From 4fe26956098bdb1d962e5b3ec8c435cc9cd999d4 Mon Sep 17 00:00:00 2001
From: TheRealAlexxx <TheRealAlexxx@users.noreply.github.com>
Date: Sat, 24 Oct 2026 09:11:53 +0100
Subject: [PATCH] Resolv: build DNS wire messages as binary strings

Fixes [Bug #19697].

`Resolv::DNS::MessageEncoder` accumulates the encoded packet in a UTF-8 buffer. When a query name contains non-ASCII characters, such as an internationalized domain name, the buffer becomes a UTF-8 string holding non-ASCII bytes, and binary writes into it start failing: assigning the request id in `ConnectedUDP#sender` (`request[0,2] = ...`) raises `Encoding::CompatibilityError` (incompatible character encodings: UTF-8 and ASCII-8BIT), so resolving such a name crashes before any packet is sent. The same hazard exists mid-encode, where `put_length16` backpatches a packed length into the buffer after writing a name.

The wire format is bytes, so keep the buffer binary throughout: start it as ASCII-8BIT and append the binary form of byte strings, labels and character-strings. (An empty buffer adopts the encoding of the first string appended to it, so the initial conversion alone is not enough.) `Message#encode` now consistently returns an ASCII-8BIT string, matching what the socket layer and `Message.decode` already work with.

The new test resolves an internationalized domain name against a stub DNS server; without the change it raises `Encoding::CompatibilityError` in `ConnectedUDP#sender`.
---
 lib/resolv.rb           |  6 +++---
 test/resolv/test_dns.rb | 27 +++++++++++++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/lib/resolv.rb b/lib/resolv.rb
index bac338c..e73121f 100644
--- a/lib/resolv.rb
+++ b/lib/resolv.rb
@@ -1654,7 +1654,7 @@ def encode
 
       class MessageEncoder # :nodoc:
         def initialize
-          @data = ''.dup
+          @data = ''.dup.b
           @names = {}
           yield self
         end
@@ -1664,7 +1664,7 @@ def to_s
         end
 
         def put_bytes(d)
-          @data << d
+          @data << d.b
         end
 
         def put_pack(template, *d)
@@ -1689,7 +1689,7 @@ def put_string(d)
             raise ArgumentError, "character-string is too long (#{s.bytesize} bytes, max 255): #{s.inspect}"
           end
           self.put_pack("C", s.bytesize)
-          @data << s
+          @data << s.b
         end
 
         def put_string_list(ds)
diff --git a/test/resolv/test_dns.rb b/test/resolv/test_dns.rb
index b8ecef2..51c69c0 100644
--- a/test/resolv/test_dns.rb
+++ b/test/resolv/test_dns.rb
@@ -567,6 +567,33 @@ def test_no_server
     end
   end
 
+  def test_internationalized_domain_name_query
+    bug19697 = '[Bug #19697]'
+    with_udp('127.0.0.1', 0) {|u|
+      _, server_port, _, server_address = u.addr
+      client_thread = Thread.new {
+        Resolv::DNS.open(:nameserver_port => [[server_address, server_port]], :search => [], :ndots => 0) {|dns|
+          dns.getresources("\uC6B0\uD3B8.\uB2F7\uCEF4", Resolv::DNS::Resource::IN::MX)
+        }
+      }
+      server_thread = Thread.new {
+        msg, (_, client_port, _, client_address) = Timeout.timeout(5) {u.recvfrom(4096)}
+        query = Resolv::DNS::Message.decode(msg)
+        reply = Resolv::DNS::Message.new(query.id)
+        reply.qr = 1
+        reply.rd = query.rd
+        reply.ra = 1
+        query.each_question {|name, typeclass|
+          reply.add_question(name, typeclass)
+        }
+        u.send(reply.encode, 0, client_address, client_port)
+      }
+      resources = client_thread.value
+      server_thread.join
+      assert_equal([], resources, bug19697)
+    }
+  end
+
   def test_invalid_byte_comment
     bug9273 = '[ruby-core:59239] [Bug #9273]'
     Tempfile.create('resolv_test_dns_') do |tmpfile|