fix: RFC correctness drive updates

- fix(packet): Name decode rejects pointer cycles (RFC 1035)
- fix(packet): EDNS exposes extendedRcode/version/doFlag; udpPayloadSize configurable (RFC 6891)
- fix(packet): Header initializes ancount; AD/CD bits split from Z (RFC 4035)
- fix(packet): ECS encoder truncates address to ceil(prefix/8) octets, adds IPv6 family (RFC 7871)

Author: msimerson

CHANGELOG.md

@@ -4,10 +4,11 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/).
 
 ### Unreleased
 
-- fix(packet): Name decode rejects pointer cycles (RFC 1035)
-- fix(packet): EDNS exposes extendedRcode/version/doFlag; udpPayloadSize configurable (RFC 6891)
-- fix(packet): Header initializes ancount; AD/CD bits split from Z (RFC 4035)
-- fix(packet): ECS encoder truncates address to ceil(prefix/8) octets, adds IPv6 family (RFC 7871)
+- fix(packet): IPv6 `::` compression for leading-zero address #123
+- fix(packet): Name decode rejects pointer cycles (RFC 1035) #124
+- fix(packet): EDNS exposes extendedRcode/version/doFlag #124
+- fix(packet): Header initializes ancount; AD/CD bits split from Z (RFC 4035) #124
+- fix(packet): ECS encoder truncates address, adds IPv6 (RFC 7871) #124
 - feat(server): PROXY protocol v1/v2 support #122
 
 ### 2.2.1 - 2026-05-25

packet.js

@@ -5,10 +5,29 @@ const BufferWriter = require('./lib/writer');
 
 const debug = debuglog('dns2');
 
-const toIPv6 = buffer => buffer
-  .map(part => (part > 0 ? part.toString(16) : '0'))
-  .join(':')
-  .replace(/\b(?:0+:){1,}/, ':');
+// Canonical IPv6 text form per RFC 5952:
+//   - lower case hex, no leading zeros per group  (handled by toString(16))
+//   - the longest run of >= 2 zero groups is replaced with "::"
+//   - on ties, the first such run is chosen
+//   - a single zero group is NOT compressed
+const toIPv6 = buffer => {
+  const segments = buffer.map(part => (part > 0 ? part.toString(16) : '0'));
+  let bestStart = -1; let bestLen = 0;
+  let curStart = -1; let curLen = 0;
+  for (let i = 0; i < segments.length; i++) {
+    if (segments[i] === '0') {
+      if (curLen === 0) curStart = i;
+      curLen++;
+      if (curLen > bestLen) { bestLen = curLen; bestStart = curStart; }
+    } else {
+      curLen = 0;
+    }
+  }
+  if (bestLen < 2) return segments.join(':');
+  const before = segments.slice(0, bestStart).join(':');
+  const after = segments.slice(bestStart + bestLen).join(':');
+  return `${before}::${after}`;
+};
 
 const fromIPv6 = (address) => {
   const digits = address.split(':');

test/packet.js

@@ -81,6 +81,28 @@ test('Package#toIPv6', function() {
   assert.equal(Packet.toIPv6([ 9734, 18176, 12552, 0, 0, 0, 44098, 10984 ]), '2606:4700:3108::ac42:2ae8');
 });
 
+test('Package#toIPv6 RFC 5952 — leading-zero addresses', function() {
+  assert.equal(Packet.toIPv6([ 0, 0, 0, 0, 0, 0, 0, 1 ]), '::1');
+  assert.equal(Packet.toIPv6([ 0, 0, 0, 0, 0, 0, 0, 0 ]), '::');
+  assert.equal(Packet.toIPv6([ 0, 0, 0, 0, 0, 0xffff, 0xc000, 0x0201 ]), '::ffff:c000:201');
+});
+
+test('Package#toIPv6 RFC 5952 — trailing-zero addresses', function() {
+  assert.equal(Packet.toIPv6([ 1, 0, 0, 0, 0, 0, 0, 0 ]), '1::');
+  assert.equal(Packet.toIPv6([ 0x2001, 0xdb8, 0, 0, 0, 0, 0, 0 ]), '2001:db8::');
+});
+
+test('Package#toIPv6 RFC 5952 — single zero group is not compressed', function() {
+  // §4.2.2: "::" MUST NOT be used to shorten just one 16-bit 0 field.
+  assert.equal(Packet.toIPv6([ 1, 0, 1, 1, 1, 1, 1, 1 ]), '1:0:1:1:1:1:1:1');
+});
+
+test('Package#toIPv6 RFC 5952 — first run wins on tie', function() {
+  // §4.2.3: when there is more than one run of equal maximum length,
+  // the first is shortened.
+  assert.equal(Packet.toIPv6([ 1, 0, 0, 1, 0, 0, 1, 1 ]), '1::1:0:0:1:1');
+});
+
 test('Package#fromIPv6', function() {
   assert.deepEqual(Packet.fromIPv6('2a04:4e42:200::323'), [
     '2a04', '4e42', '0200', '0', '0', '0', '0', '0323' ]);