DNS server expressed in terms of simple-udp-service.rkt
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Tony Garnock-Jones
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68d2195146
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#lang racket/base
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;; DNS server using simple-udp-service.rkt.
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(require racket/match)
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(require racket/udp)
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(require racket/set)
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(require racket/bool)
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(require "../racket-bitsyntax/main.rkt")
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(require "api.rkt")
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(require "codec.rkt")
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(require "zonedb.rkt")
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(require "dump-bytes.rkt")
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(require "simple-udp-service.rkt")
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;; Instantiated with a SOA record for the zone it is serving as well
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;; as a zone's worth of DNS data which is used to answer queries
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;; authoritatively. Never caches information, never performs recursive
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;; queries.
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;; Rules:
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;; - Answers authoritative NXDOMAIN answers for queries falling within
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;; its zone. (This is the only responder entitled to answer NXDOMAIN!)
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;; - Answers with referrals for queries falling in subzones. It
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;; determines subzones based on the RRs it is configured with at
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;; startup.
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(define (authoritativeness-for dn soa-rr)
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(if (in-bailiwick? dn (rr-name soa-rr))
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'authoritative
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'non-authoritative))
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;; ASSUMPTION: r1 and r2 are both DNS replies to the same query.
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;; ASSUMPTION: no response-codes other than no-error or name-error are in use.
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(define (merge-replies r1 r2)
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(dns-message (dns-message-id r1)
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'response
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'query
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(if (and (eqv? (dns-message-authoritative r1) 'authoritative)
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(eqv? (dns-message-authoritative r2) 'authoritative))
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'authoritative
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'non-authoritative)
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'not-truncated
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(dns-message-recursion-desired r1)
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'no-recursion-available
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(if (and (eqv? (dns-message-response-code r1) 'name-error)
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(eqv? (dns-message-response-code r2) 'name-error))
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'name-error
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'no-error)
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(dns-message-questions r1)
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(listset-union (dns-message-answers r1) (dns-message-answers r2))
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(listset-union (dns-message-authorities r1) (dns-message-authorities r2))
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(listset-union (dns-message-additional r1) (dns-message-additional r2))))
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(define (listset-union xs1 xs2)
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(set->list (set-union (list->set xs1) (list->set xs2))))
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;; start-server : UInt16 RR ListOf<RR> -> Void
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;; Starts a server that will answer questions received on the given
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;; UDP port based on the RRs it is given and the zone origin specified
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;; in the soa-rr given.
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(require racket/pretty)
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(define (start-server port-number soa-rr rrs)
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;; Compile the zone hash table
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(define zone (compile-zone-db (cons soa-rr rrs)))
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(pretty-print zone)
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(start-udp-service
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port-number
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(lambda (packet old-state) (values (handle-request soa-rr zone packet) old-state))
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#f
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#:packet-size-limit 512))
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(define (handle-request soa-rr zone request-packet)
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(match-define (udp-packet request-body request-host request-port) request-packet)
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(define request-message (packet->dns-message request-body))
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(define (make-error error-response-code)
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(bit-string-case request-body
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([ (id :: bits 16) (:: binary) ]
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(list (udp-packet (dns-message->packet
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(dns-message id 'response 'query
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'non-authoritative 'not-truncated
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'no-recursion-desired 'no-recursion-available
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error-response-code '() '() '() '()))
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request-host
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request-port)))
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(else
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;; We don't even have enough information in the packet to reply.
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(list))))
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(define (make-reply name send-name-error? answers authorities additional)
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(dns-message (dns-message-id request-message)
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'response
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'query
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(authoritativeness-for name soa-rr)
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'not-truncated
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(dns-message-recursion-desired request-message)
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'no-recursion-available
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(if send-name-error? 'name-error 'no-error)
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(dns-message-questions request-message)
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(set->list answers)
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(set->list authorities)
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(set->list additional)))
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(define (answer-question q make-reply)
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(let resolve ((name (question-name q)))
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;; Notice that we claim to be authoritative for our configured
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;; zone. If we ever answer name-error, that means there are no
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;; RRs *at all* for the queried name. If there are RRs for the
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;; queried name, but they happen not to be the ones asked for,
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;; name-error must *not* be returned: instead, a normal
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;; no-error reply is sent with an empty answer section.
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;;
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;; If we wanted to support caching of negative replies, we'd
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;; follow the guidelines in section 4.3.4 "Negative response
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;; caching" of RFC1034, adding our zone SOA with an
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;; appropriate TTL to the additional section of the reply.
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;;
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;; TODO: We support returning out-of-bailiwick records (glue)
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;; here. Reexamine the rules for doing so.
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(cond
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((hash-ref zone name #f) =>
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;; The full name matches in our zone database.
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(lambda (rrset)
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(define filtered-rrs (filter-rrs rrset (question-type q) (question-class q)))
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(define cnames (filter-by-type rrset 'cname))
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(define base-reply (make-reply name
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#f
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(set-union cnames filtered-rrs)
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(set soa-rr)
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(set)))
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;; Algorithm for CNAME expansion from RFC 1034 4.3.2 step 3a.
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(if (and (not (set-empty? cnames))
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(not (eqv? (question-type q) 'cname)))
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(foldl (lambda (cname-rr current-reply)
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(merge-replies current-reply
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(resolve (rr-rdata cname-rr))))
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base-reply
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(set->list cnames))
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base-reply)))
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((referral-for name soa-rr zone) =>
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;; No full name match, but a referral to somewhere beneath our
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;; SOA but within our zone.
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(lambda (ns-rrset)
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(make-reply name
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#f
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ns-rrset
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(set soa-rr)
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(additional-section/a zone (set-map ns-rrset rr-rdata)))))
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(else
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;; Neither a full name match nor a referral is
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;; available. Answer name-error (NXDOMAIN) if the queried
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;; name is in-bailiwick, or a normal no-error otherwise.
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(make-reply name
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(in-bailiwick? name (rr-name soa-rr))
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(set)
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(set)
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(set))))))
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;; TODO: check opcode and direction in request
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;; TODO: think again about multiple questions in one packet
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(map (lambda (q)
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(udp-packet (dns-message->packet (answer-question q make-reply))
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request-host
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request-port))
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(dns-message-questions request-message)))
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(start-server 5555
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(rr '(#"example") 'soa 'in 30
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(soa '(#"ns" #"example")
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'(#"tonyg" #"example")
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1
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24
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24
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30
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10))
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(list (rr '(#"localhost" #"example") 'a 'in 30 '#(127 0 0 1))
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(rr '(#"example") 'mx 'in 30 (mx 5 '(#"localhost" #"example")))
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(rr '(#"example") 'mx 'in 30 (mx 10 '(#"subns" #"example")))
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(rr '(#"google" #"example") 'cname 'in 30 '(#"www" #"google" #"com"))
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(rr '(#"roar" #"example") 'a 'in 30 '#(192 168 1 1))
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(rr '(#"ns" #"example") 'a 'in 30 '#(127 0 0 1))
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(rr '(#"hello" #"example") 'txt 'in 30 '(#"Hello CRASH"))
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(rr '(#"subzone" #"example") 'ns 'in 30 '(#"subns" #"example"))
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(rr '(#"subns" #"example") 'a 'in 30 '#(127 0 0 2))))
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