (d 5 "text " (rr-data-txt:text dns-msg)))
     ((rr-data-wks? dns-msg)
      (d 5 "data " (rr-data-wks:data dns-msg)))
     
     )))

(define *fqdn-lock* (make-lock))
(define *fqdn-cache* '())

(define (socket-address->fqdn addr cache?)
  (receive (ip32 port)
      (socket-address->internet-address addr)
    (internet-address->fqdn ip32 cache?)))

(define (internet-address->fqdn ip32 cache?)
  (if cache?
      (begin
	(obtain-lock *fqdn-lock*)
	(cond
	 ((assv ip32 *fqdn-cache*) => 
	  (lambda (pair)
	    (release-lock *fqdn-lock*)
	    (cdr pair)))
	 (else
	  (release-lock *fqdn-lock*)
	  (let ((fqdn (dns-lookup-ip ip32)))
	    (set! *fqdn-cache*
		  (cons (cons ip32 fqdn) *fqdn-cache*))
	    fqdn))))
      (dns-lookup-ip ip32)))

;;; TODO THIS USES gethostbyname
(define (host-fqdn name-or-socket-address)
  (if (socket-address? name-or-socket-address)
      (socket-address->fqdn name-or-socket-address #f)
      (internet-address->fqdn
       (car
	(host-info:addresses
	 (host-info name-or-socket-address)))
       #f)))

;;; TODO THIS USES gethostbyname
(define (system-fqdn)
  (internet-address->fqdn (car (host-info:addresses (host-info (system-name))))
			  #t))