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Slingshot

Overview

@lastshotlabs/slingshot-embeds is Slingshot’s server-side unfurling package. It fetches a URL on the server, validates it against SSRF rules, parses Open Graph and related metadata, and returns a normalized preview payload your app can render in chat, feeds, or editors.

When To Use It

Section titled “When To Use It”

Use this package when your app needs:

  • link previews for user-submitted URLs
  • server-side metadata extraction so browser clients never hold the fetch logic
  • a single internal endpoint that can enforce domain allow/block rules

Do not use it for arbitrary HTML scraping. It is optimized for preview metadata, not full-page extraction pipelines.

Minimum Setup

Section titled “Minimum Setup”

The package is effectively standalone. Register the plugin and call its unfurl endpoint from your UI or app service.

All config is optional. The defaults are:

  • mountPath: '/embeds'
  • cacheTtlMs: 300_000
  • cacheMaxEntries: 500
  • timeoutMs: 5000
  • maxResponseBytes: 1_048_576

What You Get

Section titled “What You Get”

The plugin mounts:

  • POST {mountPath}/unfurl

The route accepts a JSON body with { url: string } and returns normalized metadata such as:

  • title
  • description
  • image
  • siteName
  • favicon
  • type

It also:

  • validates protocols and blocks obvious private or reserved hosts
  • applies optional allow-list and block-list domain policy
  • uses DNS resolution checks to reduce DNS-rebinding SSRF attacks
  • caches successful unfurls in memory for the configured TTL

Common Customization

Section titled “Common Customization”

The main knobs are:

  • allowedDomains: restrict unfurling to trusted domains
  • blockedDomains: always reject known-problem domains
  • timeoutMs: keep external fetch latency under control
  • maxResponseBytes: cap how much HTML the server will read
  • mountPath: align the endpoint with your app’s route layout

If you need to alter behavior, start in:

  • src/types.ts for config defaults and response shape
  • src/lib/ssrfGuard.ts for URL validation
  • src/lib/unfurl.ts for fetch and parsing behavior
  • src/lib/htmlParser.ts for metadata extraction rules

Gotchas

Section titled “Gotchas”
  • This package is safe-by-default only if you keep the SSRF guard intact. Do not bypass it to make local or internal network URLs work.
  • Domain allow-lists are exact-domain or subdomain matches. Be deliberate about whether example.com should also allow foo.example.com.
  • The cache is in-memory and per process. It reduces repeat fetches, but it is not a shared preview store across instances.
  • The endpoint returns 502 when remote fetches fail. That is expected proxy behavior, not an application bug.

Key Files

Section titled “Key Files”
  • src/index.ts
  • src/plugin.ts
  • src/types.ts
  • src/lib/unfurl.ts
  • src/lib/htmlParser.ts
  • src/lib/ssrfGuard.ts