Via Inspect

Parse and diagnose HTTP headers and routing signals in your browser. No input is sent to a server. Use it for first-pass observation-gap troubleshooting.

Status

Runs in your browser. No input is sent to a server. Use this as a first-pass diagnostic step.

How to use

Paste Via and click “Parse”. It splits and lists entries (accepts Via: lines, multi-line paste, or full response headers).

Notes (this tool)

  • Comments are free-form and should not be used to identify vendors definitively; treat them as hints.

About this page

What does this tool do?

Split the Via header into hop entries (protocol, received-by, comments) and display them as a list.

Useful for path diagnostics like “Am I behind a CDN?” or “Where are headers being rewritten?”

Via basics

  • Via is used by intermediaries (proxies/gateways) to indicate that a message passed through them.
  • With multiple hops, it becomes a comma-separated list of entries.
  • Some systems omit or hide Via depending on configuration.

Syntax (how to read)

Via is a comma-separated list of entries like “protocol received-by (comment)”. The protocol part may be omitted.

  • Via: 1.1 vegur
  • Via: 1.0 fred, 1.1 p.example.net
  • Via: 1.1 proxy (cache)

Glossary (terms used on this page)

  • Intermediary: a proxy/CDN/gateway between client and origin.
  • Hop: one intermediate step; multiple Via entries usually indicate multiple hops.
  • Rewrite: intermediaries adding/removing/modifying headers.

Why it helps (path visibility)

Header/caching behavior depends heavily on which layers you traverse. Via provides hints that speed up root-cause analysis.

  • Even if origin headers look correct, intermediaries may rewrite them
  • CORS or cookie attributes can be modified/stripped by intermediaries

Via alone may not be enough to identify your path. Combining multiple headers improves confidence.

  • Age: seconds in shared cache (strong CDN/proxy hint)
  • Forwarded / X-Forwarded-For / X-Real-IP: forwarding chain / client info hints
  • Server / Via comments: treat as hints due to variability
  • Vary: useful when suspecting cache key differences/fragmentation

Common pitfalls

  • No Via ≠ no intermediaries (it can be hidden or signaled via other headers)
  • Do not identify vendors solely from Via (comments are optional and free-form)
  • Multiple entries often mean a longer path; you need comparisons to find where behavior changes

How to test (compare to find where it changes)

The key is comparing headers between path A and path B—e.g., CDN vs origin-bypass (or different domains/paths).

  • curl -I https://example.com/ (normal path)
  • curl -I https://origin.example.com/ (origin-bypass path)
  • Compare Via/Age/Cache-Control/ETag/Expires to infer which layer is responsible

Debugging workflow (recommended)

  • Paste response headers into Response Headers Parser and extract Via
  • Split Via here to see hop count and comments
  • Combine with Cache-Control/Expires/Age/ETag/Last-Modified to infer which layer is responsible

Troubleshooting checklist by symptom

  • Headers unexpected: check Via (path) and Age (shared cache) and suspect which layer rewrites them
  • Prod-only issues: prod has multiple Via entries while staging has none/fewer → path difference likely
  • Caching issues: check Cache-Control/Expires/Age alongside Via
  • Response Headers Parser
  • Request Headers Parser
  • Cache-Control Inspect / Expires Inspect / Age Inspect
  • ETag Inspect / Last-Modified Inspect
  • Vary Inspect

What this tool does

  • Split Via entries by commas
  • Extract protocol / received-by / comments and list them
  • Extract Via lines from full response headers

Operational notes

  • Intermediaries may rewrite headers. Compare captures from equivalent points.
  • Confirm final decisions with server logs and configuration such as trusted proxy and routing.

Referenced specs

  • RFC 9110 (HTTP Semantics)
  • MDN: Via

FAQ

Is Via a security problem?

It can hint at internal topology. If that’s a concern, control whether it’s added/exposed as a policy.

I see CDN-like behavior but no Via

Some CDNs don’t expose Via. Use Age, vendor-specific headers, DNS/cert clues, and origin vs edge comparisons.

References

  1. RFC 9110
  2. MDN: Via
  3. MDN: HTTP caching

These links are generated from site_map rules in recommended diagnostic order.

  1. Forwarded Inspect — Parse Forwarded to inspect forwarding path data
  2. X-Forwarded-For Inspect — Parse X-Forwarded-For/X-Real-IP to inspect client chain
  3. X-Forwarded-Proto Inspect — Parse X-Forwarded-Proto/Host to verify external URL inference