fix(utils/ipaddr): render the unspecified address binary as "::" - #4998

What this fixes

convertIPv6BinaryToString(0n) returns a single ":" instead of the canonical "::" for the IPv6 unspecified address (all-zero groups).

import { convertIPv6BinaryToString, convertIPv6ToBinary } from './src/utils/ipaddr'

convertIPv6BinaryToString(0n) // => ":"   (expected "::")

The resulting string is not a valid IPv6 address (net.isIP(':') is 0) and it does not round-trip: feeding it back into convertIPv6ToBinary throws Invalid IPv6 address: :.

Root cause

For the all-zero value every one of the 8 groups is "0", so the longest zero-run spans the whole address. The compression logic replaces that run with a single ":" placeholder via sections.splice(0, 8, ':'), leaving sections === [':']. Joining that with ":" yields ":", and the trailing value.replace(/:{2,}/, '::') cannot help because there is only one colon to collapse — the regex only matches runs of two or more colons.

The existing IPv4-mapped case is already handled with an early return at the top of the function; the all-zero case has the same "every group compresses" property and needs the same treatment. The recently merged RFC 5952 work (#4971 for single zero groups, #4973 for expanding ::) guards maxZeroEnd - maxZeroStart > 1, which still routes the all-zero value through the broken 8-group splice.

The fix

Add a 5-line early return for 0n in convertIPv6BinaryToString, mirroring the existing IPv4-mapped early return:

if (ipV6 === 0n) {
  return '::'
}

This also keeps the ip-restriction middleware's string rule map consistent: the unspecified address now canonicalizes to "::" rather than the bogus key ":", so a rule such as denyList: ['::'] is registered under the correct key.

Tests

Added a focused regression test asserting "::" output and a successful round-trip:

it('Should compress the unspecified address (all zero groups) to "::"', () => {
  expect(convertIPv6BinaryToString(0n)).toBe('::')
  expect(convertIPv6BinaryToString(convertIPv6ToBinary('::'))).toBe('::')
  expect(convertIPv6ToBinary(convertIPv6BinaryToString(0n))).toBe(0n)
})

Verified locally:

• tsc --noEmit — exit 0
• vitest run src/utils/ipaddr.test.ts — 41 passed
• prettier --check on the touched files — clean
• eslint on the touched files — clean

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code