Cloudflare not forwarding ETag header?

Question

I have a site (e.g. http://example.com) that uses Cloudflare. Nothing special, no weird page rules, no weird settings, free plan.

When I go to http://example.com, I get this response for the GET http://example.com request:

Cache-Control:no-cache, must-revalidate
CF-RAY:2b8d0490837f2828-SJC
Connection:keep-alive
Content-Encoding:gzip
Content-Type:text/html; charset=UTF-8
Date:Sun, 26 Jun 2016 01:52:05 GMT
Expires:0
Pragma:no-cache
Server:cloudflare-nginx
Transfer-Encoding:chunked
Vary:Accept-Encoding
X-Powered-By:Express

Notice that the response has no ETag header, even though it's enabled on my origin server.

When I go to the origin server directly, e.g. GET http://01.23.456.789/, I get this response:

Accept-Ranges:bytes
Cache-Control:no-cache, must-revalidate
Connection:keep-alive
Date:Sun, 26 Jun 2016 01:56:53 GMT
ETag:W/"4cf8-1558a5557a0"
Expires:0
Pragma:no-cache
X-Powered-By:Express

Does anyone know why Cloudflare wouldn't be forwarding the ETag header? How can I make Cloudflare forward the ETag? Cloudflare isn't caching the GET http://example.com request by default since it's an HTML page.

Answer 1

I spoke with a Cloudflare Tech Support Engineer and he helped fixed my issue. For anyone in the future who might have this same problem, here's what fixed it:

I had Email Obfuscation enabled for the URL in question (that was getting its ETag header removed). According to https://support.cloudflare.com/hc/en-us/articles/218505467-Does-CloudFlare-support-ETag-headers-,

Email Obfuscation will need to be disabled, otherwise the ETag headers
will be removed from the response. Email Obfuscation modifies the code
significantly enough it cannot be considered semantically equivalent.

Disabling Email Obfuscation still didn't fix it, though. Turns out I had an app enabled that would modify the HTML, so the ETag had to be invalidated. In the end, setting a Page Rule with Disable Security, Disable Apps, Disable Performance for that URL did the trick.