Retrieve the correct header for 'Accept-Encoding:' used by the compressed remote resource.

From: Hongyi Zhao via curl-users <curl-users_at_cool.haxx.se>
Date: Thu, 28 Jan 2021 10:13:01 +0800

Hi,

See my following testings about interacting with compressed remote
resources using curl:

werner_at_X10DAi:~$ curl --compressed -I --etag-save 111
https://www.thepolyglotdeveloper.com/css/custom.min.css
HTTP/2 200
accept-ranges: bytes
cache-control: public, max-age=0, must-revalidate
content-type: text/css; charset=UTF-8
date: Wed, 27 Jan 2021 15:05:06 GMT
etag: "95b8df8e4f25366b91b9ed4850c8269e-ssl-df"
strict-transport-security: max-age=31536000
content-encoding: br
age: 39361
content-length: 1450
server: Netlify
vary: Accept-Encoding
x-nf-request-id: 663ffd26-7b69-4c59-b0bb-50e6de8c7fa1-3429084

werner_at_X10DAi:~$ cat 111
95b8df8e4f25366b91b9ed4850c8269e-ssl-df

werner_at_X10DAi:~$ curl -I --etag-compare 111
https://www.thepolyglotdeveloper.com/css/custom.min.css
HTTP/2 200
accept-ranges: bytes
cache-control: public, max-age=0, must-revalidate
content-length: 6285
content-type: text/css; charset=UTF-8
date: Thu, 28 Jan 2021 01:59:35 GMT
etag: "95b8df8e4f25366b91b9ed4850c8269e-ssl"
strict-transport-security: max-age=31536000
age: 122
server: Netlify
x-nf-request-id: c731d3d2-19ee-4b03-91ec-b96e8d06464c-8953234

werner_at_X10DAi:~$ curl -I --etag-compare 111 -H 'Accept-Encoding: gzip,
deflate, br' https://www.thepolyglotdeveloper.com/css/custom.min.css
HTTP/2 304
date: Thu, 28 Jan 2021 02:02:00 GMT
etag: "95b8df8e4f25366b91b9ed4850c8269e-ssl-df"
cache-control: public, max-age=0, must-revalidate
server: Netlify
vary: Accept-Encoding
x-nf-request-id: ae8bba17-c715-4a61-8e00-0a85d77334e2-5196200


As you can see, I must supply appropriate 'Accept-Encoding:' string to
the server combined with the saved etag for obtaining the correct
response from the server, i.e., the 304 code for denoting that the
remote resource hasn't been changed/updated since I've retrieved the
etag last time.

But I still can't figure out how to obtain the correct sting value for
the 'Accept-Encoding:' used by the header for passing to the remote
server. Any hints will be highly appreciated.

Regards

-- 
Assoc. Prof. Hongyi Zhao <hongyi.zhao_at_gmail.com>
Theory and Simulation of Materials
Hebei Polytechnic University of Science and Technology engineering
NO. 552 North Gangtie Road, Xingtai, China
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Received on 2021-01-28