Fix WKS record protocol field encoding.

[donapieppo]

Fix WKS record protocol field encoding and decoding size according to RFC 1035 Section 3.4.2 where PROTOCOL is an 8 bit.
I spotted the errror using Dnsruby::ZoneTransfer where i got the error

Dnsruby::MessageDecoder#assert_buffer_position_valid': requested position of 1871 must be     
  between 0 and buffer size (1870). (Dnsruby::DecodeError) 

The patch just change the pack format from "n" to "C" in lib/dnsruby/resource/IN.rb

[keithrbennett]

Hi, I have no knowledge of this particular domain, but asked Claude AI about it, and had its response reviewed successfully by Qwen AI. TL/DR is that the fix is correct, and in addition this would be a good opportunity to add unit tests for the feature. Suggested test code is also included in the response:

https://claude.ai/share/e0d07f8a-f26b-4548-a0db-71b12a7ee817

(EDIT: The source code was not included in the text referred to by the link; I have pasted it in the comment following this one.)

Qwen review at: https://chat.qwen.ai/s/58977257-7fe0-4b11-8307-c4d0196c2a69?fev=0.2.64

@alexdalitz I realize some people would prefer not to receive comments like these where I sanity checked the information but did not deeply verify it. If you would prefer me not post similar posts in the future, just say so and I will refrain.

[keithrbennett]

Sorry, I could not get a link to the conversation that included the test code. Here it is, with explanation following:

# test/tc_wks.rb
require_relative 'spec_helper'

class WKSTest < Minitest::Test
  # Protocol 6 = TCP, per IANA; fits in 8 bits (0-255).
  # Bitmap marks ports 80 (HTTP) and 443 (HTTPS) as available.
  ADDR     = "192.0.2.1"
  PROTOCOL = 6
  BITMAP   = "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02".b

  def wks
    Dnsruby::RR::IN::WKS.new(
      Dnsruby::IPv4.new(ADDR),
      PROTOCOL,
      BITMAP
    )
  end

  def encode(rr)
    msg = Dnsruby::MessageEncoder.new
    rr.encode_rdata(msg)
    msg.to_s
  end

  def roundtrip(rr)
    encoded = encode(rr)
    # The encoded wire format for WKS is: 4 bytes address + 1 byte protocol + bitmap.
    # With the old "n" (2-byte) format this was 6 + bitmap; now it must be 5 + bitmap.
    assert_equal 4 + 1 + BITMAP.bytesize, encoded.bytesize,
      "wire format must be 4-byte address + 1-byte protocol + bitmap"

    decoder = Dnsruby::MessageDecoder.new(encoded)
    Dnsruby::RR::IN::WKS.decode_rdata(decoder)
  end

  def test_roundtrip_preserves_protocol
    decoded = roundtrip(wks)
    assert_equal PROTOCOL, decoded.protocol
  end

  def test_roundtrip_preserves_address
    decoded = roundtrip(wks)
    assert_equal ADDR, decoded.address.to_s
  end

  def test_roundtrip_preserves_bitmap
    decoded = roundtrip(wks)
    assert_equal BITMAP, decoded.bitmap
  end

  def test_protocol_field_is_one_byte
    # Regression: the old "n" pack format wrote 2 bytes for protocol,
    # causing MessageDecoder to overshoot the buffer by 1 byte (Dnsruby::DecodeError).
    encoded = encode(wks)
    # Byte 4 (0-indexed) must be the protocol value itself, not the high byte of a 16-bit field.
    assert_equal PROTOCOL, encoded.bytes[4]
  end

  def test_max_protocol_value
    rr = Dnsruby::RR::IN::WKS.new(Dnsruby::IPv4.new(ADDR), 255, BITMAP)
    decoded = roundtrip(rr)
    assert_equal 255, decoded.protocol
  end
end

A few notes on the choices:

test_protocol_field_is_one_byte is the regression test that would have caught the original bug — it directly asserts that byte index 4 of the wire encoding carries the protocol value, which fails with the old "n" format (which would write 0x00 there and the protocol value at byte 5).

test_roundtrip_* are the correctness tests: encode then decode and verify all three fields survive intact. Splitting them into three assertions rather than one compound one gives clearer failure messages.

test_max_protocol_value exercises the boundary (255) to confirm the 8-bit field handles the full valid range.

The BITMAP constant is intentionally non-trivial (marks real ports) to ensure bitmap content also survives the round-trip without corruption from an off-by-one in buffer consumption.