53 lines
1.7 KiB
Racket
53 lines
1.7 KiB
Racket
#lang racket/base
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(provide ones-complement-sum16 ip-checksum)
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(require bitsyntax)
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(require "dump-bytes.rkt")
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(define (ones-complement-+16 a b)
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(define c (+ a b))
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(bitwise-and #xffff (+ (arithmetic-shift c -16) c)))
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(define (ones-complement-sum16 bs)
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(bit-string-case bs
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([ (n :: integer bytes 2) (rest :: binary) ]
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(ones-complement-+16 n (ones-complement-sum16 rest)))
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([ odd-byte ]
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(arithmetic-shift odd-byte 8))
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([ ]
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0)))
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(define (ones-complement-negate16-safely x)
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(define r (bitwise-and #xffff (bitwise-not x)))
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(if (= r 0) #xffff r))
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(define (ip-checksum offset blob #:pseudo-header [pseudo-header #""])
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(bit-string-case blob
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([ (prefix :: binary bytes offset)
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(:: binary bytes 2)
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(suffix :: binary) ]
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;; (log-info "Packet pre checksum:\n~a" (dump-bytes->string blob))
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(define result (ones-complement-+16
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(ones-complement-sum16 pseudo-header)
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(ones-complement-+16 (ones-complement-sum16 prefix)
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(ones-complement-sum16 suffix))))
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;; (log-info "result: ~a" (number->string result 16))
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(define checksum (ones-complement-negate16-safely result))
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;; (log-info "Checksum ~a" (number->string checksum 16))
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(define final-packet (bit-string (prefix :: binary)
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(checksum :: integer bytes 2)
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(suffix :: binary)))
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;; (log-info "Packet with checksum:\n~a" (dump-bytes->string final-packet))
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final-packet)))
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(module+ test
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(require rackunit)
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(check-equal? (ones-complement-negate16-safely
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(ones-complement-sum16 (bytes #x45 #x00 #x00 #x54
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#x00 #x00 #x00 #x00
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#x40 #x01 #x00 #x00
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#xc0 #xa8 #x01 #xde
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#xc0 #xa8 #x01 #x8f)))
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#xf5eb))
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