MDS072 link ready

MDS072: external-link-check

Probe external http and https URLs; flag any returning a transport error or 4xx/5xx response.

This rule closes the gap with gomarklint’s external-link check (issue #47). It is off by default and opt-in, like MDS068 (link-style). Network I/O has no place on the default mdsmith check hot path. It reads the shared links: config block — the same block MDS027 and MDS068 read. So external-skip, external-timeout, and external-rate-limit sit beside site-root and style per kind.

The rule checks inline links ([text](url)), autolinks (<https://example.com>), and images (![alt](url)). It probes over the network only on native builds. The WebAssembly engine cannot reach the network, so it treats every URL as not-probed and emits no MDS072 diagnostics. It never reports a URL as healthy on faith. A future host bridge will let a WASM host such as the Obsidian plugin supply real probe results.

# Settings

Setting	Type	Default	Description
`links.external-skip`	list	`[]`	Regex patterns; a matching URL is not probed
`links.external-timeout`	string	`"5s"`	Per-request timeout as a Go duration
`links.external-rate-limit`	int	`10`	Maximum concurrent in-flight requests; minimum `1`

Each external URL is probed once per run with an HTTP HEAD request. A URL whose HEAD returns 405 (Method Not Allowed) is retried with GET. Redirects are followed; a final 2xx or 3xx passes. Results are cached per URL for the run, so the same URL across many files costs one request. A non-positive external-timeout falls back to 5s; a rate limit below 1 clamps to 1.

# Config

Enable with defaults (5s timeout, 10 concurrent requests):

rules:
  external-link-check: true

Skip intranet and example hosts, tighten the timeout, cap concurrency:

rules:
  external-link-check:
    links:
      external-skip:
        - "^https?://localhost"
        - "^https?://127\\."
      external-timeout: 10s
      external-rate-limit: 5

Disable:

rules:
  external-link-check: false

# Examples

# Good – no external URLs to probe

# No External Links

This file links to a [sibling document](good/no-external-links.md). It also
links to an [in-page anchor](#no-external-links). Neither is an external
URL. So the rule finds nothing to probe. It reports no diagnostics.

The fixture suite omits a bad example on purpose. A fixture with a live broken URL would hit the network on every go test run. The HTTP behaviour lives in rule_test.go. That test drives a local httptest.NewServer. It covers the 200, 404, 405-then-GET, transport-error, and cache-hit paths.

# Diagnostics

Condition	Message
URL returns 4xx or 5xx	`external URL returned HTTP <code>: <url>`
URL unreachable (transport error)	`external URL unreachable: <url> (<error>)`

# See also

MDS027 cross-file-reference-integrity — validates local file and heading links; shares the links: block.
MDS068 link-style — enforces link path, extension, and form style; shares links:.

# Meta-Information

ID: MDS072
Name: external-link-check
Status: ready
Default: disabled, opt-in. Network I/O keeps it off the hot path.
Fixable: no
Implementation: source
Category: link
gomarklint: external-link

View source on GitHub