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Start splitting out and deriving a proxy-server implementation from driver.rkt

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Tony Garnock-Jones 2011-12-13 11:57:42 -05:00
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commit ea0b338b0f
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#lang racket/base
;; Simple imperative DNS proxy.
(require racket/match)
(require racket/udp)
(require racket/set)
(require racket/bool)
(require "../racket-bitsyntax/main.rkt")
(require "api.rkt")
(require "codec.rkt")
(require "zonedb.rkt")
(require "dump-bytes.rkt")
;; Instantiated with a collection of trusted roots to begin its
;; searches from. Performs recursive queries. Doesn't yet cache
;; responses, but will do so in future.
;; Rules:
;; - Finds the leafmost NS record it can find in its rootset for the
;; requested name.
;; - Queries that service. If the answer is a referral, follows the
;; chain. Remembers which servers it has seen before to avoid
;; loops.
;; - If it resolves CNAMEs on the way (should it?), remembers which
;; names it has been resolving in response to any given query, to
;; avoid loops. Perhaps limit the length of the chain to avoid DoS.
;; - Only performs recursive service if so requested! (TODO)
;; - Never put CNAME records anywhere in an answer section other than
;; at the top (TODO; also check the server)
;;
;; - See RFC 1035 section 7.1.
;; DJB's rules for handling DNS responses:
;; When a cache receives a normal DNS response, it learns exactly one
;; of the following five pieces of information:
;;
;; 1. ``The query was not answered because the query name is an
;; alias. I need to change the query name and try again.'' This
;; applies if the answer section of the response contains a CNAME
;; record for the query name and CNAME does not match the query type.
;;
;; 2. ``The query name has no records answering the query, and is also
;; guaranteed to have no records of any other type.'' This applies if
;; the response code is NXDOMAIN and #1 doesn't apply. The amount of
;; time that this information can be cached depends on the contents of
;; the SOA record in the authority section of the response, if there
;; is one.
;;
;; 3. ``The query name has one or more records answering the query.''
;; This applies if the answer section of the response contains one or
;; more records under the query name matching the query type, and #1
;; doesn't apply, and #2 doesn't apply.
;;
;; 4. ``The query was not answered because the server does not have
;; the answer. I need to contact other servers.'' This applies if the
;; authority section of the response contains NS records, and the
;; authority section of the response does not contain SOA records, and
;; #1 doesn't apply, and #2 doesn't apply, and #3 doesn't apply. The
;; ``other servers'' are named in the NS records in the authority
;; section.
;;
;; 5. ``The query name has no records answering the query, but it may
;; have records of another type.'' This applies if #1 doesn't apply,
;; and #2 doesn't apply, and #3 doesn't apply, and #4 doesn't
;; apply. The amount of time that this information can be cached
;; depends on the contents of the SOA record in the authority section,
;; if there is one.
;;
;; This procedure requires an incredible amount of bug-prone parsing
;; for a very small amount of information. The underlying problem is
;; that DNS was designed to declare information in a human-oriented
;; format, rather than to support crucial operations in the simplest
;; possible way.
(define (authoritativeness-for dn soa-rr)
(if (in-bailiwick? dn (rr-name soa-rr))
'authoritative
'non-authoritative))
;; start-proxy : UInt16 ListOf<RR> -> Void
;; Starts a proxy service that will answer questions received on the
;; given UDP port based on the NS RRs it is given.
(require racket/pretty)
(define (start-proxy port-number raw-roots)
;; Compile the table of roots
(define roots (compile-zone-db raw-roots))
(pretty-print roots)
;; Set up the socket
(define s (udp-open-socket #f #f))
(udp-bind! s #f port-number)
(define (service-loop)
(with-handlers ((exn:break? (lambda (e) (raise e)))
(exn? (lambda (e)
(display "Error in DNS proxy handler:") (newline)
(write e)
(newline)
(newline))))
(read-and-process-request))
(service-loop))
(define (read-and-process-request)
(define buffer (make-bytes 512))
(define-values (packet-length source-hostname source-port)
(udp-receive! s buffer))
(define (send-error error-response-code)
(bit-string-case buffer
([ (id :: bits 16) (:: binary) ]
(udp-send-to s source-hostname source-port
(dns-message->packet
(dns-message id 'response 'query
'non-authoritative 'not-truncated
'no-recursion-desired 'recursion-available
error-response-code '() '() '() '()))))
(else
;; We don't even have enough information in the packet to reply.
(void))))
(display "----------------------------------------") (newline)
(write (subbytes buffer 0 packet-length)) (newline)
(dump-bytes! buffer packet-length)
(flush-output)
(define request-message
(with-handlers ((exn? (lambda (e)
(send-error 'format-error)
(raise e))))
(packet->dns-message (subbytes buffer 0 packet-length))))
;;(write request-message) (newline)
(define (make-reply name send-name-error? answers authorities additional)
(dns-message (dns-message-id request-message)
'response
'query
(authoritativeness-for name soa-rr)
'not-truncated
(dns-message-recursion-desired request-message)
'recursion-available
(if send-name-error? 'name-error 'no-error)
(dns-message-questions request-message)
(set->list answers)
(set->list authorities)
(set->list additional)))
(define reply-packet
(with-handlers ((exn? (lambda (e)
(send-error 'server-failure)
(raise e))))
;; TODO: check opcode and direction in request
(define questions (dns-message-questions request-message))
(if (null? questions)
#f ;; No questions -> don't send any replies
(begin
;; TODO: what if there are multiple questions in one
;; request packet? Single reply, or multiple replies?
;; Process the additional questions, or ignore them?
;; djbdns looks like it handles exactly one question per
;; request, ignoring any excess...
(dns-message->packet (answer-question (car questions) make-reply))))))
;; TODO: Truncation
(when reply-packet
(udp-send-to s source-hostname source-port reply-packet)))
(define (answer-question q make-reply)
(let resolve ((name (question-name q)))
;; Notice that we claim to be authoritative for our configured
;; zone. If we ever answer name-error, that means there are no
;; RRs *at all* for the queried name. If there are RRs for the
;; queried name, but they happen not to be the ones asked for,
;; name-error must *not* be returned: instead, a normal
;; no-error reply is sent with an empty answer section.
;;
;; If we wanted to support caching of negative replies, we'd
;; follow the guidelines in section 4.3.4 "Negative response
;; caching" of RFC1034, adding our zone SOA with an
;; appropriate TTL to the additional section of the reply.
;;
;; TODO: We support returning out-of-bailiwick records (glue)
;; here. Reexamine the rules for doing so.
(cond
((hash-ref zone name #f) =>
;; The full name matches in our zone database.
(lambda (rrset)
(define filtered-rrs (filter-rrs rrset (question-type q) (question-class q)))
(define cnames (filter-by-type rrset 'cname))
(define base-reply (make-reply name
#f
(set-union cnames filtered-rrs)
(set soa-rr)
(set)))
;; Algorithm for CNAME expansion from RFC 1034 4.3.2 step 3a.
(if (and (not (set-empty? cnames))
(not (eqv? (question-type q) 'cname)))
(foldl (lambda (cname-rr current-reply)
(merge-replies current-reply
(resolve (rr-rdata cname-rr))))
base-reply
(set->list cnames))
base-reply)))
((referral-for name soa-rr zone) =>
;; No full name match, but a referral to somewhere beneath our
;; SOA but within our zone.
(lambda (ns-rrset)
(make-reply name
#f
ns-rrset
(set soa-rr)
(additional-section/a zone (set-map ns-rrset rr-rdata)))))
(else
;; Neither a full name match nor a referral is
;; available. Answer name-error (NXDOMAIN) if the queried
;; name is in-bailiwick, or a normal no-error otherwise.
(make-reply name
(in-bailiwick? name (rr-name soa-rr))
(set)
(set)
(set))))))
(service-loop))
(start-server 5555
(rr '(#"example") 'soa 'in 30
(soa '(#"ns" #"example")
'(#"tonyg" #"example")
1
24
24
30
10))
(list (rr '(#"localhost" #"example") 'a 'in 30 '#(127 0 0 1))
(rr '(#"example") 'mx 'in 30 (mx 5 '(#"localhost" #"example")))
(rr '(#"example") 'mx 'in 30 (mx 10 '(#"subns" #"example")))
(rr '(#"google" #"example") 'cname 'in 30 '(#"www" #"google" #"com"))
(rr '(#"roar" #"example") 'a 'in 30 '#(192 168 1 1))
(rr '(#"ns" #"example") 'a 'in 30 '#(127 0 0 1))
(rr '(#"hello" #"example") 'txt 'in 30 '(#"Hello CRASH"))
(rr '(#"subzone" #"example") 'ns 'in 30 '(#"subns" #"example"))
(rr '(#"subns" #"example") 'a 'in 30 '#(127 0 0 2))))