fix: TXT decode preserves string-boundary info

Author: msimerson

CHANGELOG.md

@@ -4,6 +4,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/).
 
 ### Unreleased
 
+### [3.0.0] - 2026-05-26
+
+- **BREAKING**, TXT `data` is now always an array of strings
+- fix(packet): TXT decode preserves character-string boundaries (RFC 1035 §3.3.14)
+
 ### [2.4.0] - 2026-05-26
 
 - feat(ESM): dual published with ESM support

package-lock.json

@@ -1,6 +1,6 @@
 {
   "name": "dns2",
-  "version": "2.4.0",
+  "version": "3.0.0",
   "description": "A DNS Server and Client Implementation in Pure JavaScript with no dependencies.",
   "main": "index.js",
   "types": "ts/index.d.ts",

package.json

@@ -768,22 +768,36 @@ Packet.Resource.PTR = Packet.Resource.CNAME = {
  * @docs https://tools.ietf.org/html/rfc1035#section-3.3.14
  */
 Packet.Resource.SPF = Packet.Resource.TXT = {
+  // RFC 1035 §3.3.14: TXT RDATA is one or more length-prefixed
+  // <character-string> items. Preserve those boundaries by returning an
+  // array — joining them silently corrupts SPF/DKIM and other multi-string
+  // records whose semantics depend on segmentation.
   decode: function (reader, length) {
-    const parts = [];
+    const strings = [];
     let bytesRead = 0;
-    let chunkLength;
-
     while (bytesRead < length) {
-      chunkLength = reader.read(8); // text length
+      const chunkLength = reader.read(8);
       bytesRead++;
-
-      while (chunkLength--) {
-        parts.push(reader.read(8));
-        bytesRead++;
+      // A character-string whose length runs past the end of RDATA would
+      // make us read into the next record. Skip the remainder of the rdata
+      // before throwing so the next record decodes from the correct offset
+      // instead of cascading the error through every following RR.
+      if (chunkLength > length - bytesRead) {
+        const remaining = length - bytesRead;
+        for (let i = 0; i < remaining; i++) reader.read(8);
+        throw new Error(
+          `TXT decode: character-string of ${chunkLength} octets overruns ` +
+            `RDATA (${remaining} octets remaining)`,
+        );
+      }
+      const bytes = Buffer.alloc(chunkLength);
+      for (let i = 0; i < chunkLength; i++) {
+        bytes[i] = reader.read(8);
       }
+      bytesRead += chunkLength;
+      strings.push(bytes.toString('utf8'));
     }
-
-    this.data = Buffer.from(parts).toString('utf8');
+    this.data = strings;
     return this;
   },
   encode: function (record, writer) {

packet.js

@@ -317,7 +317,8 @@ test('Packet#encode', function () {
     type: Packet.TYPE.TXT,
     class: Packet.CLASS.IN,
     ttl: 300,
-    data: '#v=spf1 include:_spf.google.com ~all',
+    // TXT data is an array of <character-string> items (RFC 1035 §3.3.14).
+    data: ['#v=spf1 include:_spf.google.com ~all'],
   });
 
   assert.deepEqual(Packet.parse(response.toBuffer()), response);
@@ -343,10 +344,7 @@ test('Packet#encode array of character strings', function () {
     data: dkim,
   });
 
-  assert.equal(
-    Packet.parse(response.toBuffer()).answers[0].data,
-    dkim.join(''),
-  );
+  assert.deepEqual(Packet.parse(response.toBuffer()).answers[0].data, dkim);
 });
 
 test('EDNS.ECS#encode', function () {
@@ -511,9 +509,9 @@ test('Resource#TXT round-trip single string', function () {
     type: Packet.TYPE.TXT,
     class: Packet.CLASS.IN,
     ttl: 300,
-    data: 'hello world',
+    data: 'hello world', // encoder normalizes string → [string]
   });
-  assert.equal(out.data, 'hello world');
+  assert.deepEqual(out.data, ['hello world']);
 });
 
 test('Resource#TXT round-trip with utf-8', function () {
@@ -524,7 +522,57 @@ test('Resource#TXT round-trip with utf-8', function () {
     ttl: 300,
     data: 'café résumé 日本',
   });
-  assert.equal(out.data, 'café résumé 日本');
+  assert.deepEqual(out.data, ['café résumé 日本']);
+});
+
+test('Resource#TXT preserves character-string boundaries', function () {
+  // SPF/DKIM-style multi-string TXT records must not be merged on decode.
+  const chunks = ['part-one ', 'part-two ', 'part-three'];
+  const out = roundTripAnswer({
+    name: 'multi.example',
+    type: Packet.TYPE.TXT,
+    class: Packet.CLASS.IN,
+    ttl: 60,
+    data: chunks,
+  });
+  assert.deepEqual(out.data, chunks);
+});
+
+test('Resource#TXT decode rejects character-string overruns RDATA', function () {
+  // Hand-built TXT rdata: rdlength=5, but the first character-string claims
+  // length 10. Direct caller should see a thrown error.
+  const reader = new Packet.Reader(Buffer.from([0x0a, 0x61, 0x62, 0x63, 0x64]));
+  assert.throws(
+    () => Packet.Resource.TXT.decode.call({}, reader, 5),
+    /overruns RDATA/,
+  );
+});
+
+test('Resource#TXT decode error does not cascade to following RRs', function () {
+  // A malformed TXT record (rdlength=5, chunkLength=10) must consume its
+  // declared rdlength before throwing, so the A record after it parses
+  // cleanly. Without the bounds check the reader would be misaligned and
+  // the second record would be lost.
+  const pkt = Buffer.from([
+    // header: id=1, ancount=2, others 0
+    0x00, 0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00, 0x00,
+    // answer 1: name "t", TYPE=TXT, CLASS=IN, TTL=60, RDLENGTH=5,
+    //           rdata = [chunkLen=10, 4 bogus bytes]  ← chunkLen overruns
+    0x01, 0x74, 0x00, 0x00, 0x10, 0x00, 0x01, 0x00, 0x00, 0x00, 0x3c, 0x00,
+    0x05, 0x0a, 0x61, 0x62, 0x63, 0x64,
+    // answer 2: name "a", TYPE=A, CLASS=IN, TTL=60, RDLENGTH=4, 192.0.2.7
+    0x01, 0x61, 0x00, 0x00, 0x01, 0x00, 0x01, 0x00, 0x00, 0x00, 0x3c, 0x00,
+    0x04, 0xc0, 0x00, 0x02, 0x07,
+  ]);
+  const parsed = Packet.parse(pkt);
+  assert.equal(
+    parsed.answers.length,
+    1,
+    'the malformed TXT should be dropped, leaving only the A record',
+  );
+  assert.equal(parsed.answers[0].type, Packet.TYPE.A);
+  assert.equal(parsed.answers[0].name, 'a');
+  assert.equal(parsed.answers[0].address, '192.0.2.7');
 });
 
 test('Resource#SOA round-trip', function () {

test/packet.js

@@ -151,7 +151,7 @@ test('server/udp-tcp#simple-request-async-response', async () => {
   const tcpClient = TCPClient({ dns: '127.0.0.1', port: servers.tcp.port });
   const udpClient = UDPClient({ dns: '127.0.0.1', port: servers.udp.port });
   const expected = [
-    { name: 'test.com', ttl: 300, type: 16, class: 1, data: 'Hello World' },
+    { name: 'test.com', ttl: 300, type: 16, class: 1, data: ['Hello World'] },
   ];
   assert.deepEqual((await tcpClient('test.com')).answers, expected);
   assert.deepEqual((await udpClient('test.com')).answers, expected);
@@ -456,7 +456,7 @@ test('server/doh#POST end-to-end', async () => {
       type: Packet.TYPE.TXT,
       class: Packet.CLASS.IN,
       ttl: 60,
-      data: 'post-ok',
+      data: ['post-ok'],
     });
     send(response);
   });
@@ -499,7 +499,7 @@ test('server/doh#POST end-to-end', async () => {
   });
   const parsed = Packet.parse(body);
   assert.equal(parsed.answers.length, 1);
-  assert.equal(parsed.answers[0].data, 'post-ok');
+  assert.deepEqual(parsed.answers[0].data, ['post-ok']);
   server.close();
 });