How to check if a site uses a CDN

Read the response headers with curl -s -D - -o /dev/null <url> | grep -i -E 'server|via|x-cache|cf-ray', then look for a CNAME with nslookup -type=CNAME <host> 8.8.8.8. A vendor name in either place is evidence. Neither absence is proof, because both signals can be switched off.

Why check this

A tester needs this answer before writing any cache or latency test, because a CDN changes what the numbers mean. Response times come from an edge rather than the origin, headers may be added or stripped in flight, and an origin fix can sit behind a warm cache for as long as the freshness window lasts. The failure it prevents is a bug filed against the application for a response the origin never produced. The same check confirms whether a newly configured CDN is actually in the request path.

Prerequisites

• curl 7.0 or later. Any build reads headers. See the curl manual.
• nslookup, present on Windows and on most Linux images. dig does the same job where it is installed. This machine has no dig, so every command below uses nslookup.
• A public resolver address such as 8.8.8.8, so the answer does not come from a corporate resolver that rewrites records.

Steps

1. 1

   Step 1.

   Read the headers most often added by an edge. server, via, x-cache and vendor-specific request identifiers all live in the response.

   curl -s -D - -o /dev/null 'https://www.cloudflare.com/' | grep -i -E '^server|^cf-ray|^cf-cache-status|^via|^x-cache'
   

   cf-ray: a398fcd37812027a-WAW
   cf-cache-status: HIT
   server: cloudflare

   Three separate signals: a vendor name in server, a per-request identifier, and a cache status header. The -WAW suffix names the point of presence that answered.

2. 2

   Step 2.

   Run the same read against another host. A different vendor leaves a different set of headers.

   curl -s -D - -o /dev/null 'https://developer.mozilla.org/en-US/' | grep -i -E '^server|^via|^x-served-by|^x-cache'
   

   via: 1.1 google, 1.1 varnish, 1.1 varnish, 1.1 varnish
   server: Google Frontend
   x-served-by: cache-par-lfpb1150054-PAR, cache-par-lfpb1150054-PAR, cache-par-lfpb1150054-PAR, cache-fra-eddf8230067-FRA
   x-cache: MISS, MISS, HIT
   x-cache-hits: 0, 0, 1

   server names the origin platform while via lists four hops, three of them Varnish. The x-served-by entries name cache nodes in Paris and Frankfurt, so the request crossed an edge network before reaching that origin.

3. 3

   Step 3.

   Ask DNS. A hostname pointing at a vendor-operated name through a CNAME is the clearest signal available.

   nslookup -type=CNAME developer.mozilla.org 8.8.8.8 | grep -E 'canonical name|primary name server'
   

   developer.mozilla.org	canonical name = mozilla.map.fastly.net

   The target domain names the vendor. This is the one check that survives a server stripping its own headers.

4. 4

   Step 4.

   Run the same lookup on a host you already know sits behind an edge. The result is the reason a negative answer means nothing.

   nslookup -type=CNAME www.cloudflare.com 8.8.8.8 | grep -E 'canonical name|primary name server'
   

   	primary name server = jule.ns.cloudflare.com

   No CNAME exists. Step 1 already proved this host is served from an edge, and DNS shows an A record instead because the vendor also runs the authoritative nameservers. Only the nameserver name hints at anything here.

5. 5

   Step 5.

   Read the headers of a host with no edge in front. The absence of every signal at once is what a direct origin looks like.

   curl -s -D - -o /dev/null 'https://httpbin.org/' | grep -i -E '^server|^via|^x-cache|^cf-ray|^age'
   

   server: gunicorn/19.9.0

   One header, and it names an application server rather than a network. No via, no cache status, no age.

How to read the result

| What you see | What it means | What to do | | --- | --- | --- | | A vendor name in server | An edge is in the path and does not hide it | Treat every timing you measure as an edge timing, not an origin one. | | via listing several hops | The request passed through proxies or caches | Count the hops. Each one can add or strip headers between origin and client. | | x-cache or cf-cache-status present | A cache is answering | Move on to the hit or miss procedure below. | | age present with no vendor header | A cache is in the path and stays anonymous | age is standard and sufficient to prove storage, whatever the vendor. | | A CNAME to a vendor domain | The hostname is delegated to that network | The strongest single signal, because DNS is outside the application's control. | | No CNAME and no vendor headers | Nothing was found | Not the same as nothing being there. Step 4 shows a host on an edge with no CNAME. |

Common mistakes

What to check next

• How to check if CDN cache is hit or miss: the next question once an edge is confirmed.
• How to check cache-control header: the directives that edge is obeying, or overriding.
• How to check ETag header: edges rewrite or drop validators, so check the one the client actually receives.
• How to check HTTP response headers with curl: the header-printing flags used in every step above.

FAQ

How to check which cdn a website uses?

Combine step 1 and step 3. A vendor name in server, via or a proprietary header, together with a CNAME to a domain that vendor operates, is a strong case. Either signal on its own is inference and belongs in a report as such.

How to check if a cdn is working?

Confirm the edge is in the path with steps 1 to 3, then request the same URL twice and watch the cache status header change from a miss to a hit. Presence alone says nothing about whether anything is being cached.

Can a site hide that it uses a CDN?

Yes. Vendor headers can be stripped, server can be rewritten to any string, and DNS can resolve straight to an anycast address with no CNAME. age is the hardest signal to remove, because it is required of any cache that stores a response.

Does a CDN change what my tests measure?

It changes almost all of it. Response times come from an edge, headers may be added or removed in flight, and an origin fix stays invisible until the freshness window closes. Note the CDN in the test report alongside the numbers.

Verified